Photoaging of Phenolic Secondary Organic Aerosol in the Aqueous Phase: Evolution of Chemical and Optical Properties and Effects of Oxidants

WENQING JIANG, Christopher Niedek, Cort Anastasio, Qi Zhang, University of California, Davis

     Abstract Number: 205
     Working Group: Aerosol Chemistry

Abstract
While gas-phase reactions are well established to have significant impacts on atmospheric secondary organic aerosol (SOA) formation, the aqueous-phase aging of SOA remains poorly understood. This study investigates the evolution of the aqueous SOA (aqSOA) from guaiacyl acetone (GA), a semivolatile phenolic carbonyl commonly found in biomass burning smoke. The aqSOA was produced from GA reactions with hydroxyl radical (•OH-aqSOA) or a triplet excited state of organic carbon (³C*-aqSOA) and then photoaged in water under simulated sunlight. High resolution aerosol mass spectrometry (HR-AMS) and UV-vis spectroscopy were utilized to track changes in composition and light absorption of the aqSOA during formation and aging.

Compared to •OH-aqSOA, the ³C*-aqSOA is produced more rapidly and shows less oxidation, a greater abundance of oligomers, and higher light absorption. Prolonged photoaging promotes fragmentation and the formation of more volatile and less light-absorbing products. More than half of the initial aqSOA mass is lost and substantial photobleaching occurs after 10.5 hours of prolonged aging for ³C*-aqSOA and 48 hours for •OH-aqSOA. Positive matrix factorization (PMF) analysis of the combined HR-AMS and UV-vis spectral data resolved three generations of aqSOA with distinctly different chemical and optical properties. The first-generation aqSOA shows significant oligomer formation and enhanced light absorption at 340-400 nm. The second-generation aqSOA is enriched in functionalized species, while the third-generation contains more fragmented products and is the least light-absorbing. Although photoaging generally increases the oxidation of aqSOA, a slightly decreased O/C of the •OH-aqSOA is observed after 48 hours of photoaging, likely due to greater fragmentation and evaporation of highly oxidized compounds. Increased oxidant concentration accelerates the transformation of aqSOA and promotes the decay of brown carbon (BrC) chromophores, leading to faster mass reduction and photobleaching.