Emission and Evolution of Submicron Organic Aerosol in Smoke from Wildfires in the Western United States

Despite increasing incidence of wildfires in the United States, wildfire smoke is poorly characterized, with little known about particle composition and emission rates. Chemistry in transported plumes confounds interpretation of ground and aircraft data, but near-field observations can potentially disentangle the effects of oxidation and dilution on aerosol mass and chemical composition. We report the organic aerosol (OA) emission ratios from aircraft observations near the fire source for the 20 wildfires sampled during the Western Wildfire Experiment: Cloud Chemistry, Aerosol Absorption, and Nitrogen (WE-CAN) study of summer 2018. We observe no changes in submicron nonrefractory OA mass concentration, relative to CO which accounts for simple dilution, between 0.5 and up to 8 h of aging. However, static OA excess mixing ratios hide shifts in the aerosol chemical composition that suggest near-balanced, simultaneous oxidation-driven condensation and dilution-driven evaporation. Specifically, we observe significant increases in the extent of oxidation, evident by an increase in oxidation marker f₄₄ and loss of the biomass burning marker f₆₀, as the smoke ages through chemistry and dilution. We discuss the competing effects of oxidative chemistry and dilution-driven evaporation on the evolution of the chemical composition of aerosols in wildfire smoke over time.