Secondary Organic Aerosol is Formed by Unexpected Multiphase Brown Carbon Photochemistry during Winter
LAURA MARIE DAHLER HEINLEIN, Jonas Kuhn, Karolina Cysneiros de Carvalho, Brent Williams, Jochen Stutz, Cort Anastasio, University of California, Davis
Abstract Number: 334
Working Group: Aerosol Chemistry
Abstract
Subarctic cities notoriously experience severe multi-day winter pollution episodes with PM2.5 concentrations above 35 µg/m3, the EPA’s 24-hour standard. Biomass burning (BB) for residential heating is a major source of primary organic aerosol in winter, making particles rich in brown carbon and therefore potential sites for multiphase photochemistry. Despite limited sunlight, we found that brown carbon particles in winter pollution episodes in Fairbanks, Alaska produced significant concentrations of photooxidants, such as organic triplet excited states. These particle photooxidants can react with semi-volatile BB phenols, emitted abundantly in wood smoke, to produce BB secondary organic aerosol (BBSOA) in the particles. Here, using measurements made in the winter of 2021 in Fairbanks during the ALPACA field campaign, we used the PACT-1D model to predict gas and particle BBSOA formation. We find rapid daytime BBSOA formation during pollution events driven by multiphase reactions of triplet excited states with phenols, with slow nighttime BBSOA formation when photochemistry ceases. BBSOA formation is also slow during clean periods, when our model predicts lower phenol and photooxidant concentrations. We compare our modeled phenol concentrations to field measurements of gas and particulate phenols, exploring how multiphase chemistry alters their atmospheric lifetime, and assessing how our model captures complex gas-particle partitioning in cold conditions. Finally, we compare our modeled BBSOA formation to measurements of oxidized organic aerosol to explore whether BBSOA formation can explain the observed oxidative aging of BB organic aerosol in Fairbanks. BBSOA formation can increase the amount of PM pollution, as well as alter the chemical composition, volatility, and viscosity of PM pollution; understanding the impact of unexpected winter BBSOA formation is essential for accurately modeling PM during pollution episodes in cold climates.