Secondary Organic Carbon and Aerosol Yields from the Irradiations of

For α-pinene in the absence of SO2, the SOC yield of the irradiated mixture ..... and monoterpenes in Hong Kong and Guangzhou in the Pearl River Delt...
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Environ. Sci. Technol. 2006, 40, 3807-3812

Secondary Organic Carbon and Aerosol Yields from the Irradiations of Isoprene and r-Pinene in the Presence of NOx and SO2 T A D E U S Z E . K L E I N D I E N S T , * ,† EDWARD O. EDNEY,† MICHAEL LEWANDOWSKI,† JOHN H. OFFENBERG,† AND MOHAMMED JAOUI‡ National Exposure Research Laboratory, U.S. Environmental Protection Agency, Research Triangle Park, North Carolina 27711, and Alion Science and Technology, P.O. Box 12313, Research Triangle Park, North Carolina 27709

A laboratory study was carried out to investigate the secondary organic carbon (SOC) yields of R-pinene and isoprene in the presence of SO2, which produces acidic aerosol in the system. Experiments were based on irradiating each hydrocarbon (HC) with NOx in a 14.5 m3 smog chamber operated in the dynamic mode. The experimental design consisted of several multi-part experiments for each HC. In the first part of each experiment, an HC/NOx irradiation was conducted in the absence of SO2 and was followed by irradiations with the addition of SO2 in subsequent parts. Filter-based analyses for organic carbon were made using a thermal-optical approach either with an off-line instrument or in situ with an automated instrument. For isoprene in the absence of SO2, the SOC yield was approximately 0.001, a value consistent with earlier work from this laboratory. With the addition of up to 200 ppb SO2, the yield increased by a factor of 7. For R-pinene in the absence of SO2, the SOC yield of the irradiated mixture was found to average 0.096 from two experiments. With SO2 in the system, the SOC yield increased on average to 0.132. These results suggest that SO2, and by inference acidic aerosol, may play a role in increasing the yield of SOC from the photooxidation products of biogenic hydrocarbons or by the direct uptake of biogenic hydrocarbons onto acidic aerosol.

1. Introduction Sulfur dioxide (SO2) is an important constituent in the polluted atmosphere. It is mainly emitted from combustion sources using fuels that contain sulfur. Emissions of SO2 in the United States were reportedly 15 Tg in 2000 with most coming from coal and petroleum combustion (1). The primary removal mechanism of SO2 in the atmosphere is by reaction with hydroxyl radicals to produce sulfuric acid which then condenses on particulate matter (PM) to form an acidic aerosol. If gas-phase ammonia is present, the acidic aerosol can become partially neutralized to form acidic sulfate salts. * Corresponding author e-mail: [email protected]; phone: (919) 541-2308. † U.S. Environmental Protection Agency. ‡ Alion Science and Technology. 10.1021/es052446r CCC: $33.50 Published on Web 05/12/2006

