Secondary Organic Aerosol Formation from m-Xylene in the Absence

Sep 22, 2007 - Bourns College of EngineeringsCenter for Environmental. Research and ... Riverside, California 92521, and Department of Chemical and...
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Environ. Sci. Technol. 2007, 41, 7409-7416

Secondary Organic Aerosol Formation from m-Xylene in the Absence of NOx C H E N S O N G , †,‡,§ K W A N G S A M N A , † BETHANY WARREN,‡ QUENTIN MALLOY,‡ AND D A V I D R . C O C K E R I I I * ,†,‡ Bourns College of EngineeringsCenter for Environmental Research and Technology (CE-CERT), University of California, Riverside, California 92521, and Department of Chemical and Environmental Engineering, Bourns College of Engineering, University of California, Riverside, California 92521

Formation of secondary organic aerosol (SOA) from m-xylene photoxidation in the absence of NOx was investigated in a series of smog chamber experiments. Experiments were performed in dry air and in the absence of seed aerosol with H2O2 photolysis providing a stable hydroxyl radical (OH radical) source. SOA formation from this study is exceptionally higher than experiments with existence of NOx. The experiments with elevated HO2 levels indicate that organic hydroperoxide compounds should contribute to SOA formation. Nitrogen oxide (NO) is shown to reduce aerosol formation; the constant aerosol formation rate obtained before addition of NO and after consumption of NO strongly suggests that aerosol formation is mainly through reactions with OH and HO2 radicals. In addition, a density of 1.40 ( 0.1 g cm-3 for the SOA from the photooxidation of m-xylene in the absence of NOx has been measured, which is significantly higher than the currently used unit density.

Introduction Aromatic hydrocarbons, an important class of VOC emitted into the atmosphere, including benzene, toluene, xylenes, and trimethylbenzenes, have been widely investigated in smog chamber studies (1-4). Experimental results have demonstrated that these aromatic compounds have significant individual SOA formation potential (5-10). However, the chemical composition of SOA as well as the chemical reaction mechanisms leading to formation of SOA are still far from completely understood (11, 12). Ongoing research efforts have helped to improve our understanding of the SOA formation from aromatic hydrocarbons. Song et al. (8) demonstrated that SOA formation from the photooxidation of m-xylene was promoted at low NOx levels and inhibited at high NOx levels. A similar trend was observed by MartinReviejo and Wirtz (10) for the benzene-NOx photooxidation system. However, neither study was able to conclusively * Corresponding author phone: 909-781-5695; fax: 909-781-5790; e-mail: [email protected]. † Bourns College of EngineeringsCenter for Environmental Research and Technology (CE-CERT). ‡ Department of Chemical and Environmental Engineering, Bourns College of Engineering. § Now affiliated with Atmospheric Science and Global Change Division, Pacific Northwest National Laboratory; Phone: 509-3724985. 10.1021/es070429r CCC: $37.00 Published on Web 09/22/2007

