University of Amsterdam

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1 University of Amsterdam On- line chemical analysis of inorganic species in atmospheric particulate matter and their related trace gasses: Instrument recovery and validation of a new instrument Master research thesis of my 4 month research period at the Peking University Roeland Jansen July

2 On- line chemical analysis of inorganic species in atmospheric particulate matter and their related trace gasses: Instrument recovery and validation of a new instrument By Roeland Jansen, University of Amsterdam, Master Chemistry, Track Analytical Sciences Master coordinator: Doctor Wim Theodore Kok Date of research: February 14 th June 14 th 2009 Research institute: Peking University, Beijing P.R. of China Research group: College of Environmental Sciences and Engineering Daily supervisor: Professor Min HU 2

3 Abstract Atmospheric particulate matter (PM) contributes to air pollution. PM has an adverse effect on human health, it decreases visibility and contributes to climate change. Furthermore when deposition happens, species in PM can lead to changes in plant and animal communities. PM can be directly emitted in the air, primary aerosols, and it can be formed by homo- or heterogeneous reactions in the atmosphere, secondary aerosols. Both primary and secondary aerosols can origin from both natural and anthropogenic sources. The chemical composition of PM is a fundamental property related to above mentioned environmental issues and to understand which species are responsible for which effects the composition needs to be studied. The classical filter based technique for PM sampling and collecting requires long term sampling and hence results in average concentrations, leads to over- or underestimations of concentrations due to artifacts and is labor intensive to perform. This has led to the development of on- line and high sample frequency instruments for sampling, collecting and analyzing PM. An instrument called the Steam Jet Aerosol Collector (SJAC) has been developed at the Energy Research Centre of the Netherland (ECN, Petten The Netherlands). The SJAC has been successfully used in field campaigns as on- line high resolution instrument to measure concentrations of inorganic species in aerosols (NH + 4, Cl -, NO - 2, NO - 3, SO 2-4 ) simultaneously with water soluble acidic trace gasses (SO 2, HNO 3, HNO 2, HCl) and ammonia (NH 3 ). The simultaneous measurement of inorganic species in PM and acidic trace gases and ammonia is of high interest to study their relation. A version of this instrument was donated to the Peking University (PKU, Beijing P.R of China) several years ago. The condition of the instrument is not good and it has been partly recovered in this study. Further work needs to be done before this instrument can be used in field campaigns again. An instrument called the Gas Aerosol Collector (GAC) has been developed at the Peking University. The methodologies for gas and aerosol sampling are based on the principles of the ones used the SJAC. The GAC is significantly more robust and easier to use and transport, the control and data acquisition are up to date and hence it is friendlier to its users. The performance (i.e. the efficiency of the devices to collect gasses and aerosols) of the GAC has been partly validated during this study. The efficiency has been tested by sampling artificial aerosols and standard certified gases. Several artifacts were observed which consequently lead to over- or underestimation of the signals. Primarily results show a relation between aerosol concentration and aerosol loss in the inlet part, leading to underestimation in aerosol results and overestimation in the related gas results. The collection efficiency of the aerosol collector for artificial generated (NH 4 ) 2 SO 4 aerosols is larger than 99%. Further validating of the GAC needs to be performed before accurate measurements of the inorganic composition of PM and related water soluble trace gases can take place and recommendations are given to do so. The physical properties of artificial generated (NH 4 ) 2 SO 4 particles were studied by the use of a commercial aerosol generator (TSI Atomizer) coupled to a scanning mobility particle sizer (SMPS) and a condensation particle counter (CPC). Clear changes in physical properties are observed when changing the settings of the atomizer. Keywords: Inorganics, aerosols, ammonia, acidic trace gases, wet rotating denuder, aerosol collector, efficiency, artifacts, SMPS, CPC. 3

4 Preface I spent a little over 5 months in tate Key Joint Laboratory of Environmental Simulation and Pollution Control. This research is part of my master Chemistry track Analytical Sciences at the University of Amsterdam. The topics of my research are related to air pollution. By luck, Beijing is a neat place to study about air pollution and to discuss about this topic. According to the World Bank 16 of the 20 cities with the worst air pollution in the world are in China, including Beijing. Particularly alarming are particulate matter concentration, PM10 levels, 3 nearly three times the WHO standard. The 2008 Olympics can be called the first global experiment to directly observe if governmental regulations have a direct effect on air quality. It has to be mentioned though that the geographic location of Beijing is not favourable in respect to emission transport; the city is surrounded by mountains which are known for their trapping effect. And so the air quality during the Olympics was also based on luck by meteorological conditions such as northern wind from clean areas which can close factories, to convert furnaces of tens of thousands of homes from coal to gas and to decrease the number of cars in the streets. That seemed to have been a great success for the pollutants which are being held responsible for the air quality: SO 2, NO x, CO, PM10 and O 3 all decreased significantly. Although the primary purpose of my visit was of course to do research and help the Peking University to make progress in their studies, I had personal reasons also to dive in this adventure. I always wanted to spend a longer period in another country to experience a different culture and I feel really lucky I was given the opportunity to do largest population and henc of the future, in several aspects. For me attractive reasons to see, feel, experience this myself. These months I had a great time and I experienced lots of moments which will be a lifetime memory. I have been in lots of situations in which I was faced with culture differences and these situations were sometimes funny, sometimes stressful, sometimes uncomfortable, sometimes embarrassed and sometimes difficult but after all they were all moments from which I and the people I dealt with learned. We learned to appreciate our differences, I learned about the local customs.., and as a result many of the difficulties I encountered became much easier. I enjoyed talking with students, teachers, professors, locals, and expats about their view on the past, present and future of China. I want to thank all the people who made this time possible for me. Thanks to my host University, The Peking University, Professor HU Min and her students for their hospitality. Special thanks to Dingliyue for sharing her knowledge about aerosol properties, special thanks to Wu Yusheng and Tang Jingyue for helping me with practical work and interpretation of the data obtained. Special thanks to Cheng Hong when I was in desperate need for help. And special thanks to Peng Jianfei for being my personal guide at PKU these months. Thanks to the University of Amsterdam for their guidance and fundings. Thanks to Dr. Wim Kok for being my coordinator. And thanks to my parents on which I can always rely. Roeland Jansen, July

