Molecular Composition of Monoterpene Secondary Organic Aerosol at

Sep 20, 2010 - Hundreds of product molecular formulas were identified that were common to all mass loadings; however the relative intensities changed ...
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Environ. Sci. Technol. 2010, 44, 7897–7902

The molecular composition of secondary organic aerosol (SOA) from the ozonolysis of monoterpenes (R-pinene and β-pinene) was studied by liquid chromatography mass spectrometry and high-resolution Fourier transform ion cyclotron resonance mass spectrometry techniques, both employing electrospray ionization (ESI). SOA particles were generated in a flow tube reactor with a reaction time of 23 s. A microsampling assembly in combination with ESI-FTICR analysis permitted SOA with a mass loading as low as 3.5 µg/m3 to be characterized with high accuracy and precision mass analysis. Hundreds of product molecular formulas were identified that were common to all mass loadings; however the relative intensities changed significantly. In particular, a species with the (neutral molecule) formula C17H26O8 increased substantially in intensity relative to other products as the mass loading decreased. Tandem mass spectrometry (MSn) of this species showed it to be a dimer of C9H14O4 and C8H12O4, most likely pinic acid and terpenylic acid, respectively. LCMS analysis showed different elution times for the dimer and monomer species, confirming that the dimer was not an artifact of ESI analysis. The particle number concentration increased linearly with ozone concentration (the limiting reactant in the experiment), arguing against gas phase dimerization as the rate limiting step in particle formation.

example to elucidate gas phase (6) and particle phase (7) components under conditions closer to ambient air. Although general chemical characteristics have been measured for low mass loading SOA, for example average oxygen to carbon (O:C) and hydrogen to carbon (H:C) atomic ratios, detailed molecular information is still needed. In recent years, electrospray ionization (ESI) combined with either a Fourier transform ion cyclotron resononace (FTICR) or orbitrap mass analyzer for high accuracy and resolution mass analysis has been used to identify individual molecular formulas of compounds in laboratory SOA (8-10). With this approach, more than one thousand unique molecular formulas are detected in a typical sample. This molecular complexity makes understanding the details of SOA formation and growth difficult. An important observation in these studies is the prevalence of oligomeric as well as monomeric products. Oligomers are thought to be produced by several mechanisms. Oligomerization reactions of stable molecule end products of the ozonolysis reaction include noncovalently bonded dimers of organic acids (11-14), esters and anhydrides derived from carboxylic acids (15, 16), and products of carbonyl chemistry including acetals and hemiacetals (17). Oligomerization reactions involving intermediates such as peroxides, hydroperoxides, and the stable Criegee intermediate have also been postulated (18-20). Oligomers have been detected within seconds after the onset of reaction, although processes occurring over time periods of several hours also occur (18). Oligomer formation provides a means to transform relatively volatile monomers into nonvolatile macromolecules capable of migrating to the particle phase, thereby enhancing new particle formation and growth of existing particles. In this study, the molecular composition of monoterpene SOA generated in a flow tube reactor is determined under conditions where the mass loading is as close as possible to ambient conditions (less than 10 µg/m3). The study is made possible by use of a sensitive analysis procedure requiring only about 2 µg of SOA to be collected, even though on the order of a thousand individual molecular components are present in the sample. The observed change in composition with mass loading provides insight into the particle formation process.

Introduction

Experimental Section

Biogenic secondary organic aerosol (SOA), particulate matter produced by oxidation of volatile precursors such as isoprene and monoterpenes, is a major component of ambient particulate matter (1). Biogenic SOA is responsible for the “blue haze” frequently observed over forested areas (2). Ambient particles exert a significant environmental burden by adversely affecting human health and influencing Earth’s energy balance (3). Understanding the mechanism of SOA formation and growth is a key step for predicting future changes in ambient particulate matter that could result from changes in human activity and/or global climate. Numerous laboratory, field, and modeling studies have addressed this topic (1, 4, 5). In general, laboratory studies are carried out in two main ways: environmental chambers and flow tubes. The former one is used to mimic ambient air while the latter one is used mostly to study reaction mechanisms. Laboratory studies often are performed with an elevated SOA mass loading to facilitate chemical analysis. Recently, experiments have been reported with much lower mass loadings, for

