Effect of Ammonia on Secondary Organic Aerosol Formation from α

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Environ. Sci. Technol. 2007, 41, 6096-6102

Effect of Ammonia on Secondary Organic Aerosol Formation from r-Pinene Ozonolysis in Dry and Humid Conditions K W A N G S A M N A , † C H E N S O N G , †,‡ CAMERON SWITZER,† AND D A V I D R . C O C K E R , I I I * ,†,‡ Center for Environmental Research and Technology (CE-CERT), Bourns College of Engineering, University of California, Riverside, California 92521, and Department of Chemical and Environmental Engineering, Bourns College of Engineering, University of California, Riverside, California 92521

This study examines the influence of ammonia (NH3) on secondary organic aerosol (SOA) formation from the R-pinene/ ozone oxidation system for dry and humid conditions. Aerosol yield differed depending on which OH scavenger was used, with the highest yield noted for CO, followed by cyclohexane and 2-butanol. Number and volume concentrations were quickly increased within the reactor by 15 and 8%, respectively, when NH3 was added after the reaction ceased. The increase in number concentration indicated the formation of new particles resulting from gasto-particle conversion. Moreover, average particle size increased from 242 to 248 nm. The resulting aerosol growth was attributed to ammonium salts formed by the reaction between organic acids and NH3. When NH3 was added to aerosolized cis-pinonic acid in the environmental reactor, a dramatic increase in both number and volume concentrations of cis-pinonic acid was observed. This provides further evidence that NH3 can interact with gasphase organic acids forming condensable salts and thereby enhancing SOA formation. Initially present NH3 significantly enhanced aerosol yield in R-pineneozone reactions, regardless of the presence of water vapor. The role of NH3 on SOA formation in the dry and humid conditions is discussed in terms of a theoretical modeling study.

1. Introduction Ammonia (NH3) is a ubiquitous trace atmospheric gas found at widely varying ambient concentrations. NH3 is emitted to the atmosphere from a variety of natural processes and human activities. The South Coast Air Quality Management District estimated in California that livestock (e.g., beef cattle, dairy cows, hogs, and chickens) accounted for 33% of ammonia emissions in the South Coast Air Basin (SoCAB), followed by soil emissions (19%) and mobile emission sources (18%) (1). NH3 reacts with inorganic acids (e.g., sulfuric acid and nitric acid) in the atmosphere to produce secondary particulate species such as ammonium sulfates (NH4HSO4 and * Corresponding author phone: 951-781-5695; fax: 951-781-5790; e-mail: [email protected]. † Center for Environmental Research and Technology. ‡ Department of Chemical and Environmental Engineering. 6096

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(NH4)2SO4) and ammonium nitrate (NH4NO3), depending on the availability of NH3. These compounds are a potentially vital contributor to the atmospheric burden of particulate matter and contribute to exceedences of the PM2.5 National Ambient Air Quality Standards (NAAQS) as well as to visibility degradation. Potential removal mechanisms for NH3 include wet and dry deposition, and oxidation by hydroxyl radicals. NH3 has a weak reactivity in the atmosphere (2), so its removal in the atmosphere by chemical reactions in the gas phase may be insignificant. The gas-phase reaction between NH3 and inorganic acids (e.g., sulfuric acid and nitric acid) leading to significant secondary particulate formation has been well characterized (2). However, little work has been conducted with respect to secondary particulates formed from reaction between NH3 and organic acids (3). We speculated that the reaction of organic acids and NH3 produced condensable salts through acid-base reaction leading to enhanced SOA formation in ammonia-rich environments. Such a reaction could explain the significantly higher organic carbon loadings measured in areas such as western Riverside County and the San Joaquin Valley. However, the potential role of ammonia chemistry on SOA formation is still poorly understood. Organic acids are ubiquitous components, not only in the gas phase but also in the particle phase (4). In addition, organic acids are a major fraction of the total organic carbon in fog and cloudwater and in precipitation due to their high polarity. Formic and acetic acids are known as dominant organic gases in the gas phase. It was estimated by Keene and Galloway that these two acids may contribute between 16 and 35% of the free acidity of precipitation in the U.S. (5). Organic acids can be produced by direct anthropogenic emissions (6), biogenic emissions (7), and biomass burning (8). Organic acids like carboxylic acids are also produced during oxidation of atmospheric hydrocarbons such as alkene-ozone reactions. As a representative ozone-alkene reaction, the R-pinene and ozone reaction produces low volatility organic acids such as pinic acid (dicarboxylic acid) and pinonic acid (ketocarboxylic acid). Of its oxidation products, pinic acid is the most abundant species in both gas phase and particle phase (9). Kawamura et al. (10) reported the concentrations of C2-C10 diacids measured in the Los Angeles ambient air from July 1987 to June 1988. Oxalic acid (C2) was the most abundant diacid species followed by malonic (C3) and succinic (C4). These diacids comprised more than 80% of the total diacid concentrations (4-97 ng m-3). We speculate that gas phase organic acids could be converted into condensable salts by reacting with NH3. In other words, SOA formation may be enhanced when air mass containing organic acids moves into ammonia-rich areas. However, it is not necessary for the reaction between organic acids and NH3 to produce condensable salts. For example, both formic and acetic acids do not form condensable salts by combining with NH3. The objective of this study is (1) to determine whether the presence of ammonia can lead to enhanced SOA formation and (2) to explore how NH3 affects secondary aerosol formation potential in the absence and presence of water. The R-pinene/ozone system was chosen as this monoterpene is regarded as a significant source to biogenic secondary aerosol formation (11, 12, 13, 14), and its reaction mechanisms are well established (9, 15, 16, 17).

