Investigating the Reactivity of Gas-Phase Species withModel Tropospheric Aerosols: Substrate, Bulk, and Interfacial Reactions

Holly M. Bevsek

BackgroundCurrently one of the most interesting pursuits in atmospheric chemistry is the development of

an understanding of the reactivity of trace gases with tropospheric aerosols. The importance of these

heterogeneous reactions first came to light when it was discovered that SO2 is oxidized to sulfate in

cloud droplets, leading to acid rain. This was an initially surprising find since oxidation of SO2 by a

trace gas such as O3 was known to be negligible in the gas phase. However in aqueous solution this

reaction can be rapid1, illustrating the importance of gas-liquid reactions. A different heterogeneous

system–formation of HCl from HNO3 reacting with solid NaCl (a model of sea-salt aerosol)–was

studied by Finlayson-Pitts, et al.. They found that in addition to reaction of HNO3 with a bulk aqueous

solution of Na+ and Cl- a secondary mechanism utilizing surface-adsorbed water also appeared to be

occurring2. These results provide evidence that reactions of gaseous species within liquid aerosols and

at aerosol interfaces–like the NaCl-H2O interface–can occur via alternate reactive pathways. Despite

the potential atmospheric impact of these reactions–including those that may contribute to global

climate change–very little is known about them and this leads to the failure of models that simulate

atmospheric chemical processes. My research will fill this gap by determining the kinetics and

mechanisms of the reactions of tropospheric gas-phase species with aerosols. It is furthermore

anticipated that the results of this study will lead to improved atmospheric models.

Tropospheric aerosol is complex, consisting mostly of sea salt, mineral dust, organic aerosols,

and carbonaceous (soot) aerosols. This study will focus on three of these four types: sea salt, mineral

dust, and carbonaceous aerosols (organic aerosols are structurally complicated and will be studied at a

later time). Sea salt and mineral dust are important atmospheric constituents because they are the most

widespread and concentrated natural aerosols1,3. Chemically, sea salt is a potential source of

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atmospheric free radical chlorine and/or bromine4. Mineral dust may undergo electron transfer

reactions with oxidants and also acid-base reactions to form a variety of gas-phase and condensed

products. Photochemical reactions are also possible if species comprising the mineral dust have band

gaps that fall within the solar spectrum. Understanding the chemical impact of mineral dust on the

troposphere is particularly needed since it is expected to become more concentrated due to growing

land use and erosion5. Carbonaceous aerosol (CA) is a product of incomplete combustion processes

and can therefore be locally concentrated in the troposphere as the result of fossil fuel and biomass

combustion as well as in the lower stratosphere as a product of aircraft exhaust. Soot is of

considerable environmental importance since it can act as a reducing agent in the otherwise oxidizing

atmosphere6. CA has also been predicted to have a cooling effect on the global climate7.

In addition to being chemically complex, tropospheric aerosol is also structurally complex.

Depending on the atmospheric conditions inorganic aerosol can exist as ionic species in bulk liquid

water (high humidity) or as a solid kernel surrounded by a film of water (low humidity). Based on this

model, three reactive domains exist. Under high humidity conditions, one expects reaction primarily

at the air-liquid film interface or possibly within the bulk liquid. Under low humidity conditions,

reaction at the substrate-film interface should dominate (CA is similar except the condensed species

would be organic). A full understanding of aerosol reactivity must account for these three domains

and to do so one must be able to isolate them. This can be accomplished by “tuning” the film

thickness. That is, reaction at the substrate-film interface can be observed if the film is a few

nanometers thick while reaction in the bulk film can be observed if the film is several micrometers

thick. Reaction at the air-film interface can then be determined by comparing the reactivity of the film

supported by an inactive substrate with the reactivity of the bulk film.

