Exploring Conditions for Ultrafine Particle Formation from Oxidation of

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Environmental Processes

Exploring Conditions for Ultrafine Particle Formation from Oxidation of Cigarette Smoke in Indoor Environments Chen Wang, Douglas B. Collins, Rachel F. Hems, Nadine Borduas-Dedekind, Maria Antiñolo, and Jonathan P.D. Abbatt Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b06608 • Publication Date (Web): 30 Mar 2018 Downloaded from http://pubs.acs.org on March 31, 2018

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Environmental Science & Technology

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Exploring Conditions for Ultrafine Particle Formation from Oxidation of Cigarette Smoke in Indoor Environments

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Chen Wang, Douglas B. Collins, Rachel F. Hems, Nadine Borduas, María Antiñolo#, Jonathan P.D. Abbatt*

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Department of Chemistry, University of Toronto, 80 St. George Street, M5S 3H6, Toronto, ON, Canada

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#

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*To whom correspondence should be addressed: [email protected]

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TOC art

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Current address: Instituto de Investigación en Combustión y Contaminación Atmosférica. Universidad de Castilla-La Mancha, Camino de Moledores s/n, 13071, Ciudad Real, Spain

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1 ACS Paragon Plus Environment


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Abstract

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Cigarette smoke is an important source of particles and gases in the indoor environment. In this work, aging of side-stream cigarette smoke was studied in an environmental chamber via exposure to ozone (O3), hydroxyl radicals (OH) and indoor fluorescent lights. Aerosol mass concentrations increased by 1318% upon exposure to 15 ppb O3 and by 8-42% upon exposure to 0.45 ppt OH. Ultrafine particle (UFP) formation was observed during all ozone experiments, regardless of the primary smoke aerosol concentration (185 to 1950 µg m-3). During OH oxidation, however, UFP formed only when the primary particle concentration was relatively low (< 130 µg m-3) and the OH concentration was high (~1.1 x 107 molecules cm-3). Online aerosol composition measurements show that oxygen- and nitrogen- containing species were formed during oxidation. Gas phase oxidation of NO to NO2 occurred during fluorescent light exposure, but neither primary particle growth nor UFP formation were observed. Overall, exposure of cigarette smoke to ozone will likely lead to UFP formation in indoor environments. On the other hand, UPF formation via OH oxidation will only occur when OH concentrations are high (~107 molecules cm-3), and is therefore less likely to have an impact on indoor aerosol associated with cigarette smoke.

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Introduction

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Cigarette smoking often takes place in indoor environments, such as households, workplaces and vehicles, despite the negative health effects of direct and passive smoking. Cigarette smoke contains a complex mixture of particles and gases, including CO, NO, organic compounds of different functionalities and volatilities, and metals,1-5 which together are a major source of contaminants in indoor environments. Exposure to fine particles and carcinogenic species from cigarette smoke is related to multiple health problems such as lung cancer and asthma.6, 7 Indeed, there are about six million deaths annually attributed to cigarette smoke globally, and about 10% of these people are estimated to die from the effects of second-hand smoke.8

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Due to its influence on indoor air quality and associated health effects, there are a large number of studies on cigarette smoke. Earlier studies focused on the primary emissions to characterize the emitted particles and gaseous species. Many of the emissions have the potential to react with indoor oxidants such as ozone or OH radicals to form less volatile species that may contribute to secondary organic aerosol (SOA) formation and form more toxic oxidation products. Studies on the oxidative aging of cigarette smoke have taken place only in the past decade with a focus on SOA and carcinogenic species formation. These studies include secondary reactions of cigarette smoke with ozone,9 reaction of nicotine with OH,10 SOA formation from heterogeneous oxidation of nicotine and cigarette smoke on indoor surfaces,11, 12 and the reaction of nitrous acid (HONO) with surface-sorbed nicotine to form carcinogenic tobacco-specific nitrosamines.13 More needs to be determined on the secondary reactions of cigarette smoke, such as the dependence of aerosol formation, including particle size distribution and compositional change, on exposure to ozone, oxidation with OH, and indoor irradiation, all of which are relevant indoor processes. It is necessary to know which of these three processes is likely to dominate aging of second hand cigarette smoke.


