Evolution of the photosensitized production of singlet oxygen by aqueous extracts of biomass-burning aerosol.

Biomass-burning aerosol (BBA) is emitted by combustion through wildfires and prescribed burns. As BBA contains chromophoric brown carbon, this aerosol increases radiative forcing and drives particle-phase photochemistry such as generation of singlet oxygen (1O2) via photosensitization. By generating 1O2, BBA could contribute to oxidation of atmospheric aerosol constituents and facilitate photobleaching of its brown carbon. Little is known regarding the quantum yield for 1O2 production in the aqueous dissolved fraction of BBA, or the variability of 1O2 production between biomass fuel types and other factors. In this study, the quantum yield of 1O2 photosensitization in aqueous BBA extracts during UVA light exposure was determined using the molecular probe furfuryl alcohol. Quantum yields of 1O2 formation ranged from 1.2-3.4%, and BBA generated by wax myrtle was found to have a significantly higher quantum yield than five other biomass fuels, indicating a possible significant dependence on fuel type in some cases. These values lie in a similar range observed for aquatic dissolved organic matter (DOM), where 1O2 production plays an important role in the fate of aquatic contaminants. No dependence on fuel type was observed in measured BBA steady-state 1O2 concentrations or rates of light absorption. The quantum yield of 1O2 was negatively correlated with absorption at 254 and 300 nm, but no other optical index was found to correlate with 1O2 production. Additionally, dynamic changes to the UV-vis absorption spectrum indicated that optical indices used to predict size and aromaticity of aqueous DOM do not apply to BBA extracts. These results emphasize the importance of BBA photosensitization as an aerosol-phase source of 1O2 decoupled from gas-phase atmospheric oxidants. BBA thus plays a role similar to DOM photosensitization in the aqueous environment, despite differences in their formation, chemical composition, and photochemical properties.