Variations in organic aerosol optical and hygroscopic properties upon heterogeneous OH oxidation

• Published in: Journal of Geophysical Research Vol. 116; no. D15
• Main Authors: Cappa, Christopher D., Che, Daphne L., Kessler, Sean H., Kroll, Jesse H., Wilson, Kevin R.
• Format: Journal Article
• Language: English
• Published: Washington Blackwell Publishing Ltd 06.08.2011
• Subjects: Atmospheric aerosols; Atmospheric sciences; Geophysics; Hygroscopicity; Optical Properties; Organic Aerosol
• ISSN: 0148-0227; 2169-897X; 2156-2202; 2169-8996
• DOI: 10.1029/2011JD015918

Measurements of the evolution of organic aerosol extinction cross sections (σext) 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 σext values at 532 nm increase substantially as the particles undergo oxidation, exhibiting a logarithmic increase with OH exposure. The increase in σext 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 σext, 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 γext, 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, γext was zero and only increased slowly as the particles were oxidized by OH radicals. In contrast, γext for azelaic acid increased rapidly upon exposure to OH, eventually reaching a plateau at high OH exposures. In general, γext appears to vary sigmoidally with O:C, reaching a plateau at high O:C. Key Points Organic aerosol optical properties are not static Organic aerosol optical properties are directly related to the mean composition OA hygroscopicity depends on both atomic composition and molecular volume