Evolution of Secondary Organic Aerosol Composition, Volatility, and Gas-Particle Partitioning During Oxidation of Phenolic Compounds Under Conditions Relevant to Biomass Burning

Abstract:

Phenolic compounds comprise one important class of volatile organic compounds (VOCs) emitted from biomass burning. Oxidation of these compounds by the hydroxyl radical, OH, or the nitrate radical, NO₃, is known to form products of low volatility and secondary organic aerosol (SOA). In a series of chamber experiments, we simulated the oxidation of phenol, catechol, guaiacol, and cresol under conditions found in wildfire smoke, motivated by the results from the Western Wildfire Experiment for Cloud Chemistry, Aerosol Absorption and Nitrogen (WE-CAN) campaign. The oxidation products in both the gas and particle phases were measured and characterized using an I_­­^- adduct high-resolution time-of-flight chemical ionization mass spectrometer (HR-ToF-CIMS) coupled with the Filter Inlet for Gases and Aerosols (FIGAERO). We evaluate the measured SOA yield, molecular composition, and volatility in the context of currently proposed phenolic oxidation mechanisms at high concentrations of nitrogen oxide radicals (NO_x = NO + NO₂) and gas-particle absorptive partitioning theory. During extended aging of the formed SOA under photochemical and dark conditions, we measured notable shifts in both the molecular composition and volatility of the SOA consistent with either changes in particle-phase state and/or particle-phase accretion chemistry, and oxidative mass loss occurring simultaneously. We quantitatively connect the evolution of SOA composition to changes in the particle light absorption at 405 nm measured concurrently, providing insights into the atmospheric fate of brown carbon constituents as wildfire plumes dilute and photochemically evolve.