Environ. Sci. Technol. 2009, 43, 4744–4749
Intermediate-Volatility Organic Compounds: A Potential Source of Ambient Oxidized Organic Aerosol ALBERT A. PRESTO,† MARISSA A. MIRACOLO,† JESSE H. KROLL,‡ DOUGLAS R. WORSNOP,‡ ALLEN L. ROBINSON,† AND N E I L M . D O N A H U E * ,† Center for Atmospheric Particle Studies, Carnegie Mellon University, Pittsburgh, Pennsylvania and Aerodyne Research, Inc., Billerica, Massachusetts
Received November 14, 2008. Revised manuscript received February 9, 2009. Accepted March 6, 2009.
Smog chamber experiments were conducted to investigate secondary organic aerosol (SOA) formation from intermediate volatility and semivolatile organic compounds (IVOCs and SVOCs). We present evidence for the formation of highly oxygenated SOA from the photooxidation of n-heptadecane, which is used as a proxy for IVOC emissions. The SOA is consistent with multiple generations of oxidation chemistry resulting from OH radical exposure equivalent to ∼0.5 days of atmospheric processing under high-NOx and low-COA conditions. The SOA has a calculated O/C ratio of 0.59, which is higher than typical for chamber-generated SOA. The mass spectrum of the SOA, as measured with a high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS), is similar to the OOA-2 factor determined for Mexico City. SOA formed from the low-NOx, lowCOA oxidation of n-heptadecane is less oxidized because of differences in the chemical mechanism and lower integrated OH exposure. SOA formed from both the oxidation of n-heptadecane under high-NOx, high-COA conditions and the oxidation of n-pentacosane, a proxy for semivolatile organic emissions, does not produce highly oxygenated SOA, largely because of the condensation of early generation oxidation products.
Introduction Recent observations indicate that a significant fraction of both ambient (1, 2) and laboratory (3, 4) secondary organic aerosol (SOA) cannot be explained by current models. Robinson et al. (5) provided evidence that the missing SOA can be explained by the evaporation and subsequent oxidation of compounds traditionally associated with primary organic aerosol (POA). Specifically, they called attention to intermediate volatility and semivolatile organic compounds (IVOCs and SVOCs) as potential sources of SOA. IVOCs and SVOCs are defined according to their effective saturation concentration (C*, µg m-3) as 10-1 < C*SVOC < 103 and 103 < C*IVOC < 106, respectively (6). All of these species are less volatile than traditional SOA precursors such as light aromatics and monoterpenes, which are VOCs (C* > 106). * Corresponding author e-mail:
[email protected]. † Carnegie Mellon University. ‡ Aerodyne Research, Inc. 4744
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IVOCs exist almost completely as vapors at atmospheric conditions, while SVOCs partition between the vapor and condensed phases. Many of the IVOC and SVOC vapors in the atmosphere are thought to be saturated hydrocarbons. Therefore hydroxyl radical (OH) is likely the most important oxidant. While SVOC and IVOC vapors constitute only a few percent of the total hydrocarbon budget, their mass emissions are likely larger than POA emissions (5, 7). Due to their low initial volatility, aerosol mass yields from the oxidation of large organic species is high (8, 9). Therefore, we expect that IVOCs and SVOCs could be a significant source of ambient SOA. Recent results from this laboratory demonstrated the formation of oxidized organic aerosol from the photooxidation of fresh diesel exhaust (4, 10) and wood smoke (11). The oxidation of traditional SOA components, mostly light aromatics, accounted for a small fraction (8 ppbC/ppb-NOx for low-NOx experiments and 100 µg m-3 of SOA. In both cases, the contribution of m/z 44 was significantly reduced relative to low-COA conditions (Figure 1b). In addition, the contributions of m/z 29, 43, and 57 all increased. The SOA also contained significant particle mass at m/z 28. Dotriacontane is a solid; the results obtained with the dotriacontane seed suggest that it dissolved in the SOA. The data indicate that the SOA formed at high COA is less oxidized than SOA formed at low COA. This result is consistent with previous results (31) that showed a COA dependence of the m/z 44 contribution to R-pinene ozonolysis SOA. These experiments are also consistent with the results from the photooxidation of pentacosanesincreased partitioning of first-generation compounds to the particle phase leads to less oxidized SOA. This strongly suggests that the rate of heterogeneous oxidation of OA by OH radicals is significantly lower than the rate of gas-phase oxidation of organic vapors by OH. Mass transfer of the OH becomes rate limiting for heterogeneous processing. We might also expect the SOA formed in the high-NOx, high-COA oxidation of heptadecane to be more volatile than the SOA formed during high-NOx, low-COA oxidation. However, this is not the case. The SOA formed under conditions of both high- and low-COA show nearly identical behavior in the thermodenuder (Figure 4) despite significant differences in the mass spectrum. Atmospheric Implications. The results presented here have implications for our understanding of SOA formation in the atmosphere. Photooxidation of IVOCs is clearly a source of oxygenated OA at atmospherically relevant COA and time scale. The results