 2007 American Chemical Society

elucidate the chemical mechanism for the NOx effect on SOA formation. Nevertheless, both studies provide evidence that the OH radical played a much more important role in SOA formation from aromatic hydrocarbons than ozone or the nitrate radical. Johnson et al. utilized a detailed atmospheric gas-phase chemical mechanism (MCM v3.1) (13) to simulate SOA formation from photooxidation of toluene at varied NOx levels. Their results suggested that peroxyhemiacetals formed from reactions of organic hydroperoxides with aldehydes in the condensed organic phase significantly contributed to the SOA production under low NOx level conditions. The formation of organic hydroperoxides, at high NOx concentrations, could be considerably reduced, leading to lower SOA production. Such aerosol-phase reactions, which combine constituents with high vapor pressure together to form less volatile products, can bring highly volatile carbonyl products (such as glyoxal and methylglyoxal) into the condensed phase and thereby increase SOA mass. Jang et al. (14) suggested that aerosol-phase reactions involving aldehydes and ketones could proceed in the condensed phase of acidic aerosol and result in an increase of SOA mass. The aerosol-phase reactions proposed include polymerization, hydration, hemiacetal and acetal formation, and aldol condensation. Products from aerosol-phase reactions have been measured in SOA from several studies. Tobias and Ziemman (15, 16) observed peroxyhemiacetals in SOA generated from the ozonolysis of 1-tetradecene using a thermal desorption particle beam mass spectrometer. Kalberer et al. (5) identified polymers from photooxidation of 1,3,5-trimethylbenzene and NOx and showed that polymers contributed to a large fraction (50%) of the total SOA mass. However, a recent study by Barsanti and Pankow (17) argued from a thermodynamics perspective that aerosol-phase reactions for atomospheric aldehydes and ketones, such as hydration, polymerization, hemiacetal/acetal formation, and adol condensation, are not likely favored either in the atmosphere or in an acidic environment. A more complete molecular identification of SOA products of aromatic hydrocarbon photooxidation would help solve these puzzles; however, this has not been accomplished with current analytical techniques. In this work, we present a series of experiments mainly focused on the photooxidation of m-xylene without participation of NOx. Our goal is to maximize the possible formation of peroxyhemiacetals from the reaction of organic peroxides with OH radical in the condensed phase by eliminating the reaction channels involving NO, which may show different SOA yield profiles as classic m-xylene/NOx experiments did. Hydrogen peroxide (H2O2) was photolyzed to produce OH within the reaction system. Additional experiments were performed with addition of CO and NO to gain further insight into aromatic SOA formation processes.

Experimental Section All experiments were carried out in the UCR Chamber Facility, which has been described in detail elsewhere (18). All experiments were carried out under dry conditions (RH < 0.1%) and in the absence of inorganic seed aerosol, while temperature was kept at 27 °C. Prior to irradiation, m-xylene and hydrogen peroxide were each gently evaporated at 50 °C in a pure N2 stream and were directly injected into the chamber. For experiment 02/12/06B and 02/20/06A, NO at known concentration was injected from a glass bulb and was flushed into the chambers by a pure N2 stream; for experiments 01/30/06B and 02/04/06A, CO was directly injected from a gas cylinder, with concentration determined VOL. 41, NO. 21, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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by the flow rate (cc/min) and injection duration. m-Xylene concentration was monitored directly from the reactors by a Hewlett-Packard 5890 (Palo Alto, CA) gas chromatograph (GC) equipped with a DB-5 column and a flame ionization detector (FID). NOx and CO were monitored by a Thermo Environmental Instruments model 42C chemiluminescence NOx analyzer and an Environmental Corp. model 48C CO analyzer, respectively. Irradiation (blacklights) commenced after the chamber was well mixed and the initial concentrations of all reactants (except for H2O2) were determined. The size distribution and number concentration of SOA were monitored using a scanning mobility particle sizer spectrometer (SMPS) (19) located inside the temperaturecontrolled room to keep the temperature of sheath and aerosol sampling flows the same as that inside the chambers. An aerosol particle mass analyzer (APM; 20) from Kanomax (model APM-10) was used to measure aerosol density of SOA generated inside the chamber using a method similar to that developed by McMurry et al. (21) (see Supporting Information for the details of operating APM and calculating the particle density).