Flow Tube Reactor. The flow tube reactor used in this study has been previously described (8, 18, 20). Gas flows of monoterpene, either R-pinene (98% Sigma-Aldrich, St. Louis, MO) or β-pinene (99% Sigma-Aldrich), and ozone were sent concentrically into the reactor at room temperature under laminar flow conditions. The reaction time was 23 s. The flows of the two reactants were kept constant with individual flow controllers, while the total flow through the reactor was driven and controlled by a metering pump. Aerosol flow from the reactor was split to the metering pump, a microsampling assembly described below, a scanning mobility particle sizer (SMPS), and in some experiments a nano aerosol mass spectrometer (NAMS) also described below. Ozone was generated and monitored with a model 49 O3 Analyzer (Thermo Electron Corp., Waltham, MA). For the reactions studied, ozone was the limiting reactant and its concentration determined the mass loading of SOA. Ozone concentrations ranged from 200 to 700 ppbv (as listed in Table 1). Monoterpene vapor was produced by passing clean dry air over the surface of the fluid and diluting with additional air. The concentration of R-pinene (98%; Sigma-Aldrich) vapor was approximately 43 ppm whereas that of β-pinene

Molecular Composition of Monoterpene Secondary Organic Aerosol at Low Mass Loading YUQIAN GAO, WILEY A. HALL IV, AND MURRAY V. JOHNSTON* Department of Chemistry and Biochemistry, University of Delaware, Newark, Delaware 19716

Received June 1, 2010. Revised manuscript received August 20, 2010. Accepted September 8, 2010.

* Corresponding author phone: (302) 831-8014; fax: (302) 8316336; e-mail: [email protected]. 10.1021/es101861k

 2010 American Chemical Society

Published on Web 09/20/2010

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TABLE 1. Physical Properties and O:C Ratios of r-Pinene SOA and β-Pinene SOA Produced in the Flowtube Reactor r-pinene SOA initial O3 concentration, ppbv 6

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number concentration, 10 no./cm mass concentration,10-6 g/m3 mode diameter (number), nm geometric standard deviation (number) mode diameter (mass), nm geometric standard deviation (mass) measured O:C ratio (NAMS) 95% confidence interval for O:C ratio

