Variations in organic aerosol optical and hygroscopic properties upon heterogeneous OH oxidation
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Author(s)
Cappa, Christopher D.
Che, Daphne L.
Kessler, Sean Herbert
Kroll, Jesse
Wilson, Kevin R.
Date Issued
August 2011
Journal
Journal of Geophysical Research. Atmospheres
Publisher
American Geophysical Union (AGU)
Citation
Cappa, Christopher D. et al. “Variations in Organic Aerosol Optical and Hygroscopic Properties Upon Heterogeneous OH Oxidation.” Journal of Geophysical Research 116.D15 (2011). ©2011 John Wiley & Sons, Inc.
Version
Final published version
Abstract
Measurements of the evolution of organic aerosol extinction cross sections (σ[subscript ext]) and subsaturated hygroscopicity upon heterogeneous OH oxidation are reported for two model compounds, squalane (a C30 saturated hydrocarbon) and azelaic acid (a C9 dicarboxylic acid). For both compounds, the σ[subscript ext] values at 532 nm increase substantially as the particles undergo oxidation, exhibiting a logarithmic increase with OH exposure. The increase in σ[subscript ext] correlates with both an increase in the particle oxygen to carbon (O:C) atomic ratio and density and a decrease in mean molecular weight. The measurements have been used to calculate the variation with oxidation of the mean polarizability, α, of the molecules comprising the particles. The absolute α values for the two systems are shown to be related through the variation in the particle chemical composition, specifically the relative abundances of C, O, and H atoms and the mean molecular weight. Unlike σ[subscript ext], it was found that the evolution of the particle hygroscopicity upon oxidation is quite different for the two model systems considered. Hygroscopicity was quantified by measuring γ[subscript ext], which is a single-parameter representation of hygroscopicity that describes the increase in extinction upon exposure of the particles to a high–relative humidity environment (here, 75% and 85% RH). For unoxidized squalane, γ[subscript ext] was zero and only increased slowly as the particles were oxidized by OH radicals. In contrast, γ[subscript ext] for azelaic acid increased rapidly upon exposure to OH, eventually reaching a plateau at high OH exposures. In general, γ[subscript ext] appears to vary sigmoidally with O:C, reaching a plateau at high O:C.
MIT Department
Massachusetts Institute of Technology. Department of Chemical Engineering
Massachusetts Institute of Technology. Department of Civil and Environmental Engineering
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DOI of Published Version
https://doi.org/10.1029/2011jd015918
