Measurement of Non-Volatile Particle Number Size Distribution

Transcription

1 University of patras Master Thesis in Chemical Engineering Energy & Enviroment Measurement of Non-Volatile Particle Number Size Distribution Author: Georgios Gkatzelis Supervisor: Prof. Spyros Pandis July 1, 2014

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3 To my parents, my brother & in memory of my sister

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5 Acknowledgments This study took place in the laboratory of air quality studies of ICE-HT in Patras. First, I would like to thank my supervisor, Prof. Spyros Pandis, for his expert guidance and most valuable support. I was really lucky to have the opportunity to learn from him and work for the LAQS team, one of the strongest aerosol groups in Europe. Also I would like to thank Dimitris Papanastasiou, without whom this work wouldn t have been possible. His laboratory experience, passion for science and endless guidance and support motivated me to work harder and more efficiently. I would like to thank all the members of our group, especially Christos Kaltsonoudis, Kalli Florou, Vangelis Louvaris and Maria Tsiflikiotou, with whom it was a pleasure working with. I would like to thank Prof. Biskos for his supervision, support and guidance during my Erasmus placement in Delft. There I was able to work on more instrumentation oriented projects with valuable help from very good scientists like Anne Maisser and Kostis Barbounis. I really appreciate their help, endless assistance, always being cheerful and eager to work. I would also like to thank Prof. Koutsoukos for his valuable help and guidance on the Erasmus placement choices, opening new roads for my scientific life. Finally, a special thanks to my parents, my brother and my friends for always being there for me. iii

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7 Abstract A new experimental methodology was developed to measure the non-volatile particle number concentration, using a thermodenuder (TD). The TD was coupled with a highresolution time-of-flight aerosol mass spectrometer, measuring the chemical composition and mass size distribution of the submicrometer aerosol and a scanning mobility particle sizer (SMPS) that provided the number size distribution of the aerosol in the range from 10 to 500 nm. The method was evaluated with a set of smog chamber experiments and achieved almost complete evaporation (98 %) of secondary organic aerosol (SOA) as well as freshly nucleated particles, using the TD temperature of C. This experimental approach was applied in a winter field campaign in Athens and provided a direct measurement of non-volatile particles from major pollution sources. During periods in which the contribution of biomass burning sources was dominant, more than 80 % of the particles survived the intense heating, suggesting that nearly all biomass burning particles had a non-volatile core. These particles consisted mostly from black carbon (BC), while organic aerosol (OA) were responsible for another 40 %. Organics that survived through the TD were mostly biomass burning OA (BBOA) and oxygenated OA (OOA) that had not evaporated, contributing 90 % of the organic mass concentration, while the other 10 % was hydrocarbon-like OA (HOA) and cooking OA (COA). For periods that traffic contribution was dominant, mostly during the rush hour, % of the particles had a non-volatile core, while the rest evaporated at C. The remaining particles consisted mostly from BC, with an 80 % contribution, while organics were responsible for another %. Organics were mostly HOA and OOA, with a contribution of > 95 % to the organic mass concentration, while < 5 % was from BBOA and COA. v

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9 Contents Acknowledgments Abstract iii v 1 Introduction 1 2 Thermodenuder description Particle wall losses Thermodenuder characterization Evaporation of model aerosol Evaporation of SOA Biogenic SOA Anthropogenic SOA Evaporation of freshly nucleated particles Ambient measurements Instrumentation Chemical characterization of ambient and thermodenuded aerosol Biomass burning periods Traffic periods Conclusions 34

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11 List of Figures 2.1 Detailed schematic of the thermodenuder used in this study Temperature profile inside the heating section of the TD at the set temperature of 400 o C and a flow rate of 1 L min 1 as a function of position (distance from the entrance of the heating section, bottom axis) and average travel time (top axis) Average percentage of losses at 400 o C based on 9 different experiments of NaCl aerosol generated with an atomizer, using a flowrate of 1 L min 1. The uncertainty (± 2 σ) of the average loss correction is given by the vertical lines Number size distribution of (a) ammonium sulfate (b) adipic acid and (c) purified water after the bypass line (25 o C, blue) and after the TD (400 o C, red) as a function of diameter, D p. (d) Number fraction remaining (NFR) as a function of time for the three different experiments Time evolution of particle number size distribution during the dark ozonolysis of α pinene, measured after (a) the bypass line at 25 o C and (b) the TD set at 400 o C. The color scale represents particle number concentration Particle number size distribution of α-pinene SOA (at 22:00), for chamber measurements, at 25 o C (blue) and TD measurements, at 400 o C (red), for Exp. #1, from Table Time evolution of particle number size distribution during the photooxidation of toluene, measured after (a) the bypass line set at 25 o C and (b) the TD set at 400 o C. The color scale represents particle number concentration 14 ix

12 3.5 Average particle number size distribution (from 01:00 to 03:00) of toluene SOA for chamber measurements, at 25 o C (blue) and TD measurements, at 400 o C (red) (Exp. 1, Table 3.2) Time evolution of particle number size distribution during the induced ambient nucleation experiment, measured after the BP line at 25 o C: (A) the chamber background (13:30 to 15:50), (B) the ambient VOCs and particles introduced in the chamber (16:00 to 19:00), (C) the nucleation event (19:40 to 22:00) and (D) fresh SOA formation (22:00 to 12:00) Total number concentration of (B) ambient particles introduced in the chamber (18:00 to 19:35) and (C) ambient and nucleated particles (19:40 to 21:00) as a function of time The number size distribution during the nucleation event (phase C) as a function of the diameter at 25 o C (blue) and 400 o C (red). (1) The representative region of nucleation particles and (2) Ambient particles region after introduction in the chamber Temperature profile of a TD cycle during the campaign Timeseries of Athens-2013 campaign as a function of particle NFR when the TD was set at 400 o C The fractional contribution of biomass burning to the organic aerosol (PMF analysis) as a function of the number fraction remaining (given by the SMPS). Each dot is a representative of each measurement period SMPS particle number size distribution for the most dominant biomass burning period (NFR > 90 %) at ambient conditions (black) and 400 o C (blue) The fractional contribution of traffic to the organic aerosol (PMF analysis) as a function of the number fraction remaining (given by the SMPS). Each dot is a representative of each measurement period SMPS particle number size distribution for the most dominant traffic period (NFR 60 %), at ambient conditions (black) and 400 o C (blue) x