200

240

300

450

550

280

700

0.15 3.5 34 1.5 42 1.3 0.38 0.02

0.39 14 37 1.5 49 1.3 0.38 0.03

0.93 49 44 1.5 53 1.3 0.36 0.02

2.7 187 48 1.5 56 1.3 0.34 0.01

3.7 279 49 1.4 56 1.3 0.35 0.02

1.30 6 18 1.3 21 1.3

6.50 157 30 1.5 40 1.4

(99%; Sigma-Aldrich) was approximately 45 ppm. Because the monoterpene concentration could not be monitored in real time, the experiment was performed with monoterpene in excess so that fluctuations in its concentration did not significantly alter the mass loading or molecular composition. Table 1 lists the number concentration, mass concentration, and mode diameters of SOA produced under various conditions. Each experiment was repeated at least two times. Aerosol was collected with a microsampling assembly that was partially described previously (8, 21). Aerosol flowed into an aerodynamic lens assembly at a rate of 100 cm3/min; the lens assembly focused particles up to ∼500 nm diameter into a tight beam. A U-shaped collection well with dimensions 2.4 mm wide and 1.7 mm deep was machined on the surface of the stainless steel collection plate and properly positioned so that the particle beam exiting the aerodynamic lens deposited inside the well. Since the microsampling assembly was under vacuum (ca. 10-2 Torr) during the particle collection, volatile and semivolatile compounds were assumed to be evaporated quickly minimizing artifacts such as on-plate oligomerization of volatile or semivolatile monomers. Therefore, the collected aerosol was assumed to be characteristic of a 23 s reaction time independent of the length of time needed to collect a suitable mass. After the desired aerosol mass was sampled, the plate was removed from vacuum and extraction solvent was added to the well. Throughout the experiments, both the aerosol mass sampled (about 2 µg) and volume of extraction solvent added (2 µL, acetonitrile/water, 50/50 by volume) were kept constant to minimize concentration dependent artifacts in the ESI mass spectra. Acetonitrile was used rather than methanol to avoid the possibility of reaction with carbonyl or carboxylic acid groups to form hemiacetals, acetals, and esters (22). The collection time ranged from 2 to 120 h, with the highest collection time required for the smallest mass loading. To ensure that sample residence time in the vacuum was not a significant factor in mass loading dependent changes in composition, some samples were held under vacuum for an additional 30-120 h and compared to similar samples held in vacuum only long enough to collect the desired mass. No changes were observed in the mass spectra within the inherent experiment-to-experiment variation (8). Direct Infusion Mass Spectrometry. ESI in both positive and negative ion modes was performed with a Bruker Daltonics 7 T Apex Qe FTICR-MS (Billerica, MA) equipped with a Bruker NanoElectrospray ion source that required less than 1 µL of solution for analysis. The CID potential in the ion source was found to affect the relative intensities of monomers, monomer clusters, and oligomers. Monomer clusters (e.g., the dimers of pinic or pinonic acid produced by electrospray of the corresponding solution) were observed with zero potential applied, but were completely eliminated when a - 3 V potential was applied in ( ESI mode, respectively. Accordingly, a 3 V potential was used to minimize the effect of cluster formation in the source. A higher potential of 10 V was used in MSMS experiments to 7898

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enhance product ion detection. The CID source potential dependence observed in this work is similar to that reported previously using atmospheric pressure chemical ionization (23). Typically, data were acquired for 2.5 min, over which 50 spectra were summed. Mass spectra were externally calibrated with ESI tuning mix (G2421A; Agilent technologies, Santa Clara, CA) and then internally calibrated when specific chemical species were identified with the Bruker Daltonics DataAnalysis software (v3.4). The root mean-square error was less than 2 ppm. Only peaks between 150 and 1000 m/z with a signal-to-noise ratio greater than 4 and a relative intensity greater than 0.01% were analyzed. Accurate m/z of individual peaks were assigned molecular formulas with a home-developed Matlab program, the logistic of which is similar to Molecular Formula Calculator v1.0 (National High Magnetic Field Laboratory 1998, Tallahassee, FL). Assigned formulas were required to be within 5 ppm of the measured mass and contained only oxygen, carbon (12C and up to one 13 C), hydrogen, and, for positive ion spectra, up to one sodium. The average error between the measured and assigned masses was less than 1 ppm. MSMS spectra were obtained by collision induced dissociation (CID) of selected precursor ions in a collision cell upstream of the ICR cell. Product ions were analyzed in the ICR cell to perform high accuracy and precision mass analysis. Most MSMS spectra were obtained with the FTICR, except for analysis of the 171.1 m/z (-) ion, which was obtained with a nanoESI Q-Tof API-US (Waters Corporation, Milford, MA) because of its greater sensitivity. The nano aerosol mass spectrometer (NAMS) (24) was used for online measurement of the O:C atomic ratios of individual particles as described in previous experiments (17, 18, 20). For each mass loading, the spectra of one thousand 25-nm (mass normalized diameter) particles were averaged. Liquid Chromatography Mass Spectrometry (LCMS). LCMS was performed with a liquid chromatograph (LC-20AD; Shimadzu, Columbia, MD) and a quadrupole time-of-flight mass spectrometer (ESI Q-Tof API-US; Waters Corporation). Separations were performed with a C18 column (Jupiter 5 µm C18 300 Å 250 × 1.0 mm; Phenomenex, Inc., Torrance, CA). The eluents were water (A) and methanol (B), programmed as 0-7 min 5% B, 7-17 min 5-95% B, 17-27 min 95% B, then 27-45 min 5% B. The column temperature was kept at room temperature; the flow rate was 0.030 mL/min with an injection volume of 20 µL. Because LCMS analysis required much more sample than ESI-FTICR, only one mass loading was studied (49 µg/m3). About 40 µg of SOA at this mass loading was collected and dissolved by 45 µL of extraction solvent (acetonitrile/water, 50/50 by volume).

