Loza, C. L. and Chhabra, P. S. and Yee, L. D. and Craven, J. S. and Flagan, R. C. and Seinfeld, J. H. (2012) Chemical aging of m-xylene secondary organic aerosol: laboratory chamber study. Atmospheric Chemistry and Physics, 12 (1). pp. 151-167. ISSN 1680-7316 http://resolver.caltech.edu/CaltechAUTHORS:20120305-121637814
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Secondary organic aerosol (SOA) can reside in the atmosphere for a week or more. While its initial formation from the gas-phase oxidation of volatile organic compounds tends to take place in the first few hours after emission, SOA can continue to evolve chemically over its atmospheric lifetime. Simulating this chemical aging over an extended time in the laboratory has proven to be challenging. We present here a procedure for studying SOA aging in laboratory chambers that is applied to achieve 36 h of oxidation. The formation and evolution of SOA from the photooxidation of m-xylene under low-NO_x conditions and in the presence of either neutral or acidic seed particles is studied. In SOA aging, increasing molecular functionalization leads to less volatile products and an increase in SOA mass, whereas gas- or particle-phase fragmentation chemistry results in more volatile products and a loss of SOA. The challenge is to discern from measured chamber variables the extent to which these processes are important for a given SOA system. In the experiments conducted, m-xylene SOA mass, calculated under the assumption of size-invariant particle composition, increased over the initial 12–13 h of photooxidation and decreased beyond that time, suggesting the existence of fragmentation chemistry. The oxidation of the SOA, as manifested in the O:C elemental ratio and fraction of organic ion detected at m/z 44 measured by the Aerodyne aerosol mass spectrometer, increased continuously starting after 5 h of irradiation until the 36 h termination. This behavior is consistent with an initial period in which, as the mass of SOA increases, products of higher volatility partition to the aerosol phase, followed by an aging period in which gas- and particle-phase reaction products become increasingly more oxidized. When irradiation is stopped 12.4 h into one experiment, and OH generation ceases, minimal loss of SOA is observed, indicating that the loss of SOA is either light- or OH-induced. Chemical ionization mass spectrometry measurements of low-volatility m-xylene oxidation products exhibit behavior indicative of continuous photooxidation chemistry. A condensed chemical mechanism of m-xylene oxidation under low-NO_x conditions is capable of reproducing the general behavior of gas-phase evolution observed here. Moreover, order of magnitude analysis of the mechanism suggests that gas-phase OH reaction of low volatility SOA precursors is the dominant pathway of aging in the m-xylene system although OH reaction with particle surfaces cannot be ruled out. Finally, the effect of size-dependent particle composition and size-dependent particle wall loss rates on different particle wall loss correction methods is discussed.
|Additional Information:||© 2012 Author(s). This work is distributed under the Creative Commons Attribution 3.0 License. Received: 20 August 2011; Published in Atmos. Chem. Phys. Discuss.: 7 September 2011; Revised: 8 December 2011; Accepted: 20 December 2011; Published: 3 January 2012. This work was supported by the Office of Science (Biological and Environmental Research), US Department of Energy Grant DE-SC 0006626, and National Science Foundation Grant AGS-1057183. The CIMS instrument was purchased as part of a major research instrumentation grant from the National Science Foundation (Grant ATM-0619783); assembly and testing was supported by the Davidow Discovery Fund. We thank Katherine Schilling and ManNin Chan for experimental assistance and Arthur Chan for helpful discussion. We thank the reviewers for useful feedback on the discussions paper. Christine Loza, Lindsay Yee, and Jill Craven were supported by National Science Foundation Graduate Research Fellowships.|
|Official Citation:||Loza, C. L., Chhabra, P. S., Yee, L. D., Craven, J. S., Flagan, R. C., and Seinfeld, J. H.: Chemical aging of m-xylene secondary organic aerosol: laboratory chamber study, Atmos. Chem. Phys., 12, 151-167, doi:10.5194/acp-12-151-2012, 2012|
|Usage Policy:||No commercial reproduction, distribution, display or performance rights in this work are provided.|
|Deposited By:||Jason Perez|
|Deposited On:||13 Mar 2012 17:13|
|Last Modified:||26 Dec 2012 14:54|
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