13 List of Tables 3.1 Experimental conditions used in α-pinene dark ozonolysis experiments Summary of experimental conditions for the toluene photooxidation experiments Initial species concentration for the induced ambient nucleation experiment Instrumentation deployed at the sampling site during the Athens-2013 winter campaign Average mass concentration of the major species (in µg m 3 ), when the TD was set at 400 o C, given for both modes (BP for ambient conditions and TD after 400 o C), during the Athens 2013 campaign. The different source contribution in (%) for the organic aerosol fraction estimated by PMF analysis. The quoted error provides the precision (± 1 σ) for AMS (as given by HEPA filter blank measurements) and MAAP measurements Average mass concentration of the major species (in µg m 3 ) for the major biomass burning periods, given for both modes (BP for ambient conditions and TD after 400 o C), during the Athens 2013 campaign. Number fraction remaining (NFR) for each event and the average mass fraction remaining of each species are also provided Average contribution of the different pollution sources for the four representative biomass burning periods, for ambient measurements (BP) and after 400 o C (TD), based on the PMF analysis, in µg m 3. The % mass fraction remaining after 400 o C is also given xi

14 4.5 Average mass concentration of the major species (in µg m 3 ) for the representative traffic periods, given for both modes (BP for ambient conditions and TD after 400 o C), during the Athens 2013 campaign. The number fraction remaining (NFR) for each event and the average mass fraction remaining of each species and NFR are also provided Average contribution of the different pollution sources for the three representative traffic periods, for ambient measurements (BP) and after 400 o C (TD), estimated by PMF analysis, in µg m 3. The % mass fraction remaining after 400 o C is also given

15 Chapter 1 Introduction Atmospheric aerosols, also known as particulate matter (PM), are defined as a suspension of fine solid or liquid particles suspended in a gaseous medium. These particles range in size, from a few nanometers in diameter to as much as 100 µm. Aerosols consist of inorganic ions, organic compounds, oxides of most metals, elemental carbon and water with organic compounds often being the dominant fraction of atmospheric PM, with a contribution of 20 to 90 %, depending on the location site (Kanakidou et al., 2005). Atmospheric particles can be directly emitted into the atmosphere, called primary aerosol, from anthropogenic sources, through combustion processes but also from biogenic sources, to produce sea-salt and dust particles. Secondary particle formation can also occur, due to nucleation of supersaturated vapors, to generate high number particle concentrations, with particle sizes < 10 nm that partly grow to larger particles, of about 100 nm, through condensation mechanisms. Fine particles (PM 2.5 ) can affect human health by penetrating into the respiratory tract, and reaching deep into the lungs (Cohen et al., 2005; Dockery et al., 1993; Künzli et al., 2005; Miller et al., 1979; Wyler et al., 2000). Toxicological data suggests that ultrafine particles whose diameter is < 100 nm, can have adverse pulmonary and cardiovascular effects (Donaldson and MacNee, 2001). Furthermore, particles soluble in water and lipids, smaller than 100 nm, have a minor contribution to the total aerosol mass and result to minor effects on human metabolism (Kreyling and Scheuch, 2000) but insoluble particles, like black carbon (BC), could have more significant health impacts. The impacts of these insoluble particles cannot be easily determined focusing on mass but require studies of the number concentration, on where these particles dominate. This 1

16 potential size-dependent toxicity of particles due to their ability to penetrate into the human body, has shifted the interest to metrics other than mass and mainly on the number size distribution of ultrafine particles. Atmospheric particulate matter can also effect the Earth s radiative budget and global climate. Particles act as cloud condensation nuclei and form cloud droplets. Thus, also scatter light, leading to the cooling of the planet. Absorbing aerosol particles, such as black carbon (BC), contribute to warming by strongly absorbing light. BC particles, mostly emitted by anthropogenic sources through combustion, evolve to internally mixed particles, due to coagulation or condensation of vapors to their surface, leading to the formation of a coating that consists of organics, sulfates and nitrates (Scheer et al., 2005). This coating of BC particles can increase their absorption up to a factor of three (Andreae and Gelencsér, 2006) but also their hygroscopicity (Kuwata et al., 2008). The number concentration, size distribution and mixing state of BC particles are therefore essential in understanding their effect on climate. Since the number concentration of ultrafine atmospheric particles, especially of the nonvolatile BC particles, plays a key role in both health and climate effects, ways to measure these particles have been developed. Nonvolatile particles, defined as the material that is only volatile above temperatures of C, have been indirectly measured by heating up the particles through thermodenuders (TD). TDs (Burtscher et al., 2001; Wehner et al., 2002), have been used to quantify the volatility of particles at low and intermediate temperatures (An et al., 2007; Kim et al., 2010) but also the nonvolatile fraction, at higher temperatures (> 250 o C). By heating up the particles through a TD, at temperatures higher than 280 o C, sulfates, nitrates and organic acids, soluble in water as well as lipid-soluble organic compounds evaporate and only non-volatile particles and cores remain.(wehner et al., 2004) To measure the number particle size distribution in this size range scanning mobility particle sizers (SMPS) have been used upstream of a TD (Kim et al., 2010; Saleh et al., 2012; Wehner et al., 2004). For the determination of the chemical composition of these particles aerosol mass spectrometers (AMS) have been increasingly applied (Cappa and Jimenez, 2010; Lee et al., 2010; Poulain et al., 2010). In this work a new experimental methodology is developed to measure the non-volatile particle number concentration using a TD that utilizes heating at C. The TD is coupled with a high-resolution time-of-flight aerosol mass spectrometer, measuring the 2