Results and Discussion The initial conditions for all experiments are summarized in Table S1 (see Supporting Information), which includes the initial concentrations of m-xylene (HCo) and the initial concentrations of H2O2. The initial concentration of H2O2 was estimated from the simulation of m-xylene decay of each experiment using SAPRC-99 (22). For experiments 01/30/ 06B, 02/04/06A, 02/12/06B, and 02/20/06A, the initial CO and NOx concentrations are also included. Three different types of experiments were performed in this study, each with different objectives. The first type of experiment was to observe SOA formation in the absence of NOx, which would be compared with our previous study (8) in which experiments of m-xylene photooxidation were carried out in the presence of NOx. The second type of experiment was also performed without NOx, but included CO to artificially increase HO2 radical concentration within the chamber. For the third type of experiment, NOx was introduced into the smog chamber after obvious aerosol production was observed, which provided additional insight into the impact of NOx on SOA formation. SOA Density. As listed in Table S2, density of SOA from this study ranged from 1.40 to 1.41 g cm-3, significantly higher than the unit density (1 g cm-3) that is assumed to convert organic aerosol volume concentration to mass concentration for most smog chamber studies (7, 23, 24). Unit density was first estimated by Wang et al. (24) for SOA produced from photochemical oxidation of 1-octene based on measured aerosol-phase oxidation products. Following the same method, Kalberer et al. (25) estimated a density of 1.4 g cm-3 for aerosol formed from the ozonolysis of cyclohexene using the detailed chemical speciation of SOA products collected on filters. Martin-Reviejo et al. (10) measured density of SOA using a combination of TEOM and SEMS. They reported values of 1.35 and 1.55 g cm-3 for the densities of SOA from benzene photooxidation with and without the presence of NOx respectively, and the results are comparable with the density of SOA generated from m-xylene of this study. The results of this work and previous work indicate that unit density is apparently not appropriate for many systems and may result in large discrepancies when estimating mass based aerosol yields from particle mobility data. Therefore, it is imperative to carefully evaluate the densities of SOA produced when estimating aerosol mass production from aerosol precursors. The APM provides a simple and feasible measurement method to determine SOA density from chamber experiments. 7410

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TABLE 1. Experimental Results experiment

Mo (µg/m3)

∆HC (µg/m3)

Y (yield)

average OH (106 molecules/cm3)

05/19/05B 01/30/06A 02/04/06B 02/11/06A 02/12/06A 02/22/06A 02/22/06B 02/24/06B

17.3 11.9 38.0 76.0 87.3 20.0 45.0 25.6

192.9 96.6 226.9 272.4 288.2 113.2 147.3 124.3

0.089 0.123 0.167 0.279 0.303 0.177 0.306 0.206

0.55 1.05 0.84 1.14 1.20 0.80 1.29 1.18

SOA Yield in the Absence of NO and CO. Table 1 summarizes the concentrations of m-xylene consumed (∆HC), the mass concentration of SOA produced (∆Mo), the SOA yield (Y), and the average hydroxyl radical (OH) concentrations for experiments in the absence of NO and CO (except for experiments 05/19/05A, 05/20/05A, and 05/ 25/05A which were only used to measure the density of SOA). The average OH radical concentration is defined as the integrated OH radical concentrations during an experiment divided by the total time of that experiment and is calculated from the decay of m-xylene during experiment using leastsquares analysis. The calculated average OH radical concentration (5.5 × 105 to 1.3 × 106 molecules/cm3) is relatively low compared with typical polluted urban atmospheres (for instance, diurnal OH radical concentration was 5-20 × 106 molecules/cm3 in summer 2001 during a field measurement campaign in New York; 26), but still within the same magnitude of the theoretically calculated global OH concentration in the troposphere (2 × 105-106 molecules/cm3; 27). Figure 1 shows typical temporal profiles of m-xylene concentration and aerosol volume concentration from photooxidation of m-xylene in the absence of NOx. Unlike traditional smog chamber experiments in the presence of NOx, SOA formation is nearly linear. SOA yield is defined as the mass fraction of aerosol produced (∆Mo, µg m-3) from hydrocarbon reacted (∆HC, µg m-3), calculated as: Y ) ∆Mo/∆HC. SOA yield has been used to predict ambient SOA formation from experimentally obtained smog chamber data (28, 29). The SOA model commonly used is the two-product semiempirical equilibrium partitioning model described in detail elsewhere (23, 30). Experimental data are fit to an aerosol yield curve using four parameters (R1, R2, Kom,1, Kom,2) which represent two theoretical oxidation products, one of lower and one of higher volatility, that participate in the gas-particle equilibrium. The generic form of the aerosol yield equation is provided below for i species: 2

Y ) ∆Mo

RiKom,i

∑1+K i)1

om,i∆Mo

(1)

where Ri is the mass stoichiometric factor of compound i that is formed and Kom,i(m3 µg-1) is the partitioning coefficient in terms of the organic mass concentration. Figure 2 shows the SOA yield as a function of aerosol mass produced for experiments listed in Table 1. These experiments were divided into two groups based on initial m-xylene concentration. One group consisted of 3 experiments with initial m-xylene concentration of about 50 ppb; the other group consisted of 4 experiments with initial m-xylene concentration of about 100 ppb. Also shown in Figure 2 are two yield curves from our previous study that were generated from experimental data of photooxidation of m-xylene and NOx using eq 1. The two yield curves represent SOA yield of different NOx levels: high, defined as

FIGURE 1. Typical temporal profiles of hydrocarbon and aerosol volume concentration from photoxidation of m-xylene in the absence of NOx. Data are selected from experiment 02/12/06A.