5 Contents This report starts with an introduction to clarify the need to measure the concentrations of inorganic species in aerosols and their related water soluble gases with on- line and high sample frequency instruments. A detailed description of the hardware follows. Special attention is being paid to measure gas phase ammonia and ammonium in particles. The experimental part of the report covers experiments to (1) attempt to recover an instrument called the SJAC developed at the Energy Research Centre of the Netherlands which was donated to the Peking University, (2) test a new at the Peking University developed instrument named the GAC which contains devices similar to the ones used in the SJAC and (3) test and map the performance of a commercial aerosol generator from TSI which can be used to quantify the efficiency the SJAC and GAC. In the results and discussion part the experimental testing results and the observed artifacts are shown and discussed. A detailed description of the testing results is given in the conclusion part and recommendations are given for further validating of the GAC instrument before it can be used as on- line instrument in field campaigns of PKU in China. 5

6 Table of contents Abstract.. 3 Preface 4 Contents 1. Introduction Sampling, collecting and analyzing inorganics in ambient aerosols and related water soluble gasses: Detailed description of the hardware used Sampling and collecting of ambient acidic trace gasses Experimental The Introduction to validation of the methodology applied in the GAC Aerosol loss in the wet rotating denuder Physical properties of artificial (NH 4 ) 2 SO 4 (s) aerosols measured with SMPS- CPC Results and discussion The Steam Jet Aerosol Collector The Gas Aerosol Collector The TSI Atomizer Model Conclusions 52 References 55 6

7 1. Introduction 1.1. General background Atmospheric particulate matter (PM or aerosols) is the total amount of tiny particles of solid or liquid suspended in the air. It is classified according to its size, the aerodynamic diameter (Table 1). Properties of PM are important in relation to environmental issues, human health and climate change. The most important aerosols properties related to these issues are the particle size distribution and the chemical composition (Khlystov et al., 1995). Figure 1 shows a typical particle size distribution with corresponding chemical composition. Table 1 EPA Terminology for Particle sizes ( EPA description pa ) Supercoarse Total Suspended Particles PM10 PM2.5 Coarse PM Fine PM PM 0.1 or Ultrafine PM d pa pa d pa d pa 2.5 < d pa pa d pa Fig. 1 Typical particle size distribution with corresponding chemical composition. Reproduced from Central Pollution Control Board, Ministry of Environment & Forests, Govt of India, Parivesh Bhawan: 4.html 7

8 is not harmful. Smaller particles however are considered harmful since they are able to depending on concentration, size, composition and exposure time, can cause asthma, lung cancer, cardiovascular issues and premature death. Particles below 2.5 settle deep in the lungs. Particles below 100 nm may pass through the lungs and affect other organs and can result in heart attacks and other cardiovascular problems (Pope et al., 2002). Carbonaceous species such as soot particles emitted from diesel engines (Diesel Particulate Matter, DPM) are typically in the size of 100 nm. Aerosols affect the global radiative balance through light scattering effects (Sutton et al., 1994; ten Brink et al., 1996). Cloud formation is stimulated by the presence of particulate matter, since water requires a non- gaseous surface to make the transition from a vapor to a liquid. And hence because of reflection and cloud formation less radiation reaches the earth surface so aerosols have an opposite greenhouse effect (IPCC, 2007). Aerosols play a role in visibility problems; Charlson et al. (1992) showed that the level of visibility caused by aerosols depends on chemical composition and sulfate contributes most to visibility problems. Aerosols contribute to acidification and eutrophication of land and water recourses due to wet and dry deposition (Erisman et al., 2001); this can lead to changes in plant and animal communities. Particles can origin from natural sources such as dust storms, volcanoes, sea salt, forest and grassland fires. The anthropogenic human contribution origins from sources such as power plants and burning of fuels. Both sources can emit inorganic and organic species. Primary aerosols are directly emitted in the atmosphere and their sources can be both natural and anthropogenic. Secondary aerosols are a result of a homo- or heterogenic reaction in the atmosphere and can be divided in secondary inorganic aerosols (SIA), e.g. NH 4 NO 3 (s) as a result from the homogeneous reaction between its precursor gases ammonia (NH 3 ) and nitric acid (HNO 3 ), and secondary organic aerosols (SOA) as a result from oxidation of volatile organic compounds. As with primary aerosols, the precursors responsible for the secondary aerosols can origin from both natural and anthropogenic sources. The composition of a particle depends on lots of factors and changes all the time. It consists of inorganic and organic species and depending on the hygroscopic properties of the particle (which depend on its composition) it will attract a certain amount of ambient water. A typical particle composition is shown in Figure 2. 8

