aerosol flow tube study of the heterogeneous interaction

Aerosol flow tube study of the heterogeneous interaction between submicron mineral dust particles and gaseous

nitric acid

Inauguraldissertation Der Philosophisch-naturwissenschaftlichen Fakultät

Der Universität Bern

Vorgelegt von Alexander Vlasenko

von Chelyabinsk / Russland

Leiter der Arbeit:

Prof. H.W. Gäggeler Departement für Chemie und Biochemie

Universität Bern

Von der Philosophisch-naturwissenschaftlichen Fakultät angenommen.

Bern, 18.01.2006 Der Dekan:

Prof. Dr. Paul Messerli

Contents ABSTRACT ................................................................................................................................................... I ZUSAMMENFASSUNG............................................................................................................................III LIST OF FIGURES......................................................................................................................................V LIST OF TABLES.................................................................................................................................... VII LIST OF ABBREVIATIONS.................................................................................................................... IX

1 INTRODUCTION ................................................................................................................................. 1 1.1 Nitric acid in atmosphere........................................................................................................ 3 1.2 Mineral dust in atmosphere .................................................................................................... 5 1.3 Importance of nitric acid and mineral dust interaction for climate ......................................... 8 1.4 Motivation ............................................................................................................................ 11 1.5 References ............................................................................................................................ 12

2 DRY DISPERSION METHOD FOR GENERATION OF SUBMICRON ARIZONA TEST DUST AEROSOL. CHEMICAL AND HYGROSCOPIC PROPERTIES OF THE ATD PARTICLES ....................................................... 19

2.1 Abstract................................................................................................................................. 20 2.2 Introduction .......................................................................................................................... 21 2.3 Experimental......................................................................................................................... 23 2.4 Results and discussion .......................................................................................................... 27 2.5 Conclusions .......................................................................................................................... 35 2.6 References ............................................................................................................................ 36

3 NITRIC ACID UPTAKE ON MINERAL DUST AEROSOL PARTICLES AT DIFFERENT CONDITIONS ............ 39 3.1 Abstract................................................................................................................................. 40 3.2 Introduction .......................................................................................................................... 41 3.3 Experimental......................................................................................................................... 42 3.4 Results and discussion .......................................................................................................... 46 3.5 Atmospheric Implications..................................................................................................... 61 3.6 References ............................................................................................................................ 62

4 KINETIC MODELLING OF THE HETEROGENEOUS REACTION BETWEEN GASEOUS NITRIC ACID AND ARIZONA TEST DUST AEROSOL PARTICLES............................................................................................... 65

4.1 Abstract................................................................................................................................. 66 4.2 Introduction .......................................................................................................................... 67 4.3 Experimental section ............................................................................................................ 69 4.4 Results and discussion .......................................................................................................... 71 4.5 Atmospheric implication ...................................................................................................... 80 4.6 References ............................................................................................................................ 82

5 FINAL DISCUSSION AND RECOMMENDATIONS FOR FUTURE STUDIES................................................ 85 6 APPENDIX ....................................................................................................................................... 89

6.1 Production of N. PROTRAC facility at PSI13 ....................................................................... 89 6.2 SMPS calibration with PSL.................................................................................................. 90 6.3 Uptake of CH COOH on ATD and Na CO aerosol particles3 2 3 .............................................. 90 6.4 Uptake of HNO on ATD measured with the WEDD-AC system3 ....................................... 91 6.5 Technical drawings and pictures of experimental setups...................................................... 93

ACKNOWLEDGEMENT ......................................................................................................................... 95 RESUMÉ..................................................................................................................................................... 96

Abstract I

Abstract Mineral dust can affect gas phase chemistry in the troposphere by providing reactive sites for heterogeneous reactions. In the experimental laboratory studies presented here the HNO3 uptake by airborne mineral dust particles was investigated. A technique has been developed to produce aerosol particles from dust powders followed by separation of particles according to their size. To our knowledge, this is the first time such a system has been used to study heterogeneous reactions between aerosol and trace gas species. The main advantage of this method is the ability to produce airborne solid particles by dry dispersion from powders of various chemical compositions, i.e., authentic dust or chemically well-defined mineral oxides. In this study the aerosol, resuspended from the Arizona Test Dust powder, is used as a proxy for atmospheric dust. The kinetics of the heterogeneous reactions was investigated in the flow tube reactor by online measurement of particulate and gas-phase reaction products. Most experiments were carried out using the 13N tracer technique, where HNO3 was labeled with the radioactive isotope 13N (τ1/2 10.0 min) which was produced at PROTRAC facility at the Paul Scherrer Institute. The reactants and products of heterogeneous reaction (i.e., HNO3, HONO, NO2, particulate HNO3) were separated in a parallel-plate denuder train coupled with an online γ-spectroscopy detection. The experiments were performed at temperature and pressure conditions typical to real troposphere (T 298 K, p ~1 atm, RH 6-70%). The uptake of nitric acid from gas phase was found to depend on HNO3 and H2O concentrations in the gas phase. Experiments showed that the uptake coefficient increased five times, from 0.02 to 0.11, by changing the RH from 12% to 73%. The results of kinetic simulations suggest that the reaction is promoted at higher RH as the number of the reactive sites on mineral dust surfaces increases with increasing relative humidity. A reaction mechanism is suggested to describe the heterogeneous interaction, involving gas-surface diffusive transport, Langmuir type adsorption and surface reaction. Several important kinetic parameters are derived which should be used in atmospheric chemistry models to improve the reliability of the effects of relative humidity on dust aging. Theses results strongly suggest that mineral aerosol can redistribute nitrate from the gaseous to the particulate phases, modifying tropospheric photochemical cycles involving NOy.

Zusammenfassung III

Zusammenfassung Mineralstaubaerosole beeinflussen die Gasphasenchemie der Troposphäre, da sie eine Oberfläche für heterogene Reaktionen darstellen. In den hier präsentierten Laborversuchen wurde die Aufnahmekinetik von gasförmiger Salpetersäure (HNO3) auf in der Gasphase suspendierte Mineralstaubpartikel untersucht. Hierzu wurde eine Methode entwickelt, um Mineralstaubaerosol einer bestimmten Grössenklasse aus einer trockenen Pulverprobe in Luft zususpendierten. Nach unserem Wissen wurde diese Technik von uns erstmalig zur Untersuchung heterogener Reaktionen zwischen Aerosolen und Spurengasen verwendet. Einer der Hauptvorteile dieser Methode ist, dass sie zur Trockenzerstäubung von Pulverproben mit verschiedener chemischer Zusammensetzung (authentischer Wüstenstaub oder chemisch reine Mineraloxide) geeignet ist. In dieser Arbeit wurde Arizona Test Staub als Modell für atmosphärische Mineralstäube verwendet. Die Kinetik der heterogenen Reaktionen wurde in einem Flussreaktor untersucht, wobei die HNO3 Konzentration in der Gas- und Partikelphase kontinuierlich verfolgt wurde. In den meisten Experimenten wurde die 13N Tracermethode verwendet, bei der HNO3 mit dem radioaktiven Isotop (13N, τ1/2 10.0 min) markiert wurde und mit radioanalytischen Methoden detektiert wurde. Das 13N Isotop wurde mit der PROTRAC-Anlage des Paul Scherrer Institutes erzeugt. Mittels einer Serie beschichteter Denuder konnten die verschiedenen markierten Stickstoffoxide (HNO3, HONO, NO2, sowie partikuläres Nitrat) separiert und kontinuierlich detektiert werden. Die Experimente wurden unter troposphärischen Bedingungen durchgeführt (T 298 K, p ~1 atm, RH 6-70%). Die Aufnahmekinetik von gasförmiger Salpetersäure auf Mineralstaubaerosole hing von der HNO3-Konzentration und der Wasserdampfkonzentration des Gases ab. Die Ergebnisse zeigen, dass der Aufnahmekoeffizient γ mit zunehmender relativer Feuchte im Bereich von 12-73% um einen Faktor 5 von γ= 0.02 bis γ= 0.11 ansteigt. Die kinetische Simulation der Resultate deutet an, dass bei höherer relativer Feuchte mehr Reaktionszentren auf der Aerosoloberfläche für die Reaktion von HNO3 zu Verfügung stehen, was zu einer Beschleunigung der Reaktion führt. Für diese heterogene Interaktion wird ein Reaktionsmechanismus vorgeschlagen. Er umfasst den Gas-Oberflächen Diffusionstransport, eine Langmuir-Adsorption und eine Oberflächenreaktion. Die wichtigsten kinetischen Parameter, welche in atmosphärenchemischen Modellen verwendet werden sollten wurden bestimmt, um den Effekt der Luftfeuchte auf die Alterung der Mineralstaubaerosole zu repräsentieren. Diese Resultate zeigen, dass Mineralstaubaerosole die Verteilung von gas- und partikelförmigem Nitrat in der Atmosphäre beeinflussen können und somit eine Wirkung auf die troposphärischen NOy Zyklen haben.

List of figures V

List of Figures Figure 1.1. The London smog disaster of December 1952.. 1 Figure 1.2. Global mean temperature anomalies. Modified after IPCC, 2001 2 Figure 1.3. Statistics on the occurrence of high values of aerosol absorption index. 8 Figure 1.4. Change in the ozone concentrations (annular vertical mean) 10 Figure 2.1. Experimental setup to resuspend mineral dust 22 Figure 2.2. Aerosol production stability and switch on/off test. 26 Figure 2.3. Typical particle number size distribution. 27 Figure 2.4. Mineral dust generator output 29 Figure 2.5. ATD particle hygroscopic properties. 30 Figure 2.6. Electron microscope images showing some of the mineral dust particles 31 Figure 3.1. Schematic diagram of the flow reactor and detection system. 42 Figure 3.2. Size distribution of aerosol particles 44 Figure 3.3. Online record of an uptake experiment. 47 Figure 3.4. Concentration of H13NO3 leaving the flow reactor 48 Figure 3.5. HNO3 reacted with ATD particles as a function of aerosol surface area. 52 Figure 3.6. Change of the HNO3 concentration in the gas and particulate phases 53 Figure 3.7. Online record of uptake experiments 55 Figure 3.8. Uptake coefficient of nitric acid to ATD as a function of RH 57 Figure 3.9. Hygroscopic growth of ATD particles before and after processing 59 Figure 4.1. Plot of 13N-labelled nitric acid concentrations in gas and aerosol phases 71 Figure 4.2. Time profiles of the labelled and total HNO3 gas phase concentrations. 75 Figure 4.3. Time profiles of the labelled and nonlabelled HNO3 (particulate) 76 Figure 4.4. Time profiles of the uptake coefficient 77 Figure 4.5. Surface concentrations of reaction products at RH 33% 78 Figure 4.6. Surface concentrations of reaction products at RH 6% 79 Figure 4.7. Surface concentrations of reaction products at RH 60%. 80 Figure 4.8. The uptake coefficient as a function of time 81 Figure 5.1. The uptake coefficient of HNO3(g) on ATD aerosol 87 Figure 6.1. Decay analysis of activity associated with nitrogen oxides 89 Figure 6.2. Control tests of the SMPS system 90 Figure 6.3. MS measurement of acetic acid uptake on ATD 91 Figure 6.4. Uptake experiment with WEDD-AC detection 92 Figure 6.5. Scheme of the cyclone. Figure 6.6. Virtual impactor 93 Figure 6.7. Electrical precipitator. Figure 6.8. Solid Aerosol Generator 93 Figure 6.9. Parallel plate denuder 94 Figure 6.10. Aerosol flow tube reactor 94

List of Tables VII

List of Tables Table 1.1. Estimated present-day sources of tropospheric NOx 4 Table 1.2. The global annual average aerosol burden and emissions 6 Table 1.3. Parameters of dust aerosol in the atmosphere 6 Table 1.4. Chemical composition of the most important crust and clay minerals 7 Table 2.1. Standard dimensions of the cyclone 23 Table 2.2 Spectral statistics of the resuspended mineral dust aerosol 28 Table 2.3. Elemental composition of mineral dust 31 Table 2.4. Composition of water-soluble fraction of mineral dust 32 Table 3.1. Flow reactor parameters and measurement conditions. 46 Table 3.2. Conditions of the uptake experiments and the results of the fits to the data 53 Table 3.3. Uptake coefficient measured for aerosol particles of different composition 58 Table 4.1. Measurement conditions 70 Table 4.2. Reactions and rates of heterogeneous interaction between radioactive-labeled gas phase nitric acid and surface of mineral dust aerosol 72 Table 4.3. Reaction mechanism and rates of heterogeneous interaction between gas phase non-labeled nitric acid and surface of mineral dust aerosol 73 Table 6.1. Uptake coefficients for various aerosol types 91

Introduction IX

List of Abbreviations AA Acetic Acid APCI-MS Atmospheric Pressure Chemical Ionization Mass Spectroscopy ATD Arizona Test Dust CCN Condensation Cloud Nuclei CPC Condensation Particle Counter DMA Differential Mobility Analyser DRIFT Diffuse Reflectance Infra-red Fourier Transform spectroscopy EDS Energy Dispersion x-ray Spectroscopy EDX Energy Dispersion X-ray spectroscopy ESP Electrostatic Precipitator FTIR Fourier Transform Infrared spectroscopy GF Growth Factor H-TDMA Hygroscopicity Tandem Differential Mobility Analyzer ICP-OES Inductively Coupled Plasma Optical Emission Spectroscopy IFN Ice Forming Nuclei KC Knudsen Cell LT Lower Troposphere NDA N-(1-naphtyl) ethylene diamine dihydrochloride OPC Optical Particle Counter PFA PerFluoroAlcoxy fully fluorinated TEFLON® material PSL PolyStyrene Latex material (nothing to do with LaTeX) RH Relative Humidity SAG Solid Aerosol Generator SEM Scanning Electron Microscopy SMPS Scanning Mobility Particle Sizer SSA Single Scattering Albedo UT Upper Troposphere LS Lower Stratosphere WEDDAC Wet Effluent Diffusion Denude Aerosol Collector XPS X-ray Photoelectron Spectroscopy

Introduction 1

1 Introduction Climate change is among the most challenging issues that have received and continue to receive the attention of scientific community during last decades. Observations of such phenomena as the recession of glaciers, melting of permafrost and rise of sea level became the motivation to start climate research. The main part of this investigation is a study of atmospheric chemistry to improve the understanding of the anthropogenic activities that change the content of atmosphere and impact climate. The importance of the “atmospheric pollution” problem was recognized in the previous century and was preceded by the observation of such phenomena as “Great London smog”, “acid rain” and “Los Angeles photochemical smog”. These severe air pollution events clearly demonstrated the potential danger of anthropogenic influence on human health and environment. For example, the Great London smog event caused more than 4000 deaths during two weeks of December 1952. Trapped by the inversion layer, the emissions from charcoal burning were concentrated so that the concentration of particulate pollutants in air rose to 1.5mg·m-3 and strongly correlated with the increase of death rate (Figure 1.1). Although the interpretation of this particular correlation is still under debate [Bell and Davis, 2001; Johnson, 2004] there is general agreement among scientists about the origin of photochemical smog and acid rain. During the hot humid summer of 1944 a severe degradation of visibility was observed in Los Angeles city. Later it was shown that car exhausts supplied nitrogen oxides and olefins into the air and triggered the production of secondary aerosol under solar radiation [Turco, 2002].

Figure 1.1. The London smog disaster of December 1952. Death rate correlates strongly with concentrations of smoke (left graph). Visibility drop due to thick fog

formation (right photo).

The term “acid rain” has been extensively used in the literature to describe the atmospheric formation and deposition of acids to the earth’s surface [Godbold and Hüttermann, 1994; Isom et al., 1986]. Traditionally, this term has sometimes been used

2 Introduction

instead of “acidic deposition”, which includes the acid deposition not only in the form of rain (wet deposition) but also in the form of direct transport of acidic gases or aerosols to the surface (dry deposition). Eventually, these depositions are harmful for the environment causing the decline of fish populations or reduced forest growth. The eutrophication described above demonstrated the adverse effects of increasing influence anthropogenic industrial activities on a regional scale. The importance of air pollution has also been recognized from a global prospective. The Intergovernmental Panel on Climate Change (IPCC) has been established by World Meteorological Organization (WMO) and United Nations Environment Programme (UNEP) to assess scientific, technical and socioeconomic information relevant for the understanding of climate change and its potential impacts. The 3rd Assessment Report of the Working Group I of the IPCC gave an overview of the development of the Earth’s climate during the 20th century and a prospect for the evolution of the climate during the next 100 years [IPCC, 2001]. In the past 100 years the surface temperature increased by (0.6 ± 0.2)ºC on the global average (Figure 1.2), and emission scenarios result in a further increase of the surface temperature in the range of 1.4 to 5.8 ºC for the next 100 years. This temperature rise is mainly caused by an increase of the tropospheric concentrations of the so-called greenhouse gases (CO2, CH4, etc.). It was shown that the effect of greenhouse gas heating is partially tempered with the increase of tropospheric aerosol concentrations but the estimation of this effect is quite uncertain.

Figure 1.2. Global mean temperature anomalies (relative to the 1880 to 1920 mean).

Modified after [IPCC, 2001]

Apart from the global temperature forcing the scientific community highlighted other issues that have a global scale and are of concern to general public. Among them perhaps one of the most famous is the problem of “ozone hole”, which is related to the considerable ozone concentration depletion over Antarctica during spring time. Ozone is an important atmospheric species which affects human life in different ways. It protects living organisms from the harmful UV radiation by absorption most of it in the stratosphere before it reaches the surface. Tropospheric ozone can act both as a direct greenhouse gas and as an indirect controller of greenhouse gas lifetimes. As a direct greenhouse gas, it is thought to have caused about one third of all the direct greenhouse gas induced warming seen since the industrial revolution. Ozone is also known as a toxic air pollutant and oxidizing agent, which irritates human respiratory system.

Introduction 3

Different studies have shown that nitrogen oxide and nitrogen dioxide are strongly involved in O3 atmospheric chemistry [Chameides et al., 1992; Singh et al., 1996]. Due to difference in physical conditions in stratosphere and in troposphere NOx (NOx=NO, NO2) affect the ozone budget in different ways. In stratosphere NOx species are involved in the catalytic destruction of O3, while in the troposphere they enable its formation [Crutzen, 1970]. It has been shown that the regional trends in ozone concentration are not dependent on local NOx emission trends but strongly affected by transport of NOx [Lelieveld and Dentener, 2000]. Since the lifetime of of NOx in the lower troposphere where it is mainly emitted is only on the order of one day, long-range transport of NOx takes place in the form of reservoir species, namely peroxyacetyl nitrate (PAN) and nitric acid (HNO3). Therefore, the understanding of the cycling between NOx and the reservoir species is crucial for predictions of atmospheric ozone concentrations. Today a scientific gap exists: models tend to underpredict NOx concentrations and overestimate HNO3 concentrations [Chatfield, 1994; Gao et al., 1999]. This chemical imbalance could be caused by HNO3 scavenging by aerosols [Hauglustaine et al., 1996], particularly by mineral dust. In this thesis, following the suggestions of several research and modeling groups [Bauer et al., 2004; Bian and Zender, 2003; Dentener et al., 1996; Liao and Seinfeld, 2005; Tie et al., 2005], a process is investigated which could possibly influence the tropospheric concentrations of nitric acid and ozone: the removal of HNO3 from the atmosphere by heterogeneous uptake on mineral dust aerosol. This investigation has been performed in a laboratory flow reactor at conditions close to the real atmosphere. The basic kinetic parameters of HNO3 uptake reaction have been studied together with the changes of the mineral dust particles which they undergo during reaction processing by this reaction.

The following paragraphs give a brief overview of tropospheric chemistry as far as it is relevant to this work: the chemistry of nitric acid in the atmosphere, followed by an introduction to the properties of atmospheric mineral dust. The importance of the heterogeneous interaction between mineral dust and HNO3 will be discussed further to illustrate its relevance to the issue of climate change.

1.1 Nitric acid in atmosphere

Nitric acid is one of the reactive nitrogen species that can be found ubiquitously in the atmosphere. It is an important acidic air pollutant reaching typical mixing ratios of the order of several parts per billion by volume (ppbv) in polluted air and a few tens of parts per trillion by volume (pptv) in clean air [Finlayson-Pitts and Pitts, 2000; Goldan et al., 1984]. It represents an important component of the NOy family (NOy = sum of all nitrogen oxides: NO, NO2, peroxyacetyl nitrate, HNO3, HOONO, …) because it is a principal reservoir for the reactive nitrogen oxides in the upper troposphere (UT) and lower stratosphere (LS). In the LS, HNO3 represents the majority of NOy, and HNO3+NOx accounts for 80–100% of NOy. In the UT, HNO3 is less than half of NOy, and HNO3+NOx accounts for 40–100% of NOy [Neuman et al., 2001].

Nitric acid is mainly produced in situ in the atmosphere. At night the heterogeneous hydrolysis of dinitrogen pentoxide, N2O5, and reactions of the nitrate

4 Introduction

radical, NO3, are likely to account for almost 25% of the HNO3 formation [Dentener and Crutzen, 1993]. The association reaction of OH radicals with NO2 constitutes the primary source of HNO3 in the troposphere during the day time:

NO2 + OH + M → HNO3 + M (R1) where M denotes a molecule – in general molecular nitrogen or oxygen – that does not participate in the chemical reaction but stabilizes a thermo molecular reaction by taking up excess energy.

Nitrogen dioxide (NO2) is emitted into the troposphere directly together with nitrogen monoxide (NO). The fraction of NO in emissions of NOx is more than 90% [Finlayson-Pitts and Pitts, 2000]. Main sources for tropospheric NOx are fossil fuel combustion, biomass burning, biological activity and lightning (Table 1.1).

Nitrogen monoxide is rapidly converted to NO2 by the reaction with ozone [Leighton, 1961]:

NO + O3 → NO2 + O2 (R2) And NO2 is rapidly transformed back under the sunlight: NO2 + hν (λ<410 nm) → NO + O (R3) O2 + O + M → O3 + M (R4) Because of this rapid cycling, it is most appropriate to consider the budget of the NOx family as a whole. This cycling is interrupted by NO2 conversion to HNO3 (R1) or to peroxyacetyl nitratrate (CH3COONO2).

Table 1.1. Estimated present-day sources of tropospheric NOx in Tg N yr-1

[Jacob, 2000] [Finlayson-Pitts and Pitts, 2000]

Fossil fuel combustion 21 22 Biomass burning 12 5.5

Soils 6 6.7 Lightning 3 3-10

NH3 oxidation 3 Aircraft 0.5

Transport from stratosphere 0.1 Oceans 0.003 Total 46 37-44

HNO3 is relatively unreactive in the gas phase with chemical and photochemical

lifetimes of several weeks. In the stratosphere HNO3 is recycled back to NOx by photolysis and reaction with OH:

HNO3 + hν → NO2 + OH (R5) HNO3 + OH → NO3 + H2O (R6)

Introduction 5

In the troposphere nitric acid can be efficiently removed on a time scale of a few days by rain-out and surface deposition [Liu et al., 1987; Parrish et al., 1986]. In the upper troposphere, where the heterogeneous loss processes are not that effective anymore and the photolysis of HNO3 becomes more important.

