Mechanistic Insights into Radical Formation during the Photochemical Aging of Biomass Burning Secondary Organic Aerosols
LENA GERRITZ, Sergey Nizkorodov, Manabu Shiraiwa, University of California, Irvine
Abstract Number: 33
Working Group: Chemicals of Emerging Concern in Aerosol: Sources, Transformations, and Impacts
Abstract
The photochemical aging of atmospheric aerosols alters its physical and chemical composition, which may have important implications on the toxicity and climate effects of these particles. Recent work has explored the impact of photochemical aging on particle composition, but there is limited understanding on the molecular-level processes occurring. This study applies electron paramagnetic resonance (EPR) spectroscopy with a spin trapping agent BMPO and high-resolution mass spectrometry (HRMS) to study the formation of reactive oxygen species (ROS) and free radicals during the irradiation of lab generated secondary organic aerosol (SOA). This work compiles the radical formation in SOA derived from terpenes (α-pinene, α -terpineol, β-ocimene, isoprene), phenyl derivatives (phenol, styrene, toluene, guaiacylacetone) and furfural to provide insight into the aging mechanisms for different classes of SOA precursors prominent in biomass burning emissions. EPR analysis reveals that terpene SOA produced the most radicals including HO2∙, carbon centered organic radicals (R∙), and oxygen centered organic radicals (RO∙). The HRMS identification of the organic radicals correspond to common products of Norrish Type I photolysis of carbonyls, and mechanisms for the photolysis of α-pinene SOA are proposed. Phenyl derivatives produced significantly lower SOA mass and fewer radicals including only HO2∙ and R∙. The EPR results suggest photosensitized phenyl derivative SOA reacts via single electron transfer reactions with dissolved oxygen to produce superoxide and phenoxy radicals. HRMS analysis identified other types of radicals formed during photolysis, suggesting the contribution of additional photosensitized mechanisms. This presentation will summarize a comprehensive series of experiments on the potential mechanisms involved in the photochemical aging of SOA, which can help inform large-scale models and assess the evolution of chemical, physical, and toxicological properties of SOA throughout its atmospheric lifetime.