9 The geographic location as well as the metrological conditions of a certain area have direct influence on the PM concentration and composition. The composition shows a diurnal (Trebs et al., 2004) and seasonal (Hu et al., 2002) variation. If a country is located near the coast a significant amount of PM will consists of sea salt (NaCl, MgCl), which is considered not harmful and with other natural compounds may be subtracted from the total PM concentration according to legislation. Precipitation and wind are meteorological parameters which have an influence on the concentration of PM. Dry deposition as well as wet deposition lower the concentration of PM in the air, while wind will transport and dilute PM. Fig. 2 Typical chemical composition of an ambient particle. Reproduced from Tennessee Valley Authority, 2001: To improve the understanding of the adverse effect of atmospheric aerosols their properties need to be studied. Beside chemical composition and size distribution, their sources and sinks need to be studied in more detail in order to successfully use models by lack of data. This includes study of the precursors in the case of SIA and SOA, their formation - and dissociation rates and conditions. The classical way to quantify PM properties is by sampling ambient air through special impregnated filters on which particles are collected fallowed by off- line chemical analysis. The composition of particulate matter requires several analytical techniques to be qualified and quantified. After filter sampling the inorganic composition can be studied by e.g. ion chromatography, the elemental and organic carbon amount and ratio by a carbon analyzer instrument and the other organic species such as acids by HPLC and MS. This filter based method is widely accepted and used all over the world. This method however (1) needs long 9

10 term sampling (and hence average results for species in aerosols are obtained), (2) can lead to underestimation of volatile compounds when temperature and/or pressure changes (e.g. NH 4 NO 3 (s) and NH 4 Cl(s) can evaporate from the filter when they react to their gas phase precursors) and (3) is labor intensive to perform. The artifacts that lead to underestimation of volatile compounds have already been documented some years ago (Appel et al., 1980). Zhang and McMurry (1987) developed a theoretical model to calculate evaporation loss from filter samplers. Currently a correction factor is applied to compensate for this loss. The above mentioned problems have led to the development of on- line and high sample frequency instruments to measure the composition of ambient particulate matter. Some of these instruments have been commercialized. The studies performed at some universities and institutes require self developed and made instruments, and in order to accurately measure the analytes of interest validation of these instruments is required. This can happen by the use of standard generated aerosols, the use of standard certified gases and comparison studies with other well known and accepted instruments. 1.2 New particle formation Secondary particles are a result of homo or heterogeneous reactions in the atmosphere, this process is called nucleation. The reactants - precursors - can origin from natural and anthropogenic sources. Once formed, the particles will grow gradually by coagulation or aggregation and surface growth processes. Coagulation and aggregation are physical processes where particles collide with each other and the mass and size of the new particles increases. Surface growth is a process where the particle grows due to the accretion of monomers or individual molecules to the already existing particle. Ammonia, NH 3 (g), is the most abundant alkaline gas in the atmosphere (Asman et al. 1997) and an increase in anthropogenic activities with emission of ammonia has been observed (Sutton et al., 2001). A substantial part of acidic trace gasses in the atmosphere is neutralized by ammonia; a homogeneous reaction with reaction product a secondary aerosol containing NH + 4 (aq). Consequently NH + 4 is a major component in atmospheric aerosols. Common atmospheric homogeneous reactions resulting in particle formation are (Reaction 1) between NH 3 and HCl (Pio and Harrison, 1987) and (Reaction 2) between NH 3 and HNO 3 (Stelson and 10

11 Seinfeld, 1982). Also heterogeneous reactions with ammonia as precursor are responsible for aerosol formation. The reaction of ammonia with liquid sulfuric acid has been widely studied and reported (e.g. Huntzicker et al., 1980; McMurry et al., 1983; Harrison and Kitto, 1992; Warneck, 2000) and are reactions 3-5. Since the products of reactions 3-5, (NH 4 ) 2 SO 4 (l) and NH 4 HSO 4 (l), have very low vapor pressure the particle, once formed, will remain a particle while the products of reactions 1 and 2, NH 4 Cl(s or l) and NH 4 NO 3 (s or l), do have a vapor pressure and hence reactions 1 and 2 are reversible reactions. The formation, and in case of volatile species evaporation, of secondary aerosols from its precursors depends on meteorological conditions (i.e. relative humidity and temperature) and the gas phase tive humidity; a solid particle will become a saline droplet when the humidity increases to a certain point and conversely when the humidity decreases to a certain point the saline droplet will evaporate ere these processes take place depend on the chemical composition of the particle (Tang and Munkelwitz, 1994) and are called the points of deliquescence. The reaction between NH 3 (g) and H 2 SO 4 (l) is preferred over the + reactions between NH 3 (g) and HCl(g) and NH 3 (g) and HNO 3 (g) (Warneck, 2000). The NH 4 containing aerosols affects the atmospheric transport distance of regional pollutants because of the lower deposition rate of particulate matter compared to its gaseous precursors (Sutton et al., 2001). Deposition of NH 3 and NH + 4 leads to acidification and eutrophication (Erisman et al., 2001). Nitrogen deposition provides important soil nutrients but excess deposition can lead to changes in plant and animal communities. Beside NH 3 and NH + 4, NO x and its reaction products (gas phase HNO 3 and particulate NO - 3 ) are responsible for nitrogen deposition (Asman et al., 1997) Reactions of secondary inorganic aerosol formation with ammonia as precursor gas: NH 3 (g) + HCl(g) NH 4 Cl(s or l) (R.1) NH 3 (g) + HNO 3 (g) NH 4 NO 3 (s or l) (R.2) 2 NH 3 (g) + H 2 SO 4 (l) (NH 4 ) 2 SO 4 (l) (R.3) NH 3 (g) + H 2 SO 4 (l) NH 4 HSO 4 (l) (R.4) NH 3 (g) + NH 4 HSO 4 (l) (NH 4 ) 2 SO 4 (l) (R.5) 11