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Ozone is an important oxidant which drives many gas phase and heterogeneous reactions.14 Indoor ozone often arises from transport of outdoor ozone through the building envelope. In some cases, ozone can reach high mixing ratios (e.g. a few hundred ppb) in buildings with the operation of devices such as photocopiers, laser printers and air purifiers.15, 16 17 Unsaturated organic species in cigarette smoke can react with ozone to form less volatile secondary products and thus change particle size distribution and composition. Indeed, Sleiman et al. have reported an initial observation that ultrafine particle formation accompanies cigarette smoke oxidation by ozone.9 OH concentrations indoors are usually on the order of 104 or 105 molecules cm-3 according to modeling and indirect measurement studies.18-21 Ozonolysis of alkenes is often a major indoor source of OH.18 Recent direct measurements and chamber studies suggest that relatively high concentrations of OH (106 molecules cm-3) may exist indoors associated with photolysis of high concentrations of HONO.22, 23 In addition, OH as high as 107 molecules cm-3 has been observed during cleaning activities.24 A recent study on the photochemistry of indoor-relevant species suggested that photolysis occurring under indoor fluorescent irradiation can change the oxidizing capacity of the indoor atmosphere by forming OH and HO2 radicals.25 Thus, fluorescent lights may affect the photochemical aging of cigarette smoke.

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This study focuses on the secondary chemistry of cigarette smoke with indoor oxidants, including both ozone and OH radicals, as well as indoor fluorescent lights. We applied on-line particle characterization techniques to study aerosol evolution during the reaction of side-stream cigarette smoke with indoor oxidants and lights in an environmental smog chamber. In this study, we focus on the phenomenon of ultrafine particle formation, and try to better characterize the conditions under which this process occurs, while acknowledging the complexity of chemical composition of cigarette smoke and possibility of heterogeneous chemistry that may result from deposition of smoke materials on different surfaces.11,

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Experimental Method

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Cigarette Smoke Injection into Teflon Chamber

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The experiments were conducted in a 1 m3 Teflon chamber (1.18 m x 1.18 m x 0.72 m) connected to instruments to characterize particles and gases. Smoke was introduced from a research grade cigarette (1R6F Kentucky Reference Cigarette, Center for Tobacco Reference Products at University of Kentucky) with a simulated smoking process. Main-stream smoke was drawn at a flow rate of 1.05 L/min through the cigarette for 2 seconds (35 mL/puff) using a custom-built smoking machine, according to standard smoking procedures.27 The side stream smoke associated with 1 puff of the cigarette was injected directly to the chamber for 15 s through stainless steel tubing (length: 18 cm, inner diameter: 25 mm) with a clean air flow of 5 L/min (zero air generator, AADCO 737-series) to minimize loss to the walls of injection lines. Five minutes after cigarette injection, the 5 L/min zero air flow was decreased to 3.5-4.1 L/min (dilution flow) to maintain the air volume in the chamber during different experiments. In general we did not observe substantial differences in aerosol formation and composition between dry and humid conditions. Most experiments were conducted under dry conditions (< 5% RH). For experiments at humid conditions (~ 50% RH) the chamber was preconditioned with humid air by passing dry zero air through a bubbler containing deionized water.

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Ozone Oxidation Experiments

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O3 was generated by passing zero air at 2 L/min over a UV lamp (254 nm Pen-Ray Lamp, UVP, Inc.) in a flow tube for different lengths of time to introduce different amounts of O3 (~15-70 ppb) into the chamber. This wide range of ozone mixing ratios covers typical levels in various indoor environments.14 To mimic the situation in a realistic cigarette smoking event in an indoor environment containing a certain amount of oxidant, O3 was added to the chamber prior to cigarette smoke injection in some experiments. In other experiments, O3 was added to the chamber after cigarette smoke injection and mixing had occurred, to investigate changes in gases and particles due to O3 addition.

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OH Oxidation Experiments

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Gaseous hydrogen peroxide (H2O2) as an OH precursor was added to the chamber by passing 0.56 L/min of zero air through a bubbler containing approximately 20 mL of 30 % H2O2 aqueous solution for 30 min. In some cases, H2O2 was added after cigarette smoke injection (following the establishment of stable particle and gas signals, indicating well-mixed cigarette smoke in the chamber), while in other cases H2O2 was added 1 h before cigarette injection. By adding H2O2 after cigarette injection in some cases, we were able to separate the effect of H2O2 on the smoke composition (see more discussion in supporting information). The RH change during H2O2 injection was smaller than 5%. OH was generated from photolysis of H2O2 with 16 UV-B light bulbs with wavelength centered at 310 nm (Philips Lighting, TL 40W/12 RS SLV/25) surrounding the Teflon chamber. O-xylene (Sigma Aldrich, ≥98%) was used as a tracer to quantify OH concentrations under the same conditions as the OH oxidation experiments. The OH concentration was measured to be (1.1 ± 0.2) x 107 molecule cm-3 from three experiments (see supporting information for details). We recognize this is a higher concentration than commonly encountered indoors, as further discussed in the Results section.