The analysis of data from numerous flight campaigns in the UT leads to a discrepancy between the ratio NOx/NOy measured in the atmosphere and that predicted by the models [Chatfield, 1994; Gao et al., 1999; Singh et al., 1996]. This means that the current understanding of the processes governing the NOx/NOy chemistry in the UT must be incomplete. One of the possible reasons is the underestimation of heterogeneous reactions with ambient aerosols that have a potential to increase the sink of NOy species in the atmosphere [Hauglustaine et al., 1996; Neuman et al., 2001].

1.2 Mineral dust in atmosphere In general “aerosol” is defined as the ensemble of particles (solid or liquid) suspended in a gaseous media. Aerosol particle size ranges from a few nanometers to several tens of micrometer in diameter. Depending on the source of generation two types of aerosol particles are defined: primary particles (emitted directly as particles) and secondary particles (formed due to processes of nucleation or/and condensation). Aerosol particles are ubiquitous in the atmosphere and the majority of primary particles are comprised of mineral dust, sea salt, fossil fuel combustion and biomass burning aerosols. Secondary aerosol is produced in-situ in the atmosphere by the nucleation processes and comprises sulfate, nitrate and volatile organics. There is sometimes a thin line between these two aerosol categories because often primary aerosol emissions are accompanied by high concentrations of volatile gas species which rapidly nucleate, forming new aerosol particles. In the case of mineral dust aerosol it is rather evident that the main source of dust in the atmosphere is primary emissions. In contrast to sulfates, sea-salt and carbonaceous aerosols, mineral dust has not received much attention with respect to heterogeneous atmospheric chemistry until the early 1990’s [Mamane and Gottlieb, 1992; Zhang et al., 1994]. Later the importance of dust has been recognized with respect to the problem of climate change [Andreae and Crutzen, 1997; Ravishankara, 1997]. Mineral dust is a general expression for the windblown particles of crustal origin that are generated mainly in the arid areas of the planet, in particular in the great deserts. The major dust producing areas of the Earth are situated in the region that extends from the west coast of North Africa, across the Arabian Peninsula to central Asia, approximately in the 10°N-28°N latitude band [Claquin et al., 1999]. Areas of moderate dust emissions are the Southwest USA, North Mexico (Chihuahua, Coahuila), a region near the Aral Sea (Tajikistan/Uzbekistan/Kazahstan) and Lake Eyre in Australia [Claquin et al., 1999; Herman et al., 1997; Sviridenkov et al., 1993]. The most active emission regions are the Sahara-Sahel in North Africa (Chad basin) and Gobi-Taklamakan in Central Asia (Tarim basin). In these regions dust is mobilized and uplifted into the air. This process, called saltation, is a highly non-linear

6 Introduction

process and it depends strongly on wind velocity and physical conditions of the soil [Gomes et al., 1990].

Table 1.2. The global annual average aerosol burden and emissions.

Adopted mean values from [Textor et al., 2005].

Burden, Tg Emissions, Tg/a Dust 19.2 1840

Sea Salt 7.53 16600 Sulfate 2 175

Particulate Organic Matter 1.67 96.6 Black Carbon 0.24 11.9

Concentrations of mineral aerosol in the atmosphere are very variable in space and in time. In the literature, there exist several estimates of aerosol concentrations in different regions of the globe [Penner et al., 2001; Raes et al., 2000; Seinfeld and Pandis, 1998]. Perhaps, the most up-to-date information on particulate sources, sinks and lifetime has been recently published by Textor et al. (2005). In this paper, the results of simulations by sixteen global aerosol models are compared to analyze the life cycles of dust, sea salt, sulfate, black carbon and particulate organic matter. All the models presented in the study are carefully constrained by various high-quality observational data sets (in-situ measurements of aerosol concentration, size distribution, and chemical composition, lidar measurements of the vertical distribution of the aerosol extinction coefficient, sun photometer measurements). From these data one may see that mineral dust represents one of the largest mass fractions of the global aerosol (Table 1.2). The atmospheric burden of dust is the largest with the value of 19.2 Tg when compared to the other aerosol fractions. The annular dust emissions are estimated to be higher than those of sulfate, black carbon and particularly organic matter, and only smaller than the emissions of sea salt.

Table 1.3. Parameters of dust aerosol in the atmosphere.

Adopted from [Textor et al., 2005].

Parameter Unit Mean Median Stdev % Fine mass fraction <1μm % 20.80 10.80 114.00

Mass fraction > 5km % 14.10 14.10 50.80 Mass fraction in pol .regions % 1.54 1.00 102.00

Residence time days 4.15 4.04 43.30 Total removal rate 1/day 0.31 0.25 62.70 Wet removal rate 1/day 0.08 0.09 41.90 Dry removal rate 1/day 0.23 0.16 84.10

wet removal / total removal % 33.00 31.70 54.30 sedimentation / dry deposition % 46.20 40.90 66.20

convective wet deposotion / wet deposition % 44.50 46.40 51.20

Introduction 7

Summer is the season when the dust emissions are at maximum. At this time of the year dust storms are frequent events in the deserts when dust particles are efficiently lifted into the air. The majority of the particles sediment out in the vicinity due to gravity. Particles smaller than 10 μm could reach altitudes of 5-7 km, where they could have atmospheric lifetime up to weeks. The characteristic parameters of mineral dust aerosol are summarized in Table 1.3. Chemical composition of dust aerosol is closely related to the origin of the emissions. Since these particles are eroded soils, their chemical composition is similar, if not identical, to crustal rock [Goudie and Middleton, 2001]. That is why the main dust components all over the world are SiO2 (60%) and Al2O3 (10-15%). The percentages of the other oxides, such as Fe2O3, MgO and CaO vary more and depend on the source location [Gomes and Gillette, 1993]. Despite the similar chemical composition of various dusts, the mineralogy of the dust particles can be quite different. Common minerals found in airborne dust include quartz, feldspars, micas, chlorite, kaolinite, illite, smectite, palygorskite, calcite, dolomite, gypsum, halite, opal and mixed layer clays. Table 1.4 shows the general formulae of the minerals that could be found in atmospheric dust. The variations in mineralogy help researchers identifying dust particles among other types of ambient aerosols and to trace the dust origin [Falkovich et al., 2001; Reid et al., 2003]. E.g., using an elemental signature for Asian dust it was shown that a long range dust transport occurs over the Western Pacific from Asia to midlatitude North America [VanCuren and Cahill, 2002] and to the northern Pacific islands [Duce et al., 1980]. Calcium is a tracer for Saharan dust and the measurement of Ca2+ concentration is often used to demonstrate the transport of Sahara dust over the Mediterranean to Europe [Schwikowski et al., 1995] and over the Atlantic to the Eastern USA [Prospero, 1999b]. It should be mentioned that during transport over the oceans the dust particles may react with trace gases and are mixed with sea salt aerosol which makes the chemical composition of the aerosol very complicated.

Table 1.4. Chemical composition of the most important crust and clay minerals, constituting mineral dust aerosol [Kuzin and Egorov, 1976].

Mineral Chemical composition quartz

orthoclase albite

anorthite calcite

dolomite

SiO2

K[AlSi3O8] Na[AlSi3O8] Ca[Al2Si2O8]

CaCO3

CaMg[CO3]2

illite kaolinite chlorite

montmorillonite smectite

palygorskite

KAl2[AlSi3O10](OH)2

Al2[Si2O5](OH)4

(Fe,Mg)n-p(Fe,Al)2pSi4-p·xH2O; n≈5, p≈0.5-2 m{Mg[Si4O10](OH)2} x p{(Al,Fe)2[Si4O10]}; m:p=0.8-0.9

Mg[Si4O10](OH)2

(Mg,Al)2[OH|Si4O10]·2H2O + 2H2O

8 Introduction

Long-range transport of dust plumes can be well observed using satellite imaging techniques. For example Prospero et al. (1999a) have published the statistics on the occurrence of high values of aerosol absorption index using the data of the TOMS/NIMBUS-7 satellite (Figure 1.3).

Figure 1.3. Statistics on the occurrence of high values of aerosol absorption index. The shaded area shows the number of days (lightest shading, 5–10 days; heaviest shading,

25–30 days) when moderate-to-high concentrations of absorbing aerosol were detected by the TOMS/NIMBUS-7 satellite. July, 1984. Data from [Prospero, 1999a].

The high values of the aerosol absorption index on this figure correspond to the higher concentrations of mineral dust and smoke. Another perfect evidence of the persistent long-range dust transport is the findings of the dust layers in glacier ice-core records [Bory et al., 2002; Grousset et al., 2003; Sodemann et al., 2005]. During the time when dust particles travel in the air they affect the radiation balance of the planet by scattering back the incoming solar radiation. In addition, the surface of such particles represents a site for heterogeneous reactions. In this thesis we focus on the mineral dust interaction with nitric acid and highlight the importance of this particular issue in the following section.

1.3 Importance of nitric acid and mineral dust interaction for climate The importance of dust aerosol for climate forcing has been described in the IPCC 2001 report and the level of scientific understanding for this particular fraction was found to be “very low” [Ramaswamy et al., 2001]. Even today there is limited knowledge on how to assess the radiative properties of dust [Hansen et al., 2005]. There is a lack of

Introduction 9

information which could be necessary for the modelling studies to describe mineral dust influence on atmospheric radiative balance:

• Several field campaigns have certainly improved the knowledge about the dust emission sources and dust transport but there is still a large uncertainty associated with a quantitative assessment of these processes [Quinn and Bates, 2005].

• The complexity of mineral dust chemical composition and particle shape introduces a large uncertainty in the assessment of dust scattering properties [Claquin et al., 1998; Sokolik et al., 2001].

• The presence of soluble material (which may be the products of heterogeneous reactions) on the desert dust particles may convert them into large and effective cloud condensation nuclei (CCN) which may affect the cloud microphysics [Mahowald et al., 2003; Rosenfeld et al., 2001]. The influence of this effect on the climate forcing has been investigated only theoretically [Wurzler et al., 2000; Yin et al., 2002] and no experimental verification has been published so far.

To reduce the uncertainties listed the heterogeneous interactions between dust

aerosol and atmospheric trace gases should be taken into account. For instance, the in-situ atmospheric processing of the dust particles could create an outer shell of products and alter in this way the initial refractive index of dust. Additionally, the processing could increase the particle size which could increase the back scattering of the incident solar light, known as “direct aerosol effect”. The processing of dust particles could also influence their role in aerosol-cloud interactions. This process is important because the formation of additional clouds could be responsible for the global cooling (indirect aerosol effect). Recent field measurement campaigns have supported this hypothesis by showing the important role of dust as CCN [van den Heever et al., 2005]. Furthermore, the dust particles could act as ice forming nuclei (IFN) in high-altitude clouds [Cziczo et al., 2004; DeMott et al., 2003; Sassen et al., 2003]. The efficiency of this process depends on the surface properties of dust particles [Archuleta et al., 2005; Hung et al., 2003; Zuberi et al., 2002], therefore, the processing of dust particles in the atmosphere during long-range transport could change the IFN properties of dust [Ansmann et al., 2005]. Perhaps, the issue, which received most of the attention and which is related to heterogeneous reaction between dust and HNO3 is the change of global ozone budget and its influence on direct radiative climate forcing. The heterogeneous reaction of nitric acid with mineral dist particles removes the acid from the gas phase and thereby depletes its concentration. This has a significant influence on the tropospheric ozone budget because it prevents HNO3 renoxification (R5) and consequently, prevents O3 formation via reactions R3 and R4. This effect has been investigated by several modeling studies which showed a depletion of O3 and HNO3 concentration over the regions with higher dust emission [Bauer et al., 2004; Liao and Seinfeld, 2005]. Figure 1.4 shows an example of such theoretical predictions. The results of the modeling were in agreement with experimental observations made during the MINATROC field measurement campaign. The observations show that surface ozone concentrations were lower than the background values in air masses coming from North Africa, and when these air masses were also rich in coarse particles, the lowest ozone values were registered [Bauer et al., 2004; Bonasoni et al., 2004]. It should be stressed that the major reason of ozone concentration decrease

10 Introduction

is the HNO3 removal by heterogeneous reaction. Another pathway of ozone depletion could be the direct heterogeneous reaction of O3 with dust surface. The kinetics of this process has been studied in the laboratory [Hanisch and Crowley, 2003; Michel et al., 2002] and it was shown that this reaction is relatively slow and its contribution to the O3 concentration decrease is minor [Bauer et al., 2004]. The fact that nitric acid is taken up by mineral dust in the lower and upper troposphere is supported by field studies that show the correlation between the HNO3 (g) concentration drop and dust aerosol concentration increase [de Reus et al., 2000; de Reus et al., 2005; Hanke et al., 2003]. Additional evidences of the heterogeneous interaction is the increase of nitrate (reaction product) in the particulate phase which was observed at high dust loadings in the air during the Asian Pacific Regional Characterization Experiment [Maxwell-Meier et al., 2004] and in the urban air of Tokio [Ooki and Uematsu, 2005].

Figure 1.4. Change in the ozone concentrations (annular vertical mean over the

troposphere, year 2000) shown as percentage changes (100 × (HET-CTR)/CTR) .HET corresponds to the simulation which includes heterogeneous loss of O3, HNO3, NO3 and

N2O5 on the surface of mineral dust, CTR is the control simulation. Adopted from [Balkanski, 2003].

Introduction 11

1.4 Motivation

In the last two decades the interaction of HNO3 with mineral dust (or dust surrogates) has been investigated by several authors in laboratory experiments. Most of these studies have been performed in a so-called Knudsen cell reactor by measuring the depletion of HNO3(g) concentration with mass-spectrometry during interaction with mineral dust or mineral oxides as surrogates. Other detection techniques were used to detect the reaction product on the surface of dust, such as Fourier Transform Infrared Spectroscopy (FTIR) and Diffuse Reflectance Infra-red Fourier Transform spectroscopy (DRIFT). All these studies have been performed for the bulk powder samples of dust (or dust surrogates) at conditions very different from atmospheric (low pressure and water vapour concentration). The use of bulk powder samples is accompanied with a large uncertainty of the particle surface area actually exposed to the trace gas, and the way these measurements should be interpreted is somewhat under debate [Li et al., 2002; Seisel et al., 2005; Usher et al., 2003]. Therefore, from studies carried out so far no conclusive picture of the HNO3/mineral dust interaction can be drawn.

Single particle techniques are illustrative for changes in morphology and

structure, but may not be sensitive enough to address kinetic issues at the low trace gas concentrations typically encountered in the environment. As an alternative, airborne mineral dust particles at atmospheric pressure could be used in laboratory studies. This method has been established for the study of heterogeneous reactions with several other aerosol systems (Mozurkewich et al. 1987, Ammann et al. 1998, Guimbaud et al. 2002), with a variety of methods to determine mass transfer to the aerosol particles. In this work the dry dust dispersion method has been developed to study the heterogeneous interaction with gaseous nitric acid.

It should be stressed that the amount of reactants used for the kinetic study are

very small, e.g., the concentration of the gas phase nitric acid is in the range of 1011-1012 cm-3 and the aerosol surface to carrier gas volume ratio is in the order of 10-4 cm2·cm-3. In order to detect the reaction products a special sensitive analytical tool is applied. In the experiments the radioactive labeled HNO3 molecules are used. The 13N isotope is produced at the PROTRAC facility at the Paul Scherrer Institute via (16O(p,α) 13N) nuclear reaction [Ammann et al., 2003]. The use of the tracer enabled us to detect the reaction products in the aerosol phase at the concentrations lower than 1 ppt.

In chapter 2 a description of our developments of a dry dust dispersion technique

is given. A commercially available dust disperser was modified for use in laboratory studies of heterogeneous gas-aerosol interactions. The system output is characterized with respect to stability of aerosol number and surface area concentration. The particles are characterized with respect to morphology, electrical properties, hygroscopic properties and chemical composition.

The results of the laboratory observation of HNO3 uptake by airborne mineral

dust particles are presented in chapter 3. The uptake of 13N-labelled gaseous nitric acid was observed in a flow reactor on the 0.2-2 s reaction time scale at room temperature and

12 Introduction

atmospheric pressure. The amount of reacted nitric acid was found to be a linear function of aerosol surface area. The reactivity of the ATD particles was compared to the reactivity of SiO2 and CaCO3 particles. Following recommendations of other researchers [Johnson et al., 2005; Usher et al., 2003], the humidity dependence of the heterogeneous uptake on dust aerosol was determined experimentally. Chapter 3 also presents the results of dust processing experiments, which show that the reaction product forms a water soluble coating on the particles and enhances their hygroscopicity.

To describe the observed kinetics of the interaction a reaction mechanism is

suggested in chapter 4, which include gas-surface diffusive transport, Langmuir type adsorption and surface reaction. This mechanism in combination with the kinetic framework is able to reproduce the observations in a broad range of experimental conditions. A number of important kinetic parameters is derived, such as the concentration of reactive surface sites of the dust surface at different relative humidity and the values of heterogeneous kinetic constants. Chapter 5 gives a summary of the heterogeneous interaction between mineral dust and gaseous nitric acid and recommendations for future studies.

1.5 References Andreae, M.O., and Crutzen, P.J.: Atmospheric aerosols: biogeochemical sources and

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Introduction 13

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14 Introduction

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Introduction 15

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Liu, S.C., Trainer, M., Fehsenfeld, F.C., Parrish, D.D., Williams, E.J., Fahey, D.W., Hubler, G., and Murphy, P.C.: Ozone Production in the Rural Troposphere and the Implications for Regional and Global Ozone Distributions, J. Geophys. Res.-Atmos., 92, 4191-4207, 1987.

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16 Introduction

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Introduction 17

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18 Introduction

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Generation of submicron mineral dust aerosol 19

2 Dry dispersion method for generation of submicron Arizona Test Dust aerosol. Chemical and hygroscopic properties of the ATD particles

Published in: Journal of Aerosol Science and Technology, vol. 39, No.5, p.452-460. Title: Generation of submicron Arizona Test Dust aerosol: chemical and

hygroscopic properties Authors: A.Vlasenko1,2, S. Sjögren3, E. Weingartner3, H.W. Gäggeler1,2 and

M. Ammann1

1Laboratory of Radio- and Environmental Chemistry, Paul Scherrer Institute, Villigen PSI CH-5232, Switzerland 2Department for Chemistry and Biochemistry, University of Berne, Bern CH-3008, Switzerland 3Laboratory of Atmospheric Chemistry, Paul Scherrer Institute, Villigen PSI CH-5232, Switzerland

20 Generation of submicron mineral dust aerosol

2.1 Abstract This paper describes a submicron dust aerosol generation system based on a

commercially available dust disperser intended for use in laboratory studies of heterogeneous gas-aerosol interactions. Mineral dust particles are resuspended from Arizona Test Dust (ATD) powder as a case study. The system output in terms of number and surface area is adjustable and stable enough for aerosol flow reactor studies. Particles produced are in the 30-1000 nm size range with a log-normal shape of the number size distribution. The particles are characterized with respect to morphology, electrical properties, hygroscopic properties and chemical composition. Submicron particle elemental composition is found to be similar for the particle surface and bulk as revealed by X-ray photoelectron spectroscopy (XPS) and inductively coupled plasma optical emission spectroscopy (ICP-OES), respectively. A significant difference in chemical composition is found between the submicron aerosol and the ATD bulk powder, from which it was generated. The anionic composition of the water soluble fraction of this dust sample is dominated by sulfate. Resuspended dust particles show, as expected, non-hygroscopic behaviour in a humid environment. Small hygroscopic growth of about 1% (relative change in mobility diameter) was observed for 100 nm particles when the relative humidity (RH) was changed from 12% to 94%. Particles larger than 100-200 nm shrank about 1% once exposed to RH > 90%. This was interpreted as a restructuring of the larger agglomerates of dust to particles of smaller mobility diameter, under the influence of water vapor.


2.2 Introduction

Arid areas of our planet generate large amounts of dust, which is uplifted to the atmosphere. These natural sources are mainly deserts, which together with anthropogenic dust sources are considered to be responsible for one-third to one-half of annual aerosol emissions by mass [Penner et al., 2001]. Through processes of saltation and sandblasting, dust particles are uplifted into the air and may travel over long distances. While larger particles sediment out due to gravity in the vicinity, smaller particles (<10μm) could reside in the atmosphere for more than a week. The strongest regional effect of so-called “dust events” is the considerable decrease of visibility due to light-scattering enhanced by dust particles. Mineral dust aerosol could also absorb incoming solar radiation and reflect back the infrared radiation coming from the ground. Due to the complexity of dust composition and the incomplete understanding of transport and removal processes, an assessment of the influence of dust on the global radiation balance is still uncertain [Ramaswamy et al., 2001].

Recent field studies have shown dust particles acting as ice nuclei in clouds [Cantrell and Heymsfield, 2005; DeMott et al., 2003; Sassen et al., 2003]. Although large dust storms are episodic, dust particles are often present in the atmosphere and so could affect ice formation in clouds.

Heterogeneous interactions between atmospheric trace gases and mineral dust aerosol are important in several issues of atmospheric chemistry. This processing may affect the chemical and physical properties of the aerosol [Putaud et al., 2004]. On the other hand, it may also impact the global budget of important trace gas compounds [Bauer et al., 2004; Bian and Zender, 2003; Liao and Seinfeld, 2005]. Among the many potential reactions occurring, the reaction with HNO3 might be the most important as it affects the ozone budget of the upper troposphere, because there the photolysis of HNO3 is a significant source of NO and NO2, to which ozone sensitively responds. This reaction is also of significant interest because it converts barely soluble calcium carbonate into very soluble calcium nitrate, which can therefore dramatically affect the properties of dust as cloud condensation nuclei [Krueger et al., 2003; Laskin et al., 2005].

Several laboratory studies have been performed to understand and quantify these processes [Hanisch and Crowley, 2001; Mamane and Gottlieb, 1992; Underwood et al., 2001]. In these studies, dust was exposed to the trace gas of interest in the form of single particles or powder samples. The use of bulk powder samples is accompanied with a large uncertainty of the particle surface area actually exposed to the trace gas, and the way these measurements should be interpreted is somewhat under debate (Usher et al. 2003). Single particle techniques are illustrative for changes in morphology and structure, but may not be sensitive enough to address kinetic issues at the low trace gas concentrations typically encountered in the environment. As an alternative, airborne mineral dust particles at atmospheric pressure could be used in laboratory studies. This method has been established for the study of heterogeneous reactions with several other aerosol systems [Ammann et al., 1998; Guimbaud et al., 2002; Mozurkewich et al., 1987], with a variety of methods to determine mass transfer to the aerosol particles. The requirements for kinetic flow reactor studies are a well-defined size distribution and a constant output with time. Working with particle sizes below about


1µm is favorable for two reasons: mass transfer to aerosol particles is less affected by diffusion in the gas phase [Fuchs and Sutugin, 1970; Kulmala and Wagner, 2001], so that also fast uptake to aerosol particles can be addressed. Furthermore, flow reactor work is technically facilitated, because submicron particles are less subject to impaction losses in inlet/exit systems. So far, a dust aerosol generation system for this purpose has not been reported in the literature.