12 The simultaneous measurement of inorganic PM composition and its precursors is of high interest to clearly monitor their relation. Furthermore the same method for the detection of ions in the gas sample and the aerosol sample is desired so no possible offset between two different detectors can occur. Liquid coated denuders coupled to impregnated filter- packs are widely being use to study and report the concentrations of water soluble trace gasses as well as to determine the inorganic composition of aerosols as off- line method. As mentioned before this set- up gives average concentrations, is sensitive to artifacts and is labor intensive. Hence the development of on- line instruments with high sample frequency, such as the Steam Jet Aerosol Collector and the Gas Aerosol Collector. The next chapter describes in detail the hardware to simultaneous sample, collect and analyze the inorganic composition of aerosols and their related water soluble gases. 12

13 2. Sampling, collecting and analyzing inorganics in ambient aerosols and related water soluble gasses: Detailed description of the hardware used. With the instruments used in this study, gases and aerosols are being separated due to their differences in diffusion velocity. Water soluble trace gases are being collected in a wet annular rotating denuder based on the original design of Ferm (1979) and Winiwarter (1989), while particles pass through this denuder and are being collected in an environment of supersaturated steam (Krystov 1995; Slanina et al., 2001) where condensation of ultrapure water on the particles results in droplets. Both sample streams are continuously collected by use of peristaltic pumps, collected in glass vessels and chemical analysis follows by ion chromatography (ic) for the anions (Cl -, NO - 2, NO - 3, SO 2-3, SO 2-4 ) and flow injection analysis (fia) for ammonium (NH + 4 ). The sample collection time is 15 minutes with an air flow of 1 m 3 per hour. Detection takes 15 minutes per sample and hence a result for concentrations of the soluble gases and inorganic species in aerosols is obtained every half an hour which represents a 15 minutes average result. This means that 50% of the time the instrument is not collecting sample. The whole process is controlled by a central PC. Figure 3 shows are schematic sketch. A detailed description of the wet annular denuder and the aerosol collector used in this study are described below. Fig. 3 Schematic sketch of the SJAC and GAC instrument for sampling, collecting and analyzing aerosols and gasses in ambient air samples. 13

14 2.1. Design of the inlet system Sampling aerosols and gasses for simultaneous measurement requires the design of the inlet system to (1) minimize gas phase losses to the inlet material and (2) minimize aerosols losses due to non- isokinetic sampling (Trebs et al., 2004). Nitrogen containing gasses are well known to be difficult to sample. HNO 3 (g) has a high affinity to absorb on a wide range of inlet materials (e.g. Clemitshaw, 2004) often referred to as being sticky. As a result HNO 3 (g) can accumulate in the inlet and react with NH 3 (g) to form particulate NH 4 NO 3 (s), or accumulate in the inlet material and when saturated suddenly large amounts of HNO 3 can be measured. Different materials have been tested for sampling HNO 3 (g) by Neuman et al (1994). They while a loss of ~ 85% was observed when they used aluminum, steel or nylon. As can be seen in Table 1 particulate matter is classified according to its size. The size of interest can be selected by the use of a size selective device placed at the very beginning of the inlet of the instrument. Such a device uses either an impactor or a cyclone to separate particle sizes by the use of gravity. Both techniques have proven their functionality and are being used in this field. 2.2 Sampling and collecting of ambient aerosols Several commercial instruments are available for collecting particles such as the Particle Into Liquid Sampler (PILS) and Aerodyne Mass Spectrometer (AMS). The latter has the capability to measure very low particle sizes and can be used to study new particle formation. The Particle Into Liquid Sampler (Weber et al., 2001) has been designed to measure inorganics in particles with a very high sample resolution and can be used in aircrafts. The Steam Jet Aerosol Collector has been developed at the Energy Research Centre of the Netherlands in the early 90's for aerosol studies (Krystov, 1995; Slanina et al., 2001). Before entering the aerosol collector the air sample is being stripped from water soluble related trace gasses which would otherwise overestimate the concentration of inorganics in aerosols (i.e. HCl, HNO 2 HNO 3, SO 2, NH 3 ). The air sample free of water soluble gases enters an environment where supersaturated steam is being created by heating ultrapure water. The steam condenses on the particles resulting in liquid droplets. The formed droplets containing 14

15 the dissolved aerosol species are then collected by a cyclone. From the cyclone the solution is continuously pumped out by the use of a peristaltic pump. This process is similar to ambient cloud formation by cloud condensation nuclei; ambient particles that cause ambient water to make the transition from vapor to liquid. Figure 4 shows the schematic principle of the aerosol collector in the SJAC, figure 5 shows the schematic principle of the aerosol collector in the GAC. As can be seen these are slight different, where the main difference is the use of coolers and an impactor particles stripped from related soluble gasses condensation chamber to air pump di water addition cyclone to sample collection heater sensor Fig. 4 SJAC Aerosol collector based on condensation of ultrapure water steam on particles. Particles stripped from related soluble gasses condensation chamber to air pump di water addition heater impactor cyclone cooler water in cooler water out to sample collection Fig. 5 GAC Aerosol collector based on condensation of ultrapure water steam on particles. 15