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Indoor Fluorescent Light Experiments

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The influence of indoor fluorescent lights on the atmospheric chemistry of cigarette smoke was investigated by replacing the UV-B light bulbs surrounding the chamber with 16 indoor fluorescent light bulbs (Sylvania, cool white deluxe, F34/CWX/SS, 34W, Canada) in two experiments. The wavelength dependence of the photon flux inside the chamber was measured using a spectroradiometer (StellarNet Inc.) for both the UV-B and fluorescent lights as shown in Figure S2 in the supporting information.

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Particle Characterization

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The number size distribution of cigarette smoke particles was measured using a Scanning Mobility Particle Sizer (SMPS; TSI, Inc. Model 3080/3787) for particle sizes between 14 and 714 nm using a sample flow rate of 0.3 L/min and a sheath air flow rate of 3 L/min. Size distribution scan time was set to 163 s, with 15 s re-trace and 2 s wait time between scans (180 s total). Total aerosol mass concentrations were calculated by transforming the number size distribution to a volume size distribution and applying a particle density of 1.4 g cm-3. Non-refractory chemical composition for aerosol particles with vacuum aerodynamic diameter of 70-1000 nm was measured using a highresolution aerosol mass spectrometer (AMS; Aerodyne Research Inc.).28 The mass spectrometer was 4 ACS Paragon Plus Environment

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operated in V-mode, and ensemble concentrations of non-refractory aerosol components were quantified every 30 s. Ionization efficiency (IE) was calibrated using size-selected NH4NO3 aerosol.29 Concentrations of organic compound families (e.g., CxHy, CxHyO) are reported using the nitrateequivalent mass concentration, where the IE for all components was assumed equal to that of NH4NO3.

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Trace Gas Measurements

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Online measurements of trace gases were conducted with commercially available instrumentation. CO was measured using a non-dispersive infrared absorption spectroscopy (Thermo Scientific, Model 48i). O3 measurements made using UV absorption photometry (Thermo Scientific, Model 49i) showed a strong interference from components of cigarette smoke as well as H2O2. As a result, O3 measurements were only reliable in the absence of cigarette smoke and H2O2. For the experiments in which O3 was added after cigarette smoke, we are confident in the initial mixing ratio of O3 since the protocol for introducing O3 was the same as for reproducible experiments in which O3 was added to a ‘clean’ chamber. NOX was measured using a chemiluminescence analyzer (Thermo Scientific, Model 42i) that was modified from its commercial configuration by the installation of a blue light converter (BLC; Air Quality Design, Inc., wavelength: 350-420 nm) that quantitatively and specifically photolyzes NO2 to NO.30 This modification replaces the less specific molybdenum catalytic converter that can also convert various NOy species to NO.30 Therefore, measurements of NO2 reported herein are a true measure of gas-phase NO2.

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Chamber Cleaning and Control Experiments

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After each experiment, the Teflon chamber was cleaned by bubbling zero air through a 30% H2O2 aqueous solution into the chamber continuously with UV-B lights on and purging with a large flow (15 L/min) of zero air for at least 12 hr. Control experiments were conducted to verify the experiment results and included oxidant only, UV-B light only, and cigarette smoke only conditions in addition to combinations of these conditions. A comprehensive accounting of all experimental configurations is provided in Table S1.

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Results and Discussion

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Cigarette Smoke in the Chamber

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Multiple control experiments with only cigarette smoke and without added oxidants have been conducted under dark conditions (Table S1). The geometric mean diameter of the primary particles from side stream cigarette smoke ranged from 160 to 400 nm. The primary NO (23-70 ppb) and CO (0.34-2.5 ppm) peak mixing ratios also varied somewhat from each cigarette-only control experiment. The emissions of CO (38.4 ± 15.8 mg per cigarette) and NO (1228 ± 334 µg per cigarette) during triplicate cigarette smoke-only experiments are similar to reported side stream emissions from previous studies for commercial cigarettes.3, 31 The variation of the primary particle size and primary gas emissions during the laboratory experiments is presumably due to different cigarette burning conditions, despite concerted efforts to maintain consistency in the experimental method. This may also highlight the degree of variability in actual emissions from smoking activities in the environment. Although primary 5 ACS Paragon Plus Environment


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gas phase species other than CO and NO were not measured in the current study, previous studies have extensively characterized the species emitted from cigarette smoke, such as nitrogenated VOCs (amines , nitroso-compounds and nitriles), aromatic hydrocarbons, alkanes, alkenes, carbonyls and chlorinated VOCs. 1, 4, 5 Many of these emitted molecules are reactive with ozone and OH (Table S3).

