Formation and loss of light absorbance by phenolic aqueous SOA by ● OH and an organic triplet excited state

Abstract. Brown carbon (BrC) is an important component of biomass burning (BB) emissions that impacts Earth’s radiation budget. BB directly emits primary BrC as well as gaseous phenolic compounds (ArOH), which react in the gas and aqueous phases with oxidants – such as hydroxyl radical (●OH) and organic triplet excited states (3C*) – to form light-absorbing secondary organic aerosol (SOA). These reactions in atmospheric aqueous phases, such as cloud/fog drops and aerosol liquid water (ALW), form aqueous SOA (aqSOA), i.e., low-volatility, high molecular weight products. While these are important routes of aqSOA formation, the light absorption and lifetimes of the BrC formed are poorly characterized. To study these aspects, we monitored the formation and loss of light absorption by aqSOA produced by reactions of six highly substituted phenols with ●OH and 3C*. While the parent phenols absorb very little tropospheric sunlight, they are oxidized to aqSOA that can absorb significant amounts of sunlight. The extent of light absorption by the aqSOA depends on both the ArOH precursor and oxidant: more light-absorbing aqSOA is formed from more highly substituted phenols and from triplet reactions rather than ●OH. Under laboratory conditions, continued reactions with ●OH diminish sunlight absorption by aqSOA on timescales of hours, while continued reactions with 3C* reduce light absorption much more slowly. Estimated lifetimes of light-absorbing phenolic aqSOA range from 3 to 13 hours in cloud/fog drops, where ●OH is the major sink, and from 0.5 to 8 hours in ALW, where triplet excited states are the major sink.