 2006 American Chemical Society

Ambient PM is frequently found to be acidic in the Eastern United States and other regions having high summertime SO2 emissions (2,3). PM2.5 (PM with diameter less than 2.5 µm) can be directly emitted into the troposphere or produced through secondary processes involving radical-initiated chemical reactions which generate nonvolatile and semivolatile oxidation products that form secondary inorganic or organic aerosol. Major chemical components of PM2.5 include inorganic salts and acids, elemental carbon (EC) and organic carbon (OC) compounds, and liquid water. The OC compounds can be characterized using several techniques including gas or liquid chromatography with mass spectrometry to measure individual compounds, infrared spectroscopy to determine the organic functional groups present, and thermo-optical analysis to measure total organic and elemental carbon. Measurements of the organic aerosol by gas chromatography have shown the presence of both polar and nonpolar compounds. Most of the nonpolar and some of the polar compounds have been attributed to primary sources, and while quantitative measurements have indicated that compound contributions to the total mass are generally small, they can be used for source apportionment in field samples (4). Poor mass balances generally observed between the sum of individual compounds and the total organic mass may be due in large part to compounds with high molecular weights and low volatilities that make up a large fraction of the organic carbon. For example, there is recent evidence that a substantial fraction of the organic carbon is made up of organic oligomeric material which may also be referred to as humic-like substances (HULIS) or polycarboxylic acids (5-8). These compounds would typically not be detected using standard gas chromatography-mass spectroscopy (GC-MS) techniques. The formation of organic compounds has also been hypothesized as occurring through heterogeneous reactions on acidic aerosol. Several laboratory studies have examined the role of acidic aerosols in enhancing the uptake of carbonyl compounds through acid-catalyzed reactions in the condensed phase (9-11). Jang et al. (9) examined the increase in aerosol growth factors of C8-C10 aldehydes and C2-C3 dicarbonyl compounds in the presence of acidic seed aerosol through the use of differential mobility analysis. FTIR spectroscopy has also been used in a similar fashion to monitor the increase of the carbonyl band of the resultant aerosol (9). Noziere and Riemer (10) have examined the C5 dicarbonyl compound, 2,5-pentanedione, which was found to undergo substantial conversions over a wide range of acidities. Given its large enolization constant, 2,5-pentanedione was found to undergo aldol condensation at acidities as low as 20 wt %. Recently, Kroll et al. (11) reported no observed growth for the reactive uptake onto inorganic aerosols for selected C1-C8 aldehydes, hydroxy aldehydes, and dicarbonyls with the single exception being glyoxal. The heterogeneous reaction of a carbonyl compound on an acidic surface has recently been reported by Claeys et al. (12) who explained the formation of 2-methylglyceric acid found in field samples through the acid-catalyzed oxidation of methacrolein, a first generation product of isoprene, with hydrogen peroxide. The findings suggest that these or similar mechanisms could lead to the formation of increased secondary organic aerosol (SOA) from the gas-to-particle conversion of carbonyl compounds in the atmosphere over a broad range of aerosol acidities. However, the hypothesis that volatile carbonyl VOL. 40, NO. 12, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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compounds can lead to SOA formation has been questioned based on a thermodynamic evaluation of these processes by Barsanti and Pankow (13). The work sought to set limits on the likelihood for reaction of particular types of condensed-phase processes, called accreation reactions by the investigators. For example, it has been suggested that free energies of formation are generally endothermic for simple aldehydes in the gas phase (e.g., n-hexanal) and high molecular weight compounds would not form via aldol condensation in acidic media. By contrast, calculations indicate that for glyoxal and methylglyoxal, aldol condensation is thermodynamically favorable for oligomer formation. The reactions are expected to contribute to organic particulate matter provided the processes are kinetically favorable (13). Other mechanisms and precursors have also been suggested as contributing to organic aerosol mass increases. For example, laboratory studies have provided evidence for the formation of HULIS from the acid-catalyzed reaction of isoprene albeit at relatively high reactant concentrations (14). The investigators suggested that this constituent could account for approximately 1 µg/m3 of the European PM2.5 mass on average; moreover, the same process could be occurring for C10 terpenoid compounds. While the studies discussed above have focused on reactions of single carbonyl compounds, few previous studies have been conducted to examine SOA production under more atmospherically relevant conditions, such as SOA formation in acidic media from the reaction products of precursor hydrocarbons. Iinuma et al. (15) examined the SOA formation from the ozonolysis of R-pinene in the presence of acidic seed aerosol. A thermographic determination of the total organic carbon indicated that the presence of the acidic seed aerosol resulted in an increase of approximately 40% in the OC yield. Other experiments involving isoprene irradiations have been undertaken by Edney et al. (16) who presented laboratory and field measurements that indicated that an increase in aerosol mass could occur from the photooxidation of isoprene in the presence of SO2. The effect is noteworthy because isoprene had previously been found to contribute negligibly to the secondary organic aerosol formation of PM2.5 (17). Edney et al. (16) also identified a set of organic compounds (2-methylthreitol, 2-methylerythritol, and 2-methylglyceric acid) that may serve as indicators of isoprene SOA in the atmosphere, which is consistent with recent reports suggesting the importance of isoprene SOA based on field measurements (18). Overall, a systematic approach based on measured increases of gravimetric or carbon mass concentrations as a function of the SO2 should be a straightforward means of assessing the importance of the acid-catalysis processes for PM2.5 formation under more atmospherically relevant conditions. We have conducted such experiments to measure secondary organic carbon (SOC) yields from the R-pinene/ NOx and isoprene/NOx photochemical systems. Experiments have been designed to measure the SOC yields (YSOC) at progressively higher SO2 concentrations (leading to increased acidic aerosol) while leaving all other parameters unchanged. For these measurements, the chamber was operated under steady-state conditions, and filter-based or automated OC measurements were made.

