The formation and evolution of organic aerosol is a complex multiphase process that is likely to be strongly dependent on the phase in which organic species and oxidants are present. Here we report environmental chamber studies aimed at examining the role of condensed-phase oxidants in the formation of secondary organic aerosol (SOA) generated from the ozonolysis of biogenic hydrocarbons. Condensed-phase oxidant levels were systematically varied by changing a number of different seed and humidity conditions, allowing for the study of the effects of (1) phase of particles (aqueous vs. dry), (2) added oxidant source (none vs. H<sub>2</sub>O<sub>2</sub> vs. Fenton chemistry), and (3) irradiation (dark vs. UV lights). For experiments conducted in the presence of aqueous particles but without any added oxidants, UV irradiation resulted in a small but significant enhancement in O/C. For experiments carried out in the presence of aqueous oxidants, the SOA was formed with an exceedingly high level of oxidation, with O/C ratios regularly exceeding 1, most likely due to the formation of oxalate. High levels of aqueous oxidants also resulted in the loss of carbon from the condensed phase, which is consistent with the formation of volatile, highly oxidized products. These findings indicate that, when present, aqueous oxidants can have a dramatic effect on the composition, properties, and amount of SOA formed. However, the concentrations of oxidants (e.g., OH) within atmospheric aqueous particles are very poorly constrained at present, making it difficult to assess the impacts of aqueous-phase oxidation within fine particles on the loadings and oxidation state of atmospheric organic aerosol.

Oxidative aging of secondary organic aerosol within aqueous particles

250th National Meeting (2015)

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