Applying strong shear forces in expanding flows (nozzles), mechanic uplifting using a brush, or fluidized bed disintegration are typical processes for powder resuspension. While in industrial applications, usually a substantial mass output is required, control of size distribution, stability and suspension in a chemically well defined carrier gas of relatively low flow rate is required in the context of interest here. Commercially available dust generators [Blackford and Rubow, 1986; Marple et al., 1978] produce mainly coarse particles (particle diameter larger than 1 μm), which may not be completely deagglomerated to represent mineral dust aerosol typically undergoing long-range transport in the atmosphere. In this study, we present a dust aerosol generation system based on a new commercially available system. The system is characterized with respect to generation stability and some physical and chemical properties of the aerosol produced, and to what degree the product aerosol represents the chemical composition of the original powder processed. The choice of Arizona Test Dust as our test material for resuspension in this study has been motivated by the possibility to use it directly without any further pre-treatment, that it represents a naturally occurring dust type and that it has also been used previously in studies addressing related issues (e.g., [Hanisch and Crowley, 2001]).


2.3 Experimental

The material used in this study to produce airborne particles is Arizona Test Dust (Ultrafine Grade A1, Ellis Components, UK), specified in ISO12103-1 standard and obtained from naturally occurring sand from a specific area of desert in Arizona, USA. According to the manufacturer, this sand was jet milled to reduce its particle size. Jet milling uses high pressure compressed air to propel the sand particles in opposing directions so that they collide. The resulting dust was then passed through a classifier to produce the required particle size distribution of the powder.

N220 Lpm

Condensationparticle counter

Feeding belt

Powder sample

Ejector 85Krsource 1

Electrostatic precipitator

AerosolExhaust

DMA

Humidifier 85Kr source 2

Virtual impactor

Cyclone

N2purge

N2purge

Toexperiment

Figure 2.1. Experimental setup to resuspend mineral dust into aerosol form and control

the particle size distribution.

Our submicron mineral dust aerosol generation system consists of three stages:

powder dispersion, particle size separation, control of particle charge and carrier gas (nitrogen). The setup is shown in Figure 2.1. First, the sample powder is dispersed by a solid aerosol generator (SAG, TOPAS GmbH, Germany). A special belt feeds the dust to the injector nozzle, which is a dual-stream injection nozzle (similar to standard ISO 5011:2000). The defined segments of the belt warrant a small but constant and reproducible supply of powder. Shear forces created in the injector nozzle at a flow rate of 20 L·min-1 disperse and disaggregate the powder to form particles. The resulting particle number concentration can be adjusted by setting the feeding belt speed over a wide range. Particle production can be stopped without changing the gas flow through the generator by stopping the belt movement. The original construction of the SAG by TOPAS allows the powder sample to come into contact with laboratory air through the opening of the dust container. Additionally, the laboratory air is admixed to the flow of resuspended dust through the opening of the dispersion unit. These two features of the manufacture design induce a contamination of the dust aerosol by the laboratory air. To


solve this problem, we modified the original instrument by introducing a purge flow of nitrogen to the dust container, to the nozzle region and into the inner room of the housing (see Figure 2.1).

The resulting aerosol contains a significant amount of large particles. These particles readily sediment and hence could cause a problem due to deposition in the aerosol experiments envisaged with this source, which rely on a relatively narrow submicron size distribution. The coarse particles are removed by passing the 20 L·min-1 of aerosol flow through a homemade cyclone with a tangent inlet and axial discharge design (see Figure 2.1). The dimensions of the cyclone are given in Table 2.1. The cyclone is immediately followed by a virtual impactor, where a final particle separation is achieved. This impactor is built according to the design by [Marple et al., 1995] and has a cut-off diameter of 0.8 μm, removing particles above that size. The total input flow into the impactor is 2.2 L·min-1, the major exiting flow is 2 L·min-1 and the minor exiting flow is 0.2 L·min-1. The diameter of the impactor entrance nozzle is 0.8 mm and the diameter of the collection probe is 1 mm. The nozzle protrusion length and the spacing between the nozzle and the collection probe were both set to 1 mm. The construction of the cyclone and the impactor were optimised to facilitate disassembling the devices for routine cleaning.

Table 2.1. Standard dimensions [Buonicore and Davis, 1992] of the cyclone

used in this study. All units are millimeters.

Dimensions mm Body diameter 68 Inlet diameter 4 Exit diameter 4

Vortex finder length 60 Body Length 63 Cone length 150

Dust outlet diameter 22

During dust dispersion, particles acquire a high electrical charge . This could cause an irreproducible particle loss to the walls of flow tubes consisting of insulating materials such as Teflon varieties necessary in experiments with reactive trace gases. To avoid this problem the aerosol is passed through a neutralizer (4 mCi 85Kr source, denoted as Kr source no 1 in Figure 2.1, inner volume 170 cm3) to reduce the high particle charge, followed by an electrical precipitator (ESP). The ESP is a 29 cm long coaxial cylindrical condenser. A high voltage (5kV) is applied between the outer tube (3.8 cm diameter) and inner rod (1.9 cm diameter) allowing all charged particles below about 2µm diameter to deposit inside the precipitator. Only electrically conductive tubing is used to transport charged particles to this point.

The aerosol humidity could be adjusted by passing it through a vertically mounted tube with a H2O permeable Goretex membrane (150 mm length, 6 mm i.d.) immersed in


demineralised water. The relative humidity was measured by capacitance humidity sensors at room temperature.

Finally, the aerosol size distribution and number concentration was obtained by a Scanning Mobility Particle Sizer system (SMPS), which consisted of a 85Kr neutralizer (denoted as Kr source no 2 in Fig. 1), a differential mobility analyzer DMA (model 3071, TSI) and a condensation particle counter CPC (model 3022, TSI). The system was operated at 3 L·min-1 sheath and 0.3 L·min-1 sample flow rates, respectively. Optionally the sheath flow could be humidified as the aerosol flow described above. The same relative humidity is maintained in both the DMA sheath air and in the aerosol flow since the aerosol liquid water content and consequently the particle diameter may be dependent on the relative humidity. Scanning time (300s) is long enough to get good counting statistics and a smooth particle spectrum. The performance of the system to resolve particle size was tested with the help of 40 nm (particle diameter) and 150 nm PSL particles (Duke Scientific Corporation, US). The SMPS system is configured to measure aerosol sizes from 20 to 800 nm. For sizing larger particles, an optical particle counter (OPC, GRIMM Labortechnik GmbH, Model 1.108) was used. That instrument was operated at 1.2 L·min-1 to measure the aerosol size distribution in the 0.3-20 μm size range. The averaging interval of the OPC was 60s. The instrument was calibrated by the manufacturer with polystyrene latex spheres (PSL). Application of both techniques allowed the measurement of the full size spectrum, which slightly exceeded the range covered by the SMPS system. Adequate control of even low numbers of larger particles is a prerequisite in experiments on trace gas uptake to particles as it strongly depends on surface area, and the larger particles may significantly contribute to the overall aerosol surface area.

Hygroscopic properties of the ATD particles were measured with a hygroscopicity tandem DMA (H-TDMA) instrument. A detailed description of the H-TDMA setup is given elsewhere [Weingartner et al., 2002; Weingartner et al., 2001] and thus will be given here in brief. A narrow size range of the polydisperse dry dust aerosol was selected with a first DMA. These monodisperse particles were humidified, and the resulting size of the humidified aerosol was measured by a SMPS consisting of a second DMA and a CPC. The RH was determined by measurement of the system temperature and sheath air dew point using dew point sensors. Two different H-TDMA operation modes were used [Gysel et al., 2004]: During the hydration mode, the monodisperse dry particles were exposed to a well defined higher RH and experience strictly increasing RH conditions. During dehydration, the dry particles are first exposed to high RH (>95% during ~5s) and are then exposed to a lower RH (~15s) in which their final size was determined.

Resuspended dust aerosol was sampled on polycarbonate filters (Nucleopore) with 0.05μm pore size and 47 mm diameter. After collection, samples were analysed at the Laboratory of Material Behaviour of Paul Scherrer Institute using a Scanning-electron-microscope "Zeiss DSM 962". The SEM is equipped with an EDS System from Tracor Noran, the System "Voyager" with a Pioneer Detector. The electron gun has a simple tungsten filament, and the microscope can reach a resolution of 4 nm. The microscope was operated at a high voltage of 20 kV. Five sampled particles were analysed for elemental composition.


Elemental composition of ATD powder and submicron ATD aerosol was measured using inductively coupled plasma optical emission spectrometry (ICP-OES). Powder samples and dust particles collected on filters (MF-Millipore) were dissolved in HNO3-HF-H3BO3-H2O solution, heated overnight at 95°C and introduced into a VISTA-AX ICP-OES spectrometer (Varian Inc.).

The elemental composition of the particle surface was analysed with the help of X-ray Photoelectron Spectroscopy. This method is very surface sensitive due to the low escape depth of photoelectrons of a few nanometers beneath the sample surface. Resuspended particles were deposited on conductive silver fiber filters with 25 mm diameter. Several mass loadings on the filter were tested. The amount of 2-5 mg of dust per filter was found to be an optimum in having complete coverage of the silver substrate while minimising charge build-up in the sample. XPS analysis was performed on an ESCALAB 220i XL instrument (Thermo VG Scientific). Binding energies were all referenced to the C 1s peak at 284.6 eV. In principle, the peak positions are specific for the chemical environment of each element. The samples were evacuated over night, but not heated.

The water soluble fraction of mineral dust was analysed by our Wet Effluent Diffusion Denuder / Aerosol Collector system, WEDDAC [Zellweger et al., 1999]. The aerosol flow first passes through the wet effluent denuder, where water soluble components of the gas phase are absorbed. The aerosol particles pass through the denuder unaffected (due to their lower diffusion coefficient) and are then trapped in a mist chamber aerosol collector system, where the particles are exposed to a high water supersaturation. Both denuder and aerosol collector effluent aqueous solutions are concentrated on ion-exchange columns for the gas and particulate phase fractions, respectively. The columns are eluted into an ion-chromatography system (DIONEX) every 30 min. This technique provides a quasi-online analysis of the anionic fraction of the resuspended aerosol and of acidic gases eventually desorbing from the aerosol surface.


2.4 Results and discussion The output stability is an important characteristic of an aerosol generator. Some instruments require some time to reach a steady state output and may show significant variation in the output upon repeated usage. These factors could limit the instruments application for studying heterogeneous reactions in flow tubes. The dust generator was tested by continuously running it over long time periods and also with intermittent stops of the feeding belt. Figure 2.2 shows a typical on/off test series with relatively stable output during 6 hours and satisfying recovery behaviour. When the generator dust production was switched off and on again the aerosol concentration reached the same value as before within 15 minutes. The observed variation of particle number concentration of 14% (σn-1) and the aerosol surface area (11%) is higher than that of and the particle mean geometric diameter (3%).

0 1 2 3 4 5 60

280

0

1x106

0

2x1011

hours

(C)

(A)

(B)

nm

OFFOFFOFF

1/cm

3nm

2 /cm

3

Figure 2.2. Production stability and switch on/off test. Solid squares represent total aerosol surface area (panel A). Open circles represent particle number concentration

(panel B). Crosses represent the particle mean geometric diameter (panel C). The labels “OFF” indicate the time intervals when the generator feeding belt was stopped.

The particle size spectrum was measured with the SMPS system by direct sampling (without dilution) of the aerosol flow. In accord with the cut-off characteristics of the virtual impactor, particles larger than 500 nm are effectively removed. Still, the SMPS measurement has shown a small number of large particles. The tail towards larger sizes of the spectrum was observed to be unresolved due to a limit of the SMPS sizing over 800 nm. To backup the SMPS measurement the optical particle counter (OPC) was used for particles larger than 300nm. The aerosol flow from the generator was diluted with particle free air by a factor of about 15 to decrease the particle concentration to


3.4·104 cm-3 to avoid exceeding the counting limit of the OPC. The measured aerosol size spectrum from both methods is shown in Figure 2.3.

101 102 103101

102

103

104

105

dN /

dLog

D, c

m-3

particle D, nm

SMPS data OPC dataLog-Norm fit

Figure 2.3. Typical particle number size distribution measured by the SMPS system (open circles) and by the optical particle counter (solid squares). Measured data are

fitted by a log-normal function (dashed line).

It should be mentioned that the methods provide different size characteristics: optical particle diameter (OPC) and mobility diameter (SMPS). The relation between these parameters depends on particle geometrical shape and refraction index. The OPC was calibrated with the help of PSL spheres. The shape of mineral dust particles is obviously different from PSL spheres, while their refractive index (e.g., SiO2:1.49, Al2O3: 1.77) might be comparable to that of PSL (1.59). We observed that both techniques show a good agreement, at least within the logarithmic plot of Figure 2.3 in the overlapping size range of 300-800 nm with a slight overestimation for the larger particle sizes by the SMPS system. Another study [Stolzenburg et al., 1998] has also shown a good agreement of the optical and mobility diameters, though for very different kind of ambient and laboratory-generated aerosol particles. For the purpose here, the most important result is that the OPC data show that the particle size distribution does not exhibit another mode, and drops rapidly with increasing particle diameter.

The observed size spectrum was fitted with a log-normal distribution by varying the geometric mean diameter Dg and the geometric standard deviation σg. Then, the other statistical parameters of the spectrum were calculated with the help of the Hatch-Choate equations for lognormal distributions of spherical particles [Cooper, 2001]. The log-normal fit function is calculated for the combined data-set measured by SMPS and OPC. Table 2.2 shows the results in comparison with the parameters calculated by the SMPS software and based on SMPS data only.


Table 2.2 Spectral statistics of the resuspended mineral dust aerosol. All particle diameters are given in nanometers, aerosol surface area in nm2/cm3

Lognormal fit SMPS data Measurement method SMPS+OPC SMPS

Dg 180 185 Dmean 204 212 Dmode 140 195

Dmedian 180 184 σg 1.65 1.74

Surface area 6.40·109 5.22·109

The total particle surface area was calculated using the log-normal fit function for the

number size distribution and assuming spherical shape of the particles. This parameter was compared to the value given by the SMPS software, which assumes the same particle shape. The SMPS software shows a value smaller by about 25% (Table 2.2). It means that the sole use of the SMPS system would lead to a systematic 25% underestimation of particle surface area measurement due to the upper size limit of the SMPS system.

The dust generator output is controlled primarily by the speed of the conveyer belt (Figure 2.4). Increasing the belt speed obviously provides more powder to the nozzle, which leads to higher concentrations of resuspended aerosol. The size of the resuspended particles (Dg) does not change because the separation devices remove the large particles, and the conditions at the nozzle do not change. Originally (by the manufacturer) the generator was designed for high mass output that could be achieved by providing large amounts of powder by the belt to the nozzle. In contrast, our purpose was to produce submicron mineral dust, which was achieved by varing the belt speed the range 1-15% of maximum speed. Increasing the belt speed further did not lead to accordingly higher concentrations of submicron particles, but rather to higher concentrations of large particles that are removed.

Hence, the combination of cyclone and virtual impactor proved to be a good choice for the size separation, the resuspended dust particles is in the designated submicron range. The production of mineral dust is stable with a sufficiently adjustable output.

The electrical properties of the resuspended dust aerosol were studied using the electrical precipitator and the neutralizer containing radioactive Kr-85. The aerosol flow was drawn through the electrical precipitator to the condensation particle counter (CPC), which measures the particle number concentration. When the voltage is not applied to the precipitator, the CPC measures the total particle concentration (charged and neutral). The concentration of only neutral particles was measured by passing the aerosol flow through the precipitator with the voltage applied. The measurements revealed that the resuspended aerosol leaving the nozzle and separation stages turned out to be highly charged. This is in accord with the study of [Forsyth et al., 1998] reporting a high charge level of Arizona road dust dispersed by a fluidised bed dust generator. Similar to that study, we used a Kr-85 source (source 1 on Fig. 2.1) to neutralize the resuspended dust aerosol.


0 20 40 600.0

2.0x105

4.0x105

6.0x105

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0

50

100

150

200

250

parti

cle

conc

entra

tion,

cm

-3

Belt speed, %

Dg,

nm

Figure 2.4. Mineral dust generator output. Open circles present the particle number

concentration, solid squares present the geometric mean particle diameter.

Passing the aerosol flow through the neutralizer decreased the concentration by about

40%, possibly due to losses inside the neutralizer and tubing. Using the measured aerosol size distribution and the symmetric Boltzman distribution of charges for aerosol particles [Flagan, 2001], we calculated that after passing through the radioactive source the aerosol should contain 31% of neutral particles. The experimentally measured value was 22% which means that neutralization was not efficient enough (probably due to the low residence time of 5s within the neutralizer), to completely reach ideal Boltzman equilibrium distribution charges. For the purpose of the present setup to provide a stable flow of a sufficient amount of neutral particles, it was not crucial to achieve an ideal equilibrium charge distribution, and therefore no further investigation was performed into this.

The hygroscopic properties of the submicron ATD particles were studied with the H-TDMA. Experiments were performed at four different particle sizes, with dry mobility diameter D = 55, 100, 250 and 400 nm, respectively. The measured humidograms at D = 100 and 250 nm are shown in Figure 2.5. The results obtained for particles with D = 55 nm are not shown since they behaved very similar to those with D = 100 nm, i.e. they also experienced a small hygroscopic growth of about 1% at RH = 90%. For the two larger sizes (D = 250 and 400 nm) a different hygroscopic behavior was observed: During hydration, no water uptake (growth factor = 1.00) was measured up to 90 %RH. At RH > 90%, a distinct decrease of 1% in the hygroscopic growth factor was encountered. These points fit to the measurement performed in the dehydration mode. This diameter change is interpreted as a small restructuring of the larger agglomerates of dust to particles of smaller mobility diameter, under the influence of water vapor. Such a phenomenon was earlier also observed for soot particles [Saathoff et al., 2003; Weingartner et al., 1997]. The shrinking, which was more pronounced in the case of soot, was explained by capillary condensation in small angle cavities of aggregates. Capillary forces induced on any asymmetric part of the particles caused them to attain a more compact structure. In


contrast, ATD particles smaller than 100-200 nm are not agglomerates and already substantially round, which can be seen in SEM images (Figure 2.6). These particles do not undergo restructuring once exposed to RH above 90%.

0 10 20 30 40 50 60 70 80 900.980

0.985

0.990

0.995

1.000

1.005

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1.015

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0 10 20 30 40 50 60 70 80 90 100

a) 100 nm

b) 250 nm

Relative humidity RH [%]

Hydration Dehydration

Relative humidity RH [%]

Gro

wth

fact

or D

/D0

Figure 2.5. Growth factors of D0=100 and 250 nm ATD particles as a function of RH at 20°C. D0 is the mobility size of monodisperse particles at the lowest RH. Hydration curve relates to the process when the dry particles are exposed to a well defined higher RH and

experience strictly increasing RH conditions. Dehydration curve relates to the process when the dry particles are first exposed to high RH (>95%) and are then exposed to a

lower RH in which their final size was determined.

Overall, the observed hygroscopic growth of the ATD particles is very small, as

compared to aerosols composed of soluble salts. An evident explanation for such a behavior is the very small amount of soluble material associated with ATD mineral dust (see also below). Similar non-hygroscopic behavior of mineral dust particles was previously reported by [Li-Jones et al., 1998; Twomey, 1977] measured the hygroscopic properties of African dust in the marine boundary layer using an integrating nephelometer. Their data also suggest that Saharan dust particles are essentially non-hygroscopic. Weingartner et al (2001) also found that during a Saharan dust event on a high alpine site 250 nm sized dust particles experience no hygroscopic growth. Note that processing of mineral dust with atmospheric trace gases could also substantially change this picture, when barely soluble mineral components are converted into very soluble products [Krueger et al., 2003; Laskin et al., 2005].


Figure 2.6. Electron microscope images showing some of the mineral dust particles

sampled. Size and shape distributions from this individual image is not representative of the dust aerosol as a whole. Particles marked with circles were analysed by EDX for the

following crustal elements: Si, Al, Na, Mg, K and Fe.

ATD mineral dust morphology was studied using scanning electron microscopy. Fig. 6 displays images of dust particles resuspended by the setup described above and deposited on a filter. It shows a heterogeneous mix of particles of different size and irregular shape. Some particles look like agglomerates either from incomplete disintegration during resuspension, or aggregation during sampling. Other particles (smaller than 200 nm) seem to be more compact, rather representing the primary mineral particles. The pictures somewhat overemphasizes the larger particles. On average, the pictures were consistent with the size distribution measured with the SMPS, with a mode at about 200 nm. A quantitative comparison of submicron aerosol composition with that of the original powder was obtained from ICP-OES measurements shown in Table 3. The notable difference between the two is the apparent depletion in the silica content in submicron particles. Several authors [Gomes et al., 1990; Reid et al., 2003] have shown that dust silica is mainly associated with coarse particles (quartz, alumino-silicates). In line with this, in our system, Si-rich particles would be preferentially removed in the separation stages, leading to the depletion in the silica content (Table 2.3).

Table 2.3. Elemental composition of mineral dust expressed in % of atoms. * XPS data on Mg is not available because Mg anticathode was used as X-ray source.

ATD powder Particle

bulk Particle surface

ICP-OES ICP-OES XPS Na 2.3±0.2 2.9±0.2 2 Mg 2.1±0.2 4.7±0.2 * Al 8.2±0.3 15.9±0.3 24 Si 79.1±1 63±1 63 K 1.7±0.2 3.1±0.2 3 Fe 2.2±0.1 4.9±0.2 3 Ca 4±0.2 4.8±0.2 5

Energy dispersive X-ray analysis (EDX) of a few particles as those shown in Fig. 6

was roughly consistent with the ICP-OES results (data not shown). It might be interesting


to note that no substantial amount of Ca was found in the few particles analysed, while both the original powder and the average aerosol contains calcium. Obviously, the Ca containing minerals are distributed non-homogeneously among dust particles. Given the importance of calcium carbonate as a reactive component [Fenter et al., 1995; Goodman et al., 2000; Hanisch and Crowley, 2001], this reminds us that the heterogeneous reactivity of dust may be associated only with a certain particle fraction only.

XPS was employed to analyze the surface composition of a large ensemble of particles sampled into a layer a few particles thick on the silver fiber filter. A small shift in the position of the peaks (binding energy) provides information about the chemical environment of each element as the method probes the valence electrons. The analysis revealed (Table 2.3) that the samples were comparatively clean with regard to carbon contamination. Usually, samples exposed to air prior to analysis become rapidly contaminated with low volatile organic compounds resulting in the so called adventitious carbon -peak apparent with dominant intensity in all spectra [Miller et al., 2002]. This peak is then usually used as reference to refer the kinetic energies of the other peaks to. In our samples, the intensity was surprisingly low, e.g. as compared to the blank filter substrate or as compared to other samples routinely used in the same apparatus. The ATD powder was exposed to air prior to introduction into the SAG. After sampling, the filters were stored in Ar until analysis. We therefore believe that most surfaces of our submicron aerosol have not been exposed to air before disaggregation. In addition, the XPS data indicated that calcium was present on the surface in the form of carbonate (based on the binding energy) rather than phosphate or nitrate. Comparison of the elemental composition suggests enrichment of Al at the surface.

