Seasonal variations in photooxidant formation and light absorption in aqueous extracts of ambient particles

• Published in: Atmospheric chemistry and physics Vol. 24; no. 1; pp. 1 - 21
• Main Authors: Ma, Lan, Worland, Reed, Heinlein, Laura, Guzman, Chrystal, Jiang, Wenqing, Niedek, Christopher, Bein, Keith J, Zhang, Qi, Anastasio, Cort
• Format: Journal Article
• Language: English
• Published: Katlenburg-Lindau Copernicus GmbH 02-01-2024; Copernicus Publications
• Subjects: Absorption; Absorption coefficient; Absorptivity; Aerosols; Biomass; Biomass burning; Carbon; Coefficients; Dilution; Dissolved organic carbon; Electromagnetic absorption; Fog; Furans; Hydroxyl radicals; Kinetics; Light absorption; Organic carbon; Organic compounds; Organic matter; Outdoor air quality; Oxidants; Oxidation; Oxidizing agents; Oxygen; Particulate matter; Pesticides; Phenols; Photodegradation; Seasonal variation; Seasonal variations; Summer; Triplets; Water; Winter
• ISSN: 1680-7324; 1680-7316; 1680-7324
• DOI: 10.5194/acp-24-1-2024

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.