16 2.3. Sampling and collecting of ambient acidic trace gasses and ammonia Several instruments have been developed for measuring concentrations of ambient ammonia and acidic trace gases based on different techniques such as coated denuders in combination with filter pack method, spectroscopic methods and diffusion based methods. Comparison studies have been done for ambient ammonia measurements and are reported by e.g. Appel et al., (1988), Williams et al., (1992) and Norman et al., (2008). Coated denuders are widely being used for ambient trace gas sampling. Denuders are round devices and are typically made of glass which surface is coated with an absorption solution. The air sample stream is led through the denuder where water soluble gases due to their high diffusion velocity hit the wall and absorb in the solution. See figure 6. Ferm (1979) was the first who reported the application of measuring atmospheric ammonia gas with a diffusion denuder. In his studies a 0.5 meter glass tube coated with 1.5% oxalic acid in methanol was used for the offline measurement of gas phase ammonia. Gas absorption reactions for ammonia and water soluble acidic trace gases in a coated denuder (as in SJAC and GAC) are Reactions Air sample Fig. 6. Schematic principle of operation of annular denuders. G = gas, P = particles. 16

17 Gas absorption reactions for ammonia and water soluble acidic trace gases: NH 3 (g) + H 2 O(l) NH + 4 (aq) + OH - (aq) (R.6) HNO 2 (g) + H 2 O(l) NO - 2 (aq) + H 3 O + (aq) (R.7) HNO 3 (g) + H 2 O(l) NO - 3 (aq) + H 3 O + (aq) (R.8) SO 2 (g) + 2H 2 O(l) HSO - 3 (aq) + H 3 O + (aq) (R.9) 2SO 2 (g) + O 2 (g) 2SO 3 (l) (R.10a) 2SO 3 (l) + 6H 2 O(l) 2SO 2-4 (aq) + 4H 3 O + (aq) (R. 10b) The SO 2 (g) concentration in ambient air is calculated by summing up the observed signals for HSO - 3 (aq) and SO 2-4 (aq). The wet coated denuder is gaining popularity for on- line sampling with high sample flow (Trebs, et al., 2004). Many trace gasses show a diurnal variation so in order to understand atmospheric reactions a high sample frequency instrument is desirable. If one wants to study seasonal variations longer term average can be sufficient. The classical dry coated denuders for measuring ammonia require long term sampling and hence give a long term average (24 hours to one week depending on gas concentrations and sample flow) against the diffusion denuders in combination with flow injection analysis coupled to conductometric detection can result in a continuous signal. Ferm showed that the collection efficiency of his denuder can be calculated according to equation 1: out a in a = (Eq. 1) Where, a in = the concentration of ammonia in air entering the denuder, a out = the concentration of ammonia in air leaving the denuder and (Eq. 2) Where, D = the diffusion coefficient of NH 3, L = the length of the tube (m), 17

18 F = the flow rate (m 3 s - 1 ) through the tube. The equation is only valid if the flow is laminar. The residence time (i.e. the airflow of the sample, F) and the length of the tube, L, determine the actual absorption rate. The formula shows that the tube diameter is not important. This can be explained since the residence time rate though a wider denuder at a given flow is proportionately slower. Sutton et al. (2001) used Ferms design for a new diffusion denuder and their instrument has been implemented at over 50 sites across the U.K. as part of a new national monitoring network. They designed a 6 mm internal diameter, 80 mm long borosilicate glass tubes with a sample airflow of 0.35 L min - 1. From equation 1 it can be calculated the 97% of the gas phase ammonia should be captured. 18

19 2.4. Detection Depending on the analyte or analytes of interest one has to choose the appropriate way of chemical analysis. Since the wet denuder and the aerosol collector both convert the sample into solution a range of wet chemistry based instruments can be used such as HPLC, LC- MS, TOC, FIA, UV- VIS and Ion selective electrodes. The sample amount in time can be altered by changing the sample flow according to the present conditions and to the detection limit of the instrument which performs the analysis. So when high ambient concentrations are observed more sample can be obtained resulting in a higher sample frequency. Both samples in the SJAC and the GAC are being analyzed by ion chromatography for the anions and flow injection analysis for ammonia. Ion chromatography is a form of high pressure liquid chromatography (HPLC) where ions (the analytes) are being separated on an analytical column based on their difference in affinity with the stationary phase. The detection happens by measuring the resistance of the eluate expressed in micro Siemens per specific conductivity resulting in different peak areas for different ions when they have the same concentrations in the solution. Usual 3 or 5 point calibrations need to be performed for each analyte. An anion needs to be used as eluent (usually NaOH or a mixture of Na 2 CO3 and NaHCO 3 ) for the competition between the eluent and analytes with the stationary phase in order to achieve separation. As a result this eluent would have a conductivity which causes a high background signal and increases the detection limit of the analytes. Before detection takes place however the eluent is being suppressed by replacing the sodium ions by protons so water and CO 2 (l) are being formed and together are responsible for the background signal and theoretically have no conductivity. Since the difference in specific conductivity among anions is relatively small compared to the difference between water and anions, the detection limit of the analytes decreases significantly. Ammonia is being measured with flow injection analysis. After the air sample is converted in a solution it is being mixed with a solution of sodium hydroxide (NaOH), Reaction 11. This reaction is 100% (Carlson et al., 1990). The sample then passes a semi- permeable Teflon membrane where the gas phase ammonia will pass through with an efficiency of about 30% (Slanina et al. 2001). Then the ammonia will dissolve in a stream of ultrapure water (Reaction 12) which has passed through a resin in order to remove any present ions which would 19

