Grantee Research Project Results
Final Report: Formation of Secondary Organic Aerosol
EPA Grant Number: R833749Title: Formation of Secondary Organic Aerosol
Investigators: Seinfeld, John
Institution: California Institute of Technology
EPA Project Officer: Chung, Serena
Project Period: October 1, 2007 through September 30, 2010 (Extended to September 30, 2011)
Project Amount: $600,000
RFA: Sources and Atmospheric Formation of Organic Particulate Matter (2007) RFA Text | Recipients Lists
Research Category: Air Quality and Air Toxics , Particulate Matter , Air
Summary/Accomplishments (Outputs/Outcomes):
Atmospheric aerosols, worldwide, comprise about 50% organic compounds. Of this fraction, as much as 90% is the result of gas-phase oxidation of volatile organic compounds (VOCs) to yield low-volatility products that partition to the aerosol phase, so-called secondary organic aerosol (SOA). The classes of VOCs that, upon oxidation, can potentially lead to SOA include anthropogenic compounds, such as aromatics and alkanes, and biogenic compounds, such as isoprene, terpenes, and sesquiterpenes. It is well recognized that the most uncertain factor in predicting future climate is the effect on global radiative forcing attributable to aerosols. At the urban and regional scale, elevated airborne particle concentrations are associated with adverse human health effects. For these reasons, understanding the sources and formation mechanisms of SOA is considered perhaps the outstanding problem in atmospheric chemistry.
The overall goal of the present grant was to further our understanding of the formation of SOA and to translate that understanding into models that can be included in regional and global atmospheric chemical transport models to predict atmospheric aerosol levels. The principal method for studying VOC oxidation and SOA formation is the laboratory chamber, data from which constitute the “gold standard” in understanding SOA formation. The laboratory chamber is a large chemical reactor that is accessed by a number of instruments for real-time measurement of the gas and particle phases. The main effort during the present grant was to conduct a large number of carefully designed and extensively measured chamber experiments to probe the chemistry and physics of SOA formation. One of the important findings that emerged from the present grant is the key importance of carrying out simultaneous gas- and particle-phase mass spectrometer measurements during the course of chamber experiments. Such measurements allow one to track the dynamics of VOC oxidation products as they form and transfer to the aerosol phase. During the present grant, we were able to report unprecedented gas- and particle-phase mass spectrometer data for the important isoprene system.
It has been well established by studies in our laboratory, and others, that SOA formation from VOC oxidation depends critically on the level of NOx in the system. This dependence arises from the essential role played by the peroxy radicals formed in the initial OH oxidation step of the VOC. At sufficiently high NOx levels, the peroxy radicals react with NO and NO2, whereas at sufficiently low levels of NOx, peroxy radicals preferentially self-react, usually with the HO2 radical. These different routes can have profound implications in terms of the amount and chemical composition of the SOA formed. Elucidating the effect of the NOx level on SOA formed in a number of important VOC systems was one of the major accomplishments of the present grant.
The VOC systems investigated experimentally in the present grant included isoprene, glyoxal, naphthalene and alkyl naphthalenes, α-pinene, β-pinene, d-limonene, delta 3-carene, methyl vinyl betone, and methacrolein. The data reported in the publications resulting from the present grant on SOA formation in these systems have been widely used.
One of the significant challenges is to translate findings from the laboratory into atmospheric models of SOA formation. Ideally, such models will predict the amount of SOA formed from a particular VOC given conditions such as NOx level, temperature, relative humidity, prevailing aerosol mass concentration, OH, ozone, and NO3 levels. In principle, an aerosol model would include a detailed gas-phase mechanism of oxidation of each parent VOC, following the multi-generation cascade of oxidation products. The model would then include a means of estimating the volatility of each oxidation product, so that its gas-particle partitioning can be explicitly calculated. Finally, to the extent of understanding, the model would include a treatment of aerosol-phase chemical reactions, which affect the overall volatility of the SOA. Such a so-called explicit model, even if all the chemical reactions and their rate constants were known, would present a computational challenge for 3-dimensional atmospheric models well beyond any computing capability available. At the opposite extreme lie models that represent SOA formation essentially empirically, based on the correlation of chamber data; these include the original Odum model and the Volatility Basis Set. Such models are tuned to actual data and have very modest computational requirements. The current versions of the EPA CMAQ model and the global GEOS-Chem model include such empirical models to represent SOA. In the present grant, Dr. Havala Pye, now a staff member at EPA, developed the most up-to-date empirical model of SOA formation from biogenic and anthropogenic VOCs. This model was incorporated into CMAQ and GEOS-Chem and used to predict global SOA production. Dr. Pye’s work showed the large sensitivity of global estimates of SOA formation to that from semivolatile VOCs. A current area of great importance is bridging the gap between the empirical models and the highly explicit models to produce the next generation of organic aerosol models, which treat explicitly observable properties, such as O:C and H:C atomic ratios of the SOA.