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Without additional oxidants, the primary particle size distribution was stable over the timescale of an oxidation experiment (a few hours) during the cigarette-only control experiments (Figure 1). After cigarette injection, the particle concentration, NO and CO mixing ratios increased rapidly in the chamber and reached maxima after a few minutes of mixing. The first-order lifetimes of CO and NO (indicated in Figure 1) were similar to the air exchange rate of the chamber (0.25 hr-1), indicating no significant reactive loss to the walls of the chamber. The shorter lifetime of particles is due to wall loss and coagulation.

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Figure 1 Time evolution of NO and CO mixing ratios (top panel), total particle number and mass (middle panel), and the size distribution in terms of number concentration for the primary cigarette smoke particles (bottom panel). First-order loss lifetimes (τ) are indicated in the figure panels in units of seconds. Time zero is when the cigarette smoke was injected to the chamber.

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Ozone Reaction with Cigarette Smoke

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In the different ozone oxidation experiments, ozone mixing ratios varied between 15 and 70 ppb and the primary particle mass loadings ranged from 185 to 1950 µg m-3, at both dry (< 5% RH) and humid (~50% RH) conditions, covering a wide range of indoor scenarios (Table S1 and Figure S11). Upon exposure to ozone, growth of the primary particle mode and the formation of ultrafine particles were both observed in all cases. Given the extremely high concentrations of primary particles (up to 1950 µg 6 ACS Paragon Plus Environment

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m-3), it is striking that formation of ultrafine particles was observed. Normally, such new particle formation and growth only occurs in relatively clean environments. The nucleation rate of the condensable products must be high enough to overcome the large condensation sink in the chamber. The very high pre-existing aerosol surface area will rapidly scavenge low volatility gases in the chamber, however the potential for nucleation to occur was clearly faster than uptake to existing condensed phase surfaces.

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Figure 2a shows the particle number size evolution during one ozone oxidation experiment as an example (see also Table S2). During this experiment, approximately 15 ppb of O3 was added into the chamber 50 min after cigarette smoke injection when primary particles (~3.2 x 104 particles cm-3, 172 µg m-3) and NO (48 ppb) were well mixed. New particle formation (~2.5 x 104 particles cm-3) and primary particle growth was observed immediately after ozone addition. Meanwhile, NO decreased rapidly and NO2 formed. The maximum increases in mass concentration for the primary (diameter > 60 nm) and secondary particles (diameter < 60 nm) are approximately 22 and 1 µg m-3, respectively. O3, 50% RH

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Figure 2 Time evolution of NOX and CO (top panel) and particle number size distribution (bottom panel) during O3 (a) and OH (b) oxidation experiments at 50% RH. (a) The light blue line shows when 15 ppb ozone was added to the chamber, which is time zero on X-axis. (b) The yellow line shows when the UV-B lights were turned on to start the photo-oxidation (i.e. time zero on X-axis). The signals between 60 and 400 nm correspond to the primary cigarette smoke particles. The signals observed below ~60 nm correspond to new particles, which were formed after the addition of O3. The decrease of CO over time is due to dilution. CO data were not available for the OH oxidation experiment.

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Sleiman et al.9 have also observed ultrafine particle formation from reaction of ozone with cigarette smoke at a higher ozone mixing ratio (110 ppb) and with different amounts of cigarette smoke in a



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larger chamber. This work confirms that ultrafine particle formation occurs with more relevant indoor air conditions (~15 ppb O3).

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We believe that growth of primary particles and the formation of ultrafine particles arise from the fast reaction between the gas-phase species in cigarette smoke and O3. Cigarette smoke contains large quantities of unsaturated organic compounds,1, 4, 5 which are reactive with ozone and are potential aerosol precursors. Nicotine, as a major component of cigarette smoke, was observed to form SOA through reaction with ozone.9 Other species such as limonene, isoprene and some other alkenes also react readily with ozone to form SOA (see a list of gaseous species found in cigarettes smoke and their ozone reaction rate constants in Table S3).32, 33 Ozone adds to electron-rich unsaturated carbon-carbon bonds and likely produces less volatile products. In addition, nitric acid from reaction of NOX (see discussion later) can react with basic compounds such as ammonia or amines in cigarette smoke to form low volatility salts, leading to particle nucleation.9, 34 Some extremely low volatility organic nitrates may play an important role in the nucleation and growth of ultrafine particles.35 Based on the observations in these experiments, the reaction products apparently have low vapor pressures that can nucleate to form new particles despite the co-existence of a large quantity of primary cigarette particles.