2. Experimental Methods Most aspects of the experimental system have been previously described (16, 19). The smog chamber is a 14.5-m3 parallelepiped chamber fabricated from stainless steel with interior walls fused with a 40 µm TFE Teflon coating. For photolysis, the chamber uses a combination of fluorescent bulbs that provide radiation distributed over the portion of the spectrum from 300 to 400 nm and has an energy 3808

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distribution similar to that of solar radiation over the same range. For these experiments, the source intensity led to an NO2 photolysis rate of 0.17 min-1. During the experiments, the radiation intensity was continuously monitored with an integrating radiometer (Eppley Laboratory, Inc., Newport, RI). The smog chamber was operated as a continuous stirred tank reactor in a dynamic mode. Reactants were added to a mixing manifold in a continuous fashion using mass flow controllers while the effluent used for analysis was withdrawn from the chamber. Nitric oxide (NO), SO2, and isoprene were obtained from high-pressure cylinders. R-Pinene was added to the chamber by bubbling air through the thermostated, neat liquid in an impinger. The relative humidity in the chamber was maintained at a constant value of 30% with a computer-controlled peristaltic pump coupled to a hightemperature vaporizer. A chamber cooling system allowed the temperature to remain near 25 °C during the irradiation. The average residence time in the chamber was 6 h and the system was allowed to come to steady-state for each condition before sampling. Chemical and physical parameters were monitored continuously during the experiments. NO and total NOx were measured with a ThermoElectron (model 8840, Thermo Environmental, Inc., Franklin, MA) oxides of nitrogen chemiluminescent analyzer. SO2 was monitored by pulsed fluorescence detection (model 43A, TECO, Hopington, MA). Isoprene and R-pinene concentrations in the chamber and inlet were measured by gas chromatography/flame ionization detection (model 5890, Hewlett Packard, Palo Alto, CA) using a cryogenic trap for sample collection. Temperature and relative humidity were measured with an Omega digital thermo-hydrometer (model RH411, Omega Engineering, Inc., Stamford, CT). Particulate matter formed during the irradiations was analyzed for organic carbon using a single method by two different thermographic techniques. The technique for the R-pinene irradiations used a 47-mm dual quartz filter which has been previously described (19). Organic carbon collected on the back filter was subtracted from that on the front filter serving to correct for positive artifacts (20). Filter collections were conducted for periods ranging from 90 to 180 min at flow rates between 8.5 and 10.5 L/min. The analysis was performed on an off-line OC instrument from Sunset Laboratories (Tigard, OR) using the thermal-optical technique as described by Birch and Cary (21). An alternative approach used for the isoprene irradiations utilized an automated, semicontinuous elemental carbonorganic carbon instrument developed by Sunset Laboratories. (Since there is no elemental carbon in the system, the total carbon measured by the instrument was equivalent to the organic carbon.) The instrument operates with a quartz filter positioned within the oven housing used for the analysis. The pumping system drew chamber effluent through the filter at a rate of 8 L/min. A carbon-strip denuder was placed in line prior to the quartz filter to remove gas-phase organic compounds in the effluent which might interfere with the organic carbon measurement. With a sample collection time of 0.5 h and an analysis time of 0.25 h, the duty cycle for the measurement of OC was 0.75 h. Analysis of the collected organic carbon was made again using a thermal optical technique. Bae et al. (22) showed that the results by the two techniques were highly correlated for ambient samples. A linear regression of collocated measurements from the two methods showed a regression slope of 0.97 (R 2 ) 0.89) for measurements of total carbon equivalent to the mode used in the present study. Samples were also taken for gravimetric mass and inorganic ions. For the determination of the gravimetric mass, the sampling train consisted of a sodium carbonate