obtained for the n-heptadecane model system can be extrapolated to similar IVOCs. These findings raise potential challenges to either the emerging interpretation of O/C in organic aerosol based on AMS spectra (18, 21) and thermodenuder data or our current understanding of hydrocarbon oxidation, which, at least for straight-chain alkanes, tends to preserve carbon number rather than induce substantial fragmentation (8, 32). The O/C and thermodenuder data we report here for high-NOx, low-COA oxidation of heptadecane is difficult to understand without substantial fragmentation in the SOA products. Highly oxygenated C17 compounds would be much less volatile than our thermodenuder results reveal, and it is also very difficult to understand how 10 oxygen atoms could attach to a carbon backbone in such a short time as we observe. The degree of volatility and oxidation is consistent with C9-C11 products. The mass yields of this SOA are relatively low, at very low mass loadings, so it remains possible that we are observing a minor pathwayshowever, for this to be true the major pathway products would have to be more volatile than the fragmentation products. Finally, our interpretation of the O/C, while consistent with the emerging body of knowledge concerning AMS behavior, involves a considerable amount of oxygen associated with the dehydration peaks at m/z 17 and 18, as one can clearly see in 4748
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Figure 2a. It is also based on calibration of the O/C observed in the electron impact cations versus the O/C of target molecules, though that calibration appears to be relatively robust. The NOx dependence presented here has interesting implications. In a succession of recent publications, our laboratory has put forth the hypothesis that IVOC compounds coemitted with high-temperature primary aerosol emissions (i.e., combustion) are potentially important sources of OOA (3–5, 10, 11). While oxidation of traditional anthropogenic and biogenic SOA precursors at high NOx shows reduced aerosol formation, we see the reverse effect for IVOCs and SVOCs. If modern or fossil IVOC carbon is more likely to form OA under high NOx conditions, this could partly explain the greatly increased OA levels in high-NOx regions (29).
Acknowledgments This work was supported by the EPA STAR program through the National Center for Environmental Research (NCER) under grant R833748 and the NSF under grant ATM-0748402. This paper has not been subject to EPA’s required peer and policy review, and therefore does not necessarily reflect the views of the Agency. No official endorsement should be inferred. We thank J.L. Jimenez for sharing ambient data for spectral comparisons.
Supporting Information Available Brief glossary of AMS-related terms and example of the calculation of organic aerosol mass at m/z 28. This information is available free of charge via the Internet at http:// pubs.acs.org.
Literature Cited (1) de Gouw, J. A.; Middlebrook, A. M.; Warneke, C.; Goldan, P. D.; Kuster, W. C.; Roberts, J. M.; Fehsenfeld, F. C.; Worsnop, D. R.; Canagaratna, M. R.; Pszenny, A. A. P.; Keene, W. C.; Marchewka, M.; Bertman, S. B.; Bates, T. S. Budget of organic carbon in a polluted atmosphere: Results from the New England Air Quality Study in 2002. J. Geophys. Res. 2005, 110; D16305, doi: 10.1029/ 2004JD005623. (2) Volkamer, R.; Jimenez, J. L.; San Martini, F.; Dzepina, K.; Zhang, Q.; Salcedo, D.; Molina, L. T.; Worsnop, D. R.; Molina, M. J. Secondary organic aerosol formation from anthropogenic air pollution: Rapid and higher than expected. Geophys. Res. Lett. 2006, 33, L17811; doi: 10.1029/2006GL026899. (3) Grieshop, A. P.; Logue, J. M.; Donahue, N. M.; Robinson, A. L. Laboratory investigation of photochemical oxidation of organic aerosol from wood fires - Part 1: Measurement and simulation of organic aerosol evolution. Atmos. Chem. Phys. 2009, 9, 1263– 1277. (4) Weitkamp, E. A.; Sage, A. M.; Donahue, N. M.; Robinson, A. L. Organic aerosol formation from photochemical oxidation of diesel exhaust in a smog chamber. Environ. Sci. Technol. 2007, 41, 6969–6975. (5) Robinson, A. L.; Donahue, N. M.; Shrivastava, M. K.; Weitkamp, E. A.; Sage, A. M.; Grieshop, A. P.; Lane, T. E.; Pierce, J. R.; Pandis, S. N. Rethinking organic aerosols: Semivolatile emissions and photochemical aging. Science 2007, 315, 1259–1262. (6) Donahue, N. M.; Robinson, A. L.; Stanier, C. O.; Pandis, S. N. Coupled Partitioning, Dilution, and Chemical Aging of Semivolatile Organics. Environ. Sci. Technol. 2006, 40, 2635–2643. (7) Shrivastava, M. K.; Lane, T. E.; Donahue, N. M.; Pandis, S. N.; Robinson, A. L. Effects of gas particle partitioning and aging of primary emissions on urban and regional organic aerosol concentrations. J. Geophys. Res. 2008, 113, D18301; doi: 10.1029/ 2007JD009735. (8) Lim, Y. B.; Ziemann, P. J. Products and mechanism of secondary organic aerosol formation from reactions of n-alkanes with OH radicals in the presence of NOx. Environ. Sci. Technol. 2005, 39, 9229–9236. (9) Ng, N. L.; Chhabra, P. S.; Chan, A. W. H.; Surratt, J. D.; Kroll, J. H.; Kwan, A. J.; McCabe, D. C.; Wennberg, P. O.; Sorooshian, A.; Murphy, S. M.; Dalleska, N. F.; Flagan, R. C.; Seinfeld, J. H. Effect of NOx level on secondary organic aerosol (SOA) formation from the photooxidation of terpenes. Atmos. Chem. Phys. 2007, 7, 5159–5174.