17 chemical composition and mass size distribution of the PM 1 1 aerosol and a SMPS that provides the number size distribution of the aerosol in the range from 10 to 500 nm. The method was evaluated with a set of chamber experiments and applied during the Athens winter campaign to investigate the concentration of nonvolatile particles in an urban enviroment, their chemical composition and size distribution as well as the contribution of different pollution sources. 1 Particulate matter smaller than 1 µm 3

18 Chapter 2 Thermodenuder description The thermodenuder (TD) setup used in this study was based on the design of An et al. (2007). A detailed schematic of the TD apparatus is shown in Fig The TD consists of the bypass (BP) line, which operates at ambient temperature and the TD line, which was set at 400 o C in this study. Automatically operated three-way valves placed on the aerosol inlet and outlet, as shown in Fig. 2.1, allowed rapid switching between the BP and TD lines and allowed a comparison of the two, essential for particle evaporation and volatility measurements. The TD line consists of two main parts: i) the TD heating section and ii) the TD cooling section. The TD heating section has a length of 0.5 m and an inner diameter of 36.4 mm. Fine sand is used to cover the inner stainless steel tube to avoid Bypass Line Aerosol Inlet 6.35 mm Thermodenuder Line Heating Section Cooling Section Aerosol Outlet to instruments 36.4 mm 3-way valve Heating Tape Fine Sand Insulation Activated Carbon Stainless Steel Gauze 3-way valve 500 mm 500 mm Figure 2.1: Detailed schematic of the thermodenuder used in this study. 4

19 temperature fluctuations. A PID controller (model CNi, I/32, Omega) controls the TD temperature through a heating tape wrapped around the outer cylinder (D cyl =100 mm) of the TD, based on readings of a thermocouple (TJ36 Series, Omega) placed in the center of the heating tube. The heating section is insulated to minimize losses to the surroundings. The cooling section has the same dimensions as the heating section and consists of a cylindrical stainless steel gauze with an inner diameter of 36.4 mm. The stainless steel gauze is covered with an activated carbon jacket which is used to remove organic vapors that evaporated from the particle phase avoiding thus re-condensation during the cooling stage. The residence time inside the TD has been shown to be a critical parameter for particle evaporation (Riipinen et al., 2010). In this study, a flow rate of 1 L min 1 inside the TD Average travel time (s) Temperature ( o C) heating section Distance (cm) Figure 2.2: Temperature profile inside the heating section of the TD at the set temperature of 400 o C and a flow rate of 1 L min 1 as a function of position (distance from the entrance of the heating section, bottom axis) and average travel time (top axis). 5

20 line was used. The flow in the system is laminar and results in different particle speeds and thus different residence times, based on radial position. The average travel time of the particles was 15.8 s, significantly longer than most commercial and research TDs. The temperature profile of the TD when set at 400 o C, is shown in Fig The Fig. 2.2 temperature profile was measured by sliding a stainless steel thermocouple (50 cm length, Omega) through the center of the TD tube. The temperature difference between the center of the TD tube and that close to the walls was less than 10 o C. The walls of the TD were warm even before the heating section, due to conduction, resulting in a pre-heating of the particles before entering the heating tube. The aerosol sample already has a temperature of 200 o C when introduced in the heating section, which increases to the set temperature of 400 o C after 7 s, 20 cm from the TD inlet. Particles remain at the set temperature of 400 o C for more than 7 s. Similar results were reported by Wehner et al. (2002) using a similar TD setup. 2.1 Particle wall losses Aerosol loss inside the TD at 400 o C was determined in a separate set of experiments, using NaCl particles. NaCl particles are commonly used for the determination of particles wall losses in TD systems, since their evaporation is negligible even at high temperatures (Burtscher et al., 2001). NaCl aerosol was produced by atomizing (constant output atomizer, aerosol generator 3076, TSI) aqueous NaCl solutions of known concentrations at the range between 0.5 and 3 g L 1 (NaCl purity 99.5 %, Sigma). The produced NaCl droplets flowed through a silica dryer (RH < 20 %) and then the dry particles passed through the TD. Particles exiting the TD were measured with a scanning mobility particle sizer (SMPS, TSI Classifier model 3080, TSI DMA 3081, TSI Water CPC 3787) which operated with sheath and sample air flows set at 5 and 1 L min 1 respectively. The aerosol number size distribution (10 to 500 nm) was measured every 3 min (SMPS scanning time). Sampling between the TD and the BP line was achieved by automatic switching (every 3 min, synchronized with the SMPS) of the 3-way valves, as shown in Fig Experiments were conducted in a wide range of particle mass and number concentrations ranging from 50 to 300 µg m 3 and 10 4 to 10 5 cm 3, respectively. Particle wall loss inside the TD line was determined relative to the BP line using Equation (2.1): 6

21 Particle loss (%) Diameter (nm) Figure 2.3: Average percentage of losses at 400 o C based on 9 different experiments of NaCl aerosol generated with an atomizer, using a flowrate of 1 L min 1. The uncertainty (± 2 σ) of the average loss correction is given by the vertical lines. Loss(i) = [dn/dlogd p(i)] T D [dn/dlogd p (i)] BP (2.1) where D p is the diameter of the particles in the range from 10 to 500 nm. Particle wall loss was determined at 400 o C and the average result from all 9 experiments is given in Fig The results indicate that up to 70 % of the particles smaller than 30 nm, are lost on the walls while for particles larger than 50 nm the losses are around 50 %. Small particles are lost to the walls due to Brownian diffusion, while for larger particles and as temperature increases, thermophoretic losses prevail (Burtscher et al., 2001). The variability of the measurements (± 2 σ) for particles smaller than 100 nm is < 10 % while for larger particles increases up to around 20 %. The focus of this project is 7