Conclusions:
A. Isoprene
One of the most important hydrocarbon precursors to SOA is isoprene. Despite the small molecular size of isoprene (C5H8), the mechanism of oxidation of isoprene by OH is exceedingly complex and depends critically on the prevailing NOx level. Paulot et al. (2008, 2009) developed a new mechanism constrained for the first time by time-resolved observations of an extensive suite of isoprene oxidation products from chamber experiments conducted in our laboratory). A condensed isoprene oxidation mechanism was also developed for use in global chemical transport models.
A previously unrecognized compound in the process by which isoprene produces SOA has been discovered. This work on the creation and effects of these compounds, called epoxides, appeared in the journal Science. Once released into the atmosphere, isoprene gets “oxidized or chewed on” by free-radical oxidants such as OH. It is this chemistry that was the focus of our study. In particular, the research was initiated to understand how the oxidation of isoprene can lead to formation of atmospheric secondary organic aerosol. A small fraction of the isoprene becomes secondary organic aerosol, but because isoprene emissions are so large, even this small fraction is important. Up until this current grant, the chemical pathways from isoprene to aerosol were not known. We discovered that when NOx levels are low, such as in forests and remote areas, this aerosol likely forms via chemicals known as epoxides. When these epoxides bump into particles that are acidic, the epoxides precipitate out of the atmosphere and stick to the particles. Because the acidity of the aerosols is generally higher in the presence of anthropogenic activities, the efficiency of converting the epoxides to aerosol is likely higher in polluted environments, illustrating yet another complex interaction between emissions from the biosphere and from humans. We were able to make this scientific leap forward owing to our own development of a new type of chemical ionization mass spectrometry (CIMS). These new CIMS methods open up a very wide range of possibilities for the study of new sets of compounds that scientists have been largely unable to measure previously, mainly because they decompose when analyzed with traditional techniques. The Caltech group also used oxygen isotopes-oxygen atoms with different numbers of neutrons in their nucleus, and thus different masses-to gain insight into the chemical mechanism yielding epoxides. Epoxides have remained unidentified so far because they have the same mass as another chemical that had been anticipated to form in isoprene oxidation, peroxide. The oxygen isotopes separated the peroxides from epoxides and further showed that as the epoxides form, OH is recycled to the atmosphere. Since OH is the atmosphere detergent, cleaning the atmosphere of many chemicals, the recycling has important implications for the overall oxidizing capacity of the atmosphere.
B. Glyoxal
Glyoxal, (CHO)2, was not considered as an SOA precursor until recently, when it was discovered that, upon absorption into an aqueous medium, glyoxal can react to form compounds of sufficiently low volatility to remain in the particle phase as SOA. The mechanism by which this occurs is not, however, well established. Galloway et al. (2008) report a series of glyoxal uptake experiments under both dark and irradiated conditions. Glyoxal oligomer formation was observed and found to be reversible under dark conditions. Evidence of carbon-nitrogen compounds in the aerosol phase was found; this is the first report of such SOA compounds resulting from the reaction of ammonium (from the seed aerosol) with organics. Under irradiated conditions organosulfates, carboxylic acids, and organic esters were identified in the aerosol.
