Carbonaceous Aerosols

A comparison of two sampling instruments

and a case study in Lutjewad

Meis UijttewaalS2601788

Bachelor Thesis

University of Groningen

8 July 2016

First Supervisor: Dr. U. DusekSecond Supervisor: Prof. dr. H. Meijer

Abstract

Aerosol particles are present everywhere in the air and even though the global con-centrations have been decreasing over the past ten year, there are still places werethe concentrations are dangerously high [9][25]. As aerosol particles affect the bio-sphere, climate and global health, it is important to have accurate monitoring andsampling systems in order to determine the correct concentrations and origins ofparticulate matter in the air. Two samplers that are often used to collect aerosolparticles are the High Volume sampler and the impactor. By simultaneous samplingtwo samplers could be compared. It was expected that both sampling methodswould give the same results concerning the carbonaceous aerosol concentration inair. However, due to the adsorption of volatile compounds on the High Volume filter,concentrations were significantly lower on the impactor filters. No explanation couldbe found for the high concentration of these adsorbed gasses. Furthermore, fromsamples collected at a coastal station, the concentrations of carbonaceous aerosolsin air masses from sea were compared to the concentrations in air masses from land.For all particle diameters the measured concentrations were significantly higher in airmasses from the land. In air masses from the sea the contribution of small particles(<0.44µm) to the total carbon mass is larger then in air masses from the land. Thismight indicate that particles from the land are older or are emitted more frequentlyby biomass burning and that particles from the sea are younger or are emitted byfossil fuel burning. For a better understanding of the origin of the aerosols furtheranalysis of the filters would be necessary.

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Contents

1 Introduction 31.1 Aerosols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.1.1 Primary and secondary aerosols . . . . . . . . . . . . . . . . . 31.1.2 Formation and removal of aerosol . . . . . . . . . . . . . . . . 31.1.3 Carbonaceous aerosols . . . . . . . . . . . . . . . . . . . . . . 51.1.4 Effect on climate . . . . . . . . . . . . . . . . . . . . . . . . . 51.1.5 Effect on health . . . . . . . . . . . . . . . . . . . . . . . . . . 6

1.2 Research topic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2 Method 72.1 High Volume Sampler . . . . . . . . . . . . . . . . . . . . . . . . . . . 72.2 Impactor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72.3 Lutjewad . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82.4 Sampling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92.5 OC-EC Analyzer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102.6 EUSAAR 2 protocol . . . . . . . . . . . . . . . . . . . . . . . . . . . 112.7 Analysis of the samples . . . . . . . . . . . . . . . . . . . . . . . . . . 12

3 Results & Discussion 143.1 Comparison of samplers . . . . . . . . . . . . . . . . . . . . . . . . . 153.2 Size distribution of aerosols . . . . . . . . . . . . . . . . . . . . . . . 20

4 Conclusion 23

A Particle trajectories 26

2

Chapter 1

Introduction

Particulate matter, also called atmospheric aerosol particles, is present everywhere.Over the past 10 years the global concentrations have been decreasing, however, theregional dispersion of concentrations is large [9]. Aerosol particles strongly affect theair quality, which endangers the health of the world population and the environment.Furthermore, aerosol particles can have either a warming or a cooling effect on theglobal climate, depending on their characteristics[17].

1.1 Aerosols

In literature several definitions are used for aerosols. In general an aerosol is definedas ”a suspension of liquid or solid particles in a gas, with particle diameters inthe range of 10−9 - 10−4m” [17]. This definition also includes gaseous compounds,however, in the remainder of this report the term aerosols will only refer to non-volatile or semi-volatile particles. When gaseous compounds are involved they will bespecifically mentioned. The particles investigated in this research all have diameterssmaller than 2.5µm, these are classified as PM2.5.

1.1.1 Primary and secondary aerosols

There are two types of particulate matter, primary particles and secondary parti-cles. The primary particles are directly emitted by natural or anthropogenic sources.The main sources of primary particles are the combustion of fossil fuels, biomassburning, the suspension of dust, sea spray containing suspended particles and bi-ological materials [17]. Secondary particles are formed through chemical reactionsand condensation of precursor gasses. The gaseous components can be transformedinto particles in three ways. Gaseous semi-volatile organic compounds can parti-tion onto existing aerosols, also these compounds can form particles by themselvesthrough nucleation. Furthermore, volatile compounds on the surface or inside ex-isting aerosols can form non-volatile compounds through chemical reactions [8].