Table 2.4. Mass concentration (in μg⋅m-3) of identified compounds in water-soluble fraction of mineral dust aerosol generated from ATD. Concentrations of acetate and

formate are smaller than the detection limit of the method.

Compound Concentration Fluoride 0.1±0.05 Acetate < 0.3 Formate < 0.5 Chloride 0.7±0.1 Nitrate 0.2±0.1

Sulphate 41±0.5 Phosphate 3±0.3

The water soluble composition of mineral dust aerosol was measured by the Wet

Effluent Diffusion Denuder system with anion chromatography detection. The mass concentrations measured are given in Table 2.4. Sulphate was the major anion, which is similar to the finding of [Falkovich et al., 2001] in the dusts from the Sahara. In addition, Cl-, PO4

3-, small amounts of NO3- and fluoride anions were detected. Concentrations of

the organic anions formate and acetate were small, and the corresponding chromatogram peaks were the same as in blank samples. Phosphate components of mineral dust are less soluble than others. The non-soluble components were retained in pre-filters in front of the preconcentrator columns. After switching off the aerosol production the concentration


of phosphate was decreasing exponentially with time while other anions were washed out immediately. This behavior is similar to the dissolution of Sahara dust samples reported by [Desboeufs et al., 1999]. Other water soluble components of the dust aerosol are extracted efficiently since the concentrations of anions scale linearly with the mineral dust mass aerosol concentration.

If we assume equal concentrations of cations and anions, then the water soluble fraction comprises only about 2% of the resuspended dust (by mass). Such a low water solubility of dust particles observed in our experiments is consistent with the non-hygroscopic behavior described earlier (Figure 2.5). Note that also with regard to the water soluble components, the situation may dramatically change upon atmospheric processing.


2.5 Conclusions A commercially available dust disperser has been modified and extended by size separating devices to produce a submicron aerosol prepared for heterogeneous chemical reaction studies. The adjustable output of the system is relatively constant with respect to aerosol number, surface and mass concentration. Submicron particles are produced with a log-normal shape of the size distribution. The system was used to produce and analyze aerosol from Arizona Test Dust, a widely used standard material representative of desert dust. Particle elemental composition is found to be similar for the particle surface and bulk, and in agreement with expectations from the size fractionated composition of the original powder. Resuspended dust particles show nonhygroscopic behaviour in a humid environment due to the low amount of soluble material associated with unprocessed dust particles. Interestingly, particles larger than 100-200 nm shrank about 1% (in mobility diameter) once exposed to RH > 90%. This was interpreted as a restructuring of the larger agglomerates of dust to the particles of smaller mobility diameter, under the influence of water vapor. The system described in this study, together with the chemical characterization employed, is ideal to provide atmospherically relevant mineral dust aerosols (from various authentic or chemically well-defined dust powder sources) for heterogeneous reaction studies in flow reactors. Acknowledgements Valuable technical support was provided by Mario Birrer. We thank R. Brütsch for the SEM, S. Köchli for the ICP-OES and Dr. B. Schnyder for the XPS measurements.


2.6 References Ammann, M., Kalberer, K., Jost, D.T., Tobler, L., Rössler, E., Piguet, D., Gäggeler,

H.W., and Baltensperger, U.: Heterogeneous production of nitrous acid on soot in polluted air masses, Nature, 395, 157 - 160, 1998.



Blackford, D.B., and Rubow, K.L.: A small-scale powder disperser, TIZ-Fachberichte, 110, 645-655, 1986.

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Uptake of nitric acid to mineral dust 39

3 Nitric acid uptake on mineral dust aerosol particles at different conditions

Submitted to: Atmospheric Chemistry Physics, 2005 Title: Effect of humidity on nitric acid uptake on mineral dust aerosol particles Authors: A.Vlasenko1,2, S. Sjögren3, E. Weingartner3, K. Stemmler1,

H.W. Gäggeler1,2 and M. Ammann1

1Laboratory of Radio- and Environmental Chemistry, Paul Scherrer Institute, Villigen PSI CH-5232, Switzerland 2Department for Chemistry and Biochemistry, University of Berne, Bern CH-3008, Switzerland 3Laboratory of Atmospheric Chemistry, Paul Scherrer Institute, Villigen PSI CH-5232, Switzerland

40 Uptake of nitric acid to mineral dust

3.1 Abstract

This study presents the first laboratory observation of HNO3 uptake by airborne mineral dust particles. The model aerosols were generated by dry dispersion of Arizona Test Dust (ATD), SiO2, and by nebulizing a saturated solution of calcium carbonate. The uptake of 13N-labelled gaseous nitric acid was observed in a flow reactor on the 0.2-2 s reaction time scale at room temperature and atmospheric pressure. The amount of reacted nitric acid was found to be a linear function of aerosol surface area. SiO2 particles did not show any significant uptake, while the CaCO3 aerosol was found to be more reactive than the ATD. Due to the smaller uncertainty associated with the reactive surface area in the case of suspended particles as compared to bulk powder samples, we believe that we provide an improved estimate of the uptake kinetics of HNO3 to mineral dust. The uptake coefficient averaged over the first 2s of reaction time at a concentration of 1012 molecules cm-3 was found to increase with increasing relative humidity, from 0.022±0.007 at 12% RH to 0.113±0.017 at 73% RH , scaling along a water adsorption isotherm. The processing of the dust at 85% RH leads to a water soluble coating on the particles and enhances their hygroscopicity.


3.2 Introduction

Heterogeneous interactions between atmospheric trace gases and aerosols are important in several issues of atmospheric chemistry. The processing of atmospheric particles might affect the chemical and physical properties of the aerosol; on the other hand, it may also impact the global budget of important trace gas compounds. Among the many potential reactions occurring, the reaction of mineral dust aerosol with HNO3 might be very important as it affects the ozone budget of the upper troposphere, because there the photolysis of HNO3 is a significant source of NO and NO2, to which ozone sensitively responds [Bauer et al., 2004; Bian and Zender, 2003; Dentener et al., 1996].

The heterogeneous reactions of mineral dust particles are also of significant interest because they could change the particle surface properties, which can therefore affect the properties of dust as cloud condensation or ice nuclei. The importance of this issue on a global scale has been demonstrated by several modelling studies [Kärcher and Lohmann, 2003; Lohmann et al., 2004]. Laskin et al (2005) reported field evidence of complete, irreversible processing of –particles containing solid calcium carbonate and quantitative formation of liquid calcium nitrate particles, apparently as a result of the heterogeneous reaction of calcium carbonate with gaseous nitric acid. Such conversion of insoluble material to soluble material strongly affects the radiative properties of these aerosol particles as well as their ability to act as cloud condensation nuclei. Recent ambient studies have shown the ability of Saharan dust aerosol particles to form ice crystals in cirrus clouds [DeMott et al., 2003; Sassen et al., 2003]. Currently, the question to what degree processing of dust particles by trace gases affects heterogeneous ice nucleation is subject of laboratory and field investigations [Archuleta et al., 2005].

Mineral dust is a complicated mixture of different minerals, and its reactivity with trace gases obviously depends on the composition. To understand the mechanism of the heterogeneous interaction with dust one could study the reactions with each of the components. However, the reactive behaviour of complex mixtures is not only a superposition of the behaviour of their individual components. In addition, in practice it would be a hard task to complete due to the complexity of the dust composition. Therefore, in the case of HNO3, many studies have concentrated on identifying the most reactive components, among which is CaCO3, and assessing their reactivity. [Fenter et al., 1995; Goodman et al., 2001; Hanisch and Crowley, 2001; Krueger et al., 2004]. In the present work, the heterogeneous reactivity of CaCO3 and SiO2 has been compared to the reactivity of the Arizona Test Dust (ATD) aerosol particles. These components are chosen since quartz is one of the major (by weight) constituent of the dust in general and in the ATD in particular; and calcite is suggested to be one of the most reactive constituents of the dust [Usher et al., 2003]. While many other studies focussed on Ca rich authentic dusts, ATD used in the present study is among the Ca poorer, though not less abundant forms of dust.


In this study, a dry dispersion generation method was used to produce submicron mineral dust aerosol and to measure the kinetics of the heterogeneous reaction with gaseous nitric acid with the aerosol particles in gas suspension. To our knowledge it is the first time that such a method was used for the production of a surrogate for atmospheric mineral dust in combination with a kinetic flow-tube technique. This approach is an alternative to the published studies of heterogeneous interactions in a Knudsen cell or with single particle techniques.

This study concentrates on basic uptake data and its dependence on relative humidity as well as the consequences on the hygroscopic properties of the dust particles. Ongoing, more detailed kinetic experiments will be reported elsewhere.

3.3 Experimental

The experimental method used is similar to the ones reported previously [Ammann, 2001; Guimbaud et al., 2002]. Nitric acid labelled with a short-lived radioactive isotope 13N is mixed with the aerosol particles in a flow reactor. After a certain reaction time, gas phase and particulate phase products are separated and trapped in a parallel-plate denuder and in a filter, respectively. The concentration of each species is measured by counting the number of 13N decays in each trap per unit time. In this way, the loss of nitric acid from the gas phase and its irreversible uptake by the aerosol particle surface are measured simultaneously. The scheme of the setup is given in Figure 3.1. Apart from the kinetic experiments, ATD aerosol particles were also processed by gaseous HNO3 in a larger reactor. Hygroscopic properties of dust particles were studied before and after HNO3 exposure using a HTDMA system described below.

aerosolH14NO3

H13NO3

γγ

SMPS

γγ γγ γγ

NaClNa2CO3NDA

γγ

NaCl

Aerosol filter

reactionzone

trap

Figure 3.1. Schematic diagram of the flow reactor and detection system.


Production of HNO3

The production of 13N in the form of 13NO has been described in detail elsewhere [Ammann, 2001]. In brief, the 13N isotope is produced via the reaction 16O(p, α)13N in a gas-target, which is set up as a flow cell, through which 20% O2 in He pass at 1l /min stp at 2.5 atm, and which is continuously irradiated by 15 MeV protons provided by the accelerator facilities at Paul Scherrer Institute. The primary 13N molecules and radicals are reduced to 13N labelled NO over a TiC catalyst immediately after the target cell. The resulting gas is continuously transported to the laboratory through a 580 m capillary. There, a small fraction of this flow (typically 25 ml/min ) is mixed with nitrogen as carrier gas (1 lpm) in our experiments. Additional amounts of non-labelled NO can be added from a certified cylinder (10 ppm in N2) to vary the total concentration of NO within a range of 1 ppb to 1 ppm. NO is oxidized to NO2 by reaction with ozone in a flow reactor with a volume of 2 liters. Ozone is generated by passing a mixture of synthetic air in nitrogen through a quartz tube irradiated by a mercury penray UV lamp (185 nm wavelength). HNO3 is produced from the reaction of NO2 with OH radicals; the flow containing NO2 is humidified to 40 % relative humidity and irradiated by a second 172 nm excimer UV lamp to produce OH radicals, which rapidly convert a large fraction of NO2 to HNO3 (see results section).

Aerosol particle generation

In this study, two types of aerosol generation methods were employed: dry dispersion from a powder and atomisation of an aqueous solution. The dispersion of Arizona Test Dust and detailed characterisation of the resulting aerosol is published elsewhere [Vlasenko et al., 2005]. Here only a short description of the technique is given. In a first step, the sample powder is dispersed by a solid aerosol generator (Topas GmbH, Dresden, Germany). Therein, a special belt feeds the dust to an injector nozzle in order to provide a constant input. Shear forces created in the injector disperse and disaggregate the powder to form submicron particles. In a second step, the remaining coarse particles are removed by a cyclone and a virtual impactor. This method is used to produce submicron particles from Arizona Test Dust (Ellis Components, England) and silica (Aerosil 200, Degussa, Germany). Calcium carbonate aerosol was generated by nebulizing a saturated aqueous CaCO3 solution (Model 3075, TSI, USA). The resulting droplets are dried by passing the flow through a diffusion dryer. Charged particles from both aerosol sources are removed by passing the flow through an electrical precipitator. Finally, the aerosol flow is conditioned to a certain relative humidity. The humidifier is a vertically mounted tube with a H2O permeable Goretex membrane (150 mm length, 6 mm i.d.) immersed in demineralised water. The relative humidity was measured by capacitance detectors at room temperature. The aerosol number concentration, size distribution and total aerosol surface area are controlled by a Scanning Mobility Particle Sizer (SMPS, TSI, USA). The size spectra of the aerosols obtained are given in Figure 3.2. One can see that the particle concentration was largest for silica aerosol and lowest for the calcium carbonate particles.


0.01 0.1 1

102

103

104

105

106

CaCO3

ATD

SiO2

dN /

dLog

D, c

m-3

particle diameter, μm Figure 3.2. Size distribution of aerosol particles used to study the heterogeneous

reaction with gaseous nitric acid

Flow reactor for kinetic experiments

Mineral dust aerosol and nitric acid flows are mixed to react in the flow tube reactor with cylindrical geometry. The reactor is a PFA Teflon tube of 8 mm inner diameter and 10 mm outer diameter. The PFA Teflon material has been chosen to minimise the losses and retention of HNO3 on the surface [Neuman et al., 1999]. Gaseous nitric acid is introduced via an injector along the axis of the flow reactor. The injector is a PFA tube (i.d. 4mm), which could be moved along the axis of the reactor. The position of the injector determines the gas-aerosol contact or reaction time. When the injector is pushed all the way in to the maximum position inside the flow reactor then the reaction time is minimum (0.2 s) and vice versa (2 s). The end of the injector tube is supplied with a special plug so that the gas enters the flow reactor through small openings at the end of the injector, perpendicular to the flow of aerosol. This is used to facilitate rapid mixing of the flows, which is critical for exactly controlling the reaction time. The degree of mixing was checked by measurement of the aerosol particle concentration upstream and downstream of the injection point by extracting a small flow with a small capillary pushed in from the opposite end of the reactor. The particle concentration was decreased immediately after the mixing with the gas flow in accord with the dilution factor of the corresponding volumetric flows. The flow tube is operated under laminar flow conditions, and it is assumed that the laminar flow profile is established a few cm downstream of the injector. The outer flow tube is replaced after each 6 hours of operation to avoid wall losses of HNO3 driven by the particles deposited on the inner wall. The system is kept at room temperature. The relative humidity of the flow is continuously measured downstream of the reactor.

Detection system

The flow leaving the flow reactor was directly entering the parallel-plate denuder system. The latter captures the gaseous species HNO3, HONO, NO2 on different chemically selective coatings by lateral diffusion. Note that this denuder train also effectively scrubs HNO3 reversibly adsorbed to the particles. The sub-micron aerosol particles have a small diffusivity and pass through the denuder without being collected.


Gaseous nitric acid is taken up in the first denuder section coated with NaCl. HONO is collected in the next section coated with Na2CO3, while NO2 is absorbed in the third section by reaction with NDA (N-(1-naphtyl) ethylene diamine dihydrochloride) mixed with KOH. These coatings are freshly prepared after each 6 hours of operation. Generation of HNO3 by reaction of NO2 with OH is accompanied by ozone production under UV radiation. High concentrations of O3 are not desirable because ozone reacts with the NDA-coating and depletes the capacity of the coating to absorb NO2. To minimise this effect the parameters of the HNO3 generation (UV radiation exposure and amount of synthetic air) are optimised in a way to keep the output concentration of O3 at minimum (below 30 ppb).

After passing the denuder, the aerosol particles are captured by a glass fiber filter. To each trap (the coatings and filter) a separate CsI scintillator crystal with integrated PIN diode is attached (Carroll and Ramsey, USA) which detects the gamma quanta emitted after decay of the 13N atoms. The detector signal is converted to the flux of the gaseous species into the trap using the inversion procedure reported elsewhere [Guimbaud et al., 2002; Kalberer et al., 1996; Rogak et al., 1991]. This flux is proportional to the concentration of the species in the gas phase.

An additional NaCl-trap is used to monitor the concentration of gaseous HNO3 in a small side flow before entering the reactor. The trap consists of a quartz-fiber filter, soaked with a saturated aqueous NaCl solution and dried. Also to this trap, a scintillator device as that described above is attached. This measurement provides a “reference” for the generation of gaseous nitric acid and reduces the uncertainty related to the instability of the flux of 13N arriving in the laboratory. The relative counting efficiency of each detector is determined by accumulating a certain amount of H13NO3 in the “reference” trap and exposing it to each of the other detectors attached to the denuder sections and the particle filters in a way that closely mimics the geometrical configuration at each trap. The concentration of non-labelled NO and NO2 is monitored by a chemiluminescence analyser (Ecophysics, Switzerland). Further details of the preparation of the coatings, trap and filter efficiencies, and the performance of the detection system are published elsewhere [Ammann, 2001; Guimbaud et al., 2002].

Mineral dust processing and measurement of hygroscopic properties.

Apart from the kinetic experiments, ATD aerosol particles were also processed by gaseous HNO3 in a laminar flowtube reactor at room temperature and atmospheric pressure over longer time scales. The mean residence time of the aerosol in this reactor was 3 min at a flowrate of 0.3 lpm. Relative humidity in the reactor was monitored by a capacitance detector. To vary the relative humidity in the reactor chamber, the aerosol flow passes through the humidifier (identical to the one described above). The flow of HNO3 was maintained by passing a 0.2 lpm flow of nitrogen through a bubbler, which contained a nitric acid solution in H2O (0.1 M) at 12°C. Half of this flow was directed to a molybdenum converter held at 400°C and then to a NO chemiluminescence detector to monitor the concentration of nitric acid in the gas phase [Joseph and Spicer, 1978]. The


concentration of gaseous HNO3 detected in this way was 3×1013 molecules cm-3 at 298K. The other half of the HNO3 flow is mixed with the ATD aerosol flow (0.2 lpm) prior to the reactor entrance. After the reaction chamber, the aerosol flow was drawn through a NaOH coated denuder tube to remove HNO3 from the gas phase. The hygroscopic properties of the processed ATD aerosol was measured by a Hygroscopicity Tandem Differential Mobility Analyzer (HTDMA) system described elsewhere [Weingartner et al., 2002]. Briefly, in this instrument, the aerosol is first dried to a low RH (< 5%) and fed into the first differential mobility analyzer (DMA) where a monodisperse particle size fraction is selected (diameter D = Do). Then, the aerosol is exposed to higher RH during ~60 s, and the resulting new particle size distribution is determined with a second DMA combined with a condensation particle counter. This instrument is capable of measuring the hygroscopic growth factor (GF) defined as the relative particle diameter increase from dry to humidified state, D/Do. A prehumidifier (RH=95%) is included or bypassed in order to measure hygroscopic growth factors during dehydration or hydration.

Table 3.1. Flow reactor parameters and measurement conditions.

Parameter Value

Reaction time 0.2-2 s

Concentration H13NO3 (labelled) ~106 cm-3

Concentration of HNO3 (not labelled) 1011-1012 cm-3

Pressure ~ 1 atm

Relative humidity 12-73 %

3.4 Results and discussion Procedure of the kinetic experiments

The time profiles of the NO2, HNO3(g), HNO3 (γ-reference) and HNO3(aerosol) concentrations during an individual uptake experiment is illustrated in Figure 3.3 and described in detail below.

The experiment starts with equilibrating the system by running all gas flows without the admission of 13N-labelled nitrogen dioxide (0-12 min time interval). At this time the background signals of the gamma detectors are recorded.


0 20 40 60 800

1

2036

048

C

A

B

HNO3 (aerosol)

time, min

HNO3 (g)

Con

cent

ratio

n, a

rb. u

nits

aerosolpresent

NO2 (g)

Figure 3.3. Online record of an uptake experiment. Panel A: dashed line represents the signal of γ-detector at the NDA-trap and solid line corresponds to the concentration of

nitric dioxide. Panel B: the dashed line represents the “reference” gas phase concentration of HNO3 (concentration before entering the reactor) and the solid line

corresponds to the concentration after the reactor. Panel C: the solid line represents the concentration of nitric acid on the aerosol surface. The grey bar (75-90 min) corresponds

to the time when aerosol was present in the flow reactor. The HNO3 gas phase concentration in the flow tube was 1011 cm-3 and RH 33%.

Then at 12 minute, a small flow of 13NO2 is admitted to the main gas flow. The NDA coating of the denuder starts to absorb nitrogen dioxide from the gas phase, accompanied by an increasing number of decays observed in this trap (panel A, dashed line). This growing signal is inverted to the flux of 13N labelled molecules into this trap, which is proportional to the concentration of the 13NO2 in the gas phase (panel A, solid line). Because this flux is calculated based on the difference of two consecutive activity measurements only, the inverted data (solid line) show more apparent scatter than the raw activity signals (dashed line). Prior to the reactive absorption in the NDA-trap the nitrogen dioxide molecules travel along the NaCl and Na2CO3 traps. Due to reversible adsorption and some slow conversion to HNO3 and HONO, some of the 13N atoms are also being absorbed on these traps, which leads to a small increase of the corresponding detector signals (panel B: solid line). For the same reason, the signal at the “reference” trap detector is increased (panel B, dashed line). The efficiency of the NO2 absorption in the NDA-trap is not entirely 100%, so that a small fraction of NO2 may penetrate the denuder to the aerosol filter and manifests itself as a slight increase of the signal (panel C). Note that this penetrating fraction may be extremely low but may still allow a detectable signal.

At 33 min of the experiment, the production of HNO3 is started by switching on the UV lamp for OH production to convert NO2 into HNO3. As a result, the detector signal of the


NDA-trap decreases by about a factor of three (panel A, solid line). It indicates that two thirds of the labelled NO2 molecules were oxidized to HNO3. We use this conversion factor to calculate the overall concentration of nitric acid in the gas phase by applying the same factor for the conversion of non-labelled NO2, the concentration of which is measured by the chemiluminescence detector. The increase of the HNO3 concentration is detected at the NaCl denuder (panel B, solid line) and in the reference trap (panel B, dashed line).

The mineral dust particles are introduced to the flow reactor at 75 min of the experiment. The gas phase nitric acid concentration drops (panel B, solid line) due to reaction with the aerosol surface, while the concentration of the particulate HNO3 increases (panel C). As noted above, the signal associated with particulate HNO3 is due to HNO3 irreversibly taken up to the particles. HNO3 desorbing from the particles faster than 0.1 s would be detected as gas phase HNO3 in the first denuder. No increase of the signals in the other denuders has been observed during the presence of aerosol, so that not significant amounts of HNO3 desorbing on the time scale of a second while travelling along the denuder train had been associated with the aerosol. A significant loss of HNO3 from the particles on the filter on the time scale of minutes would have resulted in a lack of mass closure for HNO3.

