Chemical Aging and Cloud Condensation Nuclei Activity of Biomass Burning Aerosol Proxies in the Presence of OH Radicals

Biomass burning aerosol (BBA) can adversely impact regional and global air quality and represents a significant source of organic aerosol (OA) to the atmosphere that can affect climate. Aerosol particles can alter the transfer of radiation in earth's atmosphere directly by scattering and absorbing radiation or indirectly via cloud formation. Gas-to-particle, also termed heterogeneous, oxidation reactions can significantly alter the particle's physical and chemical properties. In turn, this can lead to the degradation of biomolecular markers for air quality-related aerosol source apportionment studies, the particles' lifetime, and modify the particles' abilities to serve as cloud condensation nuclei (CCN). However, the rates, mechanisms, and conditions by which these multiphase oxidation reactions occur and influence the CCN activity of OA is not well understood. The work presented here aims to determine the reactivity and products from the interaction of BBA surrogate-particles and trace gas-phase oxidants and to link the effects of OA chemical aging on the particles' ability to nucleate clouds.;The reactive uptake of OH by BBA surrogate-substrates and particles, including levoglucosan, nitroguaiacol, abietic acid, and methyl-nitrocatechol, was determined as a function of both OH concentration and relative humidity (RH) using chemical ionization mass spectrometry coupled to various flow reactors. OH reactive uptake decreased with increasing OH concentration, indicative of OH adsorption followed by reaction. OH oxidation led to significant volatilization, i.e. mass loss of the organic material, as determined by application of high resolution proton transfer reaction time-of-flight mass spectrometry. Volatilized reaction products were identified, providing mechanistic insight of the chemical pathways in the heterogeneous OH oxidation of BBA. The reactive uptake of OH by levoglucosan particles increased with RH due to enhanced OH and organic bulk diffusivity. In contrast, OH uptake by methyl nitrocatechol decreased with increasing RH, attributed to the adsorption of water molecules on the particle surface, which blocks surface reactive sites. The CCN activity of single-component BBA surrogate-particles can be enhanced following OH exposure; however, when mixed with more water-soluble organic or inorganic compounds, OH oxidation showed no impact on the particles' CCN activity. Ambient temperature has been shown to impact the extent of particle oxidation and CCN activity of Suwannee River Fulvic Acid particles, a surrogate Humic-Like Substance of BBA. The atmospheric implications of the results on the understanding of the OH oxidative aging of OA and their cloud formation potential are discussed.