FIGURE 1. Generation of sulfuric acid (measured as sulfate) from the OH reaction with SO2 in the r-pinene/NOx photochemical system in Experiment A2. The linear formation indicates that the addition of SO2 does not perturb the radical level in the system. The intercept represents a small amount of ammonium sulfate seed aerosol added to the mixture to aid organic aerosol formation. denuder to remove acidic gases and SO2 followed by a 47-mm, 0.45-µm Teflon filter. Procedures for the gravimetric analysis of the aerosol mass have been previously described (23). Following the mass measurements, the Teflon filters were extracted in 10 mL of deionized water and analyzed for their ionic contents with an ion chromatograph (model DX500, Dionex Corp, Sunnyvale, CA) with electrical conductivity detection. Anions were separated with an IonPac AS14A column and an isocratic 4 mM sodium carbonate/0.5 mM sodium bicarbonate eluent. This method provided a direct means of analyzing sulfate which mainly comes from sulfuric acid and acidic sulfate salts formed in the system. Cation analysis for particle-phase ammonium ions was performed with a Dionex IonPac CS15 column and an isocratic 9% acetonitrile/10 mN sulfuric acid eluent. Experiments were performed in multiple parts for each hydrocarbon. The initial part of each experiment consisted of an irradiation of R-pinene or isoprene and oxides of nitrogen (NOx) in the absence of SO2. In subsequent parts, SO2 was added to the chamber in concentrations ranging from approximately 30 to 300 ppb while leaving the reactant levels of the HC and NOx constant. Under the conditions of the experiments, the concentration of sulfate, from sulfuric acid produced in the reaction system, was linearly related to the initial SO2 concentration. This relationship is shown in Figure 1 for an experiment with R-pinene/NOx with SO2 which shows the aerosol sulfate concentration measured as a function of the added SO2. As is seen below, the addition of SO2 perturbed the radical level to a negligible extent. Thus, it is not expected that changes that may be seen in the particle phase concentrations from the addition of SO2 resulted from major changes in the gas-phase chemistry.

FIGURE 2. Inlet manifold (solid triangles) and chamber (solid squares) concentrations of r-pinene used to determine the reacted mass concentration for Experiment A1. The chamber radiation was turned on at 0 h. The SO2 concentration at irradiation times greater than 40 h was 252 ppb.

3. Results and Discussion The present study consisted of two multipart experiments for R-pinene and two for isoprene. In Experiments A1 and A2, SOA was generated from the photooxidation of R-pinene; Experiments I1 and I2 were conducted using isoprene. The initial conditions for these experiments are given in Table 1. R-Pinene Experiments. Experiment A1 was a two part R-pinene/NOx irradiation, the first part without SO2 and the second with 252 ppb SO2. In Experiment A2, SO2 was added to the reaction mixture in successive, approximate 50-ppb increments from 48 to 193 ppb. Again, the initial part of the experiment was conducted without SO2. Reacted R-pinene from Experiment A1 is shown in Figure 2. As shown in the figure, the triangles represent R-pinene measurements made in the inlet before and during the dynamic experiment. The squares in Figure 2 give the measurements of the R-pinene concentration in the chamber. From an examination of the R-pinene concentration, the reaction system is found to be at steady state after the first 20 h. With the system at steady state, samples were collected for aerosol carbon and mass. The figure also shows that the R-pinene concentrations were essentially unchanged when SO2 was added to the chamber indicating that the gas-phase organic compounds were largely unaffected by SO2, as discussed previously. The reacted R-pinene is obtained from the difference between the input and steady-state chamber values, giving reacted organic and carbon mass concentrations of 1350 µg/m3 and 1190 µgC/m3, respectively. The carbon data from Experiment A1 are given in Table 2. The back-filter correction was found to be about 35% of the carbon on the front filter both with and without SO2 present. The secondary organic carbon yield (YSOC) and

