Multiphase reactions of OH radicals are among the most important pathways of organic aerosol (OA) chemical aging in the atmosphere. However, the kinetics of mass transport and chemical reaction, the role of phase state, and the implications for cloud condensation nucleus (CCN) activation are still not well understood. We address these questions by combining novel experimental and modeling approaches. Levoglucosan (LEV), abietic acid (AC), and methyl-nitrocatechol (MNC) serve as surrogate species of biomass burning aerosol (BBA). A low pressure aerosol flow reactor coupled (AFR) is applied to examine the impact of particle phase change due to presence of water vapor (relative humidity, RH) on the reactive uptake kinetics. OH uptake by LEV increases significantly as particle viscosity decreases in response to an RH increase whereas OH reaction kinetics decrease in the case of MNC likely due to water occupying reactive sites. The kinetic multilayer model of gas–particle interactions (KM-GAP) is used to describe LEV and AC OH exposure studies. The model demonstrates that OH uptake is governed by reaction at or near the surface and can be kinetically limited by surface-bulk exchange or bulk diffusion of the organic reactants. The chemical half-life of LEV in 200 nm particles in a biomass burning plume is estimated to increase from 1 day at high relative humidity to 1 week under dry conditions. Low temperatures can result in viscous particle phases slowing bulk diffusion and thus increasing lifetimes to over 1 month. The CCN activity of single and multicomponent LEV, MNC, and potassium sulfate (KS) particles exposed to OH is evaluated by determining the hygroscopicity applying a CCN counter (CCNc) coupled to AFR. Our results suggest that when OA is dominated by water-soluble organic carbon (WSOC) or inorganic ions, chemical aging has no significant impact on OA hygroscopicity. Suwanee River Fulvic Acid (SRFA) is applied as a surrogate compound for humic-like substances in BBA. SRFA CCN activity is determined for varying temperatures above and below its glass transition temperature (T<sub>g</sub>). Our results demonstrate that the CCN activity of SRFA is sensitive to changes in particle phase following exposure to OH where the most significant enhancements in hygroscopicity following OH exposure occurred when temperatures were below T<sub>g</sub>. This is indicative of efficient chemical modification at the particle surface.

Multiphase chemical kinetics between OH and biomass burning surrogate species: The role of particle phase state and implications for cloud formation

250th National Meeting (2015)

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