1.1.2 Formation and removal of aerosol

Aerosols in the air will age with time and change continuously in composition andsize. Small particles that are emitted or formed will diffuse to other particles andstick together, a process that is called coagulation [21]. As already mentioned,

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vapours will condensate on the surface of aerosols, increasing their size. On the otherhand, particles are removed from the atmosphere. The most important removalmechanism is wet deposition, in which two processes can be distinguished. Firstly,there is rainout, a term containing all the processes which happen inside clouds whenwater condensates on particles to form cloud droplets, subsequently removing theaerosols as rain. Secondly, when aerosols are intercepted by falling rain or snow, thisis called washout [9]. Another process of removal is dry deposition through severalmechanisms such as convective transport, impaction on obstacles and diffusion andadhesion to surfaces [17]. All the processes concerning the formation and depositionof aerosols are depicted in Fig 1.1.Due to all these processes the number-size andthe mass-size distributions of aerosol particles have a specific shape, as is shown inFig 1.2.

Figure 1.1: Processes controlling numbers and size of aerosols[11]

Figure 1.2: Particle number-size and particle mass-size distribution of aerosols andthe processes controlling them[9]

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1.1.3 Carbonaceous aerosols

In this research the focus is on carbonaceous aerosols, which includes all particu-late matter containing carbon. Carbonaceous particles can be subdivided into twogroups, the organic carbon (OC) and the elemental carbon (EC) or black carbon(BC), which together form the total carbon (TC). Inorganic carbon is not takeninto account in this research, as the concentrations of this type of carbon previouslymeasured in the Netherlands are very low [18]. EC and BC are distinguished bythe way in which they are measured. When the analysis technique relies on theoptical properties of the carbon content, this carbon is defined as BC. On the otherhand, when thermochemical properties are used the carbon is classified as EC [17].The apparatus used for the analysis of filters in this research uses a thermal-opticalapproach, and therefore measures EC. Both EC and BC consist of graphite-like ma-terial which is present in combustion products such as soot. These graphite likematerials can be seen as stacks of graphene layers or polycyclic aromatics [10]. ECoriginates almost completely from anthropogenic emissions, whereas the origin ofOC depends on the location of sampling, OC particles can be both biogenic andanthropogenic [20].

1.1.4 Effect on climate

The temperature on Earth is regulated by the flux of radiation. Shortwave radiationfrom the sun is absorbed and reflected by the Earth’s surface, which then emits long-wave radiation in order to keep the temperature constant. Changes in the energyfluxes of these two types of radiation are called radiative forcings. Radiative forcingresults in a change of the average global temperature. Examples of negative forcingare reflection and scattering of radiation from clouds or aerosols, resulting in a de-crease in temperature. Absorption of radiation, on the other hand, is an exampleof positive forcing, leading to an increase in temperature [17]. Aerosols influencethe radiative forcing through direct and indirect effects. Depending on their com-position, particles either absorb or scatter radiation. Particles larger than 2µm aremore likely to absorb longwave radiation from the Earth’s surface, whereas particlesthat are smaller will reflect the radiation from the sun entering the atmosphere [21].Particles containing black carbon absorb shortwave radiation. The scattering andabsorption of radiation is a direct effect on the radiative forcing. However, aerosolscan also influence the formation, composition and lifetime of clouds, through whichthey indirectly influence the radiative forcing. Aerosols in the atmosphere formnuclei on which water vapor can condensate to form cloud droplets. If the concen-tration of aerosols increases and the cloud water vapor content remains constant,more droplets will form which are smaller and contain less water. This process en-larges the total surface area of cloud droplets, resulting in an increased reflectionof solar radiation. Also, the reduced water content of the droplets will increasethe lifetime of clouds, as the droplets need to have a minimum size before they falldown as rain. The longer lifetime of clouds results in more reflection of shortwaveradiation, hence it has a cooling effect [1]. It is difficult to determine whether theoverall effect of all aerosols on the radiative balance is positive or negative, due tothe numerous indirect and feedback effects. It is being estimated that aerosols havea negative radiative forcing effect [2].

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1.1.5 Effect on health

Air pollution poses a huge threat to global health. The WHO estimated that in 20127 million people died prematurely due to exposure to increased concentrations ofparticulate matter [7]. A lot of research has been done on the effects of air pollutionon human health. Particles enter the body through the respiratory system, whena person inhales. Depending on the size of the particles they are deposited indifferent parts of the respiratory tract. The largest particles (>10µm) will havedifficulty following fast airstreams and impact on the walls of the nose and mouth.When the air speeds are low the smallest particles will deposit on the walls throughdiffusion. Particles in the size range 0.1-1.0 µm will not be subject to impaction ordiffusion and will therefore be able to penetrate deeply into the lungs. However, thechance that they will deposit in the alveoli is small, so most of these particles willbe exhaled again [21]. The main health effects caused by exposure to particulatematter are cardiovascular and respiratory diseases. Particles that are deposited inthe lung tissue cause inflammations, due to which mediators are released in theblood circulation system. These trigger a wide range of physiologic responses, suchas the formation of atherosclerosis, a disfunctioning of the blood vessels or an alteredrheology of the blood [5]. Aerosols that are being inhaled into the alveoli can alsotake part in the gas exchange. Harmful gaseous compounds can then enter the bloodstream and spread throughout the whole body, damaging also other organs.