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In addition to reaction with organic compounds, ozone also reacts with NO in cigarette smoke to produce NO2. The lifetime for the gas-phase loss of NO due to reaction with O3 under a constant mixing ratio of 15 ppb O3 is 142 s using a second order rate constant of 1.9 x 10-14 cm3 molecules-1 s-1 at 298 K for the following reaction.36 The assumption of a constant mixing ratio of O3 likely underestimates the lifetime. In reality, the mixing ratio of O3 would decrease rapidly due to reaction with cigarette smoke.

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NO + O3 → NO2 + O2

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The exponential fit for the NO data in Figure 2a shows a lifetime of 174 ± 10 s after ozone exposure, slightly slower than reaction with O3. In Figure 2a, the initial NO was approximately 48 ppb which decreased to 10.5 ppb after adding 15 ppb of O3 into the chamber, when NO2 mixing ratio reached maximum value (12.7 ppb). Intriguingly, the amount of O3 (15 ppb) was lower than the amount of reacted NO indicating that there was likely another sink for NO. Indeed, reaction of O3 with organic compounds in the smoke can lead to radical propagation reactions (reactions 2-5). In particular, NO also reacts with RO2, and HO2 (reactions 2 and 3), which can be formed from ozonolysis of VOCs such as alkenes.37, 38 For example, reaction of one molecule of O3 with one molecule of 2-butene, an alkene emitted from cigarette smoke5, can produce sufficient radical species (e.g. RO2, HO2 and OH) to react with four NO molecules (details of the reaction mechanism are in the supporting information).37 Alkene emissions from cigarette smoke may have similar reaction mechanisms, and are summarized in Table S3.4, 5 OH radicals are expected to form from ozonolysis of alkenes and are likely involved in the evolution of cigarette smoke, although the significant difference in aerosol formation during the ozone and OH experiments (discussed in the next session) suggests that the amount of OH formed from ozonolysis and its impact on oxidation of cigarette smoke is limited.

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RO + O2 → R’=O + HO2

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The amount of NO2 formed after exposure to O3 was about 12.7 ppb, much lower than the decrease in NO (38 ppb). The missing nitrogen could be in the form of HONO and HNO3 (from reaction of NO2 on surfaces), alkyl nitrates (RONO2), peroxy nitrates (RO2NO2), nitrites (RONO), nitro-compounds (RNO2) or nitroso-compounds (RNO) (e.g. nitrosamines)13, 39, 40, all of which are potential reaction products of cigarette smoke components with NOx and HONO. Depending on their volatilities, these nitrogen containing species either contribute to particle formation or stay in the gas phase. The formation of HONO (about 7 ppb) has been confirmed with Chemical Ionization Mass Spectrometry measurements conducted during the same experiment which will be addressed in a separate publication. AMS measurements (as discussed in a later session) showed an increase of nitrate (by ~4 µg m-3, corresponding to 1.6 ppb of NO) in primary particles after O3 addition. The ultrafine particles may also contain nitrates, although the AMS is not sensitive to particles of such small size. There were reactive loss processes of NO2 in the system as evidenced by the faster loss of NO2 (first order decay lifetime τ= (6.4± 0.11) x 103 s) after NO disappeared (i.e. after there was no more production of NO2) compared to the air exchange rate (i.e. decay of CO in Figure 2a).

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Experiments with 15 and 50 ppb of ozone added after cigarette injection at dry conditions (< 5% RH) (e.g. Figure S3a and Figure S3b) observed similar NOX evolution, primary particle growth, and ultrafine particle formation. More secondary particle mass was formed after adding 50 ppb ozone compared to the experiments at 15 ppb ozone (Table S2). In other experiments, different amounts of ozone (15-70 ppb) were added to the chamber prior to cigarette injection to create an environment with well-mixed oxidant. We observed ultrafine particle formation immediately after cigarette smoke injection with different primary particle loadings (Figure S4). No particles were observed after adding ozone into the chamber in the absence of cigarette smoke during the experiments when ozone was added before cigarette smoke injection. Likewise there were no particles formed in the ozone-only control experiments (supporting information). This indicates aerosol forming species were likely not formed on the wall. Examination of all the O3 experiments (Figure 3 and Figure S5) shows that more ultrafine particles were formed when O3 mixing ratios were higher (50-70 ppb). For similar O3 mixing ratios (at 15 ppb or 50-70 ppb), there was generally more ultrafine particle formation when primary particle surface area was smaller, i.e. lower condensation sink. The number of ultrafine particles produced at ~50% RH was slightly higher than for dry experiments (< 5% RH) with similar primary particle surface area and O3, yet the dependence on RH was quite weak. Since the amount of gaseous species that can be oxidized to form ultrafine particles co-varies with the primary particle surface area (since both are derived from the same source), there was likely a higher concentration of gas phase smoke when the primary particle surface area was larger. However, this is not important in the ozone oxidation experiments, because the reaction was ozone limited (cigarette smoke was in a large excess; see a list of expected species present in the chamber and their rate constants with ozone in Table S3), so particle formation and growth should not depend on the concentration of reactive gas phase species.