TABLE 1. Initial Conditions for SOA Yields in Presence and Absence of SO2 (Multiple SO2 Concentrations Represent the Concentration Levels for Different Parts of Each Experiment) parameter

Expt A1 (r-pinene)

Expt A2 (r-pinene)

Expt I1 (isoprene)

Expt I2 (isoprene)

NO (ppb) NOx (ppb) hydrocarbon (ppmC) HC/NOx SO2 (ppb) temperature (°C) relative humidity (%)

213 242 2.55 10.5 0, 252 26.3 29

529 543 1.78 3.2 0, 64, 125, 191 24.8 30

485 492 8.04 16.3 0, 33, 64, 125, 291 24.7 30

406 475 7.99 16.8 60, 200 24.6 30

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TABLE 2. Secondary Organic Carbon Yields from r-Pinene/NOx Irradiations in the Absence and Presence of SO2 Expt

initial SO2 (ppb)

sulfate (µg m-3)

reacted r-pinene (µgC m-3)

OC (µgC m-3)

SOC yield

A1 A1 A2 A2 A2 A2

0 252 0 64 125 191

a 43.9 1.5b 17.2 20.4 30.5

1190 1190 815 815 815 815

130 172 67.3 87.0 96.1 94.3

0.109 0.145 0.083 0.107 0.118 0.116

a

Not measured.

b

Background ammonium sulfate.

secondary organic aerosol (YSOA) are then calculated from the following respective relationships

YSOC ) SOC/∆HCC

(1)

YSOA ) SOA/∆HC

(2)

where in eq 1 the SOC is the corrected organic carbon concentration and ∆HCC is the reacted carbon mass concentration of the hydrocarbon obtained from Table 2. In eq 2, ∆HC is the reacted hydrocarbon mass concentration. Through the application of eq 1, for R-pinene in the absence of SO2, YSOC is found to be 0.109. In the presence of SO2, YSOC was found to increase by 33% to 0.145 as given in Table 2. The organic aerosol yield (YSOA) from the organic mass concentration formed during the reaction was calculated using eq 2. In the absence of SO2, the gravimetric and organic aerosol mass concentrations are nearly equal except for the seed aerosol added to the chamber (ca. 1 µg/m3). In Experiment A1, the reaction produced an organic mass concentration of 215 µg/m3 for a reacted R-pinene mass concentration of 1350 µg m-3. The use of eq 2 gives a yield of 0.159. The addition of 252 ppb SO2 to the system produced an aerosol mass concentration of 352 µg/m3, which includes organic and inorganic components. Acidic sulfate salts comprised the major portion of the inorganic component and were found to be 50.5 µg/m3 from the IC measurements of sulfate and ammonium. (The cation measurement of the aerosol showed an ammonium concentration of 6.6 µg/m3 indicating that partial neutralization had occurred. The calculated ammonium-to-sulfate ratio of 1.1 indicated that the aerosol was close to the acidity of ammonium bisulfate.) Subtracting the inorganic mass concentration from the gravimetric mass concentration gives 302 µg/m3 for the organic mass concentration. Using eq 2 for YSOA gives a yield of 0.223, or an increase of 40%. The second R-pinene/NOx irradiation, A2, was conducted to examine the change in YSOC as the SO2 was varied over three different concentrations. In the present experiment, the initial HC/NOx ratio was reduced as given in Table 1. The results are presented in Table 2 for experiments labeled A2. For this experiment, the reacted R-pinene mass concentration was 815 µgC/m3. In the absence of SO2, the mass concentration of secondary organic carbon was 67.3 µgC/m3 which using eq 1 gives an SOC yield of 0.083. For each of the three conditions where SO2 was added, the SOC yield progressively increased from 0.107 for an initial SO2 concentration of 64 ppb to 0.118 at 125 ppb as shown in Figure 3. Isoprene Experiments. Experiments to measure YSOC from isoprene photooxidation products were conducted in a similar fashion. The initial conditions are given in Table 1 for the two multipart isoprene experiments. The relatively high HC/NOx ratios made the systems very reactive to ensure substantial secondary reaction of the primary products. The extent of hydrocarbon reaction for the two experiments were within 0.5% of each other. The reacted isoprene carbon 3810