Results and Discussion The positive and negative ion ESI-FTICR mass spectra of SOA from R-pinene ozonolysis are shown in Figure 1 for two mass loadings to illustrate the mass loading dependence. The mass spectra at all mass loadings were similar in character

FIGURE 1. High resolution nanoESI FTMS mass spectra of r-pinene SOA generated in the flow tube reactor at 187 µg/m3, (a) in negative mode and (b) in positive mode; and at 3.5 µg/m3, (c) in negative mode and (d) in positive mode. The stars in the negative ion spectra indicate C17H25O8- and those in the positive ion spectra indicate C17H26O8Na+. to those in the literature (8, 9, 17). Ions were observed continuously from 150 to 1300 m/z in positive mode and from 150 to 900 m/z in negative mode. All peaks were singly charged with multiple peaks detected at many nominal m/z values. Where possible, peaks were assigned a unique isotopic formula using the approach of Heaton et al. (8). Using the negative ion spectra as an example, after the formulas differing only by isotopic composition (e.g., 12C vs 13C) were combined, on average ∼950 unique molecular formulas were assigned for each sample. Focusing just on the 150 to 500 m/z range, ∼400 unique molecular formulas were assigned per sample and of these 282 were common among all samples analyzed. These 282 molecular formulas typically encompassed >90% of the total signal intensity of each sample. The nonmatching formulas could reflect sample-to-sample variations in oligomer formation chemistry or formula misassignments caused by very low signal-to-noise ratios. While there was a high degree of similarity among the assigned molecular formulas at different mass loadings, Figure 1 illustrates that the relative signal intensities changed substantially. In particular, the signal intensities of 357.1550 m/z in the negative ion spectrum and 381.1525 m/z in the positive ion spectrum increased relative to other ions in the spectra as the mass loading decreased. These ions were assigned as [C17H25O8]- and [C17H26O8Na]+, which corresponded to (M - H)- and (M + Na)+ of the same molecular formula, C17H26O8. Collision-induced dissociation of