C. SVOC and IVOC
Until recently, primary organic aerosol (OA) had been considered as nonvolatile. The Carnegie Mellon University group showed in 2007 that diesel emissions, which typify primary OA, are semivolatile and, upon volatilization, can undergo photooxidation to produce SOA. To assess the importance of this source, we undertook a study of the SOA formation from polycyclic aromatic hydrocarbons typical of diesel and other combustion exhaust. Chan et al. (2008) measured SOA production from photooxidation of naphthalene and the alklynaphthalenes in the Caltech chambers. Mass yields of SOA ranged from 20% (under high-NOx conditions) to 70% (under low-NOx conditions). Results were also extrapolated to emissions from wood combustion. We estimate that polycyclic aromatic hydrocarbons can account for over 50% of SOA formed from diesel exhaust and 80% of SOA from wood burning. The implications of this important finding to the global and regional budgets of SOA has assessed in the global chemical transport model, GEOS-Chem.
D. Mechanisms of SOA Formation (Organosulfates and nitrooxy organosulfates)
Organosulfates (e.g. hydroxy sulfate esters) and nitrooxy organosulfates are novel compounds that have only been recently detected and characterized in both laboratory-generated and ambient SOA. These compounds have been previously proposed to serve as ambient tracer compounds for the occurrence of biogenic SOA under acidic conditions in the atmosphere. Understanding the detailed chemical reaction mechanisms responsible for the formation of these compounds has been as an area of intense research, especially the last 1-2 years. We have shown that these compounds may contribute significantly to the organic mass fraction in ambient aerosol (i.e. upwards of 30% in certain locations). Since numerical models currently underpredict the amount of SOA mass observed in ambient aerosol, organosulfate and nitrooxy organosulfate formation potentially offers a missing source of SOA not currently accounted for in models. These compounds have been demonstrated by our group to form from the photooxidation, i.e. OH-initiated, nighttime (i.e. NO3-initiated) oxidation, and ozonolysis of biogenic VOCs, such as isoprene and monoterpenes, in the presence of acidified sulfate seed aerosol. Additionally, recent online aerosol mass spectrometry in our laboratory has demonstrated that the reactive uptake of volatile aldehydes, such as glyoxal, onto acidified sulfate seed aerosol forms organosulfates.
There has been some debate in the literature as to which intermediates (i.e. alcohols, epoxides, or aldehydes) primarily lead to the formation of organosulfate compounds in both laboratory-generated and ambient organic aerosol. It is possible that only one or all of these reaction intermediates lead to organosulfates; however, this is likely dependent on the reaction conditions employed (i.e. aerosol chamber experiment vs. bulk solution experiment). Much of the previous work has proposed that these compounds form from either the particle-phase esterification of a semivolatile oxidation product containing one or two hydroxyl groups with sulfuric acid, or by a semivolatile oxidation product containing an aldehyde (or keto group) that forms a gem-diol upon partitioning to the aerosol phase followed by the esterification with sulfuric acid. Some studies have also suggested that epoxides may play a role in forming these compounds. Surratt et al. (2008) demonstrated that nitrooxy organosulfates with MW 295 formed from the photooxidation (OH-initiated) and nighttime (NO3-initiated) oxidation of α-pinene in the presence of only highly acidified sulfate seed aerosol. These compounds have been detected in ambient aerosol collected from the southeastern U.S. and Europe; moreover, these compounds are sometimes the most abundant products observed in the ambient samples. Interestingly, these compounds have been previously observed to form more abundantly at night (where the NO3- initiated oxidation of α-pinene would dominate) than during the day. It was proposed by Surratt et al. (2008) that these nitrooxy organosulfates form from the reactive uptake of hydroxynitrate gas-phase oxidation products. The proposed hydroxynitrate gas-phase product was previously observed by online atmospheric pressure ionization mass spectrometry. Additionally, Surratt et al. (2008) also demonstrated that a nitrooxy organosulfate with MW 297 formed from the photooxidation of d-limonene only in the presence of highly acidified sulfate seed aerosol. This compound was proposed to form from the reactive uptake of a hydroxynitrate formed from the further oxidation of a d-limonene first-generation oxidation product (i.e. limonaketone). Surratt et al. (2008) confirmed in a separate experiment that the photooxidation of limonaketone in the presence of highly acidified sulfate seed aerosol indeed forms the observed nitrooxy organosulfate of MW 297 in d-limonaketone SOA. It was proposed that a hydroxynitrate gasphase product of limonaketone was responsible for these compounds, especially since this hydroxynitrate was observed in the gas phase by proton transfer mass spectrometry. The point is that it appears from chamber data that the particle-phase esterification of hydroxynitrates with sulfuric acid yield nitrooxy organosulfates. Ng et al. (2008) investigated SOA formation from the NO3-initiated oxidation of isoprene under dark conditions. Nitrooxy organosulfates were also also observed in these experiments.
