Singlet Oxygen Seasonality in Aqueous PM 10 is Driven by Biomass Burning and Anthropogenic Secondary Organic Aerosol.

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