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The experimental strategy utilized in these studies is to follow the reaction in time using diffuse

reflectance infrared Fourier transform spectroscopy (DRIFTS)–a technique that is sensitive to surface

species on powdered samples–for different film thicknesses. Film thickness will be measured using

optical differential reflection (ODR), an inexpensive technique that allows the determination of the

thickness of a film on a reflective substrate based on the difference in reflectance of parallel- and

perpendicular-polarized light from the surface8-12. Both of these techniques are straightforward to

implement and very suitable for undergraduates.

Experimental

Systems Investigated

Table 1 lists the type of tropospheric aerosols to be investigated, how they will be modeled, and

what reactant gases will be introduced to the system.

Aerosol Model Substrate Aerosol Film Reactant Gas

Sea Salt NaCl and NaBr Powder H2O NO2, SO2, HNO3, N2O5

Mineral Dust a- and g-Fe2O3, CaCO3,MgCO3, SiO2 Powders

H2O NO2, SO2, HNO3, NH3

Carbonaceous n-Hexane and Diesel Soot H2O, adipic acid NO2, O2

Table 1. Model aerosol systems.

Methodology

Reactions will take place inside a stainless steel vacuum chamber that is designed to fit inside

the sample compartment of a FTIR spectrometer equipped with a diffuse reflectance accessory and

MCT detector. The chamber will be evacuated by a turbomolecular pump to a base pressure of 10-7

Torr. Heating/cooling the sample in the chamber will enable a range of environmentally-relevant

temperatures to be investigated. A typical experiment will consist of:

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1. Insertion of the sample into the chamber for evacuation and bakeout,

2. Film deposition to an approximate thickness by backfilling the chamber with vapor

(required pressure determined from adsorption isotherms),

3. ODR measurement of film thickness,

4. Introduction of reactant gas, and

5. DRIFTS measurement as a function of time.

As new species are produced, IR absorption will increase at frequencies corresponding to new bonds

and decrease at frequencies corresponding to broken bonds. Recording DRIFT spectra as a function of

time will allow reaction rates and ultimately rate laws and mechanisms to be hypothesized (see below).

Analysis

Film thickness will be determined by nonlinear least squares fitting of the ODR data, with

thickness and complex refractive index as variable parameters. Measurements will be made before

and after reactant gas introduction to determine if reaction changes the thickness and/or refractive

index. Using the refractive indexes of H2O (1.33313) and graphite (5.4(real); 8.4(imaginary)14), the

ODR signal can be estimated to be approximately 1% that of the initial light intensity for a 6.5C-thick

water layer on graphite (a model for soot) at 70° incident angle and 650 nm incident wavelength,

which is easily detected by standard Si photodiodes. Feasibility calculations have also been performed

for water films on Fe2O3 and NaCl with similar results.

Different mechanisms occurring in the three reactive domains will be discerned by analyzing

the identity and rates of product formation for the three regimes. Specifically:

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Reaction at the substrate-film interface

--investigated by comparing DRIFTS data from a bare substrate to that of a substrate with a

thin liquid film.

Reaction in the bulk liquid

--investigated by comparing data in progressively thicker liquid films.

Reaction at the liquid-air interface

--investigated by comparing data from the thickest film to that from a film of similar thickness

deposited on an unreactive substrate, such as a microscope slide.

Role of Undergraduate and Graduate Student Researchers

Undergraduate and graduate student researchers will contribute to all aspects of this project:

sample preparation, data collection, data fitting, and analysis. Students will also assist in building the

vacuum system and ODR set-up. Topics students will learn about include:

· gas-handling, vacuum, laser, and FTIR spectroscopic techniques

· atmospheric chemistry

· reaction kinetics and mechanisms at interfaces

Current Status of Project

The vacuum and optics systems were designed, assembled, and tested at Susquehanna

University. The first system my (undergraduate) students and I examined was the reaction kinetics of

NO2 on vacuum-dried a-Fe2O3 (hematite) and g-Fe2O3 (maghemite) powders, which are components of

mineral dust. As we did not deposit a film of water on the Fe2O3 powders, this study targets the

“substrate-film interface” regime. These experiments have been very successful and we have made a

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number of discoveries not previously documented including reactive differences between a- and g-

Fe2O3, the permanent formation of NO2- (which has led to a refining of the accepted reaction

mechanism), and the permanent formation of NO+. The findings for g-Fe2O3 + NO2 (the data for a-

Fe2O3 are still being analyzed) are briefly discussed below. The remainder of this section may be

skipped without loss of continuity.