22 on particles smaller than 100 nm so the corresponding losses have been determined quite precisely. The average particle wall loss function determined, shown in Fig. 2.3 was fitted with a triple-exponential function (2.2). (%) Loss (D p ) = 0.64 exp( 0.11 D p )+1.28 exp( 0.1 D p )+0.47 exp( D p ) (2.2) where D p is the diameter of the particles measured from the SMPS. The resulting fit was used to correct all the measurements throughout this work. 8

23 Chapter 3 Thermodenuder characterization 3.1 Evaporation of model aerosol A set of experiments was conducted to evaluate the TD performance when particles of known volatility are introduced inside the TD. In this series of measurements aerosol was produced with a constant output atomizer using aqueous solutions of a) adipic acid, (CH 2 ) 4 (COOH) 2 (purity 99.5 %, Fluka, solution of 2.5 g L 1 ) and b) ammonium sulfate, (NH 4 ) 2 SO 4 (purity 99 %, Sigma-Aldrich, solution of 1.76 g L 1 ). A blank experiment using only water was also performed and used as reference. It should be noted that the water used in this work was first deionized passing through anion and cation exchange resins and then purified in a water purifier (model Neptune Analytical, Purite). The particle number size distribution of ammonium sulfate, adipic acid and purified water at 25 o C (BP) and 400 o C (TD), respectively, are shown in Fig Although more than 97 % of the particles evaporated, a small fraction with sizes around 20 to 40 nm remained in all cases. The number fraction remaining (NFR), that is the fraction of total particle number concentration at 400 o C (N(400 o C)) to that at 25 o C (N(25 o C)), varied from 2 to 4 % (Fig. 3.1 d). In all three experiments, the NFR had the same values regardless of the particle composition that was used. When pure water was atomized approximately 5000 particles cm 3 were produced with a mean size of 50 nm. When heated to 400 o C most of these particles evaporated (Fig. 3.1 c). However, around 2 % remained, corresponding to non-volatile impurities, probably traces of metals. This is strong evidence that in these experiments what is measured after the TD line is impurities in the water used to make the corresponding solutions. As a result neither ammonium sulfate 9

24 dn/dlogd p (cm -3 ) 20x (a) Ammonium Sulfate 25 o C dn/dlogd p (cm -3 ) 1.2x (b) 400 o C 400 o C Adipic acid 25 o C Diameter (nm) Diameter (nm) dn/dlogd p (cm -3 ) 8x (c) 400 o C Purified Water 25 o C Number Fraction Remaining (d) Adipic acid Ammonium sulfate Purified water Diameter (nm) Time (min) Figure 3.1: Number size distribution of (a) ammonium sulfate (b) adipic acid and (c) purified water after the bypass line (25 o C, blue) and after the TD (400 o C, red) as a function of diameter, D p. (d) Number fraction remaining (NFR) as a function of time for the three different experiments nor adipic acid particles survive after heating at 400 o C in our TD. 3.2 Evaporation of SOA In a second set of experiments the TD methodology developed in this study was used to test the evaporation of SOA in our system. All the experiments were performed using the indoor smog chamber facility at ICE-HT/FORTH, Patras. The chamber is a 10 m 3 Teflon reactor suspended in a temperature controlled room ( 30 m 3 ) with aluminum coated walls. The room walls are covered with UV/Vis lamps ( nm wavelength range). The photolytic rate of NO 2 was measured using the well-established chemical actinometry method (Bohn et al., 2005), giving a J NO2 = 0.59 min 1, value corresponding 10

25 to midday of a typical sunny summer day in Greece. In all experiments the temperature and the RH inside the chamber were kept constant at 25 o C and < 30 %, respectively. Volatile organic compounds (VOCs) were measured using a proton transfer mass spectrometer (PTR-MS, Ionicon), (Gouw and Warneke, 2007), and O 3 and NO x were measured with the corresponding online monitors (Model 400E Photometric Ozone Analyzer, Teledyne and Model T200 Nitrogen Oxide Analayzer, Teledyne). The TD was coupled with a high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS, Aerodyne), measuring the chemical composition and mass size distribution of the submicrometer aerosol. A scanning mobility particle sizer (SMPS, TSI) connected to the TD provided the number size distribution of the aerosol in the diameter range from 10 to 500 nm. The SMPS was operated with sheath and sample air flows set at 5 and 1 L min 1, respectively. The AMS and SMPS were synchronized to measure, either from the BP line (ambient conditions) or the TD line (400 o C) every 3 min (Fig. 2.1) Biogenic SOA For the generation of biogenic SOA a set of experiments utilizing the dark ozonolysis of α-pinene were conducted following the experimental conditions given by Pathak et al. (2007), (Table 3.1). First, ozone was introduced inside the chamber and then an injection of α-pinene initiated the production of SOA. The experiment took place at NO x concentrations < 2 ppb in the absence of particle seeds. The SOA mass concentration was determined from the measured SMPS size distribution using a density of 1.4 g cm 3 determined from a previous study from our group (Kostenidou et al., 2007). The time evolution of SOA number size distribution from α pinene ozonolysis at ambient temperature (BP) and 400 o C (TD), for experiment #1 (Table 3.1), is shown in Table 3.1: Experimental conditions used in α-pinene dark ozonolysis experiments Initial Initial Temperature RH α pinene O 3 SOA Mass a Experiment ( o C) (%) (ppb) (ppb) (µg m 3 ) a Determined from the SMPS measurements using a density 1.4 g cm 3 11