Using the procedure given here, uptake to aerosol particles can be measured as a function of reaction time, HNO3 concentration in the flow tube and relative humidity. The algorithm to derive the value of the uptake coefficient from the measurements shown in Figure 3.4 is described below.

0 10 20 30 40 50-0.4

-0.2

0.0

1011 cm-3

1012 cm-3

Ln (C

/C0)

reaction zone length, cm Figure 3.4. Concentration of H13NO3 leaving the flow reactor in absence of aerosol at different injector positions, normalized by the initial concentration. Circles and squares represent the data points measured at 1011 cm-3 and 1012 cm-3, respectively, of HNO3 in

flow tube and 33% RH. Lines are fits to the data according to the model explained in the text.


Calculation of the uptake coefficient

The uptake coefficient is usually defined as the ratio between the net flux of molecules from the gas phase to the aerosol particles and the gas-kinetic collision flux of the molecules to the surface of the particles.

coll

net

JJ

=γ (1)

The observations from an individual experiment shown in the previous section allow determining the rate of change of gas-phase and particulate phase concentrations. In principle, the mechanism leading to the net transfer of HNO3 to an irreversibly bound product in the particulate phase can be very complex. Nevertheless, to obtain first insights into the kinetics, the uptake coefficient can be estimated in a first order approach similar to that reported earlier [Guimbaud et al., 2002]. Note that this approach assumes a constant (quasi-steady state) uptake to the aerosol during the residence time of the aerosol in the flow reactor. Because, as noted above, reversibly adsorbed HNO3 is not detected in the aerosol phase, the uptake coefficient obtained this way is the probability that an HNO3 molecule colliding with the dust surface is irreversibly reacting with a dust component. Therefore, initial loss from the gas phase could be stronger than the quasi-steady state uptake coefficient assumed in this approach. The rate equation for the depletion of radioactively labelled HNO3 from the gas phase in the cylindrical flow tube is given by

gpwg Ckk

dtdC )( +=− (2)

where Cg is the average concentration of HNO3 in gas phase. kw is the constant which describes the pseudo first order loss of H13NO3 from the gas phase due to its adsorption to the walls of the reactor. kp is the constant, which describes the heterogeneous reaction between gaseous nitric acid and aerosol particles. The presence of the wall-loss is rather specific for the radioactively labelled molecules used in this study and will be discussed in detail in the next section. Integration of the Eq.(2) with respect to time gives the concentration of H13NO3 molecules in the gas phase as a function of time:

{ tkkCtC pwtgg )(exp)( 0 +−= = } (3)

where is the initial concentration at time zero. k0=tgC w can be obtained from the

measurement of the concentration of H13NO3 (g) as a function of time in absence of aerosol (kp=0). The loss rate of H13NO3 (g) in presence of the aerosol then allows determining kp.

The kinetics of appearance of H13NO3 in the particulate phase is given by:


p

w

tkpkwt

gp

kk

eCtC+

−=

+−=

1

1)()(

0 (4)

where Cp(t) is the concentration of H13NO3 in the particulate phase.

In practice, we used Eq.(4) and not Eq.(3) to calculate the constant kp, because the experimental measurement of Cp is more accurate than that of Cg. The heterogeneous constant kp is related to the effective uptake coefficient γeff according to following equation:

4ωγ peff

p

Sk = (5)

where Sp is the aerosol surface to volume ratio, ω is the mean thermal velocity of HNO3

given by ω=(8RT/(πM))1/2, R is the gas constant, T is the absolute temperature and M is the molar weight of HNO3.

The value of Sp was measured in the experiment by the SMPS and the values of kp and ω could be calculated using the equations listed above.

The value of the effective uptake coefficient, calculated in this way, depends slightly on the aerosol particle size, because gas phase diffusion affects the rate of transfer to larger particles more strongly than that to smaller particles. The diffusion correction was made using Eq.(6) and Eq. (7) [Pöschl et al., 2005].

)1(28.075.011KnKn

Kneff +

+−=

γγ (6)

pdDKn

ω6

= (7)

where D is the diffusion coefficient of HNO3, dp is aerosol particle diameter and Kn is the Knudsen number. Note that for the experiments reported in this study, the correction was always below 5% as discussed below.

Retention of H13NO3 on the flow reactor wall

The observations show that there is a steady state drop of the gas phase concentration of H13NO3 during passage through the flow reactor even without aerosol. As already pointed out by Guimbaud et al. (2002), this is not due to an irreversible chemical loss of HNO3 on the wall, but rather due to retention driven by adsorption and desorption. When considering the non-labelled HNO3 molecules, this effect leads to the


well-known slow response time of this sticky gas measured at the reactor outlet when switching it on and off. At low concentrations, the observed response time is directly related to the average residence time of individual molecules in the flow tube. If this residence time is comparable to the half-life of the radioactive 13N-tracer, 10 min., a drop in the H13NO3 concentration along the flow tube can be observed, while the concentration of the non-labelled HNO3 concentration remains constant, if equilibrium with the wall is established.

The details of lateral diffusion, adsorption, desorption, and radioactive decay are lumped into the pseudo-first order decay constant kw. Therefore, Eq.(3) with kp=0, was used to fit the experimentally observed H13NO3(g) concentration drop in absence of aerosol as shown in Fig. 4 for two examples. Typical residence times of HNO3 derived from these loss curves are about 4 min. kw obtained from these fits significantly decreased with increasing HNO3 concentration, possibly because saturation coverage of HNO3 on the PFA Teflon surface was reached above 1012 molecules cm-3. kw was also observed to increase with increasing relative humidity. This might be related to higher surface coverage by H2O molecules at higher relative humidity and the formation of surface-adsorbed nitric acid–water complexes.

Apart from these effects, kw also varied to some degree from tube to tube. Therefore, each time some parameter of the experimental system was changed or a new PFA flow tube was installed, a new measurement of kw was performed.

Effect of aerosol surface area and “diffusion resistance”on uptake

The dependence of the amount of H13NO3 taken up on the aerosol of the aerosol surface area was investigated at an HNO3 concentration in the gas phase of 1012 cm-3, RH of 33% and reaction time of 1.9 s. The aerosol surface area was varied by changing the dust generator output, which results in a change of the particle number concentration but not particle size.

Figure 3.5 shows the number of HNO3 molecules reacted per cm3 as a function of the particle surface area per cm3. The error bars represent the 1σ deviation of data about the mean. The amount of nitric acid reacted on the surface should be a linear function of the particle surface area, as long as ( 1)+ <tkk pw tkCt t 0)( =≈

pp SC, so that C , and for fixed

reaction time t, pgp

∝ . This confirms that our experiment lies well within pseudo-first order kinetics and that the availability of HNO3 is not limiting the uptake.


0 1x10-4 2x10-4

0

3x1010

6x1010

9x1010

HN

O3 o

n ae

roso

l, m

olec

ules

cm

-3

aerosol surface area, cm2 cm-3

Figure 3.5. HNO3 reacted with ATD particles as a function of specific aerosol surface area. The nitric acid gas phase concentration in the flow tube was 1012 cm-3 and RH

33%.

To estimate the limitation of uptake by the diffusion of HNO3 in the gas phase we use the expressions (6) and (7). The HNO3 diffusion coefficient D has been taken as 0.118 cm2s-1 [Durham and Stockburger, 1986], which had been measured at atmospheric pressure, 298 K and 5-95% relative humidity. The “diffusion limitation” effect is stronger for higher values of the uptake coefficient. Based on Eq.(6) and Eq.(7), the maximum correction γ/γeff is 1.5 at 1 micrometer particle diameter for γeff =0.1. When integrated over the full aerosol spectrum of ATD, the correction is about 5% or less for smaller values of γ.

Uptake coefficient on Arizona Test Dust aerosol

The experimental data of the H13NO3 (g) concentration drop and the corresponding gain of H13NO3 (p) in the aerosol phase shown in Fig. 6 was fitted using Eq.(4) and Eq.(5), with kp as independent variable. The constant kp was varied using the least square method to achieve the best agreement between the data points of the concentration in the aerosol phase and model curve, calculated by Eq.(4), because, as noted above, the changes in the aerosol phase could be detected with better accuracy than those in the gas phase. The result is given in Figure 3.6 and Table 3.2. The fit and the data agree quite well. One should notice that within accuracy of the experiment the drop of the HNO3(g) concentration due to uptake to the aerosol corresponds to the growth of the HNO3(p) signal. Most of the discrepancy between data and model has been assigned to the instability of aerosol generation. For instance, the deviation of the data points from the fit at 1.9 s reaction time in the example shown in Fig. 6 is due to an increase of the aerosol


surface area recorded by the SMPS system at that time and as a result a higher uptake to the aerosol phase and stronger depletion of the HNO3 concentration in the gas phase.

0.6

0.9

1.2

0.0 0.5 1.0 1.5 2.0

0.000.040.080.120.16

gas phase

aerosol phase

reaction time, s

C/C

0(g)

Figure 3.6. Change of the HNO3 concentration in the gas (open circles) and particulate (solid squares) phases as a function of reaction time. Experimental data are represented as the concentrations normalized by the concentration in the gas phase at reaction zero

time. The dashed lines are the model fits. The HNO3 gas phase concentration in the flow tube was 1011 cm-3 and RH 33%. The error bars represent the 1σ deviation of data about

the mean.

Table 3.2. Conditions of the uptake experiments and the results of the fits to the data.

parameter HNO3 concentration in the gas phase in flow reactor,

molecules cm-3

(1±0.5)×1011 (10±1)×1011

RH, % 33±1 33±1 Sp ×10-5, cm2 cm-3 8.6±0.5 12.9±0.3

kw, s-1 0.152±0.002 0.110±0.003kp, s-1 0.063±0.005 0.025±0.005

γeff 0.10±0.01 0.03±0.01

γ 0.105±0.01 0.03±0.01


The uptake coefficient is found to be a function of HNO3 concentration in the gas phase (Table 3.3). At higher concentration of nitric acid in the flow tube the uptake coefficient drops by more than a factor of three. Previous work on this and similar heterogeneous reaction systems indicate that the HNO3+mineral dust reaction could be considered as a two stage process: adsorption of HNO3 on the dust surface followed by a reaction of the adsorbed HNO3 with a basic surface site (surface OH—group on aluminosilicate or similar minerals or bulk CaCO3). Therefore, the decrease of the uptake coefficient, which is an average over the two seconds reaction time, could be either due to the depletion of the reaction sites or due to saturation of the adsorbed precursor [Ammann et al., 2003]. The available data points at HNO3(g) concentration 1012 cm-3 still fit the model (which assumes an average uptake coefficient over the time scale of the experiment) reasonably well, so that depletion of the reactants during the early periods of the reaction time is likely not the reason for the concentration dependence. While the amount of HNO3 found on the particle surface after two seconds of about 2×1014 molecules cm-2 could be considered close to a monolayer surface coverage, the degree with which components contained in the bulk of the particles can react and thus extend the capacity of the particles to react is not known. Therefore, this apparent non-first order behavior of the rate of product appearance indicates that the simple approach adopted here is not sufficient to retrieve a reliable parameterization of the kinetics. A more extended kinetic data set associated with proper kinetic modeling is necessary to extract the parameters describing the elementary processes of the uptake process. This will be part of a follow-up study of this, while here we concentrate on the humidity dependence.

Uptake coefficient on SiO2 and CaCO3 aerosols

In an attempt to understand the mechanism of the heterogeneous reaction between HNO3 and the ATD and for the purpose of comparison with other studies, we also made experiments of uptake of HNO3 to silica and calcite aerosol particles.

Figure 3.7 shows the uptake of the gaseous nitric acid to silica and CaCO3 aerosol particles. To simplify the discussion of this comparison, we show the raw data in the same way as discussed in section 3.1 above. The experiment starts with recording the background detector signal, as shown in Figure 3.3. After 20 minutes, the aerosol is introduced into the flowtube, adjusted to a reaction time of 1.9s. One may see that the detector signal level does not change with the introduction of SiO2 particles at a statistically significant level. This means that silica aerosol seems rather inert with respect to reaction with HNO3. Assuming the detection limit at 3σ of the background noise level of the signal of the γ-detector at the aerosol filter and taking into account the measured SiO2 aerosol surface area of 1011 cm2·cm-3, an upper limit to the uptake coefficient of 5×10-4 is obtained.


0 10 20 30 4

0.0

0.1

0

aerosolpresent

time, min

0.0

0.5

1.0

reac

ted

HN

O3 p

er a

eros

ol s

urfa

ce, a

rb. u

nits

-0.01

0.00

0.01

c. ATD

b. CaCO3

a. SiO2

Figure 3.7. Online record of uptake experiments between gaseous HNO3 and aerosol

particles of different materials. Panel a,b,c represent the reactions with the aerosol composed of silica, calcium carbonate and Arizona Test Dust, respectively. The time

period 0-20 min corresponds to the background readings of the detector. The grey bar (20-40 min) corresponds to the time when aerosol was present in the flow reactor. The

HNO3 gas phase concentration in the flow tube was 1012 cm-3 and RH 33%.

This result is in agreement with the Knudsen cell study of Underwood et al. (2001) who reported the uptake of HNO3 to a SiO2 surface “too low to be measured”. Goodman et al. (2001) studied the heterogeneous reaction of silica powder with gaseous nitric acid using transmission FT-IR spectroscopy and classified the SiO2 as a non-reactive neutral oxide with respect to this reaction. The authors also concluded that the adsorption of nitric acid on silica surface is reversible at 296 K. This is also in agreement with the data of the present study because HNO3 reversibly adsorbed to the SiO2 particles in the flowtube is desorbed in the denuder and not detected in the aerosol phase. This reversible nature of HNO3 adsorption on silica surfaces was also reported by Dubowski et al. (2004) who found no significant amounts of covalently bonded nitrate on glass and quartz surface after exposure to HNO3.

In contrast, CaCO3 particles are more reactive with respect to nitric acid than ATD, as also shown in Fig. 7. The uptake to CaCO3 is almost 4 times higher than to ATD. This is not surprising, since the reactivity of CaCO3 with HNO3 is well known, while ATD contains only little CaCO3 but much more of the less reactive silica and alumino-silicates. This is in agreement with the studies of Krueger et al. (2003, 2004), which showed the formation of Ca(NO3) in single CaCO3 and authentic dust particles as a reaction product at conditions close to the experimental conditions of this study (RH 38%, HNO3 concentration 4.6×1011 molecules cm-3).


To some extent the nature of the mineral surface under the humid conditions of the present study could be rationalized from the way how the major mineral constituents are expected to dissolve in near neutral or acidic aqueous solution [Desboeufs et al., 2003; Schott and Oelkers, 1995]:

4422 2)( SiOHOHquartzSiO ⇔+ (8)

)()(3/83/13/1+++ +⇔+ KNaalbiteedhydrogenetSiOAlHHAlbite (9)

++ +⋅⇔−++ 32223/83/13/1 3/1)()3/2( AlOnHSiOOHnHSiOAlH (10)

+−+ +⇔+ 233 )( CaHCOHcalciteCaCO (11)

This list of reactions is not complete and one should also consider the dissolution of minor components of the ATD: microcline, illite, etc. Most of these minerals dissolve similar to albite and some could additionally release Mg2+ cations. Exposure of ATD to HNO3 under humid conditions certainly helps promoting these hydrolysis processes by providing protons. Even though these hydrolysis processes might not be complete on the surface, especially at relatively low humidity, partial solvation might be enough for providing a reactive site to HNO3.

Effect of humidity on uptake

The hydrolysis reactions (8) to (11), which might promote the surface (and eventually also bulk) reactivity of dust towards HNO3, are directly suggesting that a significant humidity dependence should exist. The effect of variation in relative humidity on the uptake coefficient was investigated using a fixed HNO3 concentration of 1012 molecules cm-3 and a fixed reaction time of 1.9 s. The data displayed in Figure 3.8 indeed show that γ increases steadily from 0.022±0.007 at 12% RH to 0.113±0.017 at 73% RH. A possible explanation to this is the increasing amount of H2O adsorbed on the surface of ATD particles, which may promote the hydrolysis processes. Some information on the amount of water associated with ATD aerosol can be obtained from the hygroscopic growth of ATD aerosol particles investigated in a previous study [Vlasenko et al., 2005].


0 20 40 60 800.00

0.05

0.10

0.15

0.20

0

1

2

3

4

5

6

upta

ke c

oeffi

cien

t γ

relative humidity, %

num

ber o

f H2O

laye

rs

Figure 3.8. Uptake coefficient of nitric acid to ATD mineral dust aerosol as a function

of relative humidity. Open circles represent the experimental values of the uptake coefficient reaction time 1.9 s, the concentration of nitric acid in the flow tube was

1012cm-3). The solid line represents a BET isotherm (Eq.8, c=8) for water adsorption, scaled to match the uptake data.

The main conclusion was that the ATD particles adsorb water under increasing RH conditions, to some degree related to the presence of water soluble material. However, the small size changes did not allow retrieving a well resolved water adsorption isotherm. For bulk oxide materials it has been shown that several monolayers of water can be formed on the surfaces of SiO2, Al2O3 and CaCO3 with increasing relative humidity [Al-Abadleh and Grassian, 2003; Goodman et al., 2001; Goodman et al., 2000]. Similar behavior was shown for water adsorption on a borosilicate glass surface [Dubowski et al., 2004; Sumner et al., 2004]. These authors have adapted the BET equation [Adamson, 1982] to describe water adsorption on the surface of solids. We used the same approach to calculate the isotherm for the adsorption of water by

))1(1)(1(2 RHcRHRHс

OH −−−=Θ (12)

One can see in Fig. 8 that this isotherm can be well fitted to the experimental data of the uptake coefficient of HNO3 on ATD. This observation continues the row of “BET isotherm like” humidity dependent heterogeneous reactions on solid surfaces: HNO3(g)+NaCl(s) [Davies and Cox, 1998], HNO3(g)+CaCO3(s) [Goodman et al., 2000], NO2+1,2,10-anthracenetriol (s) [Arens et al., 2002] and NO2+borosilicate glass [Finlayson-Pitts et al., 2003], in all of which hydrolysis of the substrate provides the reactive components. This contrasts other heterogeneous processes, such as oxidation reactions [Adams et al., 2005; Pöschl et al., 2001], in which water adsorption competes with the adsorbing gaseous reactant, so that humidity has an inhibiting or no effect on the overall process. Furthermore, the increase of the uptake coefficient with humidity measured in this study is consistent with data from the ACE-Asia field campaign, where the mass accommodation coefficient of HNO3 on ambient dust was found to depend on RH [Maxwell-Meier et al., 2004].


Comparison to literature data

Several aspects should be considered, when comparing the uptake data of the present study to those currently available in the literature as listed in Table 3.3. Our data suggest a strong humidity dependence of the uptake coefficient, while the previously available kinetic studies were all performed under completely dry conditions. If we would extrapolate our data along the water adsorption isotherm to very low humidity (0.1% RH) in Fig. 8, we expect the uptake coefficient to get into the range of 10-3 for ATD, and a similar shift might be expected for the uptake on CaCO3, if we would assume a similar dependence on humidity. Hanisch et al. (2001) report an about a factor of 2 change in the uptake coefficient on CaCO3 measured under dry conditions, when water remaining after evacuation was further removed by baking the dust powder.

Table 3.3. Uptake coefficient measured for aerosol particles of different composition. KC, DRIFTS and FT are abbreviations of Knudsen Cell, Diffuse reflectance infrared spectroscopy and Flow Tube reactors, respectively. Only average values of uptake

coefficients and orders of magnitude HNO3 concentrations are given for conciseness. *values are given for nonheated and heated sample respectively. ** uptake measured at

RH 33%.

Study Reactor Sample HNO3Conc, cm-3

Composition

SiO2 CaCO3 ATD Fenter et al. 1995 KC powder 1010-1013 0.15

Goodman et al. 2001 Goodman et al. 2000

DRIFTS KC

powder powder

1014-1015

101210-9

2.5×10-4

Johnson et al. 2005 KC powder 1011 2×10-3 Hanisch et al. 2001 KC powder 1014-1015

1011-1012 0.18, 0.1*

0.06 Seisel et al. 2004 DRIFTS powder 1011-1012 0.012

this work** FT aerosol 1012

1011<5×10-4 0.11 0.03

0.11

A second aspect relates to the issue of surface area to normalize the uptake according to equation (2). The significant disagreement between the values reported by Goodman et al. (2000) and by Johnson et al. (2005) on the one hand and those reported by Fenter et al. (1995) and Hanisch et al. (2001) on the other are due to different ways of taking into account internal surface areas of the powders used. Goodman et al. (2000) measured the specific area of CaCO3 powder with a BET method and applied the Keyser-Moore-Leu model [Keyser et al., 1991] to account for the contribution by the internal surface area, while Fenter and Hanisch referred to the geometric sample surface area to calculate the collisional flux in the molecular flow regime to the external powder surface. In our study with suspended aerosol particles, we estimate the reactive surface area from the measurement of the mobility diameter (measured by SMPS) and assuming that the particles are spherical, even though it has been shown [Vlasenko et al., 2005] that the


ATD particles used in this study are not perfectly spherical and could be to some degree agglomerates, especially for particle sizes larger than 200 nm. Experimentally, the relation between the surface of aerosol agglomerates available for reaction and the surface measured by SMPS is only known for a perfectly sticking (γ=1) species, namely 211Pb atoms. Rogak et al. (1991) experimentally proved that the mobility diameter measured by SMPS is equal to the “mass transfer” diameter not only for spherical particles but also for complex agglomerates, namely soot. Other existing theoretical approaches to account for the additional internal surface of aerosol agglomerates rely strongly on empirical parameters [Naumann, 2003; Xiong et al., 1992]. In the absence of a more accurate way to evaluate the true dust surface area (available for reaction with HNO3) we used the surface area measured by the SMPS system to calculate the uptake coefficient. Bearing in mind that only part of all the particles are slightly agglomerated (particles larger than 200 nm) we guess a not more than 20% systematic underestimation of the dust surface area given by the SMPS. Therefore, the apparent agreement of the uptake coefficients observed under humid conditions of this study with those of Hanisch and Fenter for CaCO3 and Hanisch and Seisel for ATD under very dry conditions might be accidental, And our data might be in closer agreement with the Goodman and Johnson data than it would seem at first glance.

Implications for the hygroscopic properties of ATD aerosol

One of the atmospherically relevant consequences of the heterogeneous interaction between HNO3(g) and mineral dust is the associated change of hygroscopic properties of the dust particles. Figure 3.9 shows the hygroscopic growth of ATD particles before and after reaction with gaseous nitric acid and water vapor.

0 20 40 60 80 1000 20 40 60 80 100

1.00

1.02

1.04

1.06

1.08

(a)

Relative humidity, %

Gro

wth

fact

or, D

/D0

(b) nonprocessed / hydration nonprocessed / dehydration processed / hydration processed / dehydration

Figure 3.9. Hygroscopic growth of ATD particles before (circles) and after (squares)

reaction with gaseous nitric acid (3×1013 molecules cm-2) and water vapour. (a): circles represent ATD particles with D0=100nm before reaction, squares represent particles of the same size after reaction with HNO3 at 30% RH. (b): circles represent ATD particles before reaction, squares represent ATD particles after reaction with HNO3 at 85% RH. D0 is the mobility size of monodisperse particles at lowest RH at 20°C. Open and solid

symbols correspond to hydration and dehydration curves, respectively.