The nitrate products formed on the surface of g-Fe2O3 powder as a result of reaction with NO2

are similar to those formed on the surface of a-Fe2O3, as evidenced by a previous study15 (no

investigations of NO2 reacting with g-Fe2O3 exist in the literature because it is assumed that the a and

g phases react similarly). However, our studies indicate permanent nitrite and nitrosonium formation

on g-Fe2O3 at low pressures (24 mTorr), unlike the a-Fe2O3 study.

As an example of a typical data set for this study, Figure 1 shows the first twelve spectra

collected during the reaction of 109 mTorr NO2 with g-Fe2O3. Spectra similar to these were collected

for 24, 153, and 210 mTorr NO2 as well. To determine the rate law for this reaction, the area of the

band extending from 1450-1650 cm-1 (“Band 1”) was measured and plotted as a function of reaction

time for all pressures . The initial reaction rate at these pressures was then found via linear regression

and the order of reaction found from the slope of a plot of log(reaction rate) vs. log(NO2 Pressure).

We find that the order of reaction of NO2 with g-Fe2O3 is first order with a value of 1.08 ± 0.09.

Assuming the amount of active sites on the Fe2O3 powder is constant, pseudo-first order conditions

exist and the rate law for this reaction may then be written as:

d[NO3-]/dt = kN P(NO2)

1

with kN = 1.2 ± 0.3 x 10-16 cm3 mlcl-1 s-1.

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A mechanism for this reaction has been proposed for a-Fe2O3 + NO2 by Underwood et al.15 In

this mechanism, NO2, once adsorbed, reacts with the surface to form nitrite. The surface-bound nitrite

may then react with either another surface-bound nitrite group (Langmuir-Hinshelwood mechanism,

LH) or gas-phase NO2 (Eley-Rideal mechanism, ER) to form nitrate:

NO2(g) W NO2(surface) (1)

NO2(surface) 6 NO2-(surface) (2)

2 NO2-(surface) 6 NO3

-(surface) + NO(g) (3a, LH)

or

NO2-(surface) + NO2(g) 6 NO3

-(surface) + NO(g) (3b, ER)

No hypothesis was given regarding whether 3a or 3b would be more likely to occur, however one of

these reactions is thought to be the rate-limiting step.

As stated above we have found this reaction to be first order in NO2. This order does not

correspond to the LH (second order at low pressure, zeroth order at high) or ER (second order at low

pressure, first order at high; we measure our reaction rate well before saturation therefore our

conditions are closer to low pressure than to high) mechanisms. However it does correspond to

unimolecular reaction of NO2 with the surface to produce NO2-, so we propose

NO2(g) W NO2(surface)

to be the rate-limiting step. The reaction then appears to follow a LH mechanism after the initial NO2

6 NO2- conversion, based on the fact that NO2

- remains on the surface in the presence of a continually

refreshed supply of NO2.

The presence of nitrosonium (absorption at 2150 cm-1) indicates that the follow reaction16 is

also likely occurring:

2 NO2(surface) W N2O4(surface) W NO+(surface) + NO3-(surface)

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Due to the small intensity of the NO+ signal (approximately 5% of the dominant NO3- signal in Band 1)

it is clearly a minor product channel. We are currently investigating how this impacts the above

mechanism.

Future Directions

In addition to altering concentrations of trace gases, aerosols also affect the surface temperature

of the earth1,4. The scattering of light by aerosols has been well known for years and is currently a

vigorous field of climate change research, however it is unknown whether the index of refraction–and

hence the light-scattering ability–of aerosols is altered by reaction. Since this property will be

measured through ODR in this study, determining whether the index of refraction is changed upon

reaction will be an interesting side-project to pursue.

