Dependence of Heterogeneous OH Kinetics with Biomass Burning Aerosol Proxies on Oxidant Concentration and Relative Humidity

Chemical transformations of aerosol particles by heterogeneous reactions with trace gases such as OH radicals can influence particle physicochemical properties and lifetime, affect cloud formation, light scattering, and human health. Furthermore, OH oxidation can result in degradation of particle mass by volatilization reactions, altering the budget of volatile organic compounds (VOCs). However, the reactive uptake coefficient (γ) and particle oxidation degree can vary depending on several factors including oxidant concentration and relative humidity (RH). While RH can influence the extent of dissociation/ionization, it can also affect particle phase and thus oxidant diffusivity. Only one study so far has investigated the effect of RH on the rate of OH uptake to organic surfaces; however, the underlying processes affecting OH reactivity with organic aerosol under humidified conditions still remains elusive. Here, we determine the effect of RH on OH reactivity with laboratory-generated biomass burning aerosol (BBA) surrogate particles: levoglucosan and 4-methyl-5-nitrocatechol. The effect of OH concentration on γ for three common BBA molecular markers (levoglucosan, abietic acid, and nitroguaiacol) under dry conditions was investigated from [OH]≈10⁷-10¹¹ molecule cm^-3, covering both [OH] in biomass burning plumes and [OH] commonly used in particle aging studies. Furthermore, key VOC reaction products and their production pathways resulting from BBA volatilization by OH were identified. OH radicals are produced using a microwave induced plasma (MIP) of H₂ in He or Ar followed by reaction with O₂, or by photolysis of O₃ in the presence of H₂O. A cylindrical rotating wall flow-tube reactor and fast-flow aerosol flow reactor are used for conducting kinetic studies. OH is detected using a Chemical Ionization Mass Spectrometer (CIMS) and a Proton Transfer Reaction Time-of-Flight Mass Spectrometer (PTR-ToF-MS) is employed for VOC analysis. γ decreases from 0.2-0.5 at [OH]≤~10⁹ molecule cm^-3 to ~0.01 at [OH]≈10¹¹ molecule cm^-3, indicative of a Langmuir-Hinshelwood (L-H) uptake mechanism, whereby OH first adsorbs and diffuses along the surface prior to reaction. Using an L-H model, OH adsorption/desorption equilibrium constants (K_OH) and surface reaction rate constants (k_s) are derived, allowing for the first time an estimate of OH surface coverage. A suite of VOCs from OH oxidation of levoglucosan, abietic acid, and nitroguaiacol were identified indicating enhancements over background of 50% and up to a factor of 15. A fuller evaluation of the influence of RH on OH uptake to BBA and its atmospheric implications will be presented. Our results suggest that particle lifetime estimates and VOC yields are strongly influenced by [OH] and that volatilization reactions may contribute significantly to the budget of VOCs within biomass burning plumes.