When dust particles are exposed to HNO3 (3×1013 molecules cm-3) for 3 min at 30% relative humidity the hygroscopic properties do not change significantly (Fig. 9(a)). The exposure in this experiment corresponds to the integrated exposure of an atmospheric dust particle to 0.1-1ppb of HNO3 during a typical life-time of 1-10 days, even though we are aware that in view of the concentration dependence observed this might be an over-simplification. The ATD hygroscopicity is significantly changed after weathering the particles at the same concentration of HNO3 and 85% RH. Fig. 9(b) shows that after the exposure the particle diameter is increased by 7% while increasing the humidity from 10% to 78%. This finding is consistent with the kinetic data that show a strong effect of humidity on the speed and extent of processing of the dust particles by HNO3 (Figure 3.8).

Even though the formation of a liquid phase is thermodynamically not favored (for the pure HNO3-H2O system) under our conditions, we nevertheless assume that the concomitant exposure to nitric acid and high humidity of 85% over longer time scales promotes the significant dissolution of the particle surface material through reactions (9) to (12) [Desboeufs et al., 2003; Schott and Oelkers, 1995]. Desboeufs et al. (1999) studied the dissolution rates of different elements from Sahara dust at different pH and reported the increasing solubility sequence Si<Mg<Ca<K<Na. From these data we concluded that the uptake of HNO3 from the gas phase to mineral dust particles at 85% RH increases the acidity of the adsorbed water layers and strongly promotes the dissolution of major minerals. The weathering of silica is believed to be small on a time scale of our experiment (3 min). In our experiments, after the processing the aerosol was dried, and possibly separate phases of Ca/Mg/Na nitrates are formed on the particles. Laskin et al. (2005) interpreted the increase the O,N atomic content of the Ca-rich mineral dust particles after HNO3(g) exposure as formation of calcium nitrate. We use the assumption (that the reaction product is Ca(NO3)2) to calculate the amount of product built up after processing of ATD with gaseous nitric acid. Using the approach of Saathoff et al. (2003) and assuming spherical shape of particles and independent additive hygroscopic behaviour of the different components one may calculate the volume fraction of Ca(NO3)2 coating on the processed particles according to the following equation:

33)(

33

23 nonprocNOCa

nonprocproc

GFGFGFGF−

−=ε (13)

where GFproc and GFnonproc are hygroscopic growth factors of the processed and nonprocessed ATD particles, respectively. GFCa(NO3)2 is a hygroscopic growth factor of pure calcium nitrate, which is quite similar to the other soluble nitrates in dust (sodium nitrate and magnesium nitrate).

Using the measured values of the hygroscopic growth factors (Fig. 9(b)) for the ATD and the literature value for the calcium nitrate hygroscopic growth [Tang and Fung, 1997] as a proxy for the behaviour of the soluble nitrates, we calculate a 8% volume fraction of Ca(NO3)2 in the processed particles. Expressed as an external coating, this corresponds to an about 1 nm thick layer. This rough estimate shows that several


monolayers of hygroscopic reaction products could be formed on the surface of the ATD particles as a result of processing with gaseous nitric acid. Note that extrapolation of our processing experiment to atmospheric conditions is only valid under the assumption that the degree of processing is a function of the integrated exposure only and not a function of HNO3 concentration during the exposure.

3.5 Atmospheric Implications

The major atmospheric implication of this study is the experimentally determined humidity dependence of the heterogeneous uptake on the dust aerosol. It has been shown that increasing the relative humidity promotes the uptake of nitric acid. However, recent modeling studies [Bauer et al., 2004; Bian and Zender, 2003; Tang et al., 2004] consider the heterogeneous reactivity of the dust independent of relative humidity. While the order of magnitude of the uptake coefficient used by Bauer et. al .(2004) based on the data of Hanisch and Crowley (2001) obtained under very dry conditions (RH<1%) is similar to what we report here, we strongly suggest that a humidity dependent uptake coefficient scaling along a water isotherm as shown in Figure 3.8 could be used in modeling studies. Implementing this dependence into global dust models will certainly reduce their uncertainty.

Another atmospherically relevant outcome of the present study is the experimental evidence that extensive processing of mineral dust by HNO3(g) and possibly other acidic gases results in significant hygroscopic growth. The enhanced water uptake by dust particles increases their interaction with solar radiation. To illustrate this effect we estimate an increase of the dust single scattering albedo (SSA). SSA is the commonly used measure of the relative contribution of absorbing aerosol to extinction and is a key variable in assessing the climatic effect of the aerosol [Seinfeld et al., 2004]. Assuming that the processing does not change the refractive index (n=1.52, k=0.00133) of the dust shown on Fig.9(b) and only increases the particle size we estimate 3% increase of the SSA for the aged dust at 550 nm wavelength of incident light. This calculation is very crude but indicates the potential of the dust aging process to affect the radiation balance of the planet.

Acknowledgements

We would like to thank Mario Birrer for his excellent technical support. We also greatly acknowledge the staff of the PSI accelerator facility for their efforts to provide stable proton beam.


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Modelling of heterogeneous reaction kinetics 65

4 Kinetic modelling of the heterogeneous reaction between gaseous nitric acid and Arizona Test Dust aerosol particles

To be submitted to: Journal of Physical Chemistry and Chemical Physics. Title: Kinetics of the heterogeneous reaction of nitric acid with Arizona Test

Dust particles: a flowtube study Authors: A.Vlasenko1,2, T. Huthwelker1, H.W. Gäggeler1,2 and M. Ammann1

1Laboratory of Radio- and Environmental Chemistry, Paul Scherrer Institute, Villigen PSI CH-5232, Switzerland 2Department for Chemistry and Biochemistry, University of Berne, Bern CH-3008, Switzerland

66 Modelling of heterogeneous reaction kinetics

4.1 Abstract In the study presented here the heterogeneous reaction of HNO3 and mineral dust aerosol has been studied in the flow reactor at atmospheric relevant conditions (298K , ~1 atm, 6-60% RH). The uptake of nitric acid from gas phase was found to depend on HNO3 and H2O concentrations in the gas phase. A reaction mechanism is suggested to describe the heterogeneous interaction, involving gas-surface diffusive transport, Langmuir type adsorption and surface reaction. This mechanism in combination with the PRA kinetic framework [Pöschl et al. 2005] is able to reproduce the observations in a broad range of experimental conditions. Using radioactive labeled molecules it is shown that the number of reactive surface sites on the dust surface is efficiently depleted at higher HNO3 concentration in the gas phase. The number of important kinetic parameters is derived which should be used in atmospheric chemistry models to improve the reliability of the effects of relative humidity on dust aging.


4.2 Introduction

Atmospheric aerosols play a significant role in many important environmental issues including climate change, stratospheric ozone depletion, smog, acid rain and adverse health effects. Among the several types of ambient particles dust represents the largest annular average burden [Textor et al., 2005]. According to several estimates [Dentener et al., 1996; Ginoux et al., 2001] the annular dust emissions increase due to the expansion of arid areas. The main source of particles is the continuous process of soil and rock erosion combined with saltation driven by the action of strong air currents [Alfaro et al., 1997; Shao and Raupach, 1993]. Deserts are the particular places of our planet, which generate large amounts of dust and eject it to atmosphere. Once lifted into the air some particles may survive along range transport, for instance, from the Sahara to the American continent [Prospero, 1999; Reid et al., 2003].

During this several day transport, dust particles are exposed to various trace gases and may react with them. This processing might affect the chemical and physical properties of the aerosol; on the other hand, it may also impact the global budget of important trace gas compounds. Among the many potential reactions occurring, the reaction of mineral dust aerosol with HNO3 might be important as it reduces the global ozone burden by 5% [Bauer et al., 2004; Bian and Zender, 2003; Liao and Seinfeld, 2005].

The reaction of nitric acid with dust particles is also important because it provides a pathway for the formation of a water soluble coating, which is hygroscopic. It has been recently reported that mineral dust from source regions containing high levels of calcium, namely China loess dust and Saudi coastal dust, may exhibit continuous, extensive reactivity with nitric acid resulting in formation of highly hygroscopic calcium nitrate particles [Laskin et al., 2005]. The aging of dust particles renders them effective cloud condensation nuclei (CCN), which could affect the formation of “warm“ clouds [van den Heever et al., 2005]. On the other hand, the water soluble coating on dust particles could possibly suppress the formation of ice crystals and hence, decrease the efficiency to become ice nuclei [Ansmann et al., 2005; Archuleta et al., 2005]. Currently, the question to what degree processing of dust particles by trace gases affects heterogeneous ice nucleation is subject of laboratory and field investigations. Therefore, it is important to know the kinetic details of the interaction between HNO3(g) and mineral dust.

In the laboratory, the heterogeneous interaction of nitric acid with mineral dust was studied on authentic dust samples (Sahara dust, Arizona Test Dust, China loess) as well as on dust surrogates (dust clay minerals, synthetic mineral oxides). Several studies of this reaction by other researchers have been performed on bulk powder samples in a Knudsen cell reactor [Hanisch and Crowley, 2001a; Hanisch and Crowley, 2001b; Johnson et al., 2005; Usher et al., 2003] and using FTIR and DRIFTS [Borensen et al., 2000; Goodman et al., 2001; Goodman et al., 2000; Seisel et al., 2004]. Depending on the kinetics, the use of bulk powder samples may introduce significant uncertainties due to diffusion of the gas molecules into the interior. There is still an open and unresolved debate about the proper way to define the area of exposed dust, which is rather critical for


an assessment of the uptake kinetics and its extrapolation to atmospheric conditions. As an alternative, dust particles may be exposed to a trace gas as aerosol in gas-suspension, which allows standard aerosol sizing technology being used to assess the exposed surface areas. The inherent mass transport limitations induced by diffusion at high pressures may be overcome by constraining the size range to below micrometer, where the effect of gas phase diffusion on the uptake kinetics is small or negligible. Recently, we have developed a technique to generate submicron aerosol from powder samples [Vlasenko et al., 2005a]. This method has been combined with a flow reactor to measure the kinetics of the heterogeneous reaction with gaseous nitric acid. The first experiments showed a significant concentration and humidity dependence of the uptake coefficient [Vlasenko et al., 2005b], which was consistent with the earlier reported laboratory studies on calcium carbonate [Fenter et al., 1995; Goodman et al., 2000] and with findings from field measurements[Maxwell-Meier et al., 2004]. The observed behaviour of the uptake coefficient suggested a kinetics affected by adsorption and the limited availability of reactants associated with the dust aerosol surface. Similar indications were also retrieved from the investigation of the reaction of HNO3 on γ-Al2O3 surface [Seisel et al., 2004], who also invoked a mechanism involving an adsorbed precursor. In this work we present more detailed kinetic data for the system Arizona Test Dust+HNO3(g), including an analysis of time dependent features and a larger range of concentrations. To support the analysis we will use a kinetic model, which has been recently developed by Pöschl, Rudich and Ammann [Ammann and Pöschl, 2005; Ammann et al., 2003; Pöschl et al., 2005] further to referred as PRA model. The PRA model is a comprehensive kinetic model framework, which allows describing mass transport and chemical reactions at the gas-particle interface in systems with multiple chemical components and competing physicochemical processes. The present study aims to provide the key kinetic parameters of the heterogeneous reaction between mineral dust aerosol and gas phase nitric acid, which can then be used in atmospheric chemistry models in a straightforward way.


4.3 Experimental The experiments were carried out in a flow tube reactor at atmospheric pressure and room temperature. This technique has been previously used to study the kinetics of heterogeneous interactions between trace gases and different types of aerosol [Ammann, 2001; Guimbaud et al., 2002; Wachsmuth et al., 2002]. The detailed description of the experimental set-up and measurement procedures pertinent to the present study has been published elsewhere[Vlasenko et al., 2005b]. Therefore, in this study we refer only to the main features. The unique feature of these experiments is the use of short-lived radioactive isotope 13N with a half-life of 10 min facilitating detection of uptake to aerosol particles under atmospheric conditions with regard to temperature, pressure and humidity [Ammann, 2001]. Radioactively labelled nitrogen monoxide (NO) is produced at the PROTRAC facility (Paul Scherrer Institute, Switzerland). 13NO is carried in a flow of a 20 % oxygen in helium to the laboratory through a 580 m long capillary. A fraction of this flow is mixed with nitrogen buffer gas. Additional amounts of non-radioactive 14NO can be added from a certified cylinder to vary the total concentration of NO within a range of 1011 cm-3 to 1012 cm-3. Nitrogen monoxide is then oxidized to nitrogen dioxide (NO2) by reaction with O3. NO2 is further oxidized to nitric acid by reaction with OH by irradiating the humidified mixture of NO2 in nitrogen (and traces of He and O2) with 172 nm radiation using an eximer lamp. This results in a continuous flow containing a mixture of H13NO3 and HNO3. Mineral dust aerosol is produced by dry resuspension of Arizona Test Dust powder (Ellis Components, England) using a modified commercial dust generator and size-separators [Vlasenko et al., 2005a]. Particles with diameters larger than 800nm and those carrying electrical charge are removed to maintain a stable output and to prevent particle deposition on the electrically insulating reactor wall. The aerosol size, surface and number distribution is measured by a scanning mobility particle sizer (SMPS, TSI Inc.). Aerosol humidity could be adjusted by passing the flow through a tube with a H2O permeable Goretex membrane immersed in water. The relative humidity is measured using capacitance sensors. The flow tube is a reactor with cylindrical geometry. Gaseous nitric acid is introduced via an injector along the axis of the flow reactor. The injector is a PFA tube which could be moved along the axis of the reactor. The position of the injector determines the gas-aerosol contact or reaction time. When the injector is pushed in to the maximum position (0 cm) inside the flow reactor or minimum (45 cm) then the reaction time is minimum (0.2 s) and maximum (2 s), respectively.The end of the injector tube is supplied with a special plug so that the gas enters the flow reactor through small openings at the end of the injector, perpendicular to the flow of aerosol. The flow tube is operated under laminar flow conditions, and it is assumed that the laminar flow profile is established a few cm downstream of the injector. The relative humidity of the flow is continuously measured downstream of the reactor. The gas leaving the flow reactor


enters a parallel-plate denuder train. The latter captures gaseous species HNO3(g), HONO(g), and NO2(g) by a series of selective coatings due to the high diffusion mobility. Sub-micron aerosol particles have a small diffusion mobility and pass through the denuder without being collected. Passing through the denuder the flow is drawn through the filter, which retains aerosol particles. To each trap (the denuder coatings and the aerosol filter) a separate CsI scintillator crystal with integrated PIN diode is attached (Carroll and Ramsey, USA) which detects the gamma quanta emitted after decay of the 13N nuclei. The detector signal is converted to the flux of the gaseous species into the trap using the inversion procedure reported elsewhere [Rogak et al., 1991]. This flux is proportional to the concentration of the species in the gas phase.

Table 4.1. Measurement conditions

Parameter Value

Reaction time 0.2-2 s

Concentration H13NO3 ~105 cm-3

Concentration of HNO3 1011-1012 cm-3

Relative humidity 12-73 %

Total flow 0.7 Lpm

Total pressure ~ 1 atm


4.4 Results and discussion Flowtube experiment The interaction of mineral dust aerosol with gaseous nitric acid has been studied at T=298K with HNO3(g) concentrations between 1011-1012 cm-3 and relative humidity in the range 12%-73%. Fig. 4.1 shows a typical data record of the kinetic experiment. A more detailed description of the raw signals and their analysis has been given elsewhere [Vlasenko et al., 2005b]. The sequential increase of the reaction time goes along with the depletion of H13NO3 in the gas phase (panel B) and with the increase of H13NO3 or its products in the particulate phase (panel C). The observation also shows that there is a steady state drop of the gas phase concentration of H13NO3 during passage through the flow reactor even without aerosol (panel A). This is not due to an irreversible chemical loss of HNO3 to the wall, but rather due to retention driven by adsorption and desorption. When considering the non-labelled HNO3 molecules, this effect leads to the well-known slow response time of this sticky gas measured at the reactor outlet when switching it on and off. At low concentrations, the observed response time is directly related to the average residence time of individual molecules in the flow tube. If this residence time is comparable to the half-life of the radioactive 13N-tracer, 10 min., a drop in the H13NO3 concentration along the flow tube can be observed, while the concentration of the non-labelled HNO3 concentration remains constant, if equilibrium with the wall is established. Note that the flow tube was frequently exchanged to avoid build up of significant amounts of dust particles on the walls, which would lead to irreversible loss of HNO3.

0.3

0.8

0.3

0.8

-0.1

0

0.1 C

reaction time, s0

2

Det

ecto

r sig

nal,

r.u.

A

B

D

Figure 4.1. Plot of 13N-labelled nitric acid concentrations in gas and aerosol phases as a function of injector position in the flowreactor. A: the concentration in the gas phase

when no dust particles are present. B: the concentration in the gas phase when dust particles are present. C: the concentration of H13NO3 in the aerosol phase. D: the

position of the injector, 45 cm corresponds to the maximal reaction time.


Uptake mechanism and PRA model To support the analysis of our data as shown in Figure 4.1 and to determine elementary kinetic parameters of the uptake mechanism in the present case, we used the PRA framework model [Pöschl et al., 2005]. This framework assumes a double-layer surface connected to the gas and bulk phases and is entirely based on fluxes. The following features are of relevance to the present situation: volatile species can adsorb into the sorption layer and react with other species present either in the sorption layer (i.e., other adsorbing gases) or in the surface layer (i.e., bulk dust constituents). The consistency of the PRA model for such cases has been demonstrated by comparison of model simulations and experimental data for several heterogeneous uptake studies [Ammann and Pöschl, 2005; Ammann et al., 2003].

Table 4.2. Reactions and rates of heterogeneous interaction between radioactive-labeled gas phase nitric acid (A*=H13NO3) and surface of mineral dust aerosol.

Qualifiers (gs) and (s) correspond to gas phase and surface, respectively. [A*]gs and [A*]s are the average gas phase (number per unit volume) and the surface (number per unit

area) concentrations of H13NO3, respectively. [A]s and [B]s are surface concentrations of the non-labeled nitric acid (A=HNO3) and reactive sites. Gas phase concentration is

expressed in “number per unit volume”, surface concentrations are expressed in “number per unit surface”

Process Reaction Rate collision

adsorption

A*(gs)+ Surface → A*(s) *colJ = [A*]gsω/4

*adsJ = (1-σ([A*

colJ *]s +[A]s))

(1)

(2)

desorption A*(s) → A*(gs) *desJ = [A*]s

1−dτ (3)

product formation A*(s) + B(ss) → Product* PP

*= k [A*] [B]s ss (4)wall loss A*(gs) + reactor wall→ d[A*]gs/dt = -kw[A*]gs (5)

The main aim of the present PRA model application is to fit the time dependent uptake data as shown in Figure 4.1. Note that in our previous work we have made a simple, pseudo-first order analysis of data such as in Fig. 4.1 to derive uptake coefficients. That analysis does not take into account the time dependent features and was not able to explain the observed concentration dependence. As our detection method only observes the radioactively labeled molecules, we will make full use of the multicomponent capabilities of the PRA approach and explicitly treat labeled and non-labeled HNO3 molecules separately. This allows fitting the quantities associated with H13NO3 to the data, while at the same time the corresponding quantities related to non-labeled HNO3 can be inferred. The starting point of the present PRA model application is a definition of reaction mechanism. For the heterogeneous system used in this study we consider the interactions of two chemically identical gas species H13NO3(labeled) and HNO3(non-labeled) with the surface of mineral dust aerosol particles. The following processes are taken into account (Tab. 4.2): collision of gas molecules with the aerosol


surface (1), adsorption into the sorption layer (2), desorption from the sorption layer to the gas phase (3), reaction of the adsorbed molecule with a reactive component of the dust surface (4) and effective loss of the labeled gas phase H13NO3 molecules due to interaction with the reactor wall (5). Since the labeled and non-labeled HNO3 molecules are chemically identical the reaction mechanism described above can be applied also for the non-labeled HNO3 molecules (Table 4.3). The only difference is that there is no wall loss of the non-labeled molecules on the reactor wall. Following the PRA formalism the rates of the processes are defined in terms of mass fluxes (Table 4.2, Table 4.3). The flux of collisions with the surface is proportional to the average concentration of HNO3 molecules in gas phase and their mean thermal velocity ω (3×104 cm s-1).

Table 4.3. Reaction mechanism and rates of heterogeneous interaction between gas phase non-labeled nitric acid and surface of mineral dust aerosol. The explanation of the

symbols see the caption of Table 4.2.