[C17H25O8]- was performed to gain insight into the molecular structure. An advantage of FTICR for this experiment was the ability to perform accurate mass analysis of the product ions, thereby allowing molecular formulas to be assigned. As shown in Figure 2, there were two main product ions of [C17H25O8]- dissociation, and the accurate masses corresponded to [C9H13O4]- (171.0657 m/z) and [C8H11O4](185.0814 m/z). Ions with these molecular formula assignments were also found within the monomer regions of the spectra. These observations suggest that C17H26O8 is a dimer of C9H14O4 and C8H12O4. Other features of the mass spectra were consistent with this assignment. Notable minor product ions in the MSMS spectrum of Figure 2 corresponded to loss of H2O, CO2, and CO2H2 from [C17H25O8]- and secondary loss of H2O, CO2, and CO2 + H2O from each of the main product ions, [C9H13O4]and [C8H11O4]-. When [C9H13O4]- and [C8H11O4]- monomer ions were isolated and subjected to collision-induced dissociation, they also showed loss of H2O, CO2, and CO2 + H2O. Because of its low signal intensity, the [C8H11O4]monomer ion was analyzed by MSMS on the Q-ToF instrument having higher sensitivity than the FTICR. The [C9H13O4]ion has the same molecular formula as pinic acid, a commercially available product derived from R-pinene oxidation. When the [C9H13O4]- ion from ESI of a pinic acid solution was dissociated under the same conditions as the corresponding monomer ion from Figure 1, the product ion VOL. 44, NO. 20, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 2. MSn spectra of 357.2 m/z(-) from r-pinene SOA by collision-induced dissociation. The inset shows the structure of the neutral molecule reported in ref 24 and the likely location of fragmentation to produce monomer product ions. spectrum was the same with respect to molecular formula assignments and relative intensities. However, we cannot rule out that [C9H13O4]- from the SOA sample is a structural isomer of pinic acid, for example homoterpenylic acid (24). Because of low signal intensities, it was not possible to perform MSMS on other ions in the dimer region of the spectrum. However, it was possible to mathematically search for monomer combinations that could give rise to the assigned dimer ions according to the equation a + b + H ) c (for negative mode) where a and b are monomer ions observed in the spectrum, c is a dimer ion observed in the spectrum, and H represents the hydrogen ion difference between two negatively charged monomers and one negatively charged dimer. With this approach, over 70% of assigned molecular formulas in the dimer region of the 3.5 µg/m3 negative ion spectrum could be attributed to the combination of two detected monomers. Some of the more intense dimer ions were [C18H29O9]- (m/z 387.1655) from [C10H13O4]- and [C8H15O5]-, [C19H27O7]- (m/z 367.1757) from [C10H13O3]- and [C9H13O4]-, [C17H25O7]- (m/z 341.1600) from [C9H13O4]- and [C8H11O3]-, and [C19H29O5]- (m/z 337.2015) from [C10H15O3]- and [C9H13O2]-. To confirm that [C17H25O8]- is not an artifact of ESI analysis by in-source clustering, LCMS was performed on a SOA sample produced at a mass loading of 49 µg/m3. This mass loading was chosen because of the relative ease of collecting a large amount of sample (40 µg for LCMS vs 2 µg for ESIFTICR). Figure 3 shows selected ion chromatograms for 357.2 m/z(-) and its two monomer building blocks 185.1 m/z(-) and 171.1 m/z(-). All three compounds elute at different times, proving that C17H26O8 is not produced by in-source clustering of C9H14O4 and C8H12O4. The area of the dimer peak is approximately 20 times greater than the areas of each monomer peak. If it is assumed that ESI signal intensities scale approximately with concentration (25), the dimer appears to be present in much higher amounts than either monomer. Furthermore, there are two distinct dimer peaks in the chromatogram indicating the possibility of at least two molecular structures. β-Pinene SOA at Low Mass Loading. The negative ion spectra of β-pinene SOA with lower (6 µg/m3) and higher (157 µg/m3) mass loadings are shown in Figure S1 (Supporting Information). In general, the trend for β-pinene SOA was very similar to that for R-pinene SOA. A single ion assigned as [C17H25O8]- became prominent at low mass loading while the relative intensities of other oligomer ions decreased. 7900