20 otherwise increase the blank or baseline signal. The signal responsible for the baseline is caused by dissociation of water in protons and hydroxyl ions (Reaction 13). Now when ammonium is present in the sample a signal will be observed due to change in conductivity. NH 4 + (aq) + OH - (aq) NH 3 (g) + H 2 O(l) (R. 11) NH 3 (g) + H 2 O(l) NH 4 + (aq) + OH - (aq) (R. 12) H 2 O(l) H + (aq) + OH - (aq) (R. 13) 2.5. Control of the instrument. The instrument is performing the same steps every half an hour. This is sampling, selecting the sample by use of valves and analyze it, convert the data to ambient concentrations and store the data. The process is controlled by a central PC which controls the valves, the liquid sensors, the liquid pumps, the conditions of the chromatography system and the flow injection analysis. Since temperature is important in relation to conductivity measurements the software corrects for the temperature by continuous measurement of the conductivity 20

21 3. Experimental 3.1. The Steam Jet Aerosol Collector The control of the SJAC instrument is performed by an intern computer which is strongly aged. The hardware in the instrument: the wet rotating denuder, the aerosol collector, the liquid sensors, the valves, the air pump, the ionchromatograph, the flow injection cell are all being controlled by the software as well as the calculations from liquid concentrations to ambient concentrations. The control software was written in MS- DOS based Quick Basic (QB) version 4.5. The present computer is not reliable enough since it crashes all the time and hence it is no longer useful. Consequently no user interface (UI) is available to control the instrument. In order to recover the instrument a new Windows based user interface written with Microsoft Visual Basic (VB) V 6.0 language has been designed for future use. VB is the successor of QB, the main difference is that it is designed for Windows instead of MS- DOS so current computers can recognize and run the language. Since Quick Basic and Visual Basic are strongly related, the software files behind the user interface of the Quick Basic version can be used and partly copied to the new Visual Basic based user interface. The main reasons to recover this instrument are to be able to (1) measure online gas phase ammonia and particulate ammonium to study their relation and (2) compare the performance of the new designed GAC instrument with this by ECN developed well accepted, frequently used SJAC instrument Two ways to detect ammonium The SJAC can measure online gas phase ammonia and particulate ammonium with flow injection analysis coupled to a conductometric cell. Once gas phase ammonia (NH 3 ) absorbs in the coating solution of the wet denuder it forms ammonium (NH + 4 ) and hydroxyl ions (OH - ). Particulate ammonium already is in the form of ionic NH + 4 in the sample obtained from the aerosol collector. As described in chapter 2.4 the sample is being mixed with a solution of sodium hydroxide (NaOH) and any present ammonium will form gas phase ammonia (NH 3 ). In the flow injection cell the sample passes a semi permeable membrane and on the other side of the membrane a continuous stream of ultrapure water is pumped through. Now any 21

22 present gas phase ammonia will pass this membrane by diffusion. On the other side of the membrane the ammonia dissolves again in the continuous stream of ultrapure water. Detection happens by measuring the conductivity (reciprocal value of the resistance) of this ultrapure water stream. The baseline or blank of the signal is a result of the dissociation of the water molecules in protons and hydroxyl ions. When ammonia is present in the sample a change in signal will be observed due to change in conductivity. Detection of ammonia in the sample can happen in 2 ways: (1) continuously by connecting the sample collection device to the flow injection cell and measure the conductivity as is shown in figure 7 and (2) with the use of a loop and inject a known amount of sample after the flow injection cell into the conductivity detector as is shown in figure 8. In the later set- up a high pressure pump was used for the ultrapure water stream (the baseline) in order to observe any difference in noise between the peristaltic pump and the high pressure pump. Originally the SJAC is designed to measure in the continuous way but since the control is being changed now anyway, the injection- wise set- up was studied also as an alternative way of detection. The difference in detection is that for the continuous set- up any present ammonium results in a continuous change of signal and the injection wise way results in a peak shaped signal as is shown and discussed in the results and discussion chapter. Both ways of detection were tested by using ammonium standards. Fig. 7 Experimental set up for measuring continuously ammonium standards. 22

23 Fig. 8 Experimental set up for measuring injection wise ammonium standards Recovery of the instrument The PC that controls the SJAC instrument is no longer useful and has to be replaced. A new Windows based user interface written with Visual Basic (VB) language has been designed for future use. Visual Basic version 6.0 was used to design the user interface and to write the language. The signal obtained is displayed in millivolts (mv). The software will calculate to the 3 or ppb based on the calibration and displays and stores that concentration. Parameters necessary for the software to make the calculation to the actual concentration have to be measured manually or by automatic instruments and are: the airflow, the liquid flow of the NaOH solution, the liquid flow of the sample, the liquid flow of the ultrapure water stream and the temperature of the conductivity cell. 23

24 3.2. The Gas Aerosol Collector Introduction to validation of the methodology applied in the GAC instrument The GAC is a new instrument for ambient air monitoring and a new instrument requires detailed research in it properties in order to be able to measure with the required accuracy. Two separate signals summed up will be the signal observed for the analyte or analytes of interest. The first is the efficiency of the device itself, e.g. the collection efficiency of the wet rotating denuder to collect the acidic trace gases and ammonia. The second contributor to the signal is an artifact. An artifact is a non desired contributor to the signal and is caused by not proper working of the instrument. Both negative and positive artifacts can occur and result in underestimation and overestimation of the signal, respectively. To quantify the performance of the GAC instrument experiments have been done and recommendations to do more are given. Although the gas- aerosol- collector concerns one instrument, it clearly consists of several parts which all require specific attention in order to quantify their efficiencies and the relation between them. For instance less efficiency in the denuder part means underestimation in gas results and overestimation in the results obtained for the related aerosol species. Also the other way around can happen, the denuder can collect aerosols which it should not collect. More artifacts that lead to over- or underestimation of the signal are described below. The efficiency of the gas collection device, the wet rotating denuder, can be quantified by (1) Sample known concentration of standard certified gases, (2) Monitor for particulate matter loss in the denuder which should penetrate, (3) Compare results with well known and accepted instruments, Earlier testing has been done at the Peking University to quantify the efficiency of the wet denuder by sampling standards of pure certificated SO 2 gas. A collection efficiency of >98% 2-3 and this efficiency is stable from 20 to 180 ppb SO 2. When water was used as coating solution the efficiency decreased slightly from 24

