ARD Aerosol Research Discussions ARD Aerosol Research Discuss. 2940-3405 Göttingen, Germany 10.5194/ar-2026-12 Measurements and modeling of urban secondary organic aerosol formation potential as a function of precursor volatility class in the Los Angeles area during summer 2022