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MSMS of this ion was similar to that of R-pinene in Figure 2, suggesting that C17H16O8 is a dimer of C9H14O4 and C8H12O4. About 40% of assigned dimer peaks in the low mass loading spectrum could be explained by the direct combination of two compounds detected in the monomer portion of the spectrum. Particle Formation Mechanism. As described by Pankow (26), new particle formation occurs when nonvolatile organics self-nucleate from the gas phase. These molecular clusters grow quickly at first and then more slowly as the gas phase becomes depleted of lower volatility matter. For simplicity, “particle formation” in the discussion below refers to both nucleation and the early stage of growth, leading to the formation of a particle above the minimum detectable size (∼3 nm in this work). Figure 1 suggests that C17H26O8, a dimer of C9H14O4 and C8H12O4, is a key contributor to particle formation in flow tube SOA. This species is clearly favored at low mass loading and is consistent with several recent studies. Shilling et al. used AMS to characterize elemental composition changes associated with chamber SOA from R-pinene oxidation at low mass loading (7). The oxygen to carbon (O:C) atomic ratio was found to increase from 0.29 to 0.45 as the mass loading decreased from 140 to 0.5 µg/m3. Fitting the data to a four product partitioning model gave a lowest volatility component with an O:C ratio of 0.48, which matches C17H26O8 and its monomer components within experimental error. Table 1 gives the average O:C ratios of flow tube SOA studied in this work that were obtained with our nano aerosol mass spectrometer (NAMS). The O:C ratios are fairly constant, ranging from 0.34 at the higher mass loading end to 0.38 at the lower mass loading end. These values are similar to the AMS measured O:C ratio (0.38) for chamber SOA at 15 µg/m3 (7). The relatively constant ratios obtained by NAMS are not surprising considering the decrease in aerosol mass loading is primarily caused by a decrease in particle number concentration, not size distribution. In this work, NAMS was restricted to analysis of 25 nm mass normalized diameter particles (27). A previous study with NAMS of flow tube SOA from R-pinene ozonolysis showed that the O:C ratio increased with decreasing particle size, reaching a value of about 0.5 for 9 nm diameter particles. This value is consistent with the molecular formulas identified in the present work and suggests that C17H26O8 may represent a much larger portion of the total chemical composition in smaller particles. Whereas C17H26O8 increases in relative intensity with decreasing mass loading, it represents a small fraction of the total ion signal (and common molecular formulas) even at 3.5 µg/m3. Therefore, it is not surprising that the O:C ratio changes in the direction of C17H26O8 as the mass loading decreases, but only modestly. Hoffmann et al. initially reported a 357 m/z(-) species in chamber SOA from R-pinene ozonolysis that dissociated to 185 m/z(-) and 171 m/z(-) (12). More recently, Yasmeen et al. detected a 357 m/z(-) product in both chamber SOA and ambient fine particles (PM2.5) during summer nights at K-puszta, Hungary (24). In combination with authentic molecular standards and LCMS analysis, they identified that this product in chamber SOA is a diester of terpenylic acid (C8H12O4) and cis-pinic acid (C9H14O4), which are early oxidation products of R-pinene ozonolysis. The results of both studies are consistent with the molecular formulas identified in this work for C17H26O8 and its monomer components. Although much effort has been expended toward understanding average O:C ratios in laboratory and ambient aerosols, H:C ratios have also been measured and interpretation of these values has been problematic. Shilling et al. reported H:C ratios below 1.40 for chamber R-pinene SOA when the mass loading was below 15 µg/m3 (7). Fitting the

FIGURE 3. Selected ion chromatograms in LCMS negative ion analysis of the 49 µg/m3 sample of r-pinene SOA: (a) 171.1 m/z(-), 65 total counts under peak; (b) 185.1 m/z(-), 58 total counts under peak; (c) 357.2 m/z(-), 1460 total counts under peak. data to a four product partitioning model gave a lowest volatility component with an H:C ratio of 1.32. These ratios are much lower than expected for monomer and dimer products, as noted by Chan et al. who could not fit the measured H:C ratios to product-specific models of SOA formation (28). Insight into this discrepancy can be gained from Figures S2 and S3, which show van Krevelen plots of the molecular formulas observed in the positive and negative spectra of 3.5 µg/m3 flow tube SOA. These plots are similar to those obtained in our previous work with high mass loading SOA (8). Most of the signal intensity is encompassed within a composition domain associated with monomers (O:C ratio between about 0.2 to 0.6 and H:C ratio between about 1.4 and 1.8). Two other composition domains can be identified. The first encompasses O:C and H:C ratios that are both smaller than the main composition domain; the second encompasses O:C and H:C ratios that are both larger than the main composition domain. Compounds in these domains may be products of lower probability “side reactions” leading to oligomerization (8). Hardly any compounds are found in Figures S2 and S3 that have both an O:C ratio in the 0.4-0.45 range and an H:C ratio in the 1.3 to 1.4 range, and the compounds that do exist in this region of the plot have very low signal intensities in the ESI mass spectra. Figures S2 and S3 suggest that the average O:C and H:C ratios determined by Shilling et al. (7) do not arise from one or a few specific compounds but rather from a complex combination of compounds in the three composition domains. Therefore, modeling the change in H:C ratio with mass loading will likely require assessment of these minor reaction pathways. In these experiments, ozone was the limiting reactant and the particle number concentration was found to increase linearly with the ozone concentration (Table 1 and Figure S4). If it is assumed that one ozone molecule reacts with one R-pinene molecule to give an SOA precursor molecule, the relationship between number concentration and ozone concentration gives insight into the particle formation process. If particle formation was limited by production of a gas phase dimer, one would expect a quadratic relationship between the two concentrations. Therefore, gas phase dimerization does not appear to be the rate limiting step. Instead, the observed linear relationship leads us to speculate