(10) Sage, A. M.; Weitkamp, E. A.; Robinson, A. L.; Donahue, N. M. Evolving mass spectra of the oxidized component of organic aerosol: results from aerosol mass spectrometer analyses of aged diesel emissions. Atmos. Chem. Phys. 2008, 8, 1139–1152. (11) Grieshop, A. P.; Donahue, N. M.; Robinson, A. L. Laboratory investigation of photochemical oxidation of organic aerosol from wood fires - Part 2: Analysis of aerosol mass spectrometer data. Atmos. Chem. Phys. Discuss. 2008, 8, 17095–17130. (12) Kadowaki, S. Characterization of carbonaceous aerosols in the Nagoya Urban Area. 2. Behavior and origin of particulate n-alkanes. Environ. Sci. Technol. 1994, 28, 129–135. (13) Pankow, J. F.; Asher, W. E. SIMPOL.1: a simple group contribution method for predicting vapor pressures and enthalpies of vaporization of multifunctional organic compounds. Atmos. Chem. Phys. 2008, 8, 2773–2796. (14) Canagaratna, M. R.; Jayne, J. T.; Jimenez, J. L.; Allan, J. D.; Alfarra, M. R.; Zhang, Q.; Onasch, T. B.; Drewnick, F.; Coe, H.; Middlebrook, A.; Delia, A.; Williams, L. R.; Trimborn, A. M.; Northway, M. J.; DeCarlo, P. F.; Kold, C. E.; Davidovits, P.; Worsnop, D. R. Chemical and microphysical characterization of ambient aerosols with the Aerodyne aerosol mass spectrometer. Mass Spectrom. Rev. 2007, 26, 185–222. (15) Jayne, J. T.; Leard, D. C.; Zhang, X. F.; Davidovits, P.; Smith, K. A.; Kolb, C. E.; Worsnop, D. R. Development of an aerosol mass spectrometer for size and composition analysis of submicron particles. Aerosol Sci. Technol. 2000, 33, 49–70. (16) DeCarlo, P. F.; Kimmel, J. R.; Trimborn, A.; Northway, M. J.; Jayne, J. T.; Aiken, A. C.; Gonin, M.; Fuhrer, K.; Horvath, T.; Docherty, K. S.; Worsnop, D. R.; Jimenez, J. L. Field-Deployable, High-Resolution, Time-of-Flight Aerosol Mass Spectrometer. Anal. Chem. 2006, 78, 8281–8289. (17) Allan, J. D.; Delia, A. E.; Coe, H.; Bower, K. N.; Alfarra, M. R.; Jimenez, J. L.; Middlebrook, A. M.; Drewnick, F.; Onasch, T. B.; Canagaratna, M. R.; Jayne, J. T.; Worsnop, D. R. A generalised method for the extraction of chemically resolved mass spectra from Aerodyne aerosol mass spectrometer data. J. Aerosol Sci. 2004, 35, 909–922. (18) Aiken, A. C.; DeCarlo, P. F.; Kroll, J. H.; Worsnop, D. R.; Huffman, J. A.; Docherty, K. S.; Ulbrich, I. M.; Mohr, C.; Kimmel, J. R.; Sueper, D.; Sun, Y.; Zhang, Q.; Trimborn, A.; Northway, M.; Ziemann, P. J.; Canagaratna, M. R.; Onasch, T. B.; Alfarra, M. R.; Prevot, A. S. H.; Dommen, J.; Duplissy, J.; Metzger, A.; Baltensperger, U.; Jimenez, J. L. O/C and OM/OC Ratios of Primary, Secondary, and Ambient Organic Aerosols with High-Resolution Time-of-Flight Aerosol Mass Spectrometry. Environ. Sci. Technol. 2008, 42, 4478–4485. (19) Miracolo, M. A.; Presto, A. A.; Donahue, N. M.; Kroll, J. H.; Worsnop, D. R.; Robinson, A. L. Secondary organic aerosol formation from surrogates for motor vehicle emissions Environ. Sci. Technol., in preparation. (20) Atkinson, R.; Arey, J. Atmospheric degradation of volatile organic compounds. Chem. Rev. 2003, 103, 4605–4638.