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Figure 3 Plot of newly formed ultrafine particle number concentration (peak values) versus total surface area of primary particles (values just prior to oxidation or photolysis) during different experiments. Each marker represents one experiment. Circles: O3 experiments at 15 ppb (blue) and 50-70 ppb (orange) and at < 5% (solid) and ~50% (open) RH; Triangles: OH experiments at < 5% (solid green) and ~50% (open green) RH. Squares: UV-B only experiments at dry condition (< 5% RH).

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OH Reaction with Cigarette Smoke

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There was growth of the primary particle mode in every OH oxidation experiment, but ultrafine particle formation was not observed in all cases. Primary particle growth with OH radicals generally exhibited larger increase in mass and longer growth duration than with ozone (Table S2). Formation of ultrafine particles after OH exposure is more sensitive to condensation onto co-existing particle surface area (Figure 3). Figure 2b shows one OH oxidation experiment at relatively high primary particle loading and at 50% RH. We did not observe ultrafine particle formation; instead there was only growth of primary cigarette smoke particles after OH oxidation. Figure S6 shows results of OH experiments at both high and low primary particle loadings at dry conditions (< 5% RH). Ultrafine particle formation was observed only at lower primary particle loadings with relatively high OH concentrations (Figure S6b). Based on the five OH experiments in this study (Figure 3), ultrafine particle formation occurred when the total surface area was below 1.8 x 109 nm2 cm-3 (total mass concentration was 130 µg m-3). Similar to the O3 oxidation experiments, the RH had no strong influence on the results (Figure 3 and Figure S6). However, given the limited number of experiments at higher RH (Table S1), more studies are needed to test this. During particle formation and growth, there was a near instantaneous decay of NO and formation of NO2 after turning on UV-B lights. NO may be transformed to NO2, HONO, inorganic and organic nitrate with OH oxidation.

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The difference in new particle formation and primary particle growth after O3 and OH oxidative aging may in part be due to a difference in reaction mechanism and oxidation products. In particular, ozone adds to unsaturated reactive trace gases, whereas OH also participates in H-atom abstraction reactions followed by some degree of molecular fragmentation which would not necessarily lead to condensable materials. It is therefore possible that ozone oxidation of side-stream cigarette smoke forms products 10 ACS Paragon Plus Environment

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that result in a higher nucleation rate than OH reactions. For example, recent studies found ozonolysis of endocyclic alkenes such as limonene and α-pinene produces extremely low-volatility organic compounds (ELVOCs) very rapidly and with much greater yields than the OH radical-initiated oxidation.41, 42 These ELVOCs play an important role in new particle formation due to their low volatilities. In other studies, SOA formed from ozone-induced oxidation of monoterpenes has been found to be less volatile than OH-induced SOA.43 In addition, the experiments were conducted differently for the two oxidants. During the OH oxidation experiment in the presence of cigarette smoke and UV-B light, the oxidation proceeded continuously so that the primary particles were growing for a longer time as compared to O3 oxidation (Table S2). In particular, the OH concentration was in steady state and sustained for a few hours. This result was confirmed by fitting a prolonged first-order exponential decay to o-xylene (Figure S1). The ultrafine particles (Figure S6b) grew rapidly into larger sizes (>100 nm), indicating condensable species were formed in larger quantities during OH oxidation. In contrast, during the O3 oxidation experiments, O3 was only added initially and was unlikely to be produced during the reactions. Thus, O3 experiments experienced a shorter reaction time and were oxidant limited, although were observed to induce ultrafine particle formation under a wider range of conditions.

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The OH concentration in this study is on the order of 107 molecule cm-3, which is generally higher than observed indoor concentrations 22 but may still represent relevant indoor conditions under specific circumstances, e.g. during cleaning activities24 and when high concentrations of HONO are present from combustion sources such as gas stoves44 and candle burning23 or heterogeneous reactions of NO2 with various indoor surfaces.45Given that ultrafine particle formation only occurs when the condensation sink is low (i.e. low pre-existing particle loading), we believe that the formation of ultrafine particles is unlikely to occur with typical indoor OH concentrations (104-105 molecule cm-3).

