Superoxide Burst upon Photochemical Aging of Secondary Organic Aerosols Derived from Biomass Burning Precursors
LENA GERRITZ, Meredith Schervish, Sergey Nizkorodov, Manabu Shiraiwa, University of California, Irvine
Abstract Number: 94
Working Group: Aerosol Chemistry
Abstract
Biomass burning is a major source of particulate matter and volatile organic compounds (VOCs) that can be oxidized to form secondary organic aerosols (SOA), contributing to climate, air quality, and public health. The mechanisms of photochemical aging and its effects on particle toxicity are poorly understood. This work offers insight into the radical chemistry occurring during photochemical aging of secondary organic aerosols derived from precursors emitted during biomass burning events (BBSOA). Radicals are detected in-situ over the course of irradiation using electron paramagnetic resonance spectroscopy (EPR) with a spin trapping compound and trapped organic radicals are identified using high resolution mass spectroscopy. The EPR results revealed substantial superoxide (O2-•) formation, upon irradiation of BBSOA for all selected precursors with yields between 0.05-0.3%. Superoxide radical has important implications for atmospheric oxidation and particle toxicity due to its oxidative potential and its ability to interconvert with other reactive oxygen species (ROS) including hydroxyl radical and hydrogen peroxide. However, the BBSOA from aromatic precursors (phenol, guaiacylacetone, and styrene) exhibited a rapid burst of O2-• that decayed within 2 minutes, while the monoterpene derived BBSOA (α-pinene, α-terpineol, and β-ocimene) formed O2-• more gradually, peaking around 5 minutes, and furfural BBSOA peaked around 10 minutes of irradiation. The differences in the formation of O2-• were further evaluated using a kinetic model that suggested the initial burst observed in aromatic SOA was the result of a direct reaction between photosensitizers and dissolved oxygen, while the more gradual formation observed for the non-aromatic SOA was attributed to the decomposition of peroxides. Organic radicals identified using mass spectrometry were consistent with decomposition of chromophores like carbonyls in non-aromatic BBSOA, while potential photosensitizers were identified in phenol SOA. We also quantified the increase in H2O2 after irradiation and found that these photochemical processes are an important source of condensed phase ROS with important implications for particle toxicity and atmospheric aging. Elucidating these mechanisms of superoxide formation during photochemical aging provides crucial insight into the oxidant budget for largescale atmospheric models and health implications of aging.