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

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.