Singlet Oxygen and Triplet Excited State Organics Produced by Primary Biomass Burning Organic Aerosol from Wood, Dung, Straw, and Plastic Fuels: Role of Solvent-Dependent Photochemistry

Biomass burning organic aerosol (BBOA) is a significant source of brown carbon (BrC), a class of light-absorbing organic compounds that influence the climate and atmospheric oxidation chemistry. Certain BrC chromophores act as photosensitizers by producing reactive oxidants such as singlet oxygen (1O2*) and triplet excited states (3C*) upon solar irradiation, but we still cannot predict which functional groups drive the production of these excited-state oxidants. We hypothesized that since the major sink for O2*1 is solvent deactivation, water-based extractions of chromophores may (1) underestimate O2*1 production in BBOA, as well as (2) overlook water-insoluble chromophores potentially important in hydrophobic aerosol phases. To better represent the multiphase nature of atmospheric aerosols in this work, we employed a cosolvent extraction using acetonitrile and water to extract the hydrophobic fraction of BrC chromophores. Primary organic aerosol (POA) filters from the combustion of beechwood, cow dung, straw, and plastic were collected during the BAMBE campaign and subsequently analyzed using a two-probe photochemical method with furfuryl alcohol and syringol as probes to quantify O2*1 and 3C* steady-state concentrations and quantum yields, respectively, under UVA irradiation. The acetonitrile–water mixture enhanced chromophore recovery and modulated the quenching of the excited state of O2*1 back to O2, leading to up to a 6-fold increase in O2*1 concentrations. Fuel type significantly influenced oxidant yields. Beechwood and plastic extracts showed the highest O2*1 and 3C* production, up to 4.9 ± 0.1% for O2*1 and 4.2 ± 0.2% for 3C*, respectively. Complementary FIGAERO–CIMS analysis revealed compositional differences between fuels and combustion conditions, suggesting that nitrogen-containing species drove the variability in sensitization efficiency. Our findings underscore the importance of solvent selection and fuel source in assessing BrC photochemical activity and offer new insights into the oxidative potential of BBOA in atmospheric multiphase environments.