1.2 Research topic

Only since the 80’s of the last century the attention from the atmospheric sciencecommunity for the subject of aerosols has started to grow [9]. Nowadays, a lotof research is done into all the effects of aerosols. However, there is still a lot thatremains unknown. In order to determine characteristics and influences of particulatematter it is important to have reliable and accurate measurement equipment. Twosampling systems used to collect particulate matter are the High Volume Samplerand the Impactor Sampler. The goal of this research is to determine whether theconcentrations of carbonaceous aerosols found by these two sampling systems arecomparable. The hypothesis is that the two sampling methods show comparableconcentrations. The concentrations from the High Volume Sampler might be slightlyhigher due to particles bouncing of the filter in the impactor. However, an after filteris used in the impactor which also collects these particles, so the difference causedby this effect is expected to be negligible.

Furthermore, to reduce concentrations of aerosols in order to improve living con-ditions, it is important to know the origin of the particulate matter. This origin andthe direction in which the particles flow can also be used to determine their influenceon the climate. Another part of this research is to determine how large the concen-tration of carbonaceous aerosols is when the wind blows from the continent and howlarge it is when the wind blows from the North Sea. Also the size distributions ofparticles from the sea and from the land will be compared. The expectation is thatthe concentration of particles from the land will be higher, as there are more sourcesof particle on the land, for example burning of biomass and fossil fuels. Also, it isexpected that particles emitted at sea will be larger, as they are likely to be olderand have had more time to coagulate and absorb gaseous materials.

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Chapter 2

Method

2.1 High Volume Sampler

The High Volume sampler used for sampling carbonaceous aerosols is the DigitelDHA-80 [4]. The schematic set-up of this sampler is shown in Fig 2.1. The airis sucked in through the PM2.5 inlet, cutting off all particles bigger than 2.5µm.Subsequently, the air flows from the top to the bottom of the flow chamber, throughthe sampling filter that is positioned horizontally in the middle of the flow chamber.The air flows at a speed of 500 liters per minute, which results in a face velocitythrough the filter of 0.5 m/s. Located after the flow chamber is a flow meter with afloat which regulates the amount of air flowing through the sampler. The position ofthe float is monitored by a double photosensor, and with electronic controllers thecapacity of the blowers can be adapted if the float changes position, which indicatesa change in flow speed. The filter holder in the flow chamber can automatically bechanged for one of the filter holders in the stack. Each filter holder contains twofilters, one filter on top (A-filter) and a filter beneath it (B-filter). The air containingthe particulate matter is sucked through both filters, the particles are too large topenetrate the filters and are deposited on the A-filter. Gases in the air can passthrough the filter material and are adsorbed on the fibers of the filter material,this adsorption is present in both the A-filter and the B-filter. The filters used inthe High Volume sampler are Pallflex membrane filters of the type TISSUQUARTZ2500QAT-UP with a diameter of 150mm.

2.2 Impactor

The impactor used for sampling aerosols is the Model 130 High-Flow Impactor. Thisimpactor consists of an inlet and 5 impaction stages with cutpoints of 2.5, 1.4, 0.77,0.44 and 0.25 µm. The last stage body has a barbed pressure tap to determinethe pressure drop over all the stages. To collect aerosols a filter is placed on theimpaction plate with the retaining ring on top to keep it in the right position. Theimpaction plates are positioned on the impaction stages, which are all stacked ontop of each other, which in the end forms the complete impactor, as shown in Fig2.2. The air is pulled through the impactor with a speed of 100±2 liters per minute.The impactor separates particles of different sizes by making use of their differencein momentum. The flow of air through an impactor is depicted in Fig 2.3. Theair is sucked through small holes and then forced to make a sharp bend around

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Figure 2.1: Schematic drawing of the High Volume Sampler[4]

the collection plate. The momentum of large particles is too big for them to beable to follow the air stream, so they collide with the collection plate, where theyare collected on a filter and form a dotted pattern. The filter positioned after thecutpoint of 2.5µm collects 50% of the particles bigger than 2.5µm, the filter afterthe cutpoint of 1.4µm collects 50 % of the particles with a size between 2.5 µmand 1.4 µm, etc. On the after filter all particles smaller than 0.25 µm are collected.Also, any particles that have bounced off the other filters are collected on the afterfilter. The filters used in the impactor are Pallflex membrane filters of the typeTISSUQUARTZ 2500QAT-UP. The after filter has a diameter of 90mm, all theother filters are 75mm in cross section [14].