2. Experimental Method Experiments were performed in a dark indoor 18 m3 Teflon environmental chamber with a 2.5 m-1 surface-to-volume ratio. The chamber temperature was maintained at 21 ( 1°C. The reactor was flushed a minimum of 10 chamber volumes with purified compressed laboratory air (Aadco 737 series (Cleves, Ohio)). The chamber was irradiated for 6 h by 10.1021/es061956y CCC: $37.00

 2007 American Chemical Society Published on Web 07/24/2007

TABLE 1. Initial Conditions and Results Obtained from the r-Pinene/Ozone Reaction date 12/06/04 12/07/04 12/09/04 12/10/04 12/14/04 12/15/04 12/20/04 12/21/04 12/29/04 12/30/04 01/04/05 01/05/05 01/06/05 01/07/05 01/08/05 01/10/05 01/11/05 01/13/05 01/14/05 01/15/05 01/17/05 01/24/05 01/25/05 04/03/05 04/05/05 04/06/05 04/07/05 04/08/05 06/30/06 a

intial conc. NH3 RH temp. ∆O3 ∆HC ∆Mo yield (µg m-3) (ppb) (%) (°C) (ppb) (µg m-3) (µg m-3) (%) 205 178 564 556 746 1023 279 314 645 1092 1410 1331 1267 1213 1277 1264 1249 679 631 588 994 1258 1245 693 635 677 679 383 756

200 200

200

200 300 400 100 50 200 200

200 200

a 19.0 19.6 21.2 23.5 20.0 22.5 19.8 20.1 53 20.5 50 20.3 50 19.0 20.0 20.7 20.3 19.5 20.0 20.1 19.1 20.4 21.6 21.8 21.3 21.3 21.5 50 22.2 21.6 50 21.4 21.7 23.3

25 28 50 59 65 100 31 38 65 137 168 157 151 135 148 150 150 65 86 93 95 122 130 53 59 56 51 34 80

204 166 562 529 741 1006 278 274 643 1065 1361 1304 1245 1162 1192 1168 1150 671 629 586 964 1243 1229 690 633 676 678 382 755

46 55 227 283 343 486 92 128 249 455 626 696 813 818 823 748 694 301 334 290 474 667 667 332 333 374 273 143 377

22.5 33.1 42.0 53.5 46.3 48.3 33.1 46.7 38.7 42.7 46.0 53.4 65.3 70.4 69.0 64.0 60.3 44.9 53.1 49.5 49.2 53.6 54.3 48.1 52.6 55.3 40.3 37.4 49.9

RH < 2%.