26 O 3 α-pinene (a) 25 C (b) 400 C Figure 3.2: Time evolution of particle number size distribution during the dark ozonolysis of α pinene, measured after (a) the bypass line at 25 o C and (b) the TD set at 400 o C. The color scale represents particle number concentration. Fig After the introduction of ozone at 19:50 and injection of α pinene at 20:30, new particles were formed and grew rapidly up to 100 nm in less than one hour, reaching a concentration of around 2000 cm 3. These fresh SOA particles evaporated almost completely (2 % remained) after passing through the TD line at 400 o C (Fig. 3.2). The particle number size distribution in the chamber at 22:00 is shown in Fig A significant decrease of total particle number concentration, from 1700 cm 3 to 35 cm 3 (NFR 2.2%), after sampling through the TD line was observed. After new particle formation occurred the NFR immediately dropped to 2 % showing a 98 % particle evaporation in the TD line. This shows that fresh SOA formed from α pinene ozonolysis evaporate almost completely after heated at 400 o C. The few remaining particles were mainly due to preexisting particles in the chamber (there were around 20 cm 3 before the reactions started). 12

27 6000 dn/dlogd p (cm -3 ) o C o C Diameter (nm) Figure 3.3: Particle number size distribution of α-pinene SOA (at 22:00), for chamber measurements, at 25 o C (blue) and TD measurements, at 400 o C (red), for Exp. #1, from Table Anthropogenic SOA As a typical example of anthropogenic SOA, the photooxidation of toluene was examined to test the ability of the TD methodology developed in this work to completely evaporate anthropogenic SOA (asoa). Hydrogen peroxide and toluene were injected in Table 3.2: Summary of experimental conditions for the toluene photooxidation experiments Initial Initial Initial Temperature RH Toluene NO x H 2 O 2 [OH] a SOA Mass b Experiment ( o C) / (%) (ppb) (ppb) (ppm) (molecule cm 3 ) (µg m 3 ) % % a Initial [OH] after UV lights were turned on, measured by the decay of toluene. b Determined using a density 1.4 g cm 3 13

28 Η 2 Ο 2 & toluene UV lights ON (a) 25 C (b) 400 C Figure 3.4: Time evolution of particle number size distribution during the photooxidation of toluene, measured after (a) the bypass line set at 25 o C and (b) the TD set at 400 o C. The color scale represents particle number concentration the chamber and after reactants were mixed, UV lights were turned on, initiating the photochemistry. OH radicals were produced from H 2 O 2 photolysis and the OH concentration inside the chamber was determined following the decay of toluene. The experiments were conducted at NO x concentration of < 10 ppb in the absence of particle seeds. A summary of the experimental conditions is given in Table 3.2. The time evolution of the number size distribution of Experiment # 1 is shown in Fig After injecting toluene and H 2 O 2 at 18:15, UV lights were turned on (18:45), forming fresh particles at 25 o C. The growth of these particles up to 100 nm took place in less than one hour, reaching a concentration of 1200 cm 3. When heated at 400 o C these fresh particles evaporated practically completely (4 % remained). 14

29 o C dn/dlogd p (cm -3 ) o C Diameter (nm) Figure 3.5: Average particle number size distribution (from 01:00 to 03:00) of toluene SOA for chamber measurements, at 25 o C (blue) and TD measurements, at 400 o C (red) (Exp. 1, Table 3.2) The average number size distributions during particle growth (from 01:00 to 03:00), are shown in Fig Again, a significant decrease of the particle number concentration was observed, from 1750 cm 3 to 70 cm 3 (NFR=4 %). After new particle formation, NFR was less than 5 % and as particle number concentration increased NFR decreased to 2 % suggesting that 98 % of the particles evaporated at 400 o C. This shows that fresh SOA formed from toluene photooxidation evaporates almost completely after heating at 400 o C. 3.3 Evaporation of freshly nucleated particles An important aspect of this work was to understand the contribution of primary emissions and nucleation to the total number concentration during ambient measurements. To do so an experiment was performed to generate an induced ambient nucleation event following the methodology recently developed by our group (Papanastasiou et al., 2014). 15

30 Table 3.3: Initial species concentration for the induced ambient nucleation experiment. Gas Phase O 3 NO x H 2 O 2 Isoprene Toluene C8 & C9 aromatics OH a (ppb) (ppb) (ppm) (ppt) (ppt) (ppt) (molecules cm 3 ) Aerosol Phase AMS SMPS Organics Sulfate Nitrate Ammonium Number Mass b (µg m 3 ) (µg m 3 ) (µg m 3 ) (µg m 3 ) (cm 3 ) (µg m 3 ) < a Average [OH] during the experiment measured by the decay of n-butanol(d9) b Assumed density of 1.5 g cm 3 For the induced ambient nucleation experiment ambient air was introduced in the chamber and exposed to OH radicals. A metal bellows pump (Senior Aerospace MB 602) was used to introduce ambient air, from the suburban area of ICE/HT, into the chamber. H 2 O 2 photolysis was used as a source of OH radicals. The initial concentration of the different species, found in the aerosol or gas phase, were measured by AMS, SMPS, PTR-MS and monitors as given in Table 3.3. A tracer gas, n-butanol (d9), was injected in the beginning of the experiment, to determine OH radical concentration, following its decay. The time evolution of particle size distribution at ambient temperature, is shown in Fig After measuring the chamber background species (Fig. 3.6), the introduction of ambient air was initiated, that is the introduction of VOCs and particles (Fig. 3.3 B) found in ambient air. The injection lasted for 3 h (16:00 to 19:00). Particle chemical characterization from AMS analysis showed that organics dominated, representing 55 % of the total mass concentration while sulfate followed accounting for 31 %. Ammonium and nitrate contributed the other 13 % along with 1 % of chloride. Particulate organics were quite oxidized with an oxygen to carbon ratio (O/C) of about 0.6. In the gas phase, toluene and xylenes were the most abundant species (> 400 ppt) while the levels of small 16

