Brown carbon (BrC) is an important component of biomass burning (BB) emissions that directly absorbs solar radiation and impacts Earth’s radiation budget. One class of BrC precursors also emitted in BB is gaseous phenolic compounds (ArOH). ArOH react in the gas and aqueous phases with various oxidants, including hydroxyl radical (●OH) and organic triplet excited states (3C*), to form secondary organic aerosol (SOA). Phenol reactions in atmospheric aqueous phases, such as clouds, fog drops, and aerosol liquid water (ALW), form low-volatility, high molecular weight products via functionalization and oligomerization. These are important routes of SOA formation, but the light absorption characteristics of the aqueous-SOA (aqSOA) is poorly characterized. To study this, we monitored the formation and destruction of light absorption by aqSOA formed by reactions of ArOH with ●OH and 3C*. We determined mass absorption coefficients (MAC) to quantify the impact of oxidation on light absorption, which demonstrates the contrast between the initial ArOH and evolving aqSOA. ArOH absorb very little tropospheric sunlight, while the aqSOA products absorb significant amounts of sunlight. We calculated the rate of aqSOA sunlight absorption (Rabs) by combining MAC values with modeled actinic flux. We find that ●OH photobleaches aqSOA and diminishes aqSOA sunlight absorption over time. In contrast, rates of sunlight absorption by aqSOA change very little in the presence of 3C*. Finally, we determined the lifetimes of sunlight absorbing aqSOA, which ranged from 140 - 1800 min during ●OH oxidation, and 280 – 4100 min during 3C* oxidation.

Formation and bleaching of light absorbance by phenolic aqueous SOA by ●OH and organic triplet excited states

ACS Spring 2022

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