64 blacklights to aid in the cleaning the chamber. No particles were detected in the clean chamber air after addition of excess ozone or after blacklight irradiation. The R-pinene was injected into a small glass tube using a microliter syringe and was evaporated into a 5 L min-1 N2 streamflowing through a heated glass manifold. Ozone was injected by flowing 2 L min-1 purified air through a UV O3 generator. CO was used as OH scavenger (purity: 99.998%; Matheson Tri-Gas, Newark, CA). R-Pinene concentrations were measured using a Hewlett-Packard (Palo Alto, CA) 5890 Series II Plus gas chromatograph (GC) equipped with a DB-5 60 m column (J&W Scientific, Davis, CA) and a flame ionization detector (FID). Triplicate hydrocarbon measurements were obtained prior to the start of the reaction and repeated every 20 min thereafter. Experiments commenced by injecting ozone into the reactor at a rate of 2 L min-1 for 20 min to produce approximately 200 ( 5 ppb ozone. The target ozone concentration was achieved by changing the duration of ozone injection for the fixed volume of the reactor. Ozone was monitored with a Dasibi 1003-AH ozone analyzer (Dasibi Environmental Corporation, Glendale, CA). Ammonia was measured by flowing the NOx free chamber air through a thermal oxidizer set to 980 °C and detecting the oxidized ammonia as NO. The thermal oxidizer was calibrated using a certified cylinder of ammonia (Concentration: 0.3% in N2 with (2% of accuracy; Praxair, Santa Ana, CA) and has an estimated uncertainty of (10%. Aerosol size distributions and number concentrations were obtained every 75 s using a scanning electrical mobility spectrometer (SEMS). The SEMS was comprised of a TSI 3077 85Kr neutralizer (St. Paul, MN), a TSI 3081 long column cylindrical differential mobility analyzer (DMA), and a TSI 3760A condensation particle counter. The reactor was humidified by flowing purified dry air through a sparger immersed in DI water maintained at 40 °C by a mantle heater. The humidified air was then passed through a condensate collector to cool the humidified air to room temperature and prevent condensation within the reactor. Humidified water was injected until the target RH

FIGURE 1. SOA yields for r-pinene ozonolysis (200 ppb O3) as a function of organic aerosol mass at 294 ( 1 K.

in the reactor was reached. No particles were detected in the humidified air leaving the humidifier system. More detailed information about experimental methods and devices were previously described (3).

3. Results and Discussion 3.1. Yield of SOA. A list of experiments performed is provided in Table 1. Aerosol yields versus organic aerosol mass obtained from R-pinene ozonolysis utilizing three commonly used OH scavengers, CO (200 ppm), cyclohexane (250 ppm), and 2-butanol (250 ppm) are compared in Figure 1. Aerosol yield is calculated as the ratio of the mass concentration of secondary organic aerosol to the amount of hydrocarbon reacted (Y ) Mo/∆HC). The curve fit through the data in Figure 1 was generated using a semiempirical gas-particle partitioning approach, assuming a hypothetical two-product model, eq 1 based on Odum et al. (18)

Y ) Mo

(

R1K1 R 2 K2 + 1 + K1Mo 1 + K2Mo

)

(1)

where 1 and 2 designate two lumped aerosol-forming products, one of relatively high volatility and the other of low volatility. Mo is the organic aerosol mass concentration (µg m-3), Ri is the mass stoichiometric factor of compound i and Ki (m3 µg-1) is the partitioning coefficient (analogous to Henry’s coefficient) in terms of the organic mass concentration. Using nonlinear regression, the four parameters of R1, K1, R2, and K2 were estimated to be 0.3663, 0.0004, 0.4887, and 0.0184, respectively (R2 ) 0.99) in the presence of CO as an OH scavenger. Aerosol yield was the highest when CO was used as an OH scavenger while use of 2-butanol resulted in the smallest aerosol yield. The 2-butanol yield curve matches well with previously published data (19, 20). Aerosol yield obtained from the use of cyclohexane as an OH scavenger is slightly higher than that obtained from the use of 2-butanol. The difference between aerosol yields obtained using the three OH scavengers shows that OH scavengers can directly impact SOA formation chemistry. This is consistent with earlier work by Keywood et al. (21) Docherty and Ziemann and (22). It is well-known that the CO and OH radical reaction does not produce the alkylperoxy radical VOL. 41, NO. 17, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 2. Evolution of (a) number, volume, and r-pinene concentrations, and (b) average diameter of SOA obtained from 100 ppb r-pinene and 200 ppb O3 reaction before and after adding 1 ppm NH3.