Contributions to Understanding of Solutions to Environmental Problems
We have made significant progress in understanding the pathways of formation of the organic fraction of atmospheric aerosols. This understanding will allow one to relate VOC and primary organic aerosol emissions to ambient organic aerosol concentrations. This relation will aid the evaluation of emission abatement strategies to reduce ambient particulate matter levels.
Journal Articles on this Report : 22 Displayed | Download in RIS Format
Other project views: | All 37 publications | 22 publications in selected types | All 22 journal articles |
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Chan AWH, Galloway MM, Kwan AJ, Chhabra PS, Keutsch FN, Wennberg PO, Flagan RC, Seinfeld JH. Photooxidation of 2-methyl-3-buten-2-ol (MBO) as a potential source of secondary organic aerosol. Environmental Science & Technology 2009;43(13):4647-4652. |
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Chan AWH, Kautzman KE, Chhabra PS, Surratt JD, Chan MN, Crounse JD, Kurten A, Wennberg PO, Flagan RC, Seinfeld JH. Secondary organic aerosol formation from photooxidation of naphthalene and alkylnaphthalenes: implications for oxidation of intermediate volatility organic compounds (IVOCs). Atmospheric Chemistry and Physics 2009;9(9):3049-3060. |
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Chan AWH, Chan MN, Surratt JD, Chhabra PS, Loza CL, Crounse JD, Yee LD, Flagan RC, Wennberg PO, Seinfeld JH. Role of aldehyde chemistry and NOx concentrations in secondary organic aerosol formation. Atmospheric Chemistry and Physics 2010;10(15):7169-7188. |
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Chan MN, Chan AWH, Chhabra PS, Surratt JD, Seinfeld JH. Modeling of secondary organic aerosol yields from laboratory chamber data. Atmospheric Chemistry and Physics 2009;9(15):5669-5680. |
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Chhabra PS, Flagan RC, Seinfeld JH. Elemental analysis of chamber organic aerosol using an aerodyne high-resolution aerosol mass spectrometer. Atmospheric Chemistry and Physics 2010;10(9):4111-4131. |
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Chhabra PS, Ng NL, Canagaratna MR, Corrigan AL, Russell LM, Worsnop DR, Flagan RC, Seinfeld JH. Elemental composition and oxidation of chamber organic aerosol. Atmospheric Chemistry and Physics 2011;11(17):8827-8845. |
R833749 (Final) |
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Claeys M, Iinuma Y, Szmigielski R, Surratt JD, Blockhuys F, Van Alsenoy C, Boge O, Sierau B, Gomez-Gonzalez Y, Vermeylen R, Van der Veken P, Shahgholi M, Chan AWH, Herrmann H, Seinfeld JH, Maenhaut W. Terpenylic acid and related compounds from the oxidation of α-pinene: implications for new particle formation and growth above forests. Environmental Science & Technology 2009;43(18):6976-6982. |
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Galloway MM, Chhabra PS, Chan AWH, Surratt JD, Flagan RC, Seinfeld JH, Keutsch FN. Glyoxal uptake on ammonium sulphate seed aerosol:reaction products and reversibility of uptake under dark and irradiated conditions. Atmospheric Chemistry and Physics 2009;9(10):3331-3345. |
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Galloway MM, Huisman AJ, Yee LD, Chan AWH, Loza CL, Seinfeld JH, Keutsch FN. Yields of oxidized volatile organic compounds during the OH radical initiated oxidation of isoprene, methyl vinyl ketone, and methacrolein under high-NOx conditions. Atmospheric Chemistry and Physics 2011;11(21):10779-10790. |
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Galloway MM, Loza CL, Chhabra PS, Chan AWH, Yee LD, Seinfeld JH, Keutsch FN. Analysis of photochemical and dark glyoxal uptake: implications for SOA formation. Geophysical Research Letters 2011;38(17):L17811 (5 pp.). |