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FIGURE 3. Secondary organic carbon yield of r-pinene with increased levels of reacted SO2 as represented by sulfate in Experiment A2. The increase in the SOC yield between 0 and 20 µg m-3 of sulfate formed is 33%.

TABLE 3. Secondary Organic Aerosol Yields from Experiments I1 and I2 Where Isoprene was the Precursor Hydrocarbon Expt I1 I1 I2 I1 I1 I2 I1 a

initial SO2 sulfate reacted isoprene OC (ppb) (µg m-3) (µgC m-3) (µgC m-3) 0 33 60 64 125 200 291

0.2a 3.2 6.3 7.0 22.2 36.8 50.3

3600 3600 3590 3600 3600 3590 3600

4.1 8.7 16.8 14.3 19.3 24.9 25.4

SOC yield 0.0011 0.0024 0.0047 0.0040 0.0053 0.0069 0.0071

Background ammonium sulfate seed aerosol.

FIGURE 4. Secondary organic carbon yield of isoprene with increasing levels of sulfate in Experiments I1 (filled squares) and I2 (open squares). concentrations were 3600 and 3590 µgC m-3 for Experiments I1 and I2, respectively. Moreover, the steady-state NOx levels in Experiments I1 and I2 were similar with values of 0.31 and 0.28 ppm, respectively. Table 3 shows the OC mass concentrations measured for each of the seven initial SO2 concentrations. The SOC yields are calculated according to eq 1. A YSOC of 0.0011 was found for the isoprene oxidation products in the absence of SO2 which is very near the detection limit for this measurement. The OC mass concentrations and YSOC increased in a monotonic fashion with increasing sulfate concentrations as given in Figure 4. At sulfate concentrations less than 10 µg/m3, a linear increase is found between the SOC yield and the sulfate concentration. For the conditions of the present experiments, a plot of the yield as a function of the sulfate concentration reaches a maximum value at sulfate levels greater than 40 µg/m3. At the maximum in the presence of SO2, YSOC is 0.0071 which is considerably larger than that in the absence of SO2.