FIGURE 2. SOA yields from m-xylene photooxidation in the absence of NOx as a function of Mo. Solid line and dashed line are yield curves from Song et al. (8), which represent low and high NOx regime, respectively. ppbC HC/NOx > 8.0 and low, defined as ppbC HC/NOx < 5.5 (8). It should be noted that, in the present study, only densities of aerosol produced from m-xylene in the absence of NOx were measured and that the densities may be different from those of experiments involving NOx (10). Therefore, to facilitate comparison of SOA yields between this study and our previous study and other studies in which m-xylene was photolyzed with the presence of NOx, unit density was still utilized for all experiments to convert aerosol volume concentration to the mass concentration listed in Table 1. SOA yield of experiment with the absence of NOx is controlled by two parameters: the initial hydrocarbon concentration and the OH radical levels. For experiments with similar initial m-xylene concentrations, SOA yields are proportional to the OH radical concentrations and follow a monotonically increasing trend. Unlike the yield curve for experiments with NOx, the SOA yields of experiments with similar initial hydrocarbon concentration fall into a straight line and show no evidence of plateau at high aerosol mass. Experiments with lower initial hydrocarbon concentration exhibit a higher SOA formation rate than that of experiments with higher initial hydrocarbon concentration. Experiments with 50 ppb initial m-xylene have a steeper SOA formation rate than that of experiments with 100 ppb initial m-xylene

(Figure 3). Each curve in Figure 3 represents a single experiment. At similar OH radical levels, after the obvious appearance of aerosol, experiments with lower initial hydrocarbon concentration form more aerosol for a given reacted hydrocarbon concentration. For instance, experiment 02/22/06A has similar OH radical concentration to that of experiment 02/04/06B, but its formed aerosol mass concentration at a fixed reacted hydrocarbon concentration is higher. On the other hand, at similar initial hydrocarbon concentrations, experiment with higher OH radical concentration generates more aerosol for a given reacted hydrocarbon concentration (Figure 3). SOA yields of experiments without NOx present range from 0.089 to 0.302 (Figure 2) and greatly surpass those obtained from experiments with the presence of NOx. The highest yields obtained from this study are about three times higher than the estimated value from the yield curve for the same amount of formed aerosol mass concentration with NOx present. The huge discrepancy in SOA yield between experiments with and without the presence of NOx further confirms the previous observation that the existence of NOx negatively effects SOA formation from m-xylene photooxidation, or that the reactions through the NOx channels have significantly lower SOA formation potential than the reactions through VOL. 41, NO. 21, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 3. Aerosol mass concentrations as a function of reacted hydrocarbon concentration. Each curve represents a single experiment. The solid symbols represent experiments with initial hydrocarbon concentration of 50 ppb; the open symbols represent experiments with initial hydrocarbon concentration of 100 ppb. The OH concentration is represented as × 106 molecules/cm3. the OH radical. SOA yields from this study were obtained at fairly low OH radicals compared to those calculated from experiments in the presence of NOx, which indicates that the difference in SOA yield with and without NOx may further widen if hydroxyl radical levels were brought up to the levels present in the experiments performed in the presence of NOx. While there may exist other possibilities, this extraordinarily high SOA yield could be related to the enhanced formation of compounds with very low vapor pressures from the aerosol-phase reactions under NOx free conditions. Aerosol from Secondary Product. As pointed out by previous researchers (31, 32), SOA from aromatic hydrocarbon photooxidation is more likely generated from secondary or further generation reactions. Although aerosol-phase reactions may also contribute to the formation of SOA (2, 5), detailed reaction products and the chemical mechanisms leading to forming these products are still unclear. Current model simulations of SOA from aromatic hydrocarbons usually postulate SOA products based on the gas-phase reaction mechanisms (13, 28). These simulated SOA products are mainly oxidation species with multiple functional groups from reactions of second or higher generation products and OH radical. It is noted that in the presence of NOx, products with NO2 group and products from reactions of intermediate products and NO3 radical could also contribute to SOA. Hurley et al. (32) have developed an analytical method to simulate SOA profiles from toluene/NOx photooxidation assuming that SOA is generated from secondary reaction and tried to explain NOx effect on SOA formation. Here we adopt Hurley et al.’s method to analyze SOA formation without the presence of NOx. Our purpose is not to accurately describe the aerosol profiles of m-xylene photooxidation; instead, it is proposed to capture key features of SOA formation from the series of experiments we have performed, which may in turn provide some key insights into SOA formation from this system. In Hurley et al.’s analysis, only oxidations through reactions with OH radical were considered:

HC + OH f RP + products; P + OH f S;

k2

k1

(R1) (R2)

where HC is the parent hydrocarbon, P is the primary product that can be further oxidized, S is the secondary product that can partition between gas and aerosol phase, and R is the 7412

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yield of product P. Since reations with NO3 could also be important, this assumption is likely inappropriate in the presence of NOx and O3. However, in the case of this study, it is more reasonable to use the simplified mechanism. We also assume that the entire primary product is converted to the secondary product:

{

[S] ) Rx -

(1 -R k)(1 - x)[(1 - x)

(k-1)

}

- 1] [HC]o

(2)

where x is defined as the conversion of parent hydrocarbon, ∆[HC]/[HC]o and k is the rate constant ratio, k2/k1. Equation 2 clearly states that the secondary product is a function of the initial parent hydrocarbon concentration and its reacted portion. Figure 4 shows the secondary product profile as a function of reacted hydrocarbon generated from eq 2 using the values R ) 0.3, k ) 10, and varying [HC]o (arbitrary unit). From Figure 4, as long as the initial parent hydrocarbon concentration is fixed, the secondary product will always follow the same curve no matter how the OH radical level varies. Using a similar analysis for tertiary or higher order reactions, one can easily draw similar conclusions. If aerosolphase products are only from reactions of intermediate products with OH radical, the observation from Figure 4 should also be applied to SOA, which indicates that SOA formation rate will be the function of initial and reacted hydrocarbon concentrations only. However, the observed SOA profiles with similar initial parent hydrocarbon concentration varied with the OH radical levels (Figure 3): the higher the OH radical level, the higher the rate of SOA formation. Although the above model analysis is highly simplified, it does capture the chemical kinetics that the production rate of secondary or higher generation products is not dependent on OH radical level. Since H2O2 was utilized as OH radical source in this study, the dependence of SOA profile on OH radical level is more likely the dependence on H2O2 level. Thus, the difference between Figures 4 and 5 strongly indicates that reactions of OH radical and intermediate products are not the only sources for SOA; other reactions should also be involved in the SOA-producing process and these additional reactions should be dictated by H2O2 levels. Such reactions could be in the gas phase or in the aerosol phase. One of the candidate reactions is the aerosol-phase reactions involving organic hydroperoxides species as mentioned previously. During the photolysis of

FIGURE 4. Secondary product as a function of reacted hydrocarbon calculated from eq 2. The values used are r ) 0.3, k ) 10, and initial hydrocarbon concentrations of 25, 50, and 100 in arbitrary units.

FIGURE 5. (a) Comparison of aerosol mass concentration as a function of reacted hydrocarbon concentration for experiment 01/30/06A (initiated with 39 ppb m-xylene and 470 ppb H2O2) and 01/30/06B (initiated with 39 ppb m-xylene, 1200 ppb H2O2, and 15 ppm CO). (b) Comparison of aerosol mass concentration as a function of reacted hydrocarbon concentration for experiment 02/04/06A (initiated with 112 ppb m-xylene, 2400 ppb H2O2, and 25 ppm CO) and 02/04/06B (initiated with 113 ppb m-xylene and 950 ppb H2O2). H2O2, other than OH radical, HO2 radical is also generated (33) and its concentration is strongly dictated by the H2O2 level. As organic peroxides are formed from the reactions of organic peroxy radicals with HO2 radical, the higher H2O2 level should lead to higher organic peroxides formation rate. A simple simulation using SAPRC-99 (22) (Figures S2 and S3, see Supporting Information) supports our above analysis of HO2 and organic peroxides. Therefore, the SOA profiles (Figure 3) may be related to the level of organic peroxides present.

