Photosensitized Heterogeneous Oxidation Reactions of Organic Biomass Burning Aerosol Surrogates by Ozone Using a Novel Irradiation-Permitting Rectangular Channel Flow Reactor

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 heavily on biomolecular markers such as levoglucosan. However, these compounds can react heterogeneously with trace gases, which may cause source strength underestimation. The presence of light absorbing material known as photosensitizers can cause biomolecular markers to react more efficiently with trace gases when exposed to radiation. In this study, the heterogeneous kinetics between ozone and compounds typical of organic biomass burning aerosol particles are determined in the absence and presence of a photosensitizing compound. The effect of visible or UV radiation on the heterogeneous kinetics is investigated. Levoglucosan and nitroguaiacol serve as surrogates for organic biomass burning aerosol and Pahokee peat serves as a surrogate for HuLIS (humic-like substances). The latter is known to be a photosensitizer and can be found in biomass burning aerosol particles. The reactive uptake experiments are conducted with a newly designed rectangular channel flow reactor that allows controlled visible and UV irradiation of the organic substrates. The absolute irradiance of the visible and UV light sources is characterized using a calibrated fiber optic spectrometer. Reactive uptake coefficients are determined by monitoring the gas-phase loss of ozone to the organic substrate using a custom-built chemical ionization mass spectrometer (CIMS). The heterogeneous kinetics are derived in the presence of atmospherically relevant O₃ and O₂ concentrations and total pressure is about 2-3 hPa, ensuring negligible diffusion limitations. Reactive uptake experiments are also performed as a function of total incoming photon flux and ozone concentration. Uptake coefficients of a 1:1 by mass mixture of Pahokee peat:levoglucosan are on the order of 10^-6 in dark and irradiated conditions. Uptake coefficients of a 1:1 by mass mixture of Pahokee peat:nitroguaiacol are on the order of 10^-6 in dark conditions and on the order of 10^-5 under irradiated conditions. The uptake coefficients of a 1:10 by mass mixture of Pahokee peat:nitroguaiacol are on the order of 10^-6 in dark conditions and on the order of 10^-5 under irradiated conditions.