that a single SOA precursor molecule “activates” particle formation by inducing heterogeneous dimer formation within a small cluster of molecules. The activation mechanism for particle formation was originally suggested by Kulmala (29) to explain the sometimes observed linear relationship between gas phase sulfuric acid concentration and particle concentration. Recently, there has been great interest in the potential contribution of new particle formation to ambient concentrations of cloud condensation nuclei (30). A growing body of evidence suggests that organic compounds contribute substantially to particle growth in the sub-10 nm size range (31). While most monomer products of monoterpene oxidation are too volatile to induce new particle formation, the oligomers characterized in this experiment and elsewhere possess the requisite properties for inducing particle formation. Experimental Artifacts. An important concern in work of this type is the possible role of ESI artifacts such as signal quenching and in-source ion-molecule reaction (clustering). A variation in signal quenching, or signal suppression of certain analyte ions due to insufficient charge on droplets, is unlikely to be the source of the mass loading dependence in Figure 1 as the same mass was collected in each experiment and identical sample preparation/data acquisition procedures were used. Noncovalent molecular complexes are known to form by in-source clustering and have been discussed in detail for terpenylic acid and related compounds (11). However, for the experimental conditions used in the present work (CID potential of 3 V), the signal intensity ratios of dimers to monomers from the SOA samples are at least 1-2 orders of magnitude higher than the ratio of in-source clusters to monomers from samples such as pinic and pinonic acids (see Experimental Section). Furthermore, LCMS analysis shows that the specific dimer of interest, C17H26O8, is not an artifact of in-source clustering. A second concern is the relationship between laboratory and ambient SOA. Problems associated with interpreting laboratory experiments in the context of ambient aerosol have been discussed by Rudich et al (5). Performing laboratory experiments at a high organic mass loading runs the risk of introducing new reaction channels that are caused by increased partitioning of semivolatile compounds to the VOL. 44, NO. 20, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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particle phase relative to ambient aerosol. This problem was minimized in the present study by characterizing laboratory aerosol at a total organic mass loading relevant to ambient aerosol. Another problem is the high reactant concentration inherent to a flow tube study. In the present study, high monoterpene and ozone concentrations were used to increase the reaction rate. The inherent assumption is that a fast reaction rate process studied over a short reaction time adequately mimics a slow reaction rate process that proceeds over a long period of time. These two types of experiments are most likely to be equivalent under pseudofirst-order reaction conditions where higher order chemistry does not occur. In the present study, the experiment was designed to keep one reactant, the monoterpene vapor, in excess with ozone as the limiting reactant. The linear relationship between particle number concentration (the “product”) and ozone concentration (limiting reactant) argues against the influence of deleterious higher order chemistry. The prevalence of C17H26O8 in both chamber SOA (studied previously, 12, 24) and flow tube SOA (studied here) illustrates the important role it plays in particle formation over a wide range of experimental conditions.

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Acknowledgments This research was supported by NSF Grant CHE-0808972.

Supporting Information Available

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Four figures referenced in the text. This information is available free of charge via the Internet at http://pubs. acs.org/.

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