31 (A) (B) UV lights ON (D) (C) Figure 3.6: Time evolution of particle number size distribution during the induced ambient nucleation experiment, measured after the BP line at 25 o C: (A) the chamber background (13:30 to 15:50), (B) the ambient VOCs and particles introduced in the chamber (16:00 to 19:00), (C) the nucleation event (19:40 to 22:00) and (D) fresh SOA formation (22:00 to 12:00). oxygenated VOCs (CH 3 OH, CH 3 CHO etc.), ranged between 5 and 30 ppb. During the filling phase of the chamber with ambient air the aerosol number concentration in the chamber, increased to 400 cm 3 while the concentration after the TD, at 400 o C, decreased to 160 cm 3 (Fig. 3.7) indicating that 60 % of the ambient particles evaporated in the TD. A detailed discussion on the evaporation of aged aerosols is given in a subsequent section (Section 4.2). Induced ambient nucleation has been studied in detail by Papanastasiou et al. (2014). The authors observed high production of organics along with small increase of sulfate and ammonium mass, suggesting that these type of simulated ambient nucleation experiments have similarities and same trends with field observations. During the nucleation event the particle number concentration increased from 400 cm 3 to 4000 cm 3 in less than half an hour (Fig. 3.7) while after the TD, particle concentration was stable, suggesting that freshly formed particles evaporated completely after heated at 400 o C. Fig. 3.8 shows the number size distributions when the nucleation event occurred. Nucleation mode particles (< 20 nm) evaporated completely in the TD while larger particles, ambient particles introduced inside the chamber, evaporated partially (NFR=40 %). In the last phase of the experiment, fresh particles coming from nucleation grew up 17

32 Total Number Concentration (cm -3 ) (B) (C) 25 o C 400 o C Nucleation event 18:30 19:00 19:30 20:00 20:30 21:00 Local Time Figure 3.7: Total number concentration of (B) ambient particles introduced in the chamber (18:00 to 19:35) and (C) ambient and nucleated particles (19:40 to 21:00) as a function of time to 100 nm, growing for more than 10 h (Fig. 3.6). AMS analysis showed that most of the mass produced (> 90 %) was organic. After the nucleation event particle number concentration was stabilized and then started decreasing due to wall losses inside the smog chamber. Particles reached a maximum of 6000 cm 3 and during aging decreased to 1000 cm 3 inside the chamber. The particle number concentration for this aged aerosol decreased again significantly (NFR=2 %) after the TD, suggesting that more than 98 % of the particles evaporated at 400 o C. This is an indication that fresh SOA particles evaporate completely after passing through our TD. Overall, from this set of chamber experiments, the experimental method proposed was able to evaporate almost completely (> 97 %) particles formed from traditional biogenic and anthropogenic SOA precursors and most important, particles coming from a nucleation event. These suggest that during field measurements the surviving particles will be 18

33 PHASE C (1) (2) 25 C 400 C Figure 3.8: The number size distribution during the nucleation event (phase C) as a function of the diameter at 25 o C (blue) and 400 o C (red). (1) The representative region of nucleation particles and (2) Ambient particles region after introduction in the chamber coming from primary emission sources alone. 19

34 Chapter 4 Ambient measurements 4.1 Instrumentation During the winter of 2013 (January 7 to February 5) an intensive field campaign was conducted at the premises of the National Observatory of Athens, located in Thissio (37 o N, 23 o E). The major objective of this campaign was to determine local and regional air pollution sources in the Greek capital (3.8 million people) during wintertime with a main focus on air pollutants originating from the increasing burning of biomass for residential heating. The measurement site was in the center of Athens, 200 m from the nearest road and is considered an urban background site. The measurements took place on top of a rocky cliff, opposite of the temple of Thissio. A suite of instruments was used to characterize both the gas and aerosol phases, as described in Table 4.1. Briefly, VOCs were measured using a PTR-MS (details provided by Kaltsonoudis et al. (2014)) while online monitors were measuring NO x and NH 3 (Model T200 Nitrogen oxide analyzer, Teledyne), O 3 (Model 400E photometric ozone analyzer, Teledyne), SO 2 (Model 100E, UV fluorescence SO 2 Analyzer, Teledyne), CO 2 (Model T360 Carbon dioxide analyzer, Teledyne), and CO (Model 300E/EM Carbon monixide analayzer) mixing ratios. For the aerosol phase, our TD was coupled with a high resolution time of flight aerosol mass spectrometer (HR-ToF-AMS, Aerodyne), measuring the chemical composition and mass size distribution of PM 1 aerosol. A scanning mobility particle sizer (SMPS, TSI) was also connected to the TD providing the number size distribution of the aerosol in the range from 10 to 500 nm. The SMPS was operated with sheath and sample air flows set at 5 and 1 L min 1, respectively. A multi-angle absorp- 20