2.3 Lutjewad

All the samples were collected at the atmospheric measurement station Lutjewad.This station is located on the northern coast of the Netherlands at the coordinates 6◦

21’ E, 53◦ 24’ N. The Waddensea is located just to the North of Lutjewad, as can be

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Figure 2.2: Impactor [15] Figure 2.3: Schematic drawing of the sam-pling principle of an impactor [13]

seen in Fig 2.4. The High Volume sampler and impactor samplers were positionedon a scaffolding at a height of 11 m. This was necessary to reach an airstreamthat had not interfered with the dike behind which the samplers were situated. Atthe same height as the samplers, also the wind direction, wind speed, temperature,humidity and precipitation rate were monitored.

2.4 Sampling

For this research, samples were taken during three periods. The first period is thetest period, during which the High Volume sampler and one impactor were runningsimultaneously. This period lasted from 2nd of March 2016 until the 17th of March2016. The first period during which real samples were taken, hereafter called winterperiod, lasted from the 15th of February 2016 until the 2nd of March 2016. It must benoted that the concentrations measured during this period are not representative forthe average concentrations during winter. During the winter period the High Volumesampler was sampling continuously, starting only at the 17th of February due to afailure in the pump. The two impactors were sampling on and off, depending on thewind direction. Impactor 1 was running when the wind was directed from the sea,from an angle of 240◦ to an angle of 30◦. The other impactor was running whenthe wind blew from the land, from an angle of 90◦ to an angle of 185◦. Also duringthe second period, herafter called spring period, which lasted from the 19th of Apriluntil the 4th of May, the High Volume sampler was running continuously and theimpactors were sampling according to the wind direction. Again, the concentrationsmeasured are not representative for the average concentrations in spring. Duringthe sampling periods, the filters in the High Volume sampler were changed when thewind direction changed from land to sea or vice versa, therefore there are several

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Figure 2.4: Location of the atmospheric measurement station Lutjewad

High Volume filters for each period. At the end of each sampling campaign, blankfilters were placed in the High Volume sampler and the impactor for two weeks.During this period the samplers were turned off, so the blank filters only collectedthe background concentration. The sampling data and times for the different filtersare represented in table 2.1, this table also contains the average temperature, averageprecipitation, average humidity and average wind direction for each filter. Theair mass history was determined from the air mass back-trajectories, which wereobtained using NOAA’s HYSPLIT atmospheric transport and dispersion modelingsystem [16]. The trajectories belonging to each filter can be found in the Appendix.

2.5 OC-EC Analyzer

The carbonaceous material collected on the filters was analyzed using a thermal-optical OC-EC analyzer (produced by Sunset Laboratory Inc). The OC-EC analyzerconsists of two ovens: in the front oven the temperature can be changed, in the backoven the temperature remains constant. After inserting a 1.5 cm2 of the filter sam-ple into the front oven, the oven is first flushed with helium, while the temperatureis raised in four steps. During each step a different fraction of the OC material onthe filter is thermally desorbed off. These particles are flushed into the manganesedioxide oxidizing oven, where all the carbonaceous constituents are converted intoCO2. This CO2 gas is transported by the helium gas into the non-dispersive in-frared detector system (NDIR), which measures the amount of CO2 present in thegas. After increasing the temperature in the front oven in the presence of helium,the temperature is raised also while flushing the oven with a mixture of helium andoxygen. During this part of the analysis also the EC constituents are burned offand transported to the back oven, after which they are detected by the NDIR de-tector. Finally, a fixed volume of a known methane concentration is injected intothe system, in order to calibrate the system after each measurement and normalize

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Table 2.1: Sampling periods and weather conditions for the High Volume and im-pactor filters

Sample nameSamplingdates

Samplingtime (min)

Averagetemp. (◦C)

Averageprec. (L/h)

Averagehum.(%)

Averagewind dir.

WITest HV1 02/3-06/3 5825 3.9 0.55 67.5 SeaWITest HV2 06/3-09/3 3376 4.7 0.55 66.5 SeaWITest HV3 09/3-17/3 12100 4.8 0 66.1 LandWI16 HV1 17/2-20/2 4291 3.9 0.41 68.1 LandWI16 HV2 20/2-23/2 4463 7.4 1.26 65.1 SeaWI16 HV3 23/2-26/2 3939 3.9 0.32 67.3 SeaWI16 HV4 26/2-01/3 5901 3.9 0.36 67.3 Sea/landWI16 HV5 01/3-02/3 1516 4.8 1.82 66.6 Land/seaSP16 HV1 19/4-26/4 9756 7.0 0.82 64.3 SeaSP16 HV2 26/4-28/4 2992 5.3 2.04 63.9 Land/seaSP16 HV3 28/4-02/5 6557 8.4 1.03 62.4 Sea/localSP16 HV4 02/5-04/5 2305 10.9 1.81 60.2 Sea/localWITest IM 02/3-17/3 21390 4.5 0.31 66.6 Sea/landWI16 IML 15/2-02/3 5746 3.0 0.36 68.6 LandWI16 IMS 15/2-02/3 4616 4.6 0.47 66.9 SeaSP16 IML 19/4-04/5 2468 92 1.03 61.6 LandSP16 IMS 19/4-04/5 14125 7.4 1.06 64.0 Sea

