Singlet Oxygen Seasonality in Aqueous PM₁₀ is Driven by Biomass Burning and Anthropogenic Secondary Organic Aerosol

The first excited state of molecular oxygen is singlet-state oxygen (¹O₂), formed by indirect photochemistry of chromophoric organic matter. To determine whether ¹O₂ can be a competitive atmospheric oxidant, we must first quantify its production in organic aerosols (OA). Here, we report the spatiotemporal distribution of ¹O₂ over a 1-year dataset of PM₁₀ extracts at two locations in Switzerland, representing a rural and suburban site. Using a chemical probe technique, we measured ¹O₂ steady-state concentrations with a seasonality over an order of magnitude peaking in wintertime at 4.59 ± 0.01 × 10^-13 M and with a quantum yield of up to 2%. Next, we identified biomass burning and anthropogenic secondary OA (SOA) as the drivers for ¹O₂ formation in the PM₁₀ aqueous extracts using source apportionment data. Importantly, the quantity, the amount of brown carbon present in PM₁₀, and the quality, the chemical composition of the brown carbon present, influence the concentration of ¹O₂ sensitized in each extract. Anthropogenic SOA in the extracts were 4 times more efficient in sensitizing ¹O₂ than primary biomass burning aerosols. Last, we developed an empirical fit to estimate ¹O₂ concentrations based on PM₁₀ components, unlocking the ability to estimate ¹O₂ from existing source apportionment data. Overall, ¹O₂ is likely a competitive photo-oxidant in PM₁₀ since ¹O₂ is sensitized by ubiquitous biomass burning OA and anthropogenic SOA.