Effect of Organic Hydroperoxides on SOA Formation. Since organic hydroperoxides might be important precursors of SOA, two sets of experiments (01/30/06A, 01/30/06B; and 02/04/06A, 02/04/06B) were designed to gain experimental evidences to show the effect of organic hydroperoxides on SOA formation. Each set of experiments had the same initial m-xylene concentrations with different initial H2O2 concentrations. CO was added into the experiment with higher H2O2 concentration. The reaction of CO with OH radical would raise the HO2 radical concentration. CO and H2O2 concenVOL. 41, NO. 21, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 6. Profile of aerosol mass concentration as a function of reacted hydrocarbon concentration for experiment 02/20/06A with addition of 80 ppb NO after aerosols appear. trations were carefully adjusted so that the temporal decay rate of m-xylene would be similar for both experiments. As a result, both experiments had the same OH radical levels with one experiment (with CO) having higher HO2 concentration. Since the higher HO2 concentration could lead to higher efficiency of producing organic hydroperoxides, the experiment with CO is expected to produce more aerosols per reacted hydrocarbon. Figure 5a shows the temporal m-xylene decay profiles and aerosol mass concentration profiles as a function of reacted m-xylene concentrations for experiments 01/30/06A and 01/30/06B with similar initial m-xylene concentration of about 39 ppb. Experiment 01/30/06A has a slightly higher m-xylene decay rate than 01/30/06B (with CO), due to the excess amount of CO in 01/30/06B. Figure 5a shows that although OH concentration is higher in experiment 01/30/ 06A, the aerosol production rate is obviously higher for 01/ 30/06B. In Figure 5b, m-xylene decays for experiments 02/ 04/06A and 02/04/06B (both experiments had 112 ppb initial m-xylene) exhibit the same trend with larger aerosol formation rates seen in 02/04/06A with CO present. The differences of aerosol production in Figure 5a and b are about 20% and 25%, respectively. Since HO2 radical measurements were not available for this study, SAPRC-99 (22) was utilized to estimate average HO2 concentrations during the experiments. Simulated HO2 concentrations of experiments 01/30/06B and 02/ 04/06A are about 3.3 and 3.8 times higher than corresponding HO2 concentrations of experiments 01/30/06A and 02/04/ 06B, respectively. Figure 5a and b clearly shows that elevated HO2 radical level can promote the SOA production efficiency. From the current knowledge of chemistry of aromatic hydrocarbons, the only way for HO2 radical to participate in the gas-phase chemistry related to SOA formation would be to oxidize organic peroxy radicals to form organic hydroperoxides. Organic hydroperoxides could then further react with OH radical in the gas phase or react with aldehyde species in the aerosol phase to form compounds with lower vapor pressures that could enhance the formation of SOA. The experimental evidence included in this section is still not able to provide direct evidence of the occurrence of aerosol-phase reaction; however, it should be strong enough to state that organic hydroperoxides are one of the key precursors of SOA formation in the m-xylene photooxidation system. NOx Effect on SOA Formation. In the environment where NOx concentration is greater than 0.5 ppbv, peroxy radicals predominantly react with NO, suppressing the formation of organic hydroperoxides (34). Thus, during classic smog chamber experiments with the initial presence of NOx, 7414

