Photosensitized Heterogeneous Oxidation Reactions of Biomass Burning Aerosol Surrogates with OH Radicals under UV and Visible Irradiation

Organic aerosol particles are ubiquitous in the atmosphere and can influence the climate both directly through scattering and absorption of radiation and indirectly through modification of cloud properties. Biomass burning is a major source of organic aerosol particles to the atmosphere. Source apportionment of biomass burning plumes relies on biomolecular markers that are inert with respect to transport in the atmosphere. However, these compounds can react heterogeneously with atmospheric gas-phase oxidants, which may cause source strength underestimation. The presence of light absorbing material, also known as photosensitizers, which can transfer excitation energy to a neighboring non-light absorbing molecule, has been shown to enhance the heterogeneous kinetics of several organic compounds with O₃ and NO₂ in the presence of ultraviolet or visible (UV/VIS) irradiation. The effect of UV/VIS irradiation on the heterogeneous kinetics between biomass burning aerosol and OH radicals in the presence of a photosensitizer has not yet been investigated. OH concentrations have been shown to be about an order of magnitude larger in biomass burning plumes compared to the background atmosphere. In this study, the heterogeneous kinetics between OH radicals and compounds typical of organic biomass burning aerosol (BBA) particles such as levoglucosan, 5-nitroguaiacol, nitrocatechol, 4-methoxyphenol, and benzo[a]pyrene are determined in the absence and presence of a photosensitizing compound. Pahokee peat serves as a surrogate for humic-like substances (HULIS), which have been shown to possess photosensitive properties. The effect of UV/VIS irradiation on the reactive uptake of OH is investigated, and the presence of volatilized products formed due to reaction with OH is measured. The reactive uptake experiments are conducted with an irradiated rectangular channel flow reactor that allows controlled UV/VIS irradiation of the organic substrates. Reactive uptake coefficients are determined by monitoring the gas-phase loss of OH to the organic substrate using a chemical ionization mass spectrometer. OH is generated via microwave discharge of H₂ atoms in the presence of O₂ using a high frequency microwave generator. The heterogeneous kinetics are derived in the presence of atmospherically relevant O₂ concentrations and [OH]~10⁸ molecule cm^-3, and at total pressures of about 2-5 hPa, ensuring negligible diffusion limitations. Volatile products from the reaction of BBA surrogates, with and without the presence of a photosensitizer, with OH under UV/VIS irradiation are measured using a high resolution proton transfer reaction time-of-flight mass spectrometer operated at atmospheric pressure.