out any variations in the performance of the system. The difficulty in determiningthe fractions of OC and EC lies in the pyrolyzing properties of OC. Not all OC isevaporated from the filter while flushing the oven with helium, as part of the carbonpyrolyses and is left on the filter as char. This char does come off in the presence ofoxygen, which results in it being detected as EC. In order to correct for this pyrolysiseffect, the OC-EC analyzer makes use of the difference in light-absorbing propertiesof OC and EC. OC particles do not absorb much light, whereas EC particles arehighly absorbent. A tuned diode laser with a wavelength of 660 nm is focused onthe filter and passes through it. At the beginning of the analysis the transmittanceof the filter sample is measured. During the helium mode, when the OC pyrolyses,the transmittance decreases due to the char that has formed on the filter. Duringthe helium/oxygen mode, the transmittance increases again, as the char is burnedoff and also the EC is oxidized. The point at which the transmittance is back at itsinitial value is called the split point. In calculating the concentrations, the carbonthat has desorbed off the filter before the split point is taken as OC and the carbondetected after the split point is taken as EC [19].

2.6 EUSAAR 2 protocol

Using the light absorbing properties of the carbonaceous constituents to distinguishbetween OC and EC particles, either one of the two following assumptions hasto be correct. Firstly, pyrolitic carbon (PC) formed during the helium-mode isassumed to have the same specific attenuation cross section as EC desorbed in thehelium/oxygen mode. In this case, regardless of the order in which PC and EC aredesorbed off the filter, the amount that evolves after the split point is considered as

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EC. The second assumption is that PC always evolves before EC in the helium/oxidemode, ensuring that all carbon evolving after the splitpoint is actually EC. However,it has been shown that both assumptions are often not valid [6], resulting in over-or underestimation of the OC- and EC-fractions. In order to reduce these errorsconsiderably the EUSAAR 2 protocol was used. This protocol has been designed inorder to optimize four criteria, compared to the previously used protocols IMPROVEand NIOSH [6]. Firstly, charring of OC is reduced and volatilization is increased byadding and prolonging the steps at low temperature in the helium mode. Secondly,the maximum temperature in the helium mode is lowered to prevent prematuredesorption of EC. However, this maximum temperature should not be too low, whichis the third criteria, as this prevents the OC from completely volatilizing or it causesit to pyrolize, in both cases resulting in the release of OC during the helium/oxidemode. The fourth criteria is minimizing the uncertainty in the position of the splitpoint by increasing the number of desorption steps in the helium/oxide mode. Thesecriteria led to following temperature steps: First in helium: 200 ◦C for 120s, 300 ◦Cfor 150s, 450 ◦C for 180s and 650 ◦C for 180s. Then in helium/dioxide: 500 ◦C for120s, 550 ◦C for 120s, 700 ◦C for 70s and 850 ◦C for 80s. The temperature steps aredepicted in Fig 2.5.

Figure 2.5: Graph indicating the temperature steps in the EUSAAR 2 protocol(purple line). The blue line indicates the amount of carbon detected. The red lineshows the transmittance of the filter.[3]

2.7 Analysis of the samples

At the start of each day of measurements the OC-EC analyzer first had to be pre-pared and the calibration had to be checked. This was done in the following order.After the OC-EC analyzer had been taken out of the standby mode, the oven was

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cleaned by heating it to ±850◦C. Then a clean test filter was inserted and measuredwith the EUSAAR 2 protocol, this measurement was called First Of the Day (FOD).Immediately afterwards, the same filter was measured again without opening theoven in between. This measurement provided the background concentration mea-sured with each run, the so called instrument blank. To check the calibration ofthe instrument, 10µL sucrose solution of a known concentration was pipetted on thefilter already in place from the instrument blank measurements. After drying thefilter, it was analyzed using the EUSAAR 2 protocol. If the amount of carbon deter-mined by the OC-EC analyzer was equal to the amount pipetted onto the filter, themeasurements could be started. Otherwise several more sucrose measurements hadto be performed to rule out any errors and determine a new calibration constant.All the instruments used to handle the filters were cleaned with acetone and ethanolto remove any contamination, after which they were left to dry for 10 minutes. Atthe end of the day the OC-EC analyzer was turned back into the standby modeagain.