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formation of organic hydroperoxides is not expected to be significant until NO is depleted, which is consistent with observations that aerosols usually do not first appear until NO has been mostly consumed in aromatic photooxidation chamber experiments. In this section, we will attempt to display the direct effect of competitions between NO and HO2 and OH radicals on SOA formation. Therefore, a different type of chamber experiment was conducted (experiment 02/ 20/06A) where the reaction began with only m-xylene and H2O2 initially present and then 70 ppb NO was added to the chamber only after aerosol formation commenced. Figure 6 shows the aerosol mass concentration as a function of m-xylene consumed for experiment 02/20/06A. The slope of the curve can be viewed as an aerosol formation rate (aerosol mass formed per unit hydrocarbon reacted). Before injecting NO, a constant aerosol formation rate was observed (shown as a straight line fit). The addition of NO immediately and significantly reduced the rate of aerosol formation, indicating that NO dominates the intermediate species required for SOA formation at this stage. The aerosol formation rate is then seen to increase as NO concentration decreases. Finally, a constant aerosol formation rate appears when the entire NO has effectively been converted to NO2. The aerosol formation rates before the addition of NO and after the depletion of NO are nearly unchanged (0.147 vs 0.153). This provides evidence that different chemical reaction channels are competing with each other for the intermediate compounds required for SOA production. The presence of NO seems to have considerably lowered the SOA formation potential by competing with other channels such as OH channel and HO2 channel. Furthermore, it indicates that the NO channel leads to insignificant aerosol production. It was reported that organic nitrate compounds, normally formed through reactions of organic peroxy radicals and NO, have been identified in organic aerosol from photooxidation of aromatic hydrocarbons (2) and have even been considered as major SOA compounds in a few studies (35). During these studies, high concentrations of NOx are usually employed, which may greatly facilitate the reactions through NO channel and the formation of organic nitrate compounds. Under low NOx conditions, as discussed here, organic nitrate compounds may be formed only at a very low level and not be a major contributor to SOA. Due to the addition of NO, a considerable amount of O3 is formed in the reaction system, as shown in Figure S4 (see Supporting Information). NO3 radical should also be built up through the reaction of O3 and NO2. However, the fact that aerosol formation rate remains the same before the addition of NO and after the consumption of NO suggests that O3 and NO3 radical have no effect on SOA formation of

m-xylene photooxidation, consistent with observations from our previous study (8). For a series of m-xylene/H2O2 photooxidation experiments, we showed experimental evidence that HO2 radical is associated with SOA generation possibly through the formation of organic hydroperoxides that may give rise to the production of less volatile species. It is also demonstrated that the presence of NO greatly hinders SOA formation by apparently consuming the SOA related intermediate compounds through reactions with low SOA production. In the polluted urban atmosphere where NOx is usually kept at relatively high levels, SOA formation from aromatic hydrocarbons should be greatly suppressed and the fraction of SOA from HO2 related reactions should be insignificant. As the urban plume is transported to a clean region and NOx concentrations drop to sufficiently low level by reactions with HO2, OH, and peroxy radicals, reactions with HO2 and organic hydroperoxides become an important contributor to SOA formation. Organic hydroperoxides have been identified from SOA formed from ozonlysis of monoterpenes by Docherty et al. (36) and the aerosol-form organic hydroperoxides are believed to be part of peroxyhemiacetals. To support our observation in this study, it is imperative that similar techniques are utilized to acquire the information of the existence of organic hydroperoxides in SOA from photooxidation of aromatic hydrocarbons.

Acknowledgments We gratefully acknowledge funding support from National Science Foundation grant ATM-0234111 and ATM-0449778. We thank Kurt Bumiller, Irina Malkina, and Jean Wang for help with experiment setup and measurement, and William P. L. Carter for helpful discussions. We also gratefully acknowledge Tyler J. Beck (TSI Incorporated, Shoreview, MN, ETATS-UNIS) and Atsushi Kano (Kanomax USA, Inc.) for loaning the APM-10 for this study.

Supporting Information Available Initial conditions of experiments (Table S1), particle density measurement by Kanomax APM-10, APM measurements of PSL and SOA (Figure S1), measured SOA density (Table S2), simulated temporal HO2 radical profile (Figure S2) and organic hydroperoxides profile (Figure S3) using SAPRC-99, and temporal profiles of O3, NO, and hydrocarbon (Figure S4). This material is available free of charge via the Internet at http://pubs.acs.org.

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Received for review February 19, 2007. Revised manuscript received June 26, 2007. Accepted July 11, 2007. ES070429R