Process Reaction Rate collision

adsorption

A(gs)+ Surface → A(s) Jcol = [A]gsω/4 Jads = Jcol(1-σ([A*]s +[A]s))

(6)

(7)

desorption A(s) → A(gs) Jdes = [A]s 1−dτ (8)

product formation A(s) + B(ss) → Product P= k [A]s[B]ss (9) The PRA framework also contains a correction factor applicable to the collision fluxes (1) and (6) to correct for the effect of gas phase diffusion. . However, for the net uptake coefficients and particle sizes pertaining to the present experiments, the gas diffusion resistance correction is small (less than 5%) and for this reason is not considered in the present work. The primary interaction of HNO3 with the dust surface is assumed to be a reversible Langmuir-type adsorption mechanism. This type of mechanism was suggested earlier for the reaction of gaseous nitric acid with the solid surface of NaCl [Davies and Cox, 1998]. This approximation involves two parameters: the effective molecular cross section of the HNO3 molecule in the sorption layer σ and the characteristic desorption lifetime τd. The reaction of the adsorbed molecules leading to the product formation is assumed to be a second order reaction with a constant k. The effective loss of the labeled gas phase H13NO3 molecules due to interaction with the reactor wall can be approximated as an apparent first-order process on the basis of the disappearance of H13NO3 in absence of aerosol in the reactor (Fig. 4.1 A). Surface-to-bulk exchange (e.g., by diffusion) transport is considered to be negligible on the time scale (maximum reaction time 2s) of our experiments. The surface concentration of nitric acid and the formation of the surface products are governed by the following mass balance equations: d[A*]s/dt = - -P*

adsJ *desJ * (10)

d[A]s/dt = - -P (11) adsJ desJd[Products*]/dt = P* (12) d[Products]/dt = P (13)


This set of coupled differential equations together with the equations from Tab. 4.2 and Tab. 4.3 is solved numerically by iterative integration using a standard spreadsheet program (Microsoft Excel). The model outputs are the concentrations and the uptake coefficients as a function of the reaction time. The concentration time profiles can be compared to the experimental data. Required input parameters are the initial concentrations of gas phase nitric acid (radioactive-labeled and non-labeled) and the corresponding mass transport and reaction rate coefficients. As much as possible, the model parameters are constrained by the experimental observations. The retention losses of the H13NO3 are directly measured in absence of aerosol in the flow tube. Those parameters which are not measured in our experiments are estimated on the basis of data taken from the published studies of heterogeneous interactions of HNO3. The effective molecular cross section σ of HNO3 in the sorption layer used in this study is 3×10-15cm-2. This value is comparable to the molecular diameter of HNO3 molecule (6×10-15cm-2) used by Davies et al. [Davies et al. 1998]. to describe the reversible adsorption of nitric acid on the NaCl surface. To our knowledge the desorption lifetime τd of HNO3 on the surface of mineral dust is not known. For estimation we used the published data of Baltensperger et al. [Baltensperger et al., 1993] who studied the adsorption of HNO3 on several materials similar to mineral dust. Using the published adsorption enthalpy of HNO3 on silica powder (-53kJ mol-1) we calculate the characteristic desorption lifetime by a Frenkel-type equation: τd = τ0 exp (-ΔHads/RT) (14) where τ0 is a characteristic time of adsorbate-surface vibration (~10-13s), R is the molar gas constant and T is absolute temperature. The calculated value is 10-4 s, much smaller than that of used by Fenter et al. [Fenter et al. 1995] to estimate the desorption rate of HNO3 from the surface of sea salt (~1s). We believe that the four orders of magnitude difference comes from the fact that the measurements on silica were done at low temperature conditions (230-263 K) and extrapolation of this enthalpy value to 298 K temperature could result in a significant error. In the present study we used an intermediated value of 10-1s for the characteristic desorption lifetime τd of HNO3 on the surface of mineral dust. Time dependence of uptake The time profiles of the gas phase concentrations of labeled and non-labeled HNO3 are given in Fig. 4.2 for two different total concentrations of nitric acid in absence of aerosol. In the simulations, the concentration of the non-labeled nitric acid is kept constant since in the absence of aerosol a constant steady state HNO3(g) concentration profile is established within a few minutes. The time profile of the H13NO3(g) concentration is fitted to the experimental data by varying the kw constant. This constant depends on the total HNO3(g) concentration in the reactor and relative humidity. For example, the loss of the H13NO3 increases as a function of time due to stronger adsorption of nitric acid molecules on the surface of the flow tube. The increase of the total


concentration of HNO3 (g) in the reactor goes along with the increase of the H14NO3(g)/ H13NO3(g) ratio and therefore, with the higher coverage of the wall by non-labeled molecules. As a result the labeled HNO3 molecules have less probability to compete for adsorption sites at the wall and hence more labeled molecules “survive” the transport through the flow tube.

0.0 0.5 1.0 1.5 2.0

8x104

1x105

0.0 0.5 1.0 1.5 2.00

5x1011

1x1012

0.0 0.5 1.0 1.5 2.00

5x1010

1x1011

0.0 0.5 1.0 1.5 2.0

8.0x104

1.0x105

reaction time, s

B

A

gas

phas

e co

nc.,

cm-3

reaction time, s

Figure 4.2. Time profiles of the labelled and total HNO3 gas phase concentrations. No aerosol present in the flow reactor. The total concentration of nitric acid (solid line) is 1011 cm-3 (panel A) and 1012 cm-3 (panel B). The relative humidity is 33%. Dashed line and open squares represent the model fit and experimental data for the labeled HNO3,

respectively.

The results of the simulated and measured concentration time profiles in the presence of aerosol are given in Figure 4.3. The concentrations of the labeled nitric acid in the gas phase as function of time is taken from the data shown in Figure 4.2. The concentration the non-labeled nitric acid in the gas phase is kept constant, similar to the case when the aerosol is absent. This approximation significantly simplifies the procedure of the numerical simulation and is justified since the experimentally measured depletion of the gas phase H13NO3 concentration due to uptake to the particles is less than 10% (Figure 4.1). Two independent experimental data series of H13NO3(p), which were conducted at the same conditions except different total concentration of HNO3(g), are fitted by varying the values of the surface reaction constant k and the number of reactive sites on the aerosol surface. The best fit is found with the following parameters: k=4×10-15 cm2s-1· molecule-1 and [B]s=5×1014/molecule·cm-2. In general, the agreement between the simulations and the experiments is good. Once the set of the parameters is found it is used to simulate the time patterns of the particulate concentrations of non-labeled nitric acid. One may notice a difference in the time profiles of particulate HNO3 concentrations for the different total HNO3 concentrations. While the concentration increases linear with time at smaller HNO3(g) total concentration, at the higher gas-phase concentration (1012cm3) the particulate concentration reaches a saturation after about 1 s of the reaction


time. This behavior is observed both for the labeled and non-labeled molecules. While for the labeled molecules, the decay of 13N in response to the retention of H13NO3 on the walls is explaining part of this saturation, for the non-labelled molecules, this saturation might be due either to the depletion of reactive surface sites on the surface or to saturation of the surface with adsorbed HNO3, or a combination of both. The concentration time profiles, shown in Figure 4.2, illustrate the kinetic competition between the labeled and non-labeled HNO3 molecules on the reactor walls. Similarly, on the aerosol surface, the competition starts already at the stage of adsorption when the non-labeled molecules dominate the process of dust surface coverage due to an overwhelming superiority in number concentration. The tenfold increase of the gas-phase concentration of the non-labeled molecules corresponds to a double amount of the reacted non-labeled HNO3 molecules. At the same time, the absolute number of the H13NO3, reacted with the surface is decreased from 2×108 molecule·cm-2 to 4×107 molecule·cm-2, indicating the predominant surface coverage by the non-labeled molecules.

0.0 0.5 1.0 1.5 2.00

2x108

4x108

0.0 0.5 1.0 1.5 2.00

2x1014

4x1014

0.0 0.5 1.0 1.5 2.00

1x1014

2x1014

0.0 0.5 1.0 1.5 2.0

5.0x107

1.0x108

reaction time, s

B

A

aero

sol p

hase

con

c., m

olec

cm

-2

reaction time, s

Figure 4.3. Time profiles of the labelled and nonlabelled HNO3 aerosol phase concentrations. The total gas phase concentration nitric acid is 1011 molecule·cm-3 (panel A) and 1012 molecule·cm-3 (panel B). The relative humidity is 33%. Solid and dashed lines represent the model fits for the non-labelled and labeled HNO3(p) concentrations, respectively. Open circles represent experimental H13NO3(p) data points.

The uptake coefficient γ is a key parameter to describe the heterogeneous interaction between the gas phase and a reactive surface. However, the uptake coefficient is not an elementary kinetic parameter but may depend on time and many other parameters such as gas phase concentration. Following the formalism of the PRA model and usual practice in atmospheric chemistry, the uptake coefficient is defined as the probability that a collision of a molecule with the particle surface leads to its net uptake to the condensed phase. Since the labeled and non-labeled molecules of nitric acid are treated independently, the uptake coefficients are calculated according to the following equations:


γ* = ( - ) / (15) *

adsJ *desJ *

colJγ = ( - ) / JadsJ desJ col (16) where γ* and γ are the uptake coefficients of the labeled and non-labeled HNO3 molecules, respectively. These equations are not identical in the mathematical sense. However, the simulations show that the uptake coefficients, calculated for both types of the molecules are the same, even though the time profiles of e.g. surface coverage are different. This is in agreement with the fact that the kinetic parameters of HNO3 and H13NO3 molecules are the same. So the time profiles of the uptake coefficient showed in Figure 4.3 correspond both to the labeled and non-labeled HNO3 molecules.

0.1 1 10

0.01

0.1

γ

time / s

1011 / molec·cm-3

1012 / molec·cm-3

Figure 4.4. The uptake coefficient of HNO3 on ATD aerosol particles as a function of

reaction time. The total gas phase concentration of nitric acid is 1011 cm-3 (solid line) and 1012 cm-3 (thin line). Relative humidity is 33%. The dotted line represents the modeling

results which lie beyond the time scale of the experiment.

The uptake coefficient decreases with time in both cases. For the higher concentration of total HNO3 in the flow tube the uptake coefficient is smaller and starts to drop earlier. As described above the reason for this is the depletion of the reactive surface sites which starts earlier at the higher total concentration of HNO3 in the flow tube. The decrease of the uptake coefficient with time observed in this study is in qualitative agreement with the theoretical predictions and experimental studies on bulk powders [Ammann et al., 2003; Johnson et al., 2005; Laskin et al., 2005]. However, the time scale of the uptake drop in this work and those in literature are different.


Concentration and humidity dependence of the uptake Figure 4.5 shows the surface concentrations of labeled and non-labeled reaction products as a function of nitric acid gas-phase concentration in the reactor. These curves are calculated by the model using the parameters established for the same conditions by matching the simulated and experimental concentration time profiles (k=4×10-15 cm2s-1· molecule-1 and [B]s=5×1014/molecule·cm-2). The results of the simulation for the labeled molecules agree well with the experimental data. With the increase of [HNO3](g) the concentration of non-labeled surface product (HNO3 irreversibly reacted with the ATD) linearly increases, reaching the plateau (saturation coverage) at [A]s=5×1014 molecule·cm-2. This value equals to [B]s, the initial number of reactive surface sites available for reaction with HNO3(s). It is interesting to compare this value with the literature data. Goodman et al. [Goodman et al., 2001; Goodman et al., 2000] calculated the values of saturation coverages for HNO3 uptake on several types of oxide powders (SiO2, α-Al2O3, TiO2, γ-Fe2O3, CaO, MgO) and calcium carbonate by measuring the pressure drop in the FT-IR cell upon expansion of nitric acid from a known volume. The value measured in our work (5×1014 molecule·cm-2) is within the range of values reported by Goodman et al. (7×1013-7×1015/molecule·cm-2). This agreement is not surprising since the main chemical constituents of the ATD aerosol is silica and Al-rich minerals.

1.E+12

1.E+13

1.E+14

1.E+15

1E+10 1E+11 1E+12 1E+13

HNO3 (g) / molecule cm-3

[B]ss

/ m

olec

ule

cm-2

1E+6

1E+7

1E+8

1E+9

[B]ss

* / m

olec

ule

cm-2

Figure 4.5. Surface concentrations of labeled (dashed curve) and non-labeled (solid

curve) reaction products as a function of total [HNO3](g), calculated by the PRA model. Open squares represent the experimentally measured surface concentrations of labeled

reaction products. Reaction time 2 s, RH 33%.

Keeping all the other parameters of the model fixed (k, τd, ω, σ) and by variation of only parameter [B]s it was possible to match the simulations with the experimental data for different relative humidity conditions in the flow reactor. The results of the simulations are shown in Fig. 4.5 and Fig. 4.6. The agreement between the model results


and experimental data for the labeled molecules is satisfactory. The concentration coverages of non-labeled HNO3 molecules were not measured in our experiment. For comparison we took data from the literature [Seisel et al., 2004], who studied the heterogeneous reaction of HNO3 on the surface of the ATD powder and published the formation rate of NO3

- as a function of HNO3 gas-phase concentration. From these data we calculated the surface concentrations of the reaction product for 2 s reaction time and compared to the results of the model (Fig. 4.6). Good agreement is found when the total gas-phase HNO3 concentration is around 1011 cm-3. Some difference is observed at [HNO3](g) concentrations higher than 1011 cm-3. While our model predicts the saturation of the surface, the concentration dependence, calculated from the data of Seisel et al. [Seisel et al., 2004], exhibits a linear increase. This discrepancy could be possibly explained by the fact that Seisel et al. used powder samples, which have a large surface area compared to the surface area of aerosol mineral dust and thus is hardly saturated by reactant HNO3.

1.0E+12

1.0E+13

1.0E+14

1.0E+15

1E+10 1E+11 1E+12 1E+13

HNO3 (g) / molecule cm-3

[B]ss

/ m

olec

ule

cm-2

1E+5

1E+6

1E+7

1E+8

1E+9

1E+10

[B]ss

* / m

olec

ule

cm-2



reaction products. Solid circles refer to the data of Seisel et al. (2004). Reaction time 2 s, RH 6%.

The modeling yields the following values for the parameter [B]s: 8×1013/molecule·cm-2 for RH 6%, 5×1014/molecule·cm-2 for RH 33% and 1.5×1015/molecule·cm-2 for RH 60%. Therefore, the results of the simulation show that the number of the reactive sites on mineral dust is non-linearly increased with increasing relative humidity. The most likely reason for this is the increased amount of adsorbed H2O molecules which form additional reactive sites on the surface of mineral dust. Similar argument was brought up by Davis et al. [Davies and Cox, 1998] to explain the observed water enhancement of HNO3 uptake on solid NaCl surface. Goodman et al. [Goodman et al., 2001] also observed the enhancement of reactive uptake of HNO3 on the α-Al2O3 and CaO surfaces as a function of relative humidity and suggested that


adsorbed H2O molecules could provide a medium for the dissociation of nitric acid molecules.

1.E+12

1.E+13

1.E+14

1.E+15

1.E+16

1E+10 1E+11 1E+12 1E+13

HNO3 (g), molecule cm-3

[B] s

s / m

olec

ule

cm-2

1.0E+06

1.0E+07

1.0E+08

1.0E+09

[B] s

s* /

mol

ecul

e cm

-2



reaction products. Reaction time 2 s, RH 60%.

4.5 Atmospheric implication

One of the major atmospheric implication of this study is humidity and gas-phase concentration dependence of the uptake coefficient. The current modeling studies [Bauer et al., 2004; Bian and Zender, 2003; Tang et al., 2004] consider the heterogeneous reactivity of the dust independent of relative humidity and [HNO3](g) concentration. In this work it is shown that both RH and gas-phase concentrations of nitric acid do have a strong influence on the uptake. To illustrate this we use the parameters (k,σ), retrieved from the HNO3 interaction with ATD mineral dust particles, and simulate the time profiles of γ for atmospheric relevant conditions. The ambient concentration of nitric acid density varies strongly in space, time and altitude. For estimation we used two values: 1010 molecule·cm-3 for clean remote areas [Neuman et al., 2001] and 1011 molecule·cm-3 for the high concentration in polluted atmosphere [Kondo et al., 2004; Neuman et al., 2001]. The ambient concentration of water vapor also varies quite significantly. For the estimate we used the two RH values 6% and 60%, respectively because firstly, for these conditions the concentrations of the surface reactive sites were determined in our experiment and secondly, this range of RH is ubiquitous in atmosphere. The results of the simulations are given in Fig. 4.8. The dependence of γ with time is not linear, the value of


the uptake coefficient is higher for the higher relative humidity and drops faster with time for the higher concentration of HNO3 in the gas phase. It should be mentioned that the model used here does not include the surface to bulk transport in the dust particle. This process is of minor importance on the time scale of the experiment described, however this effect could play a significant role in atmosphere, considering the lifetime of dust particles. Another problem is the variability of reaction parameters under real atmospheric conditions. For example, once saturated with HNO3 dust particles may partially recover its surface reactivity after exposure to H2O, being brought to high relative humidity [Seisel et al., 2004]. These issues should be addressed in future studies to improve the result of atmospheric chemistry modeling.

10-3 10-2 10-1 100 101 102 10310-6

10-5

10-4

10-3

10-2

10-1

100

[HNO3](g) cm-3 RH,%

1012 6 1012 60 1010 6 1010 60

upta

ke c

oeffi

cien

t γ

time /s

Figure 4.8. The uptake coefficient as a function of time for different values of relative humidity and gas-phase concentration of nitric acid. The curves are calculated by

numerical integration of the system with the following initial conditions: k=4×10-15

/cm2s-1·molecule-1, σ=3×10-15cm-2, [B]s=8×1013 and 1.5×1015/molecule·cm-2, [HNO3](g)= 1×1010 and 1×1011/molecule·cm-3.


4.6 References Alfaro, S.C., Gaudichet, A., Gomes, L., and Maille, M.: Modeling the size distribution of

a soil aerosol produced by sandblasting, J. Geophys. Res.-Atmos., 102, 11239-11249, 1997.

Ammann, M.: Using 13N as tracer in heterogeneous atmospheric chemistry experiments, Radiochim. Acta, 89, 831-838, 2001.

Ammann, M., and Pöschl, U.: Kinetic model framework for aerosol and cloud surface chemistry and gas-particle interactions: Part 2 – exemplary practical applications and numerical simulations, Atmos. Chem. Phys. Discuss., 5, 2193–2246, 2005.

Ammann, M., Pöschl, U., and Rudich, Y.: Effects of reversible adsorption and Langmuir-Hinshelwood surface reactions on gas uptake by atmospheric particles, Phys. Chem. Chem. Phys., 5, 351-356, 2003.

Ansmann, A., Mattis, I., Muller, D., Wandinger, U., Radlach, M., Althausen, D., and Damoah, R.: Ice formation in Saharan dust over central Europe observed with temperature/humidity//aerosol Raman lidar, J. Geophys. Res.-Atmos., 110, doi:10.1029/2004JD005000, 2005.


Baltensperger, U., Ammann, M., Bochert, U.K., Eichler, B., Gäggeler, H.W., Jost, D.T., Kovacs, J.A., Turler, A., and Scherer, U.W.: Use of Positron-Emitting N-13 for Studies of the Selective Reduction of NO by NH3 over Vanadia-Titania Catalyst at Very-Low Reactant Concentrations, J. Phys. Chem., 97, 12325-12330, 1993.



Borensen, C., Kirchner, U., Scheer, V., Vogt, R., and Zellner, R.: Mechanism and kinetics of the reactions of NO2 or HNO3 with alumina as a mineral dust model compound, J. Phys. Chem. A, 104, 5036-5045, 2000.

Davies, J.A., and Cox, R.A.: Kinetics of the heterogeneous reaction of HNO3 with NaCl: Effect of water vapor, J. Phys. Chem. A, 102, 7631-7642, 1998.

Dentener, F.J., Carmichael, G.R., Zhang, Y., Lelieveld, J., and Crutzen, P.J.: Role of mineral aerosol as a reactive surface in the global troposphere, J. Geophys. Res.-Atmos., 101, 22869-22889, 1996.

Fenter, F.F., Caloz, F., and Rossi, M.J.: Experimental Evidence for the Efficient Dry Deposition of Nitric-Acid on Calcite, Atmos. Environ., 29, 3365-3372, 1995.

Ginoux, P., Chin, M., Tegen, I., Prospero, J.M., Holben, B., Dubovik, O., and Lin, S.J.: Sources and distributions of dust aerosols simulated with the GOCART model, J. Geophys. Res.-Atmos., 106, 20255-20273, 2001.


Goodman, A.L., Bernard, E.T., and Grassian, V.H.: Spectroscopic study of nitric acid and water adsorption on oxide particles: Enhanced nitric acid uptake kinetics in the presence of adsorbed water, J. Phys. Chem. A, 105, 6443-6457, 2001.

Goodman, A.L., Underwood, G.M., and Grassian, V.H.: A laboratory study of the heterogeneous reaction of nitric acid on calcium carbonate particles, 105, 29053-29064, 2000.


Hanisch, F., and Crowley, J.N.: Heterogeneous reactivity of gaseous nitric acid on Al2O3, CaCO3, and atmospheric dust samples: A Knudsen cell study, J. Phys. Chem. A, 105, 3096-3106, 2001a.

Hanisch, F., and Crowley, J.N.: The heterogeneous reactivity of gaseous nitric acid on authentic mineral dust samples, and on individual mineral and clay mineral components, Phys. Chem. Chem. Phys., 3, 2474-2482, 2001b.


Kondo, Y., Morino, Y., Takegawa, N., Koike, M., Kita, K., Miyazaki, Y., Sachse, G.W., Vay, S.A., Avery, M.A., Flocke, F., Weinheimer, A.J., Eisele, F.L., Zondlo, M.A., Weber, R.J., Singh, H.B., Chen, G., Crawford, J., Blake, D.R., Fuelberg, H.E., Clarke, A.D., Talbot, R.W., Sandholm, S.T., Browell, E.V., Streets, D.G., and Liley, B.: Impacts of biomass burning in Southeast Asia on ozone and reactive nitrogen over the western Pacific in spring, J. Geophys. Res.-Atmos., 109, 2004.

Laskin, A., Wietsma, T.W., Krueger, B.J., and Grassian, V.H.: Heterogeneous chemistry of individual mineral dust particles with nitric acid: A combined CCSEM/EDX, ESEM, and ICP-MS study, J. Geopyhs. Res., 110, doi:10.1029/2004JD005206, 2005.


Maxwell-Meier, K., Weber, R., Song, C., Orsini, D., Ma, Y., Carmichael, G.R., and Streets, D.G.: Inorganic composition of fine particles in mixed mineral dust-pollution plumes observed from airborne measurements during ACE-Asia, J. Geophys. Res.-Atmos., 109, 2004.

Neuman, J.A., Gao, R.S., Fahey, D.W., Holecek, J.C., Ridley, B.A., Walega, J.G., Grahek, F.E., Richard, E.C., McElroy, C.T., Thompson, T.L., Elkins, J.W., Moore, F.L., and Ray, E.A.: In situ measurements of HNO3, NOy, NO, and O3 in the lower stratosphere and upper troposphere, Atmos. Environ., 35, 5789-5797, 2001.

Pöschl, U., Rudich, Y., and Ammann, M.: Kinetic model framework for aerosol and cloud surface chemistry and gas-particle interactions: Part 1 – general equations, parameters, and terminology, Atmos. Chem. Phys. Discuss., 5, 2111–2191, 2005.


Prospero, J.M.: Long-range transport of mineral dust in the global atmosphere: Impact of African dust on the environment of the southeastern United States, Proc. Natl. Acad. Sci. U. S. A., 96, 3396-3403, 1999.


Rogak, S.N., Baltensperger, U., and Flagan, R.C.: Measurement of mass transfer to agglomerate aerosols, Aerosol Sci. Technol., 14, 447-458, 1991.

Seisel, S., Borensen, C., Vogt, R., and Zellner, R.: The heterogeneous reaction of HNO3 on mineral dust and gamma-alumina surfaces: a combined Knudsen cell and DRIFTS study, Phys. Chem. Chem. Phys., 6, 5498-5508, 2004.

Shao, Y., and Raupach, M.R.: Effect of Saltation Bombardment on the Entrainment of Dust by Wind, J. Geophys. Res.-Atmos., 98, 12719-12726, 1993.

Tang, Y.H., Carmichael, G.R., Kurata, G., Uno, I., Weber, R.J., Song, C.H., Guttikunda, S.K., Woo, J.H., Streets, D.G., Wei, C., Clarke, A.D., Huebert, B., and Anderson, T.L.: Impacts of dust on regional tropospheric chemistry during the ACE-Asia experiment: A model study with observations, J. Geophys. Res.-Atmos., 109, 2004.

Textor, C., and 40coauthors: Analysis and quantification of the diversities of aerosol life cycles within AeroCom, Atmos. Chem. Phys. Discuss., 5, 8331–8420, 2005.


van den Heever, S.C., Carrió, G.G., Cotton, W.R., and Straka, W.C., The impacts of Saharan dust on Florida storm characteristics, in 16th Conference on Planned and Inadvertent Weather Modification, San Diego, USA, 2005.