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Chapter 3

Results & Discussion

The OC-EC analyzer was used to measure the carbon concentration on the filters.The new data are reported as µg/cm2 for the samples from the High Volume filterand as µg/sample for the impactor samples and the sucrose measurements. Thecarbon amount was first corrected by subtracting the carbon amount on the instru-ment blanks and the blank filters. Subsequently the concentrations of carbonaceousaerosols in air (Ca) were calculated. For the High Volume filters this was done bymultiplying the concentration per cm2 (Cf ) with the total area of the filter piece(Af ) and dividing this by the flow rate (f) of the High Volume sampler times thesampling time (t).

Ca =Cf ∗ Af

f ∗ t(3.1)

For the impactor samples the concentration in air (Ca) is calculated by dividing thecarbon mass on the sample (Cf ) by the number of dots on the sample (ds) to getthe carbon mass per dot. This value is then multiplied with the total number ofdots on the filter (df ) to get the total amount of carbon on the filter. After dividingby the flow rate (f) multiplied with the sampling time (t) the concentration in airis obtained.

Ca =(Cf

ds) ∗ df

(f ∗ t)(3.2)

The uncertainties in the concentrations of carbon on the filters after subtracting thecarbon masses on the instrument blank and the blank filter are calculated as follows:

ErrorCf2 =√ErrorCf12 + Errorib2 + Errorbf 2 (3.3)

Where ErrorCf2 is the error of the corrected carbon concentration and ErrorCf1,Errorib and Errorbf are, respectively, the errors for the measured concentration onthe filter, the instrument blank and the blank filter provided by the sunset.

In order to determine the error in the concentration in air the following formulais used:

ErrorCa =

√√√√(∂Ca

∂Cf

)2 ∗ ErrorCf22 + (∂Ca

∂f)2 ∗ Errorf 2 (3.4)

Here Errorf2 is the error in the flow rate. The errors in the number of dots on thesample and filter, in the sampling time and in the filter area are negligibly small, sothey are not taken into account.

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3.1 Comparison of samplers

Figure 3.1: The TC, OC and EC concentrations in air derived from aerosol depositson High Volume filters (A/B filters) and impactor filters for the test period.

In Fig 3.1 the TC, OC and EC concentrations on the A-filter, the B-filter and thesum over the TC concentrations on all impactor filters are shown. It can be seen thatthe TC concentrations on the A-filter are much higher than the concentrations on theimpactor filters. A possible explanation for this difference is given by the presence ofvolatile organic compounds. In the High Volume sampler the air is pulled throughthe filter, so gases get in contact with the filter fibers and can adsorb to them. In theimpactor the air is pulled around the filters, so the gases do not get in contact withthe filter material. This difference in adsorption could result in a higher carbon loadon the High Volume filters. The B-filter, which is placed underneath the A-filterin the High Volume sampler only collects these gases and the difference betweenTC on the A- and B-filter is representative of the collected particles. The value iscomparable to TC found on the impactor filters. The ratio between the TC valueson the B-filter and the A-filter is 0.5, indicating that half of the TC on the A-filter isvolatile organic material. Similar ratios have been measured before [22][23]. Howeverratios around 0.5 were not expected in this study, as during previous measurementsin the Netherlands much lower ratios were found [24]. In order to say with certainty

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whether adsorption of volatile compounds causes the difference in concentrations onHigh Volume filters and impactor filters, it is necessary to do more measurementswere the impactor and the HV sampler are running simultaneously.

Not only during the testperiod was the ratio between TC on the B-filter and TCon the A-filter very high. Fig 3.2 and Fig 3.3 show the comparison between the A andthe B filter for the different samples taken during the winter period and the springperiod. The ratios between the concentration of TC on the B-filter and the A-filterfor all the filters are shown in tables 3.1 and 3.2. Especially the samples from thewinter period also show relatively high concentrations of volatile compounds, whichcan be explained by the fact that the test period and the winter period where bothin winter, hence the air might have contained the same type of particles and gasescommon in winter. The ratios between the TC on the B- and the A-filter in thespring period are mostly lower as can be seen in Fig 3.3 and table 3.2. This result isin conflict with what is found in literature [23]. Usually, the influence of adsorptionis larger for filters with lower carbon concentrations, because the adsorption reachesa saturation point, whereas the aerosol particles continue to accumulate. Thereforeit would be expected that the filters from the spring period show relatively higherratios of volatile compounds, as they contain less carbonaceous compounds overall.An explanation for this deviation from previous observations might be that in bothperiods the concentration of volatile compounds was too low to reach saturation onthe filter. Another theory that might explain the difference in volatile compoundratio is that the average temperature during sampling might have an influence. Ahigher temperature could either increase the presence of volatile compounds in theair, or decrease the chance of the volatile compounds binding to the filter fibers.Even though the sample for which the average temperature was the highest duringsample, has the lowest ratio of volatile compounds (SP16 HV4), there does not seemto be a clear relationship between the average temperature during sampling and thevolatile compound ratio. For sample WI16 HV2 (7.4◦C) the average temperaturewas higher than for sample SP16 HV2 (5.3◦C), however their respective ratios forvolatile compounds were 0.36 and 0.11. Also the average precipitation, the averagewind direction and the average humidity, which are shown in table 2.1, do not seemto influence the amount of volatile compounds adsorbing to the filters. Anotherexplanation for the difference in the ratios of volatile compounds could be that thegases present during sampling differ in nature, hence the adsorbing properties of thegases vary. It would be necessary to analyze the B-filters with other techniques inorder to confirm this hypothesis.