Future experiments will investigate the reactivity of O3 and other radical species such as OH,

HO2, and Cl with the three types of model aerosol described above. The possibility of photochemistry

on substrates with band gaps in the visible region (such as Fe2O3) will also be examined. The effect of

a surfactant layer on sea salt aerosols is yet another interesting question that will be investigated, as

field studies have found marine aerosols that have an organic component17. The reactivity of a

polycyclic aromatic hydrocarbon layer on soot towards oxidation will also be investigated to more

closely approximate reactivity of atmospheric aerosols.

Funding

As indicated in the overview, there are a number of private and government funding sources,

both in chemical and environmental divisions, for this project. In fact, while at Susquehanna

university I submitted a proposal to the ACS-PRF GB program to investigate the reaction of NO2 with

soot. Although the reviews were very favorable, the project was not funded, mostly because of my

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temporary status. Since then I have used the reviewers comments to strengthen the proposal and

submitted it for a Susquehanna University Research Grant (funded internally, reviewed externally).

Reviewer comments were very positive–they recommended the proposal be submitted to the PRF–and

I was awarded the grant in 2003. I intend to submit an updated version to the Research Corporation

for a Cottrell College Science Award in the near future.

References

1. Seinfeld, J. H.; Pandis, S. N. Atmospheric Chemistry and Physics; John Wiley and Sons, Inc.:

New York, 1998.

2. Finlayson-Pitts, B. J.; Hemminger, J. C. Journal of Physical Chemistry A 2000, 104, 11463.

3. Barnaba, F.; Gobbi, G. P. Journal of Geophysical Research 2001, 106, 3005.

4. Finlayson-Pitts, B. J.; James N. Pitts, Jr. Atmospheric Chemistry: Fundamentals and

Experimental Techniques; John Wiley and Sons: New York, 1986.

5. Sheehy, D. P. Ambio 1992, 21, 303.

6. Ravishankara, A. R. Science 1997, 276, 1058.

7. Liousse, C.; Penner, J. E.; Chuang, C.; Walton, J. J.; Eddleman, H.; Cachier, H. Journal of

Geophysical Research 1996, 101, 19.

8. Dvorak, J.; Borguet, E.; Dai, H.-L. Surface Science 1996, 369, L122.

9. Jin, X. F.; Mao, M. Y.; Ko, S.; Shen, Y. R. Physical Review B 1996, 54, 7701.

10. Kwon, S.; Russell, J.; Zhao, X.; Vidic, R. D.; Johnson, J. K.; Borguet, E. Langmuir 2002, 18,

2595.

11. McIntyre, J. D. E.; Aspnes, D. E. Surface Science 1971, 24, 417.

12. Wong, A.; Zhu, X. D. Applied Physics A 1996, 63, 1.

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13. CRC Handbook of Chemistry and Physics; 71 ed.; Lide, D. R., Ed.; CRC Press: Boca Raton,

1990.

14. Handbook of Optical Constants of Solids; Palik, D., Ed.; Academic Press: Boston, 1991; Vol.

2.

15. Underwood, G. M.; Miller, T. M.; Grassian, V. H. Journal of Physical Chemistry 1999, 103,

6184.

16. Marie, O., Malicki, N., Pommier, C., Massiani, P., Vos, A., Schoonheydt, R., Geerlings, P.,

Henriques, C., Thibault-Starzyk, F., Chemical Communications, 2005, 28, 1049.

17. Mueller, P. K.; Mosley, R. W.; Pierce, L. B. Journal of Colloid and Interface Science 1972, 39,

235.

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Figure 1. DRIFT spectra of g-Fe2O3 as a function of NO2 exposure time. P(NO2) = 109 mTorr. Spectra were recorded at 15 s intervals.