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

Abstract

[1] Measurements of the evolution of organic aerosol extinction cross sections (s ext ) and subsaturated hygroscopicity upon heterogeneous OH oxidation are reported for two model compounds, squalane (a C 30 saturated hydrocarbon) and azelaic acid (a C 9 dicarboxylic acid). For both compounds, the s ext values at 532 nm increase substantially as the particles undergo oxidation, exhibiting a logarithmic increase with OH exposure. The increase in s 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, a, of the molecules comprising the particles. The absolute a 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 s 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 g 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, g ext was zero and only increased slowly as the particles were oxidized by OH radicals. In contrast, g ext for azelaic acid increased rapidly upon exposure to OH, eventually reaching a plateau at high OH exposures. In general, g ext appears to vary sigmoidally with O:C, reaching a plateau at high O:C.

Secondary organic aerosol formation from semi‐ and intermediate‐volatility organic compounds and glyoxal: Relevance of O/C as a tracer for aqueous multiphase chemistry

“…This results in a lower critical supersaturation at which the aerosol can behave as a cloud condensation nuclei [Petters and Kreidenweis, 2007]. There is additional evidence that the aerosol O/C is correlated with aerosol scattering cross-sections [Cappa et al, 2011]. Thus, accurately modeling aerosol O/C is important not only to test our understanding of SOA formation mechanisms, but also for predictions of the aerosol direct effect (scattering incident radiation) and the aerosol indirect effect (cloud formation).…”

Dependence of Real Refractive Indices on O:C, H:C and Mass Fragments of Secondary Organic Aerosol Generated from Ozonolysis and Photooxidation of Limonene and α-Pinene

“…However, O:C versus m r trends in one direction for the two lower HC/NO x ratios; m r s increase with decreasing O:C whereas the highest HC/NO x ratio (lowest NO x ) has the opposite trend. Cappa et al (2011) showed in a previous study that heterogeneous OH oxidation of the squalene and azelaic acid increases refractive index with addition of oxygen (increasing O:C). However, our other cases (HC/NO x = 6.6,14) show the opposite relationship, suggesting that O:C ratio is not the only factor controlling the refractive index.…”

Broadband measurements of aerosol extinction in the ultraviolet spectral region

“…In addition, the optical cross sections of the multiple-charged particles are typically greater than that of the singly charged particles, with a fractional contribution to the optical cross section that is greater than their fractional abundance. A common method used in CRDS retrievals of complex refractive index is to calculate and subtract the extinction due to doubly and triply charged particles from the total measured extinction (Fierz-Schmidhauser et al, 2010;Cappa et al, 2011;Flores et al, 2012). The disadvantage of this method is that it requires an assumed refractive index for the multiple-charged particles.…”