FIGURE 3. Effect of NH3 on the formation SOA for cyclohexene and isoprene ozonlysis after the equilibrium reaches ((a) 200 ppb cyclohexene, 200 ppb O3, and 500 ppb NH3; (b) 300 ppb isoprene, 300 ppb O3, 1000 ppb NH3).

(RO2) but only produces HO2. By contrast, the OH radical and 2-butanol or cyclohexane reaction produces RO2 as well as HO2. It is likely that the reaction of the Criegee intermediate in the presence of CO enhances the formation of lower volatility products than that which occurs in the presence of 2-butanol or cyclohexane. RO2 formed from the reaction between OH and its scavenger may suppress aerosol formation in R-pinene ozonolysis. Similarly, Keywood et al. (21) suggested that for cyclohexene ozonolysis, decreased concentrations of RO2 promote aerosol formation. This is in contrast with Docherty and Ziemann (22) who reported that for β-pinene ozonolysis, increased HO2/RO2 ratios inhibits aerosol formation. Therefore, it is hard to generalize the effect of OH scavengers on SOA formation for different alkeneozone reactions, but it is probable that different OH scavengers lead to differences in HO2/RO2 ratios, which may affect the subsequent radical chemistry. 3.2. Effect of Added NH3 on SOA Formation in Dry Conditions. A 100 ppb solution of R-pinene was mixed with 200 ppb O3 and allowed to react until aerosol formation ceased. Next, 1000 ppb NH3 was added to the reactor. Evolution of size distribution (Figure S1, Supporting Information) as well as both aerosol mass and number concentrations, R-pinene concentration, and average diameter of aerosols formed before and after NH3 addition (Figure 2) are shown. After particle growth from R-pinene ozonolysis ceased, NH3 was injected resulting in an increase in the number and mass concentration by 15 and 8%, respectively. An increase in number concentration indicated the formation of new particles resulting from gas-to-particle conversion. Moreover, average size of particles increased from 242 to 248 nm after adding NH3. This increase in number and volume concentrations is evidence that NH3 affects SOA formation in an R-pinene/ozone system. This increase in

particle size may be due to the condensation of newly produced particles on the surface of preexisting particles. The resulting particles may be ammonium salts formed by the reaction between gas-phase organic acids (e.g., pinic acid, pinonic acid) (9) and NH3. As another example of enhancing effect of NH3 on SOA formation, the changes in number and volume concentration obtained from 200 ppb cyclohexene and a 200 ppb ozone reaction after adding 500 ppb NH3 (Figure 3a). Added NH3 significantly increased both number and volume concentration. This shows that NH3 drives gasphase organic acids (e.g., adipic acid, glutaric acid) (23) into particle-phase organics, leading to elevated number and volume concentrations. It is worth noting that not all gasphase organic acids produced from ozonolysis experience gas-to-particle conversion through this type of reaction. Organic acids (e.g., 2-methylglyceric acid, pyruvic acid) (24, 25) formed by 300 ppb isoprene and 300 ppb ozone reaction do not significantly form particles by reacting with 1 ppm NH3 (Figure 3b). To investigate the impact of initial NH3 on SOA formation, six different concentrations of ammonia were added to a 200 ppb R-pinene and 200 ppb ozone system at the beginning of the experiment. Aerosol formation increases as the amount of NH3 added to the system increases from 0 to 200 ppb NH3 (Figure 4). However, no additional aerosol formation is noted as ammonia concentration exceeds 200 ppb. This likely indicates that the gas-phase acid organics present in this reaction, in terms of NH3, have been consumed. A simplified reaction mechanism for R-pinene ozonolyis is presented (Figure 5) to examine how condensable salts are formed by adding NH3 to the R-pinene/ozone system. The original mechanism was proposed by Jang and Kamens (9). The reaction pathway between the excited Criegee

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FIGURE 4. Effect of initial NH3 concentration on SOA formation (initial r-pinene: 200 ppb, O3: 200 ppb, RH < 2%)

FIGURE 6. Changes in number and volume concentrations obtained from 0.02 M of aerosolized cis-pinonic acid before and after adding 2 ppm NH3 (not corrected for wall loss) ((a) gas phase dominant cis-pinonic aicd; (b) particle phase dominant cis-pinonic acid).

