A Missing Pool of Particle Photooxidants: Photochemistry of Water-Insoluble Chromophores from Wildfire Smoke Particles

LAURA MARIE DAHLER HEINLEIN, Sydney Ma, Murat Onder, Monika Madhiyan, Mingsheng Fang, Jiayun Zhao, Yue Zhang, Kyle Moore, Cort Anastasio, University of California, Davis

     Abstract Number: 303
     Working Group: Aerosol Chemistry

Abstract
Wildfire smoke particles are rich in brown carbon (BrC), organic molecules that can absorb light to form photooxidants like triplet excited states (3C*) and singlet molecular oxygen (1O2*). In particles, 3C* can form secondary organic aerosol (SOA) and secondary BrC, alter particle viscosity, and produce secondary oxidants like 1O2* and hydroxyl radical which drive additional multiphase chemical processing. Most measurements of 3C* and 1O2* are limited to BrC filter samples extracted into water, but because only 40% of organic carbon is water soluble, these studies neglect water-insoluble BrC as a potential photooxidant source. In this work, we adapt widely used water-based probe methods for measuring 3C* and 1O2* concentrations to target photooxidant generation from the mostly ignored pool of water-insoluble BrC. We report the first measurements of 3C* and 1O2* concentrations from water-insoluble BrC from wildfire smoke particles. We compare photooxidant generation in aqueous and organic phases by comparing steady-state concentrations, production rates, quantum yields, and rate constants of 3C* and 1O2* with important SOA precursors. We observe rapid decay of phenolic SOA precursors in organic solvents with 3C* and 1O2* photosensitizers. We also find that 1O2* photochemistry is distinct in the organic phase: high O2 concentrations and slow 1O2* deactivation rates suggest 1O2* concentrations may be roughly 10-times higher in an organic phase compared to the aqueous phase. Overall, we demonstrates that current estimates of 3C* and 1O2* concentrations neglect water-insoluble BrC as an important source of photooxidants. Moreover, we challenge the assumption that BrC photochemistry is limited to aqueous aerosols and suggest that BrC photooxidants may drive chemical processing in organic phases in the atmosphere.