35 Table 4.1: Instrumentation deployed at the sampling site during the Athens-2013 winter campaign. Instrument Size Range Measure Resolution (min) Aerosol Phase Particle number SMPS nm size distribution 3 Chemical characterization and AMS PM 1 mass size distribution 6 MAAP PM 2.5 BC mass concentration 5 CCN nm Cloud condensation nuclei 4 APS µm Number size distribution 6 H-TDMA nm Hygroscopicity and mixing state 3 Gas Phase Online monitors ppb Concentration of O 3, SO 2, CO 2, CO, NO x 1 PTR-MS ppb Concentration of VOCs 1 MAAP : Multi-Angle Absorption Photometer, CCN : Cloud Condensation Nucleus Counter APS : Aerosol particle sizer, H-TDMA: Hygroscopic tandem differential mobility analyzer PTR-MS : Proton-transfer-reaction mass spectrometer tion photometer (MAAP, model 5012, Thermo-Scientific) was used to measure the PM 2.5 black carbon mass concentration. The TD was operating at different temperatures, ranging from 40 o C to 400 o C as shown in Fig More specifically seven temperature steps were chosen for the campaign (40, 70, 100, 120, 200 and 400 o C). One cycle, from 40 o C to 400 o C and back to 40 o C, required 10 h, which resulted in roughly two temperature cycles per day. The starting time of each cycle changed from day to day, to early morning (02:30-05:00), morning (07:00-10:00), evening (16:00-18:00) and midnight (21:00-00:00) so that measurements at each temperature were performed during different periods of the day. The temperature of interest for the present work is 400 o C and during the campaign there were 32 periods of measurements at this temperature, resulting in 20 h of data. AMS analysis and source apportionment of the PM 1 organic fraction for the Athens 21

36 Temperature ( 0 C) :00 03:00 06:00 09:00 Local time Figure 4.1: Temperature profile of a TD cycle during the campaign campaign is given in detail by Florou et al. (2014). Briefly, AMS analysis was preformed with the analysis software SQUIRREL v1.51 C and the standard HR-ToF- AMS data analysis software Peak Integration by Key Analysis (PIKA v1.10 C) adapted in Igor Pro 6.22 A (Wavemetrics). Positive matrix factorization (PMF) was applied to the ambient measurements of the AMS organic spectra, following the approach of Ulbrich et al. (2009), to estimate the contributions of the various sources to the organic aerosol (OA) levels. Major anthropogenic emission sources, like traffic and biomass burning, were investigated as well as the contribution of aged OA and cooking aerosol to the total OA. 4.2 Chemical characterization of ambient and thermodenuded aerosol The 32 measurement periods when the TD was set at 400 o C, were analyzed with a focus on the NFR, resulting in the timeseries shown in Fig The NFR was highly 22

37 1.0 SMPS Number Fraction Remaining Early morning (02:30-05:00) Morning (07:00-10:00) Afternoon (16:00-18:00) Midnight (21:00-00:00) Jan Feb Date Figure 4.2: Timeseries of Athens-2013 campaign as a function of particle NFR when the TD was set at 400 o C. variable, especially during the midnight and early morning hours with values ranging from 0.3 to 0.9. The morning measurements ranged from 0.2 to 0.7 and depended on the traffic tensity (discussed in Section 4.4). The afternoon hours were more stable giving a NFR ranging from 0.3 to 0.5. The average mass concentration of the major species during the time that TD was set at 400 o C combined with the PMF analysis of the organics, for ambient (BP line) and thermodenuded conditions (TD line), is given in Table 4.2. During ambient measurements (BP mode), organics dominated the PM 1 mass concentration with 70 % contribution, while black carbon (BC) was responsible for another 17 %. The remaining 13 % consisted of sulfate, nitrate and ammonium. Since organics dominated the total mass, further characterization was performed using the PMF analysis. Organics were separated in four categories: hydrocarbon-like OA (HOA) coming from traffic, biomass burning OA (BBOA) due to biomass burning, oxy- 23

38 Table 4.2: Average mass concentration of the major species (in µg m 3 ), when the TD was set at 400 o C, given for both modes (BP for ambient conditions and TD after 400 o C), during the Athens 2013 campaign. The different source contribution in (%) for the organic aerosol fraction estimated by PMF analysis. The quoted error provides the precision (± 1 σ) for AMS (as given by HEPA filter blank measurements) and MAAP measurements. Mode Organics Sulfate Nitrate Ammonium BC 10 ± ± ± ± ±0.2 BP BBOA HOA OOA COA 31 % 23 % 29 % 17 % 2.1 ± ± ± ± ±0.2 a TD BBOA HOA OOA COA 13 % 23 % 58 % 6 % a Assuming zero evaporation of BC at 400 o C. genated organic aerosol (OOA) corresponding to long range transport OA, and cooking organic aerosol (COA). During ambient measurements biomass burning aerosol, contributed 30 % of the organic mass, due to increased burning of wood for domestic heating. BBOA had high levels mostly during the night. A high contribution of aged organic aerosol (28 %) was observed during low concentration periods. During these periods, mostly rainy days, most of the OA is transported to Athens from other areas. Aerosol emitted by traffic was 25 % of the total organic mass, occurring mostly during the morning and afternoon weekday rush hour traffic. Finally cooking organic aerosol contributed with 17 %. These high levels were probably due to the restaurants close to the measurements site, cooking during daytime. The aerosol composition changed, when particles passed through the TD. Assuming BC does not evaporate at 400 o C, its concentration dominated the remaining PM 1 mass, with a contribution of 51 %, while organics were responsible for another 45 %. The remaining < 5 % consisted mainly of sulfate and small amounts of nitrate. The organics mass fraction remaining (MFR) was 20 %, suggesting that part of the organics was prob- 24