FIGURE 5. A simplified mechanism of condensable salt formation in the r-pinene/O3 reaction. intermediate (CI) and the organic acids has been omitted. In brief, the mechanism proceeds via addition of ozone to the carbon-to-carbon double bond, leading initially to formation of an energy-rich ozonide. This primary ozonide decomposes rapidly by two possible channels, each forming a carbonyl-substituted C10-CI that also possess excess energy. Pinic acid, norpinic acid, pinonic acid, and norpinonic acid were reported to be major oxidation products formed from the R-pinene ozone reaction (9). These compounds exist in both the gas- and particle-phase. Added NH3 may drive gasphase organic acids to become particle-phase salts through acid-base reactions. This may be responsible for the elevated number concentration of SOA when NH3 is added as shown in Figure 2a. To further investigate if gas-phase organic acids are converted into particle-phase salts by reacting with NH3, we performed a reaction between NH3 and cis-pinonic acid (purity: 98%, Aldrich, St. Louis, MO), an oxidation product of R-pinene ozonolysis that exists in both the gas and particle phase (9). The aerosolized cis-pinonic acid was generated by atomizing ∼0.02 M pinonic acid in purified water, followed by drying in a diffusion dryer. Prior to injecting the cis-pinonic acid into the chamber, independent atomized purified water and NH3 were injected into the reactor to see if the water and NH3 generate particles. No detectable particles were mea-

sured for these two compounds. Number and volume concentration increased after NH3 injection (Figure 6a). Aerosolized cis-pinonic acid injected formed a small number of aerosol (∼120 cm-3) particles. Five min after completion of the acid injection, 2 ppm NH3 was injected, resulting in approximately 2300 cm-3 particle number concentration (total suspended mass concentration ∼1.1 µg m-3). This increase in the number concentration after adding NH3 shows the formation of new particles. This is an evidence that NH3 drives gas-phase cis-pinonic acid into condensable salt through acid-base reaction. It is not clear from the chamber experiments whether particle-phase organic acids can also contribute to elevated SOA formation through reaction with ammonia. Thus, an additional experiment to isolate this reaction was performed. cis-Pinonic acid was aerosolized and passed through a multi carbon-filter embedded denuder to reduce the gas-phase organic acid content. The denuded sample was immediately reacted with NH3 in a mixing chamber. Number and volume concentrations of aerosolized cis-pinonic acid were monitored before and after ammonia were added to the minireactor (Figure 6b). Number concentration increases by 30% after the addition of ammonia indicating new particle formation. This may be attributed to incomplete removal of gas-phase cis-pinonic acid through the denuder. Simultaneously, the particle volume concentration doubled. After the ammonia addition to the gas- and particle phase cispinonic acid mixture, number and volume concentrations increased by 15 times and 6 times more than before NH3 was VOL. 41, NO. 17, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 7. Aerosol yields obtained from r-pinene ozonolysis (200 ppb O3) in the presence and absence of water vapor and 200 ppb NH3. added, respectively (Figure 6a). This increase is far higher than the increase observed for the denuded particle-phase cis-pinonic acid-NH3 reaction. This indicates that newly formed SOA by the addition of NH3 is caused dominantly by gas-to-particle conversion. However, the particle phase cispinonic acid may still partially contribute to higher observed volume concentrations for the R-pinene-ozone-ammonia reaction system. To examine the effect of NH3 on aerosol forming potential, aerosol yields obtained in the presence and absence of 200 ppb NH3 are compared (Figure 7). It can be seen that added NH3 enhances aerosol yields. The yield is significantly higher in the presence of NH3 than those in the absence of NH3. Based on the two-product model, the four parameters of R1, K1, R2, and K2 in the presence of 200 ppb NH3 were estimated to be 0.3333, 0.0132, 0.3289, and 0.0129, respectively (R2 ) 0.98). 3.3. Effect of Added NH3 on SOA Formation in Humid Condition. Water vapor is known to affect the formation of organic acids. For instance, Fick et al. (26) found that the formation of pinic acid and pinonaldehyde increased with increasing relative humidity (RH), while the formation of pinonic acid showed the opposite trend. Since they did not distinguish between gas- and particle-phase acids in their study, it is unknown whether RH increased particle-phase pinic acid. To investigate the effect of NH3 on SOA formation in a humid environment, dry (RH < 2%) and humid (RH ) 50%) conditions were selected. Prior to this investigation, the influence of water vapor on SOA formation in the absence of NH3 was studied to obtain a reference for comparison. It can be clearly seen (Figure 7) that aerosol yields obtained in the humid condition is lower than those obtained in the dry condition. The reason for this will be explained and discussed in section 3.4 by means of a kinetic model. 3.4. Kinetic Modeling. To further explore the effect of water vapor on gas-phase and particle-phase products in R-pinene/ozone system, kinetic modeling was performed. The kinetic model used was developed by Kamens et al. (27). This kinetic model comprises of 18 gas-phase reactions and 24 particle-phase reactions. We incorporated six reactions related to CO and OH reaction into the Kamens’ mechanism to eliminate the effect of OH on R-pinene/ozone reaction in 6100