25 98% for 20 ppb of SO 2 to 94% when 120 ppb of SO 2 was being sampled. Ambient SO 2 concentrations in China are typically ppb. Both negative and positive artifacts can occur and they result in underestimation and overestimation of the signal, respectively. Particulate matter loss due to non- isokinetic sampling results in a positive artifact for e.g. NH 3 results. Detection of ions that should not be present in the denuder sample because they do not have a vapor pressure such as sodium, potassium, magnesium and calcium, but are observed prove that aerosol loss occurs in the denuder. An example of a negative artifact is described by Krystov et al. (2009) and states that volatile species evaporate from the denuder coating after absorption because of the hygroscopic properties of the particles that pass the denuder. In order to efficiently collect all the water soluble gasses in the denuder, the humidity is close to 100%. As mentioned before the hygroscopic properties of aerosols depend on their composition and hence the effect observed will be different depending on the composition of the particles. To model and compensate for this artifact seems to be difficult because the composition of the aerosol is alternating and not predictable. Furthermore results can be compared with other instruments such as a Thermo SO 2 monitor in case of SO 2. The efficiency of the particle collector device can be quantified in several ways: (1) By generating and sampling a known composition and concentration of aerosols, (2) Measuring the breakthrough by use of a filter at the outlet of the aerosol collector, (3) Comparing results of the aerosols collector with other well known and accepted instruments, As with the denuder, both negative and positive artifacts can occur resulting in underestimation and overestimation of the signal respectively. Physical processes in the aerosol collector such as partitioning of volatile species might result in underestimation of the signal. An insufficient efficiency of the denuder can result in overestimation of the signal, since soluble gasses will dissolve in the saturated environment of the aerosol collector and contribute to the signal observed from the aerosol sample. 25

26 Aerosol loss in the wet rotating denuder The TSI Atomizer, Model 9302, was used to generate artificial aerosols to observe (1) any loss of aerosols in the wet denuder and (2) to test the collection efficiency of the aerosol collector. Ammonium sulfate, (NH 4 ) 2 SO 4 (s), particles were generated for these experiments because it is the main inorganic component in aerosols and it has no vapor pressure (and hence will not evaporate to form gas phase species). The atomizer instrument generates polydisperse aerosols of an unknown size range by spraying a solution containing the salt through a nozzle under a manually selected pressure (figure 9). According to the TSI manual the physical properties of the artificial particles, the particle size and particle number, can be changed by alternating: (1) the concentration of the solution and (2) the sample flow through the nozzle (pressure). Different concentrations of (NH 4 ) 2 SO 4 (l) were used to generate artificial aerosols. A constant low aerosol flow was mixed with a flow of zero air generated by a Sabio zero air source model The zero air flow was controlled by a Thermo mass flow controller from the Dynamic gas calibrator model 146i. The zero air flow and the generated aerosol flow were mixed in a 4.5 liter polypropylene mixing tank and samples were taken continuously from there by the GAC. The total flow in which contains the artificial aerosols exceeds the sample flow of the GAC which is 16.7 L/min (under standard conditions); a vent in the tank releases the excess of sample. The GAC air sample flow was measured during these experiments with a Sensidyne Gilian Gilibrator 2. After 15 min sampling and collecting artificial aerosols, on- line chemical analysis of SO 2-4 (aq) followed for both samples separately by a Dionex ion chromatography system model ICS- 90. To determine the aerosol loss in the denuder the signal obtained from the denuder was compared with the signal from the aerosol collector. Peak areas were used for calculations. The set up for these tests is shown in figure 10. Fig. 9 TSI Atomizer Model 9302 to generate polydisperse aerosols by the use of a saline solution. 26

27 Fig. 10 Experimental set- up for particle penetration test and collection efficiency of the aerosol collector Collection efficiency of the aerosol collector To measure the efficiency of the aerosol collector Whatman Schleicher & Schuell Filters of PTFE with a 46.2 mm diameter and a pore size of 2 m were placed after the aerosol collector and before the pump (figure 10). Any not collected aerosols by the aerosol collector will be collected on this filter. After sampling, the filters were substracted in ultrapure water by use of an ultrasonic bath and off- line analysis for SO 2-4 was performed within the same chromatography instrument as mentioned above. To determine the aerosol collection efficiency of the aerosol collector the signal obtained from the aerosol collector was compared with the signal from the filters. Peak areas were used for calculations. 27

28 To get more information about the relation between any particle loss in the denuder and the physical properties of the particles generated a TSI Model 3080 Scanning mobility particle sizer (SMPS) and a TSI Model 3775 Condensation particle counter (CPC) were installed and connected to the mixing tank. The set up for these tests is shown in figure 11. The particle number size distribution is measured between 20 and 700 nanometer with 5 minutes time resolution. Again different concentrations of (NH 4 ) 2 SO 4 (l) were used to generate particles with different physical properties. Any particle loss observed (i.e. signal from the gas samples) can now be compared with particle number size distributions; in this way any loss can be subscribed to particle number and/or particle size. Fig. 11. Experimental set- up for detailed testing of particle size dependant loss in the denuder 28