Comparison with Other Studies. The results of this study appear to be consistent with those of another study that has examined enhanced SOA formation from the heterogeneous reactions of R-pinene oxidation products in acidic sulfate aerosol. Iinuma et al. (15) examined products from the R-pinene/ozone reaction in the presence of ammonium sulfate and acidic sulfate aerosols generated from dilute sulfuric acid-ammonium bisulfate solutions. By the use of a thermographic method, similar in principle to that described here, an increase in total YSOC of 40% was detected in the aerosol with acidic seed present as compared to that with ammonium sulfate. Individual R-pinene/ozone products consistent with those detected by Yu et al. (24) were reported including pinonaldehyde and cis-pinonic acid. While the photo oxidation of R-pinene also produces organic aerosol mainly from the ozone reaction with R-pinene, the first generation products from that system can continue to undergo further oxidation reactions in the present study. Nonetheless, it appears that the two systems show similar increases for YSOC. Changes in YSOC for isoprene due to the presence of acidic sulfate aerosol were also studied by Edney et al. (16). In that work only a single SO2 concentration was studied at a lower HC/NOx ratio. YSOC in the absence of SO2 was essentially identical to that (0.001) found in the present work even though the values in both studies have high uncertainties since the measured OC is within a factor of 2 of the background value in both studies. However, a recent study by Kroll et al. (25) suggests that conditions can be found where isoprene SOA yields are produced that are substantially greater than those previously observed. The conditions in that laboratory study were considerably different (temperature, seed aerosol concentration, and other parameters) than those used in the present study. It is also possible that with NO and NOx being substantially lower in the Kroll et al. (25) study, the RO2 + RO2 reactions may have led to higher SOA yields than observed here in the absence of SO2. Finally, in examining the effect of SO2 on isoprene SOA yields, Edney et al. (16) found the increase in YSOC to be a factor of 11, or 70% higher than in the present work at the highest SO2 values. This difference is probably due to a combination of factors including uncertainties in the determination of YSOC in the absence of SO2 and differences in the initial hydrocarbon and NOx concentrations. Additional research, particularly in the yield of isoprene SOA in the absence of SO2, will need to be conducted. Previous work has suggested that increases in mass concentration due to acid-catalyzed reactions in the gas phase may be attributed to the presence of carbonyl compounds formed by photooxidation reactions. Jang et al. (9) report increases in particle volume from 30% for simple carbonyl compounds to nearly a factor of 10 for unsaturated carbonyl and dicarbonyl compounds. Other indicators of increased mass for acidic particulate matter include the appearance of a strong carbonyl band in the FTIR spectrum of particulate matter (9) and the formation of oligomeric products. The oligomers have been postulated to form as dimers of pinonaldehyde (6). Gas-phase carbonyl products from the photooxidation of R-pinene have recently been reviewed by Atkinson and Arey (26). Pinonaldehyde was determined to be the predominant carbonyl product formed in the system with most of the data suggesting a yield of about 30%. There is no consensus for the formation of other high-molecularweight carbonyl compounds, although Jaoui and Kamens (27) report a broad range of oxidation products from the R-pinene/NOx system. In addition to pinonaldehyde, other carbonyl compounds found to form include norpinonaldehyde, R-campholenal, pinalic-4-acid, and pinonic acid. Tolocka et al. (6) have shown mechanisms for the dimerization of pinonaldehyde through aldol condensation and

gem-diol formation. These types of reactions with pinonaldehyde and other carbonyl products could also be occurring in the present study, although there is no direct evidence for it. While it appears that carbonyl compounds may be of some importance as reactants in heterogeneous acid-catalyzed reactions in aerosols, as noted, Barsanti and Pankow (13) have challenged these finding based on thermodynamic arguments. The results of this study potentially have atmospheric implications for areas impacted by biogenic oxidation products and SO2. Much of the acidic sulfate compounds measured in ambient PM2.5 is found as ammonium bisulfate (2). Organic carbon and sulfate salts make up a sizable portion, and frequently the majority, of the measured PM2.5 mass concentration during the summer. For example, at a site in the Eastern United States during summer, organic carbon both from primary and secondary sources has been found to make up approximately 40% of the PM2.5 mass concentration (28). Moreover, the study by Edney et al. (28) also confirmed the presence of secondary organic aerosol due to the photooxidation of R-pinene through the identification of selected indicator compounds. To understand the importance of acid-catalyzed reactions in PM2.5, it is likely that similar indicator compounds will have to be found that are specific to the acid-catalyzed system for both isoprene and R-pinene SOA. Therefore, considerably more research will be required to be able to extrapolate to ambient atmospheres the role acidity plays in increasing the OC mass of PM2.5.

Acknowledgments The U.S. Environmental Protection Agency through its Office of Research and Development funded and collaborated in the research described here under Contract 68-D-00-206 to Alion Science and Technology. The manuscript was subjected to external peer review and has been cleared for publication. Mention of trade names or commercial products does not constitute an endorsement or recommendation for use.

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Received for review December 6, 2005. Revised manuscript received February 15, 2006. Accepted April 11, 2006. ES052446R