Measurements and Modeling of Urban Secondary Organic Aerosol as a Function of Precursor Volatility Class in the Los Angeles Area During Summer 2022
MELISSA MORRIS, Benjamin Schulze, Andrew Jensen, Douglas A. Day, Pedro Campuzano-Jost, Anne V. Handschy, Melinda Schueneman, Seonsik Yun, Dongwook Kim, Donna Sueper, Harald Stark, Benjamin Murphy, T. Nash Skipper, Havala Pye, John Crounse, Joost A. de Gouw, John Seinfeld, Paul Wennberg, Jose Jimenez, CIRES & University of Colorado Boulder
Abstract Number: 227
Working Group: Aerosol Chemistry
Abstract
Urban secondary organic aerosol (SOA) contributes significantly to degraded air quality with the potential for human health risks. Air quality in Los Angeles, CA has improved over the last decade, but is now plateauing. Regulations on vehicle emissions have increased the importance of volatile chemical products and cooking emissions in the formation of SOA. These sources emit a range of gas-phase SOA precursors, from volatile organic compounds (VOCs) to semi- and intermediate-volatility compounds (S/IVOCs). In summer 2022, measurements were made to quantify the amount of SOA formed by VOCs vs. by lower volatility precursors. To our knowledge, these are the first direct measurements of ambient SOA production from discrete volatility ranges.
The measurement setup included dual oxidation flow reactors (OFRs, which simulate ~2 days of atmospheric aging in ~3 minutes). Both ingested ambient air; one OFR was equipped with an electrically conductive polymer inlet that denuded lower volatility species, and the other was run without an inlet. Previous work in our group has characterized volatility-based absorption of gas species by polymer tubing (Morris et al, AMT, 2024). We used a Vocus 2R-proton-transfer-reaction time-of-flight mass spectrometer, an aerosol mass spectrometer and a scanning mobility particle sizer.
The amount and diurnal cycles of SOA and SOA potential were similar between 2022 and 2010. Gas-phase concentrations have changed, indicating a shift in the relative importance of precursor sources. The measurements suggest that ~40% of the SOA formed in the OFR is from compounds with volatilities in the SVOC and lower IVOC range. Results are compared to a constrained box model developed at Caltech, and to the CMAQ photochemical transport model, which runs the chemical mechanism CRACMM, developed at U.S. EPA. Both models overpredict the SOA formation potential of ambient air, and rely too heavily on higher volatility species to generate SOA.