Vlasenko, A., Sjogren, S., Weingartner, E., Gäggeler, H.W., and Ammann, A.: Generation of submicron Arizona test dust aerosol: Chemical and hygroscopic properties, Aerosol Sci. Technol., 39, 452-460, 2005a.

Vlasenko, A., Sjogren, S., Weingartner, E., Stemmler, K., Gäggeler, H.W., and Ammann, A.: Effect of humidity on nitric acid uptake on mineral dust aerosol particles, Atmos. Chem. Phys. Discuss., submitted, 2005b.

Wachsmuth, M., Gäggeler, H.W., von Glasow, R., and Ammann, M.: Accommodation coefficient of HOBr on deliquescent sodium bromide aerosol particles, Atmos. Chem. Phys., 2, 121-131, 2002.

Appendix 85

5 Final discussion and recommendations for future studies

In chapter 2-4 experimental data are presented, that describe some aspects of the complex interaction of gaseous nitric acid with mineral dust aerosol. The results of this work are discussed below in connection to atmospheric implications and future studies. The most widely used experimental method to study the heterogeneous reaction between nitric acid and mineral dust is the Knudsen cell reactor. There is a specific problem related to the application of this technique for the uptake measurement, namely the ambiguity in the determination of the reactive surface area exposed to the trace gas. The problem is that the observed loss of the trace gas species from the gas phase must be normalized to the surface area, on which the loss process occurs. On a powder sample, this requires that diffusion of the trace gas beyond the apparent sample surface into the interior of the sample must be taken into account. This leads to a time dependent behaviour of the uptake, from which it is not straightforward to extract elementary parameters of the surface process as such. (see Chapter 1). This is an unresolved issue and often controversely discussed [Seisel et al., 2005]. An alternative experimental method used in this study and described in Chapter 2 seems to overcome this problem due to the better defined surface area measurement by aerosol technology. A technique has been developed to produce aerosol particles from dust powders followed by separation of particles according to their size. To our knowledge, this is the first time such a system has been used for heterogeneous kinetic studies between dust aerosol and trace gas species. The main advantage of this method is the ability to produce airborne solid particles by dry dispersion from powders of various chemical compositions, e.g., authentic dust or chemically well-defined mineral oxides. The performance of the technique is described in chapter 2 with respect to aerosol output properties (particle number concentration and size distribution) and stability. The Arizona Test Dust (ATD) aerosol, which is the aerosol used mostly in this study, was characterized with respect to morphology, electrical properties, hygroscopic properties and chemical composition. The kinetics of the reaction of nitric acid with mineral dust and silica has been studied using the 13N tracer technique. This special analytical tool has been successfuly applied earlier to study heterogeneous interactions of various types of surfaces (soot, deliquescent salt particles, ice, snow) with nitrogen-containing species relevant to atmospheric chemistry [Ammann et al., 1998; Arens et al., 2001; Bartels-Rausch et al., 2002; Guimbaud et al., 2002; Kalberer et al., 1996]. The method of dust aerosol production developed here in combination with the tracer method could be used for further studies of heterogeneous chemistry on such surfaces as TiO2, authentic Sahara dust, authentic Asian Kosa dust, clay materials (illite, kaolinite, montmorillonite). However, the question remains to which extent the submicrometer sized dust particles, used in this study, are representative for the atmosphere. Mineral dust particle that survive a long-range transport are of micron and submicron size [Penner et al., 2001]. Most likely the submicron particles, which have been studied in this work, represent only minor part of the atmospheric dust. However the obtained results could be used to predict

86 Appendix

the heterogeneous reaction kinetics for the whole population of atmospheric dust particles bearing in mind the differences in chemical composition of larger particles and the larger diffusion resistance term described in chapter 3 of this thesis.

As was mentioned in chapter 2, the mineral dust is a ubiquitous atmospheric aerosol, which is mainly composed of crustal soil material. On one hand the origin of the dust particles and the resemblance in chemical composition allow to classify them as a separate class of atmospheric aerosol. On the other hand the minor differences in chemical composition call for the specification of dusts from various regions. For example, in Chapter 3 it was shown that pure CaCO3 is more reactive than ATD with respect to heterogeneous reaction with HNO3. Therefore, the chemical composition of dust is important for its heterogeneous chemistry. This calls for further experimental studies on authentic dusts different from ATD, such as Sahara and Asian dusts.

Due to the complexity of the chemical composition of mineral dust the detailed

chemical mechanism of the reaction with nitric acid remains unclear. The suggested reaction scheme is given in chapter 4 and involves the description of HNO3 transport to the surface of dust particles, adsorption/desorption interaction and irreversible reaction at the surface. It was shown that the concentration of the reactive surface sites is a very important parameter for quantitative kinetic description of the process with Langmuir-Hinshelwood type kinetics. However, the chemical nature of the reactive sites has not been revealed explicitly. Therefore, further investigations with techniques that could resolve the product surface species (FTIR, DRIFTS, XPS) are necessary.

One of the probably most significant result of this study is the experimentally

measured dependence of HNO3 uptake on mineral dust as a function of relative humidity. This dependence might be of major importance to modeling studies that attempt to simulate the influence of heterogeneous chemistry on a global scale, where it is important to follow correctly the environment during their entire life cycle, which includes often includes both humid and dry periods. The kinetic experiments presented in this work were done at a relative humidity varied from 12 to 73%, which covers the range relevant to the atmosphere. Therefore we recommend incorporating the humidity dependence of the uptake kinetics in atmospheric chemistry models. In the literature, there are number of reports related to the measurements of HNO3 uptake on authentic dust samples and mineral oxides that were performed under very dry conditions. The difference in water vapor concentration hampers a reasonable comparison with the data reported in this work. To overcome this difficulty the measurements in the aerosol flowreactor should be performed also at well controlled humidity low enough to become comparable to those in other studies. In our experiments, we were somewhat limited in this regard, because water vapor was needed produce the OH radicals to convert NO2 into HNO3. A rough comparison could be done by extrapolating our data to low humidity. Figure 5.1 shows the data measured in the experiment with the 13N-tracer and one data point (solid square) measured in an experiment with a wet effluent diffusion denuder aerosol collector WEDD-AC (see Appendix). Note that in principle, these data could not be compared directly, because the measurements were performed at different reaction times, 2 sec for the tracer technique and 1 min for WEDD-AC method (in principle you could take into

Appendix 87

account the time dependence derived in chapter 4). Nevertheless, both data sets could be well fitted with the range of isotherms expected for the adsorption of water on a mineral surface (Eq.12, chapter 2). One may notice that if the behavior of the uptake coefficient is extrapolated to even lower humidity according to the isotherm, then at 0.1% RH the uptake coefficient would be expected at 10-3 by order of magnitude. This value is of the same order magnitude than the value for HNO3 uptake on CaCO3 measured in the Knudsen cell at low humidity [Johnson et al., 2005]. This comparison is rather an illustration to motivate further experimental studies in order to provide a link between the uptake measurements in the aerosol flow reactor and in the Knudsen cell.

0.1 1 10 1001E-3

0.01

0.1

0.01

0.1

1

10

upta

ke c

oeffi

cien

t γ

relative humidity, %

num

ber o

f H2O

laye

rs

Figure 5.1. The uptake coefficient of HNO3(g) on ATD aerosol measured as a function of relative humidity. Open circles represent the measurement with the tracer technique with 2s reaction time. The solid square represents the measurement with the WEDD-AC method with a reaction time of 1min. Dashed lines represent the calculated isotherms for the adsorption of water on a solid surface (chapter 2, Eq.12, coefficients c=3,10,20). See

explanations in the text.

The reaction kinetics in this study was measured on the 0.2-2 seconds time scale.

For atmospheric conditions, this is a very short scale, because the average lifetime of dust particles is about 4 days. One the other hand the concentrations of gaseous nitric acid used for the measurements were higher than those observed in the atmosphere. So the exposure (HNO3 concentration times reaction time) for the measurement and atmospheric conditions are estimated to be of the same order of magnitude. However, the effect of dust surface saturation due to adsorption, described in the chapter 3, does in principle not allow employing this simple approach directly for assessing atmospheric implications. Additional support should be provided by modeling as described in chapter 4 or experimental studies in chambers to allow for better estimates over longer time scale. Furthermore, such long term exposures would allow investigating the effect of surface – bulk exchange of HNO3 and mineral basic cations as briefly addressed in chapter 3.

Currently, the “state-of-the-art” atmospheric models treat mineral dust as a

heterogeneous sink for gaseous HNO3 in the upper troposphere. The uptake of nitric acid to dust is considered to prevent renoxification of HNO3 by in situ photolysis in the

88 Appendix

atmosphere. The models however do not consider the possibility of the reaction products (nitrates) photolysis on the surface of mineral dust. To improve the description of the role of mineral dust as a heterogeneous sink for nitric acid, photolysis of the nitrates on the surface of the dust aerosol should be investigated as no data are available so far. References Ammann, M., Kalberer, K., Jost, D.T., Tobler, L., Rössler, E., Piguet, D., Gäggeler,

H.W., and Baltensperger, U.: Heterogeneous production of nitrous acid on soot in polluted air masses, Nature, 395, 157 - 160, 1998.

Arens, F., Gutzwiller, L., Baltensperger, U., Gäggeler, H.W., and Ammann, M.: Heterogeneous reaction of NO2 on diesel soot particles, Environ. Sci. Technol., 35, 2191-2199, 2001.

Bartels-Rausch, T., Eichler, B., Zimmermann, P., Gäggeler, H.W., and Ammann, M.: The adsorption enthalpy of nitrogen oxides on crystalline ice, Atmos. Chem. Phys., 2, 235-247, 2002.



Kalberer, M., Tabor, K., Ammann, M., Parrat, Y., Weingartner, E., Piguet, D., Rössler, E., Jost, D.T., Türler, A., Gäggeler, H.W., and Baltensperger, U.: Heterogeneous chemical processing of 13NO2 by monodisperse carbon aerosols at very low concentrations, J. Phys. Chem., 100, 15487-15493, 1996.

Penner, J.E., Andreae, M., Annegarn, H., Barrie, L., Feichter, J., Hegg, D., Jayaraman, A., Leaitch, R., Murphy, D., Nganga, J., and Pitari, G., Aerosols, their direct and indirect effects, in Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Assessment Report of the Intergovernmental Panel on Climate Change, edited by J.T. Houghton, Y. Ding, D.J. Griggs, M. Noguer, P.J. van der Linden, X. Dai, K. Maskell, and C.A. Johnson, pp. 291-348, Cambridge University Press, Cambridge, United Kingdom and New York, USA, 2001.

Seisel, S., Borensen, C., Vogt, R., and Zellner, R.: Kinetics and mechanism of the uptake of N2O5 on mineral dust at 298K, Atmos. Chem. Phys. Discuss., 5, 5645–5667, 2005.

Appendix 89

6 Appendix

6.1 Production of 13N. PROTRAC facility at PSI The PROTRAC (PROduction of TRacers for Atmospheric Chemistry) facility was built at the Paul Scherrer Institute to provide the short-lived radioactive isotopes 13N and 11C to the chemistry laboratories as tracers for experiments pertaining to atmospheric chemistry [Ammann et al., 2003]. It uses a target installed at the isotope production station IP-2 and new transport lines for on-line transport of these isotopes in the gas-phase. In the target, a 16O gas target is bombarded with 10µA of 15 MeV protons. The predominant nuclear reaction is 16O(p,α)13N under these conditions. Another radioactive isotope, 11C, is produced in the target as a byproduct from the reaction of protons with traces of N2 in the carrier gas (14N(p,α)11C). The isotopes 11C(T1/2 = 20.39 min.) and 13N(T1/2 = 9.965 min.) are both positron emitters. Annihilation of the positrons leads to emission of two 511 keV gammas. Experimentally, the two isotopes are recognized through their half-lives. The gas is blown together with the 13N labelled NO through a pipe 580 m in length across the river Aare bridge into the new research laboratory, where experiments are carried out. The decay analysis displayed in Fig. 6.1 indicated that at least 95% of the activity was due to 13N, and less than 5% was due to 11C.

-7.000

-6.000

-5.000

-4.000

-3.000

-2.000

-1.000

0.000

0 10 20 30 40 50 60 70 80 90

decay t ime, min

Ln(C

/Co)

cps Det1

cps Det3

13N

11C

Figure 6.1. Decay analysis of activity associated with nitrogen oxides (squares) and of activity remaining after separation of nitrogen oxides (less than 5% of overall activity,

diamonds). The solid lines are expected decay curves for 13N (10 min half-life, bold line) and for 11C (20 min half-life, thin line) [Ammann et al., 2003].

90 Appendix

6.2 SMPS calibration with PSL The aerosol size distribution and number concentration was obtained by a Scanning Mobility Particle Sizer system (SMPS), which consisted of a 85Kr neutralizer, a differential mobility analyzer DMA (model 3071, TSI) and a condensation particle counter CPC (model 3022, TSI). The system was operated at 3 L·min-1 sheath and 0.3 L·min-1 sample flow rates, respectively. Scanning time (300s) is long enough to get good counting statistics and a smooth particle spectrum. The performance of the system to resolve particle size was tested with the help of 40 nm (particle diameter) and 150 nm PSL particles (Duke Scientific Corporation, US). Figure 6.2 shows that PSL size peaks could be easily recognized from the background spectra of water spray residue and the size positions of the peaks correspond to the certified PSL particle sizes within the 5% error.

100

102

103

104

PSL 150 nm

dN /

dLog

D, c

m-3

particle D, nm10 100

0.0

5.0x103

1.0x104

1.5x104

residue after MILLI-Qwater spray

PSL 40nm

particle D, nm Figure 6.2. Control tests of the SMPS system. Left panel represents the measured particle size spectrum for the dispersed water solution of PSL particles with D 150 nm. Right panel represents the measured particle size spectrum for the dispersed water solution of PSL particles with D 40 nm and the measured spectrum of water spray residue.

6.3 Uptake of CH3COOH on ATD and Na2CO3 aerosol particles Additional experiments were performed to measure the reactivity of gaseous acetic acid with the surface of ATD and Na2CO3. ATD particles were produced as described in chapter 2. Sodium carbonate aerosol was produced by nebulising an aqueous solution of Na2CO3. The reactant gas was prepared by slowly passing nitrogen over CH3COOH at –45°C to become saturated. This flow was then diluted by nitrogen to give the final concentration of acetic acid (AA) in the ppb range. The reaction kinetics was studied in the aerosol flow reactor. Flows of aerosol and the reactant gas were mixed and let to react for 48 seconds. The relative humidity was 2±1%. The gas phase concentration of acetic acid was continuously detected by chemical ionisation mass-spectrometry (CIMS) at

Appendix 91

atmospheric pressure using proton transfer as ionising method. The description of the MS and the design of CI region is given elsewhere [Guimbaud et al., 2003].

0.16

0.18

0.2

0.22

0.24

510 520 530 540 550 560 570

time, min

CH

3CO

OH

sig

nal,

r.u.

Figure 6.2. Trace of acetic acid concentration in the gas phase recorded during the

uptake experiment at 2% RH.

The experiment was started with equilibrating the AA concentration (510-516 min, Fig.6.3) inside the reactor. Then the injector was inserted to the position corresponding to the minimum reaction time (1 s). At this moment the AA concentration was increased due to desorption of AA from reactor walls (516 min). After equilibrating the AA concentration to its initial value (517-522 min) the aerosol was added (532-546 min). No loss was observed. At 547 min the injector was pulled out to the position corresponding to the maximum reaction time. The AA concentration initially drops due to adsorption of AA by the reactor walls and then recovers to the lower value due to the uptake by mineral dust. The concentration of acetic acid was depleted by reaction by only 4%, in spite of a significant aerosol surface to volume ratio of 3 10-4cm-1. Additional uptake experiments were performed with sodium carbonate aerosol. Pure Na2CO3 is more basic than mineral dust and was expected to be more reactive with acetic acid. Indeed, the experiment showed much stronger uptake of AA to sodium carbonate aerosol (Table 6.1).

Table 6.1. Uptake coefficients measured for aerosol particles of different composition.

Heterogeneous reaction RH,% Reaction time Uptake coefficientCH3COOH(g)+Mineral Dust(s) 2 48 s 10-3

CH3COOH (g) + Na2CO3(s) 2 48 s 6×10-2

HNO3 (g) + Mineral Dust(s) 12 2 s 2x10-2

6.4 Uptake of HNO3 on ATD measured with the WEDD-AC system As an alternative to the uptake measurement using the 13N-tracer another technique was applied. The ATD aerosol was produced as described in chapter 1. The flow containing nitric acid was prepared by passing the N2 flow through a HNO3 permeation source. The flows are mixed to react during 1 minute in the laminar flow reactor. The reaction products are detected by the wet effluent diffusion aerosol collector system. The detailed

92 Appendix

description of the detection system is given elsewhere [Vlasenko et al., 2003; Zellweger et al., 1999].

d u s t O F F d u s t O N0

2

4

H N O 3

A T D in N 2 c a rr ie r ( D u s t O n )o r N 2 c a r r ie r (D u s t O F F )

re a c to r W E D D -A CN 2 c a r r ie r

A T D in N 2 c a r r ie r ( D u s t O n )o r N 2 c a r r i e r ( D u s t O F F )

r e a c t o r W E D D - A C

BA

N O 3- ( g a s )

N O 3- ( p a r t i c u la t e )

d u s t O F F d u s t O N0 . 0 0

0 . 0 4

0 . 0 8

NO

3- con

cent

ratio

n, μ

g m

-3

Figure 6.4. The measurement of HNO3(g) reaction with ATD aerosol. Left panel A represents the measurement without gaseous nitric acid. The right panel B (note the

different y-axis scale) represents the measurement when gaseous HNO3 in the nitrogen carrier gas is mixed with the ATD aerosol in the N2 carrier (Dust ON) or with the N2 flow

without aerosol (see explanation in text). The results of the measurements are shown in Figure 6.4. Small concentrations were detected for the gas-phase and particulate-phase concentrations of water soluble nitrate while passing N2 carrier gas without aerosol (panel A, Dust OFF). One may see that the particulate nitrate signal increases due to water soluble nitrate in the ATD particles prior to the reaction (panel A, Dust ON). When the HNO3 flow was added to the reactor and no aerosol was present then most of nitrate is detected in the gas phase (panel B, Dust OFF). Once the ATD was added to the reactor, the concentration of HNO3 in the gas phase dropped together with the increase of NO3 concentration in the particulate phase (panel B, Dust ON). References Ammann, M., Birrer, M., and Vlasenko, A., PROTRAC-production of tracers for

atmospheric chemistry: first experiments, in Annual Report 2002. Labor für Radio-und Umwelt Chemie .PSI and University Bern, pp. 10, Paul Scherrer Institute, Villigen, 2003.

Guimbaud, C., Bartels-Rausch, T., and Ammann, M.: An atmospheric pressure chemical ionization mass spectrometer (APCI-MS) combined with a chromatographic technique to measure the adsorption enthalpy of acetone on ice, Int. J. Mass Spectrom., 226, 279-290, 2003.

Vlasenko, A., Ammann, M., Gäggeler, H.W., and Fisseha, R., Anionic composition of resuspended mineral dust aerosol particles, in Annual Report 2002. Labor für Radio-und Umwelt Chemie .PSI and University Bern, pp. 24, Villigen, 2003.

Zellweger, C., Ammann, M., Hofer, P., and Baltensperger, U.: NOy speciation with a combined wet effluent diffusion denuder-aerosol collector coupled to ion chromatography, Atmos. Environ., 33, 1131-1140, 1999.

Appendix 93

6.5 Technical drawings and pictures of experimental setups. Dimensions are in mm.

D1

Dout

D2

D in

D3

H1

H3

H4

H2

Din 4

Dout 4

D1 68

D2 22

D3 58

H1 350

H2 63

H3 150

H4 10

Figure 6.5. Scheme of the cyclone. Figure 6.6. Virtual impactor

d1d2

L

Flow

5 kVhigh-voltage

supply

5 kVhigh-voltage

supply

d1 d2 L

19 38 290

Figure 6.7. Electrical precipitator. Figure 6.8. Solid Aerosol Generator

Ejector

Scraper

Feed Belt Driving Pulley

94 Appendix

Figure 6.9. Parallel plate denuder.

Figure 6.10. Aerosol flow tube reactor.

Acknowledgement 95

Acknowledgement I am very grateful to Heinz Gäggeler for giving me the opportunity to conduct PhD study at PSI and Uni Bern, the valuable comments to paper and thesis manuscripts and certainly, for organizing fascinating ski tours. I would like to express my gratitude to Markus Ammann for supervising my work and providing a challenging project, for numerous ideas and motivating enthusiastic discussions. For giving me opportunity to attend international conferences and sharing his knowledge in the field of heterogeneous chemistry. I am thankful to Ernest Weingartner, Staffan Sjögren, Rebeka Fisseha and David Imhof for pleasant and fruitful collaboration in the field of aerosol research. I would like to thank Mario Birrer for excellent technical support and engineering solutions. Many thanks to Christophe Guimbaud for introducing me into the field of APCI mass spectrometry, french wine and hunting. My very special thanks to Surface Chemistry Group members (present and alumni) Olya Vesna, Thomas Huthwelker, Konrad Stemmler, Hanna Frånberg, Thorsten Bartels, Lukas Gutzwiller and Elfy Rössler for pleasant working atmosphere and helping me in the lab. I am grateful to Ruth Lorenzen for her administrative support, for her great help with organizing visa and cheerful mood every morning. I would like to thank Thorsten Bartels, David Bolius and Dave Piguet for software and hardware computer support. I am thankful to Roland Brütsch, Berndhard Schnyder and Silvia Köchli for performing the analysis of my “dusty” samples with powerful analytical super machines. I also would like to thank all my colleagues from Laboratory of Radio- and Environmental Chemistry for great working spirit and constant help. Many thanks to the staff of PSI Accelerator Facilities for their efforts to keep stable beam generation. And finally I would like to thank my wife Dascha and my parents for their moral and psychological support during working on this thesis.

96 Resumé

Resumé Surname Vlasenko Name Alexander Lvovich Born 24.09.1973 in Chelyabinsk, Ural region (Russia) 1980-1990 Secondary School No.127 in Chelyabisk-70, Russia 1990-1996 Chemistry studies at Novosibirsk State University, Novosibirsk, Russia 1995 Diploma in Chemistry at Novosibirsk State University “The deliquescence behavior of ambient aerosol, governed by ammonium nitrate and ammonium sulphate” 1996 Magister Diploma in Physical Chemistry at Novosibirsk State University “A study of chemical composition of ambient aerosols on the territory of Western Siberia” 1996-2001 Junior Scientist at the Institute of Chemical Kinetics and Combustion, Siberian Branch of Russian Academy of Sciences, Novosibirsk, Russia 2001-2006 PhD studies at Paul Scherrer Institute and University of Bern, Switzerland “Aerosol flow tube study of the heterogeneous interaction between submicron mineral dust particles and gaseous nitric acid”

aerosol flow tube study of the heterogeneous interaction

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