(21) Aiken, A. C.; DeCarlo, P. F.; Jimenez, J. L. Elemental Analysis of Organic Species with Electron Ionization High-Resolution Mass Spectrometry. Anal. Chem. 2007, 79, 8350–8358. (22) Zhang, Q.; Alfarra, M. R.; Worsnop, D. R.; Allan, J. D.; Coe, H.; Canagaratna, M. R.; Jimenez, J. L. Deconvolution of hydrocarbonlike and oxygenated organic aerosols based on aerosol mass spectrometry. Environ. Sci. Technol. 2005, 39, 4938–4952. (23) Huffman, J. A.; Ziemann, P. J.; Jayne, J. T.; Worsnop, D. R.; Jimenez, J. L. Development and characterization of a faststepping/scanning thermodenuder for chemically-resolved aerosol volatility measurements. Aerosol Sci. Technol. 2008, 42, 395–407. (24) An, W. J.; Pathak, R. K.; Lee, B.-H.; Pandis, S. N. Aerosol volatility measurement using an improved thermodenuder: Application to secondary organic aerosol. J. Aerosol Sci. 2007, 38, 305–314. (25) Grieshop, A. P.; Miracolo, M. A.; Donahue, N. M.; Robinson, A. L. Using thermal denuder and dilution data to characterize gas-particle partitioning of combustion aerosols. Environ. Sci. Technol. 2009, submitted. (26) Jordan, C. E.; Ziemann, P. J.; Griffin, R. J.; Lim, Y. B.; Atkinson, R.; Arey, J. Modeling SOA formation from OH reactions with C8-C17 n-alkanes. Atmos. Environ. 2008, 42, 8015–8026. (27) Kroll, J. H.; Seinfeld, J. H. Chemistry of secondary organic aerosol: Formation and evolution of low-volatility organics in the atmosphere. Atmos. Environ. 2008, 42, 3593–3624. (28) Song, C.; Na, K.; Cocker, D. R. Impact of the Hydrocarbon to NOx Ratio on Secondary Organic Aerosol Formation. Environ. Sci. Technol. 2005, 39, 3143–3149. (29) Zhang, Q.; Jimenez, J. L.; Canagaratna, M. R.; Allan, J. D.; Coe, H.; Ulbrich, I.; Alfarra, M. R.; Takami, A.; Middlebrook, A. M.; Sun, Y. L.; Dzepina, K.; Dunlea, E.; Docherty, K.; DeCarlo, P. F.; Salcedo, D.; Onasch, T.; Jayne, J. T.; Miyoshi, T.; Shimono, A.; Hatakeyama, S.; Takegawa, N.; Kondo, Y.; Schneider, J.; Drewnick, F.; Borrmann, S.; Weimer, S.; Demerjian, K.; Williams, P.; Bower, K.; Bahreini, R.; Cottrell, L.; Grffin, R. J.; Rautiainen, J.; Sun, J. Y.; Zhang, Y. M.; Worsnop, D. R. Ubiquity and dominance of oxygenated species in organic aerosols in anthropogenically-influenced Northern Hemisphere midlatitudes. Geophys. Res. Lett. 2007, 34, L13801; doi: 10.1029/ 2007GL029979. (30) Luxenhofer, O.; Schneider, M.; Dambach, M.; Ballscmiter, K. Semivolatile long chain C6-C17 alkyl nitrates as trace compounds in air. Chemosphere 1996, 33, 393–404. (31) Grieshop, A. P.; Donahue, N. M.; Robinson, A. L. Is the gasparticle partitioning in alpha-pinene secondary organic aerosol reversible? Geophys. Res. Lett. 2007, 34, L14810; doi: 10.1029/ 2007GL029987. (32) Aumont, B.; Szopa, S.; Madronich, S. Modelling the evolution of organic carbon during its gas-phase tropospheric oxidation: development of an explicit model based on a self generating approach. Atmos. Chem. Phys. 2005, 5, 2497–2517.
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