Atmospheric oxidation of volatile organic compounds and its contribution to aerosol formation

Abstract

The atmospheric oxidation of volatile organic compounds (VOC) produces lower-volatility products that can contribute to the formation of secondary organic aerosol (SOA) via gas-to-particle transformation. SOA accounts for a significant fraction of sub-micron particles in the atmosphere, with these particles adversely affecting air quality and human health, as well as playing a complex role in aerosol-climate interactions. Our understanding of VOC oxidation, along with the contribution to SOA formation, remains limited due to complex formation processes and difficulties in molecular-level quantification. The research described in this thesis primarily utilized a chemical ionization mass spectrometer combined with a filter inlet for gases and aerosols (FIGAERO-CIMS), an atmospheric simulation chamber, and an oxidation flow reactor to study the chemical compositions of SOA under various chemical and environmental conditions. The research underlying this thesis also involved ambient observations of oxygenated products from both primary emissions and the oxidation of anthropogenic and biogenic VOC. A key property which influences the formation of SOA from oxidation products detected both in the laboratory and in the field is vapor pressure. The research presented in this thesis applied various estimation methods to determine the vapor pressure of compounds, such as carboxylic acids and organic nitrates, to ultimately improve our understanding of gas-particle partitioning. The presented laboratory work focused on the O3 and NO3 initiated oxidation of the monoterpene Δ3-carene (C10H16), which is important due to high emissions in boreal forests and a higher yield of SOA compared to the structurally similar 𝛼𝛼-pinene. The investigation of Δ3- carene ozonolysis included measurements of the formation of oxidized products and subsequent gas-to-particle partitioning at three temperatures (0°C, 10°C, and 20°C) under dry (RH<2%) conditions and at 10°C under humid (RH=78%) conditions. The equilibrium partitioning coefficient (Kp,𝑖) calculated for the most dominant products increased as the temperature decreased from 20°C to 10°C, as well as from 10°C to 0°C, and as RH increased from <2% to 78%. The temperature dependence of saturation vapor pressure (psat) derived from Kp,𝑖𝑖, can be used to directly calculate the enthalpy of vaporization for oxidized products without needing pure standards. The study of NO3 initiated oxidation is relevant as this serves as a key pathway for the nocturnal formation of organic nitrates. The NO3 initiated oxidation of Δ3-carene study characterized that C10 monomers (organic nitrates with 10 carbons) were the dominant nitrated products of Δ3-carene (e.g., C10H17NO5 and C10H15NO7) in the gas phase. The gas-phase product distribution changed under various conditions, e.g., the fraction of products C9H15NO6 and C10H15NO7 increased as the oxidant level increased. The particle-phase organic nitrate products were predominantly dimers (C20H32N2O8-12), along with C10 monomers and C9 fragments. The field work described in this thesis included ambient observations of oxygenated products from both primary emissions and the oxidation of anthropogenic and biogenic VOC in the Chinese megacities of Xi'an and Beijing. Oxygenated organic compounds (CHO) and nitrogen containing oxygenated compounds (CHON) both contribute to fine particulate matter (PM2.5) loading in urban environments. In winter, particularly during severe haze episodes, concentrations of CHO and CHON increased in Xi'an. Sources related to biomass burning accounted for 61% and 68% of total CHON in autumn and winter, respectively, while secondary formation was the primary source of CHON in spring (70%) and summer (79%). In urban Beijing, the observations of concerning SOA tracers indicated that both anthropogenic and biogenic SOA play an important role during winter and summer. The increased biogenic SOA levels witnessed in the summer were primarily linked to high isoprene emissions, while the tracers for biogenic SOA observed in the winter most likely originated from the combustion of biomass and unexpectedly from fossil fuels.