334 335 336 337 338 339 340 341 342

The OH experiments were conducted with UV-B irradiation, which is rarely available in indoor environments, although we note that a number of commercial UV air cleaners using such lamps are available.46, 47 The influence of UV-B irradiance on cigarette smoke was investigated by two control experiments with cigarette smoke exposed to UV-B lights in the absence of H2O2 at low and high primary particle loadings (see Figure S7 and discussion in supporting information). In both experiments, primary particles grew under "UV-B-only” conditions (by approximately 15-22% in mass, Table S2), on the time scale of the OH oxidation experiments. There was no ultrafine particle formation even when primary particle surface area was relatively low (1.7 x 109 nm2 cm-3) under “UV-B-only” conditions (Figure S7a), different from the OH oxidation experiments (Figure S6b).

343

Aerosol Composition Change during O3 and OH Oxidation

344 345 346 347 348 349

Particle composition change was measured with an AMS during O3 and OH oxidation experiments at 50% RH (corresponding to the data in Figure 2a and Figure 2b). The AMS is not sensitive to newly-formed ultrafine particles since particles with diameters smaller than ~70 nm are not transmitted efficiently within the instrument.48 Therefore, measurements of compositional change only reflect changes in the primary smoke particles. A higher uncertainty is likely associated with the nitrogen-containing fragments of the aerosol because the AMS is unable to fully resolve the signals when detected as a mixture with



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other oxygenated and reduced organic compounds. Figure 4 thus only shows the trend of change for each fragment family.

352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367

After O3 oxidation (Figure 4a), there was an increase of CxHyO ion fragments, a small increase of CxHyO>1 ion fragments and a small decrease of CxHy ion fragments. A decrease in the CxHy family was more visible after a longer reaction time (Figure S9). The CxHyO and CxHyO>1 signals reached a maximum value about 2 and 4 min after adding O3, respectively. This result is consistent with O3 being depleted rapidly in the system and with oxidation ending quickly. The nitrate signal rapidly increased (for ~2 min) after O3 addition, which coincided with the increase of the reduced nitrogen NxHy signal, suggesting the presence of ammonium nitrate or aminium nitrate salts that have been observed in SOA from ozone reaction with amines and cigarette smoke.9, 49 The rapid initial increase of nitrate was possibly due to radical reactions before O3 has been consumed, which was on a similar timescale as the increase of the CxHyO family and NO2 in the gas phase. The nitrate signal continued to increase slightly, possibly due to reaction of NO2 on particle surfaces, which produced nitrate after O3 had been consumed. With a softer ionization technique compared to the electron impact ionization approach used in the current study, Sleiman et al.9 have observed an increase of high molecular weight products after oxidation of secondhand smoke with ozone, indicating possible formation of oligomeric species and nitrogenated by-products. Similar species such as oligomeric species and nitrogenated compounds are likely formed in the present work at lower ozone mixing ratios.

368 369 370 371 372 373 374 375

An increase of CxHyO and CxHyO>1 mass fragments and a decrease of CxHy and CxHyNz fragments were observed during OH oxidation (Figure 4b), indicating oxidation of reduced organic compounds and formation of oxygenated species. Nitrate and NxHy family signals increased rapidly after turning on the lights. The “UV-B-only” control experiment in the absence of H2O2 (Figure S7) also showed an increase of nitrate signal after UV-B irradiation. Formation of nitrate may be from reaction of NOX in the gas phase with OH, RO2 or RO radicals. By comparing the panels in Figure 4, there was a more dramatic composition change in the OH experiment than in the ozone experiment, likely due to the prolonged exposure of the cigarette smoke to OH radicals.

200 5 0

CxHy CxHyNz CxHyO CxHyO>1 CxHyONz CxHyO>1Nz NxHy nitrate

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-3

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-00:15

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3

organic family mass fraction nitrate equivalent mass -3 (µg m )

-3

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family NxHy (µg m )

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nitrate mass (µg m )

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family NxHy (µg m )

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(b)

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nitrate mass (µg m )

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UVB on

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377 378 379 380 381 382

Figure 4 Time profiles of different ion fragment families measured with the AMS showing aerosol composition change after oxidation by O3 (a) and OH (b) at 50% RH, corresponding to experiments in Figure 2a and 2b, respectively. Top panels are nitrate mass and nitrate equivalent mass concentrations for different fragment families. Bottom panels are mass fractions for each fragment family. The different colors indicate the families of fragments detected by the AMS. Time zero on X-axis shows when the reaction started.

