Grantee Research Project Results
2009 Progress Report: Formation of Secondary Organic Aerosol
EPA Grant Number: R833749Title: Formation of Secondary Organic Aerosol
Investigators: Seinfeld, John , Flagan, Richard
Current 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 Period Covered by this Report: October 1, 2008 through September 30,2009
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
Objective:
The goal of this project is to measure secondary organic aerosol formation in the laboratory and develop models to represent this process in atmospheric models.Progress Summary:
The overall goal of this project is to identify all significant sources of atmospheric secondary organic aerosol and to elucidate the chemical routes of SOA formation for each important organic precursor.
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) develop 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 is also being developed for use in global chemical transport models.
A previously unrecognized player in the process by which gases produced by trees and other plants become aerosols-microscopically small particles in the atmosphere-has been discovered. This work on the creation and effects of these chemicals, 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 this study. In particular, the research was initiated to understand how the oxidation of isoprene can lead to formation of atmospheric particular matter, so-called 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 now, 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 from 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. The research team was able to make this scientific leap forward thanks to their 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.
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.
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 is being assessed in the global chemical transport model, GEOS-Chem.
Future Activities:
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 under-predict 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 recently 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 these 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) recently demonstrated that nitrooxy organosulfates with MW 295 formed from the photooxidation (OH-initiated) and nighttime (NO3-initiated) oxidation of a-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 a-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 only formed from the photooxidation of d-limonene 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 gas-phase 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 a. (2008) recently investigated SOA formation from the NO3-initiated oxidation of isoprene under dark conditions. Nitrooxy organosulfates were also also observed in these experiments.
Proposed Reactive Uptake Experiments:
a-pinene system
- a-pinene oxide + dry (NH4)2SO4 seed (dark and dry conditions)
- a-pinene oxide + wet (NH4)2SO4 seed (dark and humid conditions)
- a-pinene oxide + wet (NH4)2SO4 + H2SO4 seed (dark and humid conditions)
- Pinanediol + dry (NH4)2SO4 seed (dark and dry conditions)
- Pinanediol + wet (NH4)2SO4 seed (dark and humid conditions)
- Pinanediol + wet (NH4)2SO4 + H2SO4 seed (dark and humid conditions)
a-pinene oxide is formed at a higher yield by NO3-initiated oxidation when compared to OH-initiated oxidation. This may be consistent with why one sees higher concentrations of certain organosulfates are observed at nighttime in ambient aerosol samples.
isoprene system:
- 2-methyl-2-vinyloxirane + dry (NH4)2SO4 seed (dark and dry conditions)
- 2-methyl-2-vinyloxirane + wet (NH4)2SO4 seed (dark and humid conditions)
- 2-methyl-2-vinyloxirane + wet (NH4)2SO4 + H2SO4 seed (dark and humid conditions)
The goals of these experiments are:
- Identify potential reactive intermediates that form prominent organic constituents found in isoprene and monoterpene SOA.
- Establish RH likely plays a role in SOA yields and the formation of SOA constituents (like that of the 2-methyltetrols found in isoprene SOA).
- Evaluate RH enhances the ability of bisulfate to react with reactive intermediates to form compounds observed in laboratory-generated and ambient organic aerosol samples (e.g. organosulfates).
- Evaluate acidity plays a role in forming organosulfates.
- Investigate the role of ammonium nitrate and HNO3 on organic nitrate formation. This could be done in a similar manner as to the experiments outlined above. Additionally, it might be interesting to evaluate if a seed aerosol containing both nitrate and sulfate could lead to nitrooxy organosulfate formation.
Organic Nitrate Formation
The role of heterogeneous chemistry on the formation of organic nitrates is an important unexplored issue. It has been established that acidic to highly acidified sulfate seed aerosol is needed to make organosulfates of monoterpenes. An interesting question is-what about the role of nitric acid or nitrate in the aerosol? Intermediate products needs to be examined further as they could explain differences in particle-phase chemistries observed for various systems. If the aerosol is acidic, sulfate or nitrate could both act as nucleophilies. For example, they both could attack an epoxide intermediate to make an organic nitrate or an organosulfate, especially considering how unstable the carbons are in the epoxide ring once the oxygen is protonated. The branching ratio for organic nitrate formation in the gas-phase isoprene photooxidation is quite low (5-8%); however, organic nitrates are observed in the particle-phase. We have even detected an oligoester of an organic nitrate with the 2-methylglyceric acid monomers. Organic nitrates detected in the particle-phase arrive there by either by semivolatile partitioning of a gas-phase organic nitrate product (as previously proposed), by a heterogeneous route, or by both. Proposed experiments:
- reactive uptake experiments - exposing intermediate products (i.e. epoxide, diol, peroxide) to acidified nitrate aerosol (maybe have it wet due to volatility of nitrate)
- generate isoprene NOx-free SOA by nucleation - turn off lights and inject large amounts of HNO3 in the gas-phase to see if organic nitrate formation occurs.
SOA Formation from Methoxyphenol Compounds
Biomass burning contributes ~ 90% of primary particulate organic carbon from combustion sources. Wildfires contribute 20% of total PM2.5 in the US (EPA, 2000). The primary POA component is levoglucosan. But, 80% of SOA is not explained by traditional precursors.
Major compounds in biomass burning emissions are:
2-methoxyphenol (guaiacol)
2,6-dimethoxyphenol (syringol)
4-methylguaiacol
Methoxyphenols are the second most abundant organic polymer on Earth. These compounds constitute 25%-33% of the dry mass of word, confer mechanical strength to the cell wall, and decay slowly-responsible for most of humus. Methoxyphenols are used as tracers for residential wood combustion.
Experimental design:
- Guaiacol, syringol, 4-methyl guaiacol SOA formation
- Effect of aerosol loading (yield curves)
- Effect of NOx level
- Full chemistry/mechanism for guaiacol only
SOA Potential of Non-alkene Terpenoids
Recent work from Europe has shown that oxidation of 1,8-cineole can lead to aerosol in large quantities (16-20%) despite the fact that this compounds lacks a double bond. Interestingly, this compound produces an SOA coumpound that was also detected in ambient aerosol collected from Australia. This 1,8-cineole has a C10 backone with one oxygen in the molecule, located in an ether function (ROR). Emissions data from Austrialia suggest that this compound is more abundantly emitted than a-pinene, b-pinene, and limonene due to the indigenous plant species (i.e. eucalyptus). The question arises whether biogenic VOCs are missing in emission inventories, such as those containing oxygen but without double bonds (that may lack detection by GC methods).
References:
Journal Articles on this Report : 6 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. |
R833749 (2008) R833749 (2009) R833749 (Final) |
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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. |
R833749 (2008) R833749 (2009) R833749 (Final) |
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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. |
R833749 (2009) R833749 (Final) |
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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. |
R833749 (2008) R833749 (2009) R833749 (Final) |
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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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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. |
R833749 (2009) R833749 (Final) |
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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.