29 3.3. TSI Atomizer Model Physical properties of artificial (NH 4 ) 2 SO 4 (s) aerosols measured with SMPS- CPC Artificial aerosols were generated to observe the particle number size distributions under different settings of the aerosol generator. Ammonium sulfate, (NH 4 ) 2 SO 4 (s), particles were generated for these experiments because it is the main inorganic component in aerosols and it has no vapor pressure (and hence will not evaporate to form gas phase species). The TSI Atomizer, Model 9302 was used to generate aerosols. This instrument generates polydisperse aerosols by spraying a solution containing the salt through a nozzle under a manually selected pressure (see figure 9). Two settings can be changed in this instrument which are responsible for the physical properties (size, number) of the aerosols generated: (1) the concentration of the solution and (2) the sample flow through the nozzle (by a needle valve to set the pressure). The TSI Model 3080 scanning mobility particle sizer (SMPS) and the TSI Model 3775 condensation particle counter (CPC) were used to measure the aerosol number size distributions. The set up for these experiments is shown in figure 12. The aerosol flow was mixed with a flow of zero air generated by a Sabio zero air source model The zero air flow was controlled by a Thermo mass flow controller from the Dynamic gas calibrator model 146i. The zero air flow and the generated aerosol flow were mixed in a 4.5 liter polypropylene mixing tank and samples were taken continuously from there by the SMPS- CPC. The total flow in which contains the artificial aerosols exceeds the sample flow of the SMPS- CPC, a vent in the tank releases the excess of sample. 29

30 Fig. 12 Experimental set- up for measuring physical particle properties generated by the atomizer 30

31 4. Results and discussion 4.1. The Steam Jet Aerosol Collector Ammonium standards were measured both continuous and loop wise (Fig. 7 and 8 respectivey). A 6- way peristaltic pump was used to mix a 250 ppb ammonium standard with a solution of 0.5 M NaOH into the flow injection cell. Both flow rates were about 0.3 ml/min. The ultrapure water sample stream on the other side of the flow injection cell was about 1 ml/min. The ultrapure water outlet of the flow injection cell was connected to a conductivity cell of a Dionex ionchromatograph model ICS- 90. The Dionex software, Chromeleon, was used to monitor the signal, store the data and to control the conditions of the conductivity detector as well as to control the high pressure pump and the injection valve used later. The temperature of the conductivity cell was kept constant at 30 C. Two ways of detection of ammonium were tested: (1) continuously measurement by connecting the outlet of the flow injection cell to the conductivity detector and monitor the signal with and without ammonium standard (Figure 7) and (2) injection wise with the use of a connected after the flow injection cell and could be injected manually by the software (Figure 8). In set up (1) the peristaltic pump was used for the ultrapure water stream and in set up (2) the ic high pressure pump was used so any difference in noise between the peristaltic pump and the high pressure pump could be observed. Figures 13 and 14 show the signals observed for the 250 ppb ammonium standard for the continuous and loop wise set- up respectively. 31

32 Fig. 13 Baseline and signal observed in the continuously set up for 250 ppb ammonium standard. Fig. 14 As can be seen in figures 13 and 14 both ways give a clear signal different from the baseline for an ammonium standard of 250 ppb. Since these signals were obtained by just one standard concentration the signals were stable. However since the ammonia and ammonium concentration in ambient air sample will not be stable the signals in reality will be alternating. For the continuous set up this will result in a continuously alternating signal different from the baseline, where for the loop wise set up this will result in peaks with different concentrations calibration need to happen. The 32

33 continuous set up is more representative for the ambient concentrations since the signal is being measured on a continuous base while the loop wise set up will result in average concentrations, although the sample injection frequency can be high since the sample volume can be little and the peaks are not broad. A higher noise was observed when the peristaltic pump was being used for the baseline signal compared to the use of a high pressure pump. Figures 15 and 16 show the noise observed when using the peristaltic pump for the baseline with and without sampling standard respectively. Figure 17 shows the noise observed for the baseline when using the high pressure pump of the Dionex ionchromatograph model ICS- 90. Fig. 15 Baseline noise observed with the use of the 6- way peristaltic pump for the detection ultrapure water stream. Fig. 16 Baseline noise observed with the use of the 6- way peristaltic pump for the detection of 250 ppb ammonium standard. 33

34 Fig. 17 Baseline noise observed with the use of the high pressure pump for the detection of 250 ppb ammonium standard. Although the noise of the baseline with the use of the high pressure pump is significant lower, which will theoretically result in a lower detection limit, the integration of the peaks is difficult for the software due to another observed noise. Before an injection the baseline goes down a little and after elution it takes some time before the signal is stable again. This is the result of the equilibrium between the ultrapure water and protons and hydroxyl ions which is disturbed by the ammonia. Any present ammonia will first react with the protons from the water equilibrium and since ammonium ions have a lower specific conductivity than protons the conductivity goes down. The water equilibrium is trying to recover the equilibrium but the excess of ammonia reacts with all the present protons first and after all the protons are gone water is next. After all the ammonium eluted the water equilibrium needs time to recover and to form protons again which results in the increase in the baseline observed. 34

35 The new designed program to control the SJAC instrument for on- line measurement of gas phase ammonia and particulate ammonium is not ready to be used yet. A user interface has been designed in Microsoft Visual Basic version 6.0 as is shown in figure 18. The files behind the user interface however, the actual language files, still have to be completed and adjusted to the controls of the user interface. This work need to be done by anyone with more knowledge about the software language than me. Fig. 18 The control of the SJAC for gas phase ammonia and particulate ammonium measurements by a new designed user interface written in Windows Visual Basic V