Online Molecular Characterization of Oxygenated Organic Compounds in Gas and Particle Phases in Winter Fresno: Influences From Residential Wood Burning and Aqueous Phase Processing

WENQING JIANG, Christopher Niedek, Anita Avery, Harald Stark, Manjula Canagaratna, Qi Zhang, University of California, Davis

     Abstract Number: 369
     Working Group: Urban Aerosols

Abstract
Fresno, located in California's Central Valley, consistently experiences high levels of particulate matter pollution and poor air quality due to its complex emission sources and unique geographical and meteorological conditions. To investigate this, we performed an online gas and particle phase molecular characterization using an iodide-adduct chemical ionization mass spectrometer (CIMS) coupled with a Filter Inlet for Gases and AEROsols (FIGAERO) during the winter of 2023/2024 at the California Air Resource Board’s Garland air monitoring site in Fresno. A wide variety of oxygenated organic compounds (OOC), representing both primary and secondary emissions sources, were detected. Notably, levoglucosan was one of the most prevalent OOC, with an average particle-phase concentration of 8.0 µg/m3 during the campaign. Levoglucosan and a group of phenols (e.g., guaiacol, syringol and vanillin) showed strong diurnal patterns, peaking between 9 pm and 1 am, which indicates that residential wood burning is an important source of PM in Fresno during the winter. Nitroaromatics (e.g., nitrocatechol, nitroguaiacol and nitrosyringol) reached their highest concentrations at night, with a secondary peak around 9 am. This pattern suggests that these nitroaromatics are primarily associated with nighttime biomass burning, with a minor contribution from vehicular traffic. Additionally, there are indications that the nighttime oxidation of phenols by nitrate radicals may also contribute to the secondary formation of nitroaromatics. Furthermore, the Fresno area experienced frequent fog events during the campaign, during which the concentrations of organic acids (e.g., oxalic acid and malonic acid) increased significantly. This rise in organic acids levels during foggy conditions suggests that aqueous-phase processing within fog droplets or hydrate aerosols plays a crucial role in the formation of SOA during the winter months in Fresno.