Measurements 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 Handschy, Melinda Schueneman, Seonsik Yun, Dongwook Kim, Donna Sueper, Harald Stark, Joost de Gouw, John Seinfeld, Paul Wennberg, Jose-Luis Jimenez, University of Colorado, Boulder

     Abstract Number: 472
     Working Group: Aerosol Chemistry

Abstract
Urban secondary organic aerosol (SOA) significantly contributes to decreased air quality and poses human health risks, especially in metropolitan areas. Many research studies have been conducted in Los Angeles, contributing to our collective understanding of its complex atmospheric chemistry. However, one lingering question has prevented the development of stronger policies on SOA precursor emissions: what compounds are the major gas-phase precursors of SOA? Conflicting model results from the 2010 CalNex analyses were highlighted by policymakers, as there is disagreement on which gas-phase precursors are the most important: volatile organic compounds (VOCs) or semi- and intermediate-volatility compounds (S/IVOCs). To our knowledge, the constraints on this problem are very indirect (through models) and there are no direct experimental constraints.

This work presents measurements at Caltech in the summer of 2022, to address this question. Dual oxidation flow reactors (OFRs, which simulate 1-2 days of atmospheric aging in ~3 minutes) oxidized ambient air, and their outflow was sampled by an aerosol mass spectrometer, a Vocus 2R-proton-transfer-reaction time-of-flight mass spectrometer, and a scanning mobility particle sizer. Both OFRs ingested ambient air, but one was deployed without an inlet, while the other had an inlet made of electrically conductive polymer tubing. Previous work in our group has demonstrated how polymer tubing can be used to denude gas-phase compounds according to their volatility, covering the range of interest. Ambient and OFR results for the 2022 study are similar to those from CalNex 2010, including the amount and diurnal cycles of precursors, SOA, and SOA potential. We demonstrate how this approach can be used to separate ambient SOA as being due to precursors of different volatility classes. Preliminary results suggest that at least half of the SOA formed in the OFR is from compounds with volatilities in the SVOC and lower IVOC range.