39 ably of extremely low volatility during the campaign. Sulfate had a MFR of 10 %, that may be due to larger particles that evaporated and were introduced to the detection range of AMS. The MFR of ammonium and nitrate is subject to a larger relative error since the corresponding concentration values after the TD were close to the detection limit of the instrument. From the PMF analysis, the organics after 400 o C were mostly OOA, around 60 %, an expected result, since usually the more oxygenated organic compounds in the aerosol tend to have lower volatility. HOA coming from traffic sources (23 %) was less volatile than BBOA, coming from biomass burning (13 %) while COA had the lowest contribution, at 6 %. The increased OOA contribution to the TD organic mass could also be due to oxidation of the organic particles when passing through the TD at 400 o C. This could also lead to artifacts in the measurement of the contribution of nonvolatile particles with our system. This could be the result of: 1) Pyrolysis of lower volatility organic species at 400 o C, leading to an overestimation of the non volatile number and mass concentration; 2) oxidation of BC and non-volatile particles leading to a possible underestimation. These processes have been examined in detail by Novakov and Corrigan (1995). There work suggested that constituents such as potassium can act as catalysts for the combustion processes and therefore lower the combustion temperature up to 100 o C. Applying these to our measurements, for high concentrations of potassium (5 % of total mass) we estimated a < 5 % overestimation of the nonvolatile particle mass concentration due to pyrolysis and < 10 % underestimation due to oxidation. These results suggest that what is measured after the TD is not a product of oxidation or pyrolysis of particles, making this method relatively precise for the estimation of non-volatile particle number concentration. Given the variability of the measured NFR we focus next on periods when the site was dominated by aerosol from a specific source, to gain insights into the behavior of the corresponding particles. When the fractional source contribution to organic aerosol was higher than 50 % (with other pollution sources contributing each less than 15 %) the measurement periods were considered representative of the emission source. Two major anthropogenic sources were investigated: 1) biomass burning and 2) traffic. 25

40 4.3 Biomass burning periods The correlation between NFR and biomass burning contribution to OA, is shown in Fig As the BBOA fraction increased the NFR increased also. For periods when the BBOA was dominant NFR exceeded 90 %, an indication that biomass burning particles did not evaporate completely in the TD system at 400 o C. The average NFR of the representative biomass burning periods was higher than 80 % suggesting that most of the biomass burning particles had a non-volatile core. The particle number size distribution from the most dominant biomass burning period (NFR > 90 %) at ambient conditions and at 400 o C is shown in Fig It should be noted that all the results are corrected for wall losses based on Section 2. During this biomass burning event there was a significant shift of the size distribution mode during heating, 1.0 SMPS Number Fraction Remaining Biomass Burning Fractional Source Contribution to OA Figure 4.3: The fractional contribution of biomass burning to the organic aerosol (PMF analysis) as a function of the number fraction remaining (given by the SMPS). Each dot is a representative of each measurement period. 26

41 from 100 nm to 40 nm, but only a few particles evaporated completely. This is a strong indication that biomass burning particles have a non-volatile core that survived after 400 o C, and are coated with volatile compounds that evaporated through the system and led to a decrease of particle size. Although > 90 % of the particle number concentration was in the size range from 10 to 100 nm the chemical characterization of these particles was based on the mass concentration, that reflects mainly particles larger than 100 nm. This asymmetry may add uncertainties to this characterization. The mass concentration of different species for the four major biomass burning events is given in Table 4.5. During ambient measurements (BP mode) organics dominated the PM 1 mass concentration with a % contribution, while black carbon (BC) was responsible for another 20 %. The remaining 5 10 % consisted of sulfate, nitrate, and ammonium. The aerosol composition changed, when particles passed through the 140x C Ambient dn/dlogd p (# cm -3 ) Diameter (nm) Figure 4.4: SMPS particle number size distribution for the most dominant biomass burning period (NFR > 90 %) at ambient conditions (black) and 400 o C (blue). 27

42 Table 4.3: Average mass concentration of the major species (in µg m 3 ) for the major biomass burning periods, given for both modes (BP for ambient conditions and TD after 400 o C), during the Athens 2013 campaign. Number fraction remaining (NFR) for each event and the average mass fraction remaining of each species are also provided. Date Mode Organics Sulfate Nitrate Ammonium BC a NFR 1. BP 47 (75 %) 1.1 (2 %) 1.8 (3 %) 0.8 (1 %) 12 (19 %) (13.Jan :45) TD 9 (42 %) 0.3 (1 %) 0.2 (1 %) 0.05 (< 1 %) 12 (56 %) 2. BP 65 (76 %) 2.4 (3 %) 2.1 (2 %) 1.5 (2 %) 14.7 (17 %) (13.Jan :45) TD 11 (41 %) 0.7 (3 %) 0.3 (1 %) 0.2 (< 1 %) 14.7 (55 %) 3. BP 6.7 (62 %) 0.7 (6 %) 0.5 (5 %) 0.45 (4 %) 2.4 (23 %) (14.Jan :05) TD 1.2 (31 %) 0.2 (4 %) 0.09 (2 %) 0.05 (1 %) 2.4 (62 %) 4. BP 16.2 (71 %) 1.2 (5 %) 1.0 (4 %) 0.6 (2 %) 4 (18 %) (24.Jan :15) TD 2 (32 %) 0.2 (3 %) 0.13 (2 %) 0.04 (< 1 %) 4 (62 %) a Assuming zero evaporation of BC at 400 o C Fraction remaining (%) TD. Assuming BC does not evaporate at 400 o C, its concentration dominated the PM 1 mass, with a contribution of 60 %, while organics were responsible for another %. The remaining < 5 % consisted mainly of sulfate and small amounts of nitrate. The percentage of sulfate was 25 % after 400 o C, possibly coming from larger particles that evaporated within the measurement range of AMS. The organic mass fraction remaining is 17 %, suggesting that roughly 80 % of the organics evaporated through the TD. The PMF analysis results for the four biomass burning events were averaged, for ambient measurements (BP mode) and after 400 o C (TD mode) and the results are shown in Table 4.5. BBOA was dominant in the BP and TD measurements with a 75 % and 50 % contribution to the organic mass, respectively. HOA and OOA mass fractions remaining were the highest with 33 % and 75 %, respectively, suggesting that although BBOA particles dominate in the TD mass, HOA and OOA are the hardest to evaporate. COA was relatively volatile, with 97 % of the COA mass evaporating in the TD. 28