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Figure 3.2: The TC concentrations in air derived from the aerosol deposits on HighVolume filters during the winter period. The letters indicate the main wind di-rection: L(land), S(sea), M(mix of land and sea). The red lines indicate the TCconcentrations derived from the aerosol deposits on the impactor filters.

Figure 3.3: The TC concentrations in air derived from the aerosol deposits on HighVolume filters during the spring period. The letters indicate the main wind direction:L(land), S(sea), S+local(mostly from sea, but with deposits from local sources). Thered lines indicate the TC concentrations derived from the aerosol deposits on theimpactor filters.

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To determine what the difference is between deposits on the filters it is interestingto have a look at the fraction of OC from the total OC mass that has charred duringthe analysis in the OC-EC analyzer. These fractions are shown in Fig 3.4 and Fig3.5 for all the High Volume filters from both periods. There does not seem to be arelation between the ratios between TC on the B-filter and TC on the A-filter (asshown in table 3.1 and table 3.2) and the fraction of OC that has charred. ThereforeFig 3.4 and Fig 3.5 do not provide more information about the difference in the ratioof volatile compounds for the different filters. However, these graphs do show thatthe composition of the particles is different in air masses from the sea compared toair masses from the land. The green bars in the figures indicate the fraction of OC ingaseous compounds that has charred, whereas the yellow bars indicate the fractionof OC in particles that has charred. In both the winter and the spring period, theparticles in the air masses from sea do not char, whereas the particles in air massesthat have passed mostly or partly over land do char.

In order to compare data collected by the High Volume sampler and the impactor,it would also be useful if it is possible to analyze OC and EC on the impactor sam-ples. Previous research shows that analysing impactor filters with a thermal-opticalmethod yields results that are comparable to values found for OC and EC concentra-tions on High Volume filters [12]. However, the results in Fig 3.1 show that for thisresearch the values for the OC and EC concentrations derived from the impactor fil-ters are not comparable to the concentrations derived from High Volume filters. Theamount of OC on the impactor filter is overestimated, whereas the amount of ECis underestimated. This deviation in the concentrations is caused by the depositionpattern of aerosols on the impactor filters. The particles are pulled through smallholes, resulting in a dotted pattern with locally high particle concentrations. Dueto this non-uniform distribution, the laser beam cannot determine the transmissioncorrectly, resulting in a shift of the split point, causing the OC and EC concentra-tions to be calculated incorrectly. Hence, it will not be possible to determine theOC and EC fractions per size interval.

Table 3.1: Ratio between TC on the B-filter and TC on the A-filter and the averagewind direction per filter, winter periodSample name WI16 HV1 WI16 HV2 WI16 HV3 WI16 HV4 WI16 HV5Ratio TC 0.37 0.36 0.41 0.25 0.42Wind direction L S S M M

Table 3.2: Ratio between TC on the B-filter and TC on the A-filter and the averagewind direction per filter, spring period

Sample name SP16 HV1 SP16 HV2 SP16 HV3 SP16 HV4Ratio TC 0.40 0.11 0.12 0.04Wind direction S L S+local S+local

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Figure 3.4: The fraction of OC that has pyrolysed during the analysis in the OC-ECanalyzer for all the High Volume filters from the winter period

Figure 3.5: The fraction of OC that has pyrolysed during the analysis in the OC-ECanalyzer for all the High Volume filters from the spring period

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3.2 Size distribution of aerosols

Figure 3.6: The mass distribution as a function of the particle diameter, derivedfrom deposits on impactor filters, sampling for different wind directions in the winterperiod and the spring period

