(673e) OH-Initiated Heterogeneous Aging of Oxidized Organic Aerosol

The oxidative evolution (“aging”) of organic species in the atmosphere is thought to have a major influence on the composition and properties of organic particulate matter, but remains poorly understood, particularly for the most oxidized fraction of the aerosol. Here we measure the kinetics and products of the heterogeneous oxidation of a wide range of model systems, with an aim of better constraining atmospheric aging processes of oxidized compounds, which are found primarily in the condensed phase. In separate experiments, submicron particles composed respectively of reduced hydrocarbons, moderately oxidized sugars, and oxidized organic acids—1,2,3,4-butanetetracarboxylic acid (C₈H₁₀O₈), citric acid (C₆H₈O₇), tartaric acid (C₄H₆O₆), and Suwannee River fulvic acid—were oxidized by gas-phase OH in a flow reactor, and the masses and elemental composition of the particles were monitored as a function of OH exposure. The effects of additional oxidation reactions on total particle mass were found to depend strongly on the average oxidation state of the aerosol. Carbon content of the aerosol always decreased somewhat, indicating volatilization caused by oxidation, typically through fragmentation pathways. The estimated reactive uptake coefficients of the reactions range from 0.2 to 0.9 and indicate that such transformations can occur at rates corresponding to less than two weeks in the atmosphere, suggesting their importance in describing the atmospheric lifecycle of organic particulate matter.