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FIGURE 8. Comparison between experimental data and calculated data. the calculation. To simplify the kinetic mechanism, they defined six generalized semivolatile products. As input data, 120 ppb R-pinene, 200 ppb ozone, 50% RH, 293 K, and 100 ppm CO were applied to the calculation. Simulated aerosol formed and the onset of aerosol formation differs from observations (Figure 8). Onset of SOA in the model calculation started around 3 min versus 15 min after ozone addition in the experiment. This difference is attributed to how initial ozone concentrations are handled in the model calculation. In the model calculation, ozone and R-pinene are uniformly and initially present. Thus, when the calculation starts, R-pinene reacts with ozone. In contrast, in the experiment, ozone was added for 20 min in the presence of R-pinene to achieve an experimental concentration of 100 ppb. The model calculation underestimated the total amount of SOA formation by 15% as compared to measurement. The kinetic modeling results indicate that water vapor enhances gas-phase organic acid formation for gas- and particle-phase products with carbon number greater than or equal to 9. (Figure 9a). Thus, the enhancing effect of NH3 on SOA formation may be larger in humid conditions than in dry conditions. As an experiment to support the notion that humid conditions enhance formation of organic acids, acidity was compared for humid and dry conditions in the 200 ppb R-pinene and 200 ppb ozone system. A 100 mL 0.01 M NaOH impinger solution (pH 9.8) was used to collect acidic oxidation products at a rate of 0.9 L min-1 for 3 h from the chamber after particle growth commenced for the R-pinene/ ozone reaction. During the humid experiment, the final pH of the solution was 7.7, which is lower than the 9.2 obtained from dry conditions. This indicates that for humid conditions, the formation of overall acidic products is enhanced. This result is consistent with the kinetic modeling result shown in Figure 9a. Even though the kinetic modeling does not exactly match the experimental data on an absolute scale, the modeling seems to predict the trends in the formation of gas-phase organic acids in humid and dry condition. This discrepancy could be explained in terms of the model overpredicting the amount of gas-phase organic acids produced. To further support the enhancing effect of water vapor on acidic organics in the R-pinene/ozone system, the degree of increase of SOA yield obtained in the humid condition was compared (Figure 9b) with that obtained in the dry condition after the addition of NH3 to R-pinene ozonolysis. Higher aerosol yields were estimated in the presence of NH3 regardless of the presence of water vapor. The extent of

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FIGURE 9. (a) Effect of water vapor on the formation of gas- and particle-phase compounds produced from 120 ppb r-pinene and 200 ppb O3 reaction, (b) Effect of NH3 on SOA formation in dry and humid conditions (100 ppb r-pinene, 200 ppb O3, 200 ppb NH3, 50% RH). increase in SOA yield is higher in humid conditions (31%) than that in the dry condition (23%). This may be because that water vapor increases gas-phase organic acid formation, which is subsequently converted into particle-phase organics by reacting with NH3. In contrast, water vapor suppresses particle-phase organic acid formation, leading to the lower measured SOA formation for humid conditions.

Acknowledgments The authors wish to acknowledge technical support from Mr. Kurt Bumiller and Dr. Chen Song. This article is based upon work supported by the National Science Foundation under Agreement Nos. ATM-0449778 and ATM-0234111. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.

Supporting Information Available Sample size distributions of SOA formed during R-pinene ozonolysis experiment. This material is available free of charge via the Internet at http://pubs.acs.org.

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Received for review August 14, 2006. Revised manuscript received April 26, 2007. Accepted June 13, 2007.

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