Fig 3.6 shows the mass distribution of carbon as a function of the particle diameter.The mass is normalized to the logarithm of the size interval, for the particle diameterthe geometric mean of each size interval was determined. For the smallest diametera size range of 0.05µm up to 0.25µm was taken and for the largest diameter thesize range was taken to be 2.5µm up to 10µm. In this graph it can be seen thatthe concentration of carbonaceous aerosols in air from the North Sea sector is muchlower than the concentration in air from the land sector. The concentration in the airfrom the sea sector remained constant over winter and spring. The concentration inthe air from land, however, is higher in winter than in spring. This could be causedby a higher amount of biomass burning or fossil fuel burning for heating. In order todetermine whether this is really the case a 14C analysis would be necessary. Anotherpossible explanation for the lower concentrations found for the land sector in thespring period can be found in the particle trajectories included in the appendix. Inthe winter period, air masses from the land sector, also had passed sea for severaldays before ending up at the sampling site. During the spring period, it was oftenthe case that the wind circled around the Netherlands. Therefore, the wind locally

20

might come from the land sector, but the air masses had actually spent most timeover sea, as is the case for the dark blue line and the yellow line in Fig A.11.

Fig 3.7 shows the percentage contribution of carbon contained in particles ina certain size range to the PM2.5 carbon mass. In the winter period, the maindifference in the distribution of the particle sizes from land and sea, is the muchhigher contribution of the very small particles to the total carbon concentration.These smallest particles are often emitted by fossil fuel burning. It might be possiblethat these particles were emitted by sea ships crossing the North Sea close to thecoast of the Netherlands. The large contribution of particles in the size range of0.44-0.77µm is expected as this is the accumulation range in which particles areformed through coagulation.

Also in the spring period the contribution of the smallest particles to the totalcarbon mass is larger in the air masses from sea than in air masses from land. Thecontribution of particles in the accumulation size range is very small, which mightindicate that the particles are emitted recently and have not had the chance todiffuse and coagulate to other particles. The particles from land are mostly larger,which may indicate that they are older or that they were emitted by the burningof biomass. In order to get a better indication of the origin of the carbonaceousaerosols it is necessary to do a 14C analysis.

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Figure 3.7: The percentage contribution of carbon contained in particles in a certainsize range to the PM2.5 carbon mass for the two impactors from both periods:(a):winter period, wind from land section, (b): winter period, wind from sea section,(c): spring period, wind from land section, (d): spring period, wind from sea section

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Chapter 4

Conclusion

It can be concluded that TC found on the A-filter of the High Volume sampler andthe sum of TC on all impactor filters are not comparable due to a high positiveartifact of adsorbing volatile compounds. When concentrations on the B-filter, con-taining only these gases, are subtracted from the concentrations on the A-filter, thisgives the concentration of the carbonaceous particles on the A-filter. This concen-tration is comparable to the concentrations on the impactor filters, to which thevolatile compounds do not adsorb. In order to confirm that the volatile compoundscause the large difference between the High Volume filters and the impactor filtersit is necessary to perform more measurements.

Even though the TC values seem to be comparable after subtracting the B-filterconcentrations, the OC and EC concentrations cannot be compared, as the analysismethod is nog suitable.

From the impactor samples from the winter and spring period, it can be con-cluded that the carbonaceous aerosol concentration for all particle sizes is signifi-cantly higher in air masses from land, compared to air masses from the North Sea.In the winter period the concentration in air from land was higher than in the springperiod, which may be caused by rapidly changing wind directions, which makes itlook as if the particles were emitted on land, while they actually originated at thesea. It is also possible that the concentrations were actually higher in winter time,caused by increased amounts of fossil fuel or biomass burning due to lower averagetemperatures.

Looking at the percentage of carbon concentrations in each size interval, it canbe concluded that in air masses from sea small particles make a higher contributionto the total carbon mass than in air masses from land. This might be due tothe emission of particles by sea ships burning fossil fuels. Bigger particles took arelatively large share in air masses from the land sector. This might indicate thatthe particles in air masses from the land were older and had more time to coagulateor that these particles were formed by biomass burning. In order to gain a betterunderstanding of the origin of the particles it is necessary to further analyse thefilters with a 14C analysis.

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Appendix A

Particle trajectories

Figure A.1: Trajectory of particles on filter WITest HV1

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Figure A.2: Trajectory of particles on filter WITest HV2

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Figure A.3: Trajectory of particles on filter WITest HV3

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Figure A.4: Trajectory of particles on filter WI16 HV1

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Figure A.5: Trajectory of particles on filter WI16 HV2

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Figure A.6: Trajectory of particles on filter WI16 HV3

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Figure A.7: Trajectory of particles on filter WI16 HV4

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Figure A.8: Trajectory of particles on filter WI16 HV5

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Figure A.9: Trajectory of particles on filter SP16 HV1

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Figure A.10: Trajectory of particles on filter SP16 HV2

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Figure A.11: Trajectory of particles on filter SP16 HV3

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Figure A.12: Trajectory of particles on filter SP16 HV4

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