Aqueous Oxidation of Biomass-Burning Furans by Singlet Molecular Oxygen (¹O₂*)

Furans are abundant emissions from biomass burning that can react with gas-phase oxidants to produce secondary organic aerosol (SOA). Furans might also react with aqueous photooxidants, such as singlet molecular oxygen (¹O₂*), to form aqueous SOA (aqSOA), but this has not been studied. To investigate the aqueous reactivities of furans and their potential to make low-volatility products, we first measured the reaction kinetics for singlet oxygen with 17 furans. The resulting second-order rate constants vary widely with chemical substitution, ranging from 10⁵ to nearly 10⁹ M^-1 s^-1. Inorganic salts can decrease or enhance the first-order loss of furans by singlet oxygen. To investigate whether furan-¹O₂* reactions might produce particulate matter, we measured SOA mass yields for three furans: furoic acid, furfuryl alcohol, and 2-methylfuran-3,4-dicarboxylic acid (MFDCA). The resulting mass yields span a huge range, with values of ∼0, 51, and 125%, respectively. Finally, we estimated rates of gas- and aqueous-SOA formation from reactions of MFDCA over a range of conditions, from cloud and fog drops to aerosol liquid water. Results suggest that aqueous reactions of highly substituted furans with ¹O₂* could be a significant source of aqSOA in biomass-burning plumes but that aqueous reactions of triplet excited states with phenols are more important.