UC Davis

Fog/cloud drops and aerosol liquid water are important sites for the transformations of atmospheric species, largely through reactions with photoformed oxidants such as the hydroxyl radical (OH), singlet molecular oxygen (1O2 -), and oxidizing triplet excited states of organic matter (3C -). Despite their importance, few studies have measured these oxidants or their seasonal variations. To address this gap, we collected ambient PM2.5 from Davis, California, over the course of a year and measured photooxidant concentrations and light absorption in dilute aqueous extracts. Mass absorption coefficients (MACs) normalized by dissolved organic carbon range from 0.4-3.8 m2 per gram C at 300 nm. Concentrations of OH, 1O2 -, and 3C - in the extracts range from (0.2-4.7) × 10-15 M, (0.7-45) × 10-13 M, and (0.03-7.9) × 10-13 M, respectively, with biomass burning brown carbon playing a major role in light absorption and the formation of 1O2 - and 3C -. Extrapolating photooxidant kinetics from our dilute particle extracts to concentrated aerosol liquid water (ALW) conditions gives an estimated OH concentration of 7 × 10-15 M and ranges for 1O2 - and 3C - of (0.6-7) × 10-12 M and (0.2-1) × 10-12 M, respectively. Compared to the results in Kaur et al. (2019), our ALW predictions show roughly 10 times higher OH, up to 5 times higher 3C, and 1O2 - concentrations that are lower by factors of 20-100. These concentrations suggest that 3C - and 1O2 - in ALW dominate the processing of organic compounds that react quickly with these oxidants (e.g., phenols and furans, respectively), while OH is more important for less reactive organics.