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Hallquist M, Wenger JC, Baltensperger U, Rudich Y, Simpson D, Claeys M, Dommen J, Donahue NM, George C, Goldstein AH, Hamilton JF, Herrmann H, Hoffmann T, Iinuma Y, Jang M, Jenkin ME, Jimenez JL, Kiendler-Scharr A, Maenhaut W, McFiggans G, Mentel TF, Monod A, Prevot ASH, Seinfeld JH, Surratt JD, Szmigielski R, Wildt J. The formation, properties and impact of secondary organic aerosol:current and emerging issues. Atmospheric Chemistry and Physics 2009;9(14):5155-5236. |
R833749 (2009) R833749 (Final) R833746 (2008) R833746 (2009) R833746 (2010) R833746 (Final) |
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Kautzman KE, Surratt JD, Chan MN, Chan AWH, Hersey SP, Chhabra PS, Dalleska NF, Wennberg PO, Flagan RC, Seinfeld JH. Chemical composition of gas and aerosol-phase products from the photooxidation of naphthalene. The Journal of Physical Chemistry A 2010;114(2):913-934. |
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Kroll JH, Seinfeld JH. Chemistry of secondary organic aerosol:formation and evolution of low-volatility organics in the atmosphere. Atmospheric Environment 2008;42(16):3593-3624. |
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Loza CL, Chan AWH, Galloway MM, Keutsch FN, Flagan RC, Seinfeld JH. Characterization of vapor wall loss in laboratory chambers. Environmental Science & Technology 2010;44(13):5074-5078. |
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Paulot F, Crounse JD, Kjaergaard HG, Kurten A, St. Clair JM, Seinfeld JH, Wennberg PO. Unexpected epoxide formation in the gas-phase photooxidation of isoprene. Science 2009;325(5941):730-733. |
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Paulot F, Crounse JD, Kjaergaard HG, Kroll JH, Seinfeld JH, Wennberg PO. Isoprene photooxidation: new insights into the production of acids and organic nitrates. Atmospheric Chemistry and Physics 2009;9(4):1479-1501. |
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Pye HOT, Chan AWH, Barkley MP, Seinfeld JH. Global modeling of organic aerosol:the importance of reactive nitrogen (NOx and NO3). Atmospheric Chemistry and Physics 2010;10(22):11261-11276. |
R833749 (2010) R833749 (Final) R833370 (Final) |
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Pye HOT, Seinfeld JH. A global perspective on aerosol from low-volatility organic compounds. Atmospheric Chemistry and Physics 2010;10(9):4377-4401. |
R833749 (2010) R833749 (Final) R833370 (2009) R833370 (Final) |
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Surratt JD, Chan AWH, Eddingsaas NC, Chan M, Loza CL, Kwan AJ, Hersey SP, Flagan RC, Wennberg PO, Seinfeld JH. Reactive intermediates revealed in secondary organic aerosol formation from isoprene. Proceedings of the National Academy of Sciences of the United States of America 2010;107(15):6640-6645. |
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Yasmeen F, Szmigielski R, Vermeylen R, Gomez-Gonzalez Y, Surratt JD, Chan AWH, Seinfeld JH, Maenhaut W, Claeys M. Mass spectrometric characterization of isomeric terpenoic acids from the oxidation of α-pinene, β-pinene, d-limonene, and Δ3-carene in fine forest aerosol. Journal of Mass Spectrometry 2011;46(4):425-442. |
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Yee LD, Kautzman KE, Loza CL, Schilling KA, Coggon MM, Chhabra PS, Chan MN, Chan AWH, Hersey SP, Crounse JD, Wennberg PO, Flagan RC, Seinfeld JH. Secondary organic aerosol formation from biomass burning intermediates:phenol and methoxyphenols. Atmospheric Chemistry and Physics 2013;13(16):8019-8043. |
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Zuend A, Marcolli C, Peter T, Seinfeld JH. Computation of liquid-liquid equilibria and phase stabilities: implications for RH-dependent gas/particle partitioning of organic-inorganic aerosols. Atmospheric Chemistry and Physics 2010;10(16):7795-7820. |
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Progress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.