Oxidation of African Biomass Burning Emissions within Cloud Water Mimics as a Potential Source of Aqueous Secondary Organic Aerosol and Brown Carbon

HALEY ROYER, Vivian Campbell, Cade Christensen, Megan McRee, Marc Fiddler, Solomon Bililign, Barbara Turpin, Jason Surratt, University of North Carolina at Chapel Hill

     Abstract Number: 292
     Working Group: Burning Questions of Aerosol Emissions, Chemistry, and Impacts from Wildland-Urban Interface (WUI) Fires

Abstract
Wildfires, which are increasing globally due to climate change and land use changes, emit water-soluble organic gases (WSOGs) that can partition into cloud water. Reactive species such as the hydroxyl radical (^•OH) are common in cloud droplets, creating an opportunity to oxidize WSOGs and alter their chemical and optical properties, including the formation of semi-absorptive brown carbon (BrC). However, few existing studies have explored this mechanism in a controlled laboratory setting with real fuel samples. Studies are especially scant for African fuels, despite African wildfires accounting for nearly half of biomass burning emissions globally. In this study, we monitor changes in cloud-water mimics consisting of ^•OH and WSOGs from the combustion of common African vegetation to explore how the chemical and optical properties of wildfire emissions change as they are oxidized. Common African biomass burning fuels were first combusted in a smog chamber, during which WSOGs were collected into mist chamber samplers and subsequently analyzed for total organic carbon (TOC) concentrations. We then produced cloud water mimics consisting of diluted mist chamber samples (to mimic TOC concentrations in clouds influenced by wildfire plumes) and hydrogen peroxide. Cloud water mimics were exposed to a 254 nm ultraviolet lamp to generate ^•OH and facilitate oxidation of WSOGs. Over the course of 3-hour experiments, aliquots were collected at 14 time points and analyzed via ion chromatography (IC) for soluble short-chain organic compounds and reverse-phase liquid chromatography coupled to both diode array detection and to a high-resolution quadrupole time-of-flight mass spectrometer equipped with electrospray ionization (RPLC/DAD-ESI-HR-QTOFMS) for characterization of BrC species and long-chain organics, respectively. Results show the evolution of short-chain and long-chain organics as they form or decay over the course of each experiment. Corresponding optical measurements provide insight into the potential of organics to evolve into semi-absorptive secondary BrC compounds.