383

Influence of Fluorescent Lights on Cigarette Smoke Particles and NOX

384 385 386 387 388 389 390 391 392 393 394

After turning on the fluorescent lights (Figure S10), NO started to decrease, but at a slower rate than in the previous experiments and NO2 started to form. The larger decrease of NO compared to the increase of NO2 implies formation of other nitrogen-containing species. Formation of NO2 suggests the presence of OH, HO2 or RO2 radicals which are able to convert NO to NO2 and likely to HONO (data to be published) as well. According to the irradiance measurement in Figure S2, the emission wavelength of the fluorescent light is mainly between 400 and 700 nm, in the visible range of the spectrum, with a small amount of UV radiation. Studies have suggested that photon fluxes from indoor fluorescent lamps may be sufficient to photolyze HONO, formaldehyde (HCHO), acetaldehyde (CH3CHO) and initiate HOX chemistry.25 Cigarette smoke contains HONO, HCHO and CH3CHO,4 and HOx is potentially produced upon irradiation. The production rates of HOx under fluorescent light irradiation were estimated and discussed in the supporting information (Table S4).

395 396 397 398 399

There was no change in particle size distribution and concentration after turning on the fluorescent light bulbs. No ultrafine particles were formed, consistent with low radical concentrations. The much slower NO decay as compared to that for the OH or O3 oxidation experiments or the UV-B-only experiments (see HOx production rate in Table S4) also suggests lower radical concentrations under indoor fluorescent light irradiation.

400

Environmental Implications

401 402 403 404 405 406 407

These experiments employed concentrations of cigarette smoke particles and ozone that are realistic for smoking events in indoor environments. Ultrafine particle formation and changes in NOX chemistry are expected to occur with ozone concentrations typical of indoor environments. Specifically, more ultrafine particles will arise with higher ozone exposure and with lower primary particle abundance (see Figure 3). The precise conditions that will lead to this phenomenon need to be carefully evaluated, however. For example, high ozone likely occurs with open windows, in which case the time for reaction with smoke VOC precursors may also be shortened.

408 409 410 411 412 413 414

The OH oxidation chemistry in a typical indoor environment will be considerably slower than in the present study due to lower OH levels (~105 molecules cm-3). However, the influence of the high condensation sink on secondary ultrafine particle formation will still prevail with typical indoor OH concentration. This will limit the likelihood of ultrafine particle formation with OH, except for specific situations arising from cooking or cleaning that may have significantly elevated OH concentrations. With both ozone and OH, there will be an increase in the mass of the primary cigarette aerosol due to oxidation of gas-phase species followed by gas-to-particle partitioning. Oxidation will lead to fewer 13 ACS Paragon Plus Environment


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reduced hydrocarbon components of the aerosol, and the formation of oxygenated and nitrogencontaining species in the particles.

417 418 419 420

Formation of NO2, and as a result HONO, is likely to occur in real indoor environments when cigarette smoking takes place with low light intensity associated with fluorescent lights. Our findings suggest that indoor lights will neither directly lead to formation of ultrafine particles nor to more primary particle mass.

421 422 423 424 425 426 427 428 429 430

Cigarette smoke is well known to have harmful health effects on both smokers and people exposed to second-hand smoke. We have observed a mass increase of primary particles after oxidative aging with both OH radicals and ozone, while ozone can also lead to formation of indoor ultrafine particles. For both the primary particles and the newly-formed ultrafine particles from second-hand smoke, health effects could arise after the particles are inhaled. Although the majority of mass increase occurred to the primary particles during cigarette smoke oxidation, exposure to the newly formed ultrafine particles may lead to significant health impacts due to the smaller particle size and higher deposition efficiencies in the lower respiratory tract.50 Further research is needed to determine if the oxidation of cigarette smoke introduces negative effects to human health beyond those that arise from the primary emissions alone.

431

Acknowledgement

432 433 434

We acknowledge funding support from the Alfred P. Sloan Foundation and from Natural Sciences and Engineering Research Council of Canada (NSERC). M. Antiñolo would like to thank UCLM (Plan Propio de Investigación) for funding.

435

Supporting Information

436 437 438 439 440 441

The supporting information contains information about: Summary of all experiments (Table S1 and S2, Figure S11); OH concentration measurement (Figure S1); Irradiance spectrum measurements(Figure S2); Details of control experiments; Chemical species in cigarette smoke (Table S3); Other O3 oxidation experiments (Figure S3, S4, and S5); Other OH oxidation experiments (Figure S6); UV-B experiments (Figure S7 and S8); AMS data for O3 oxidation experiment (Figure S9); Indoor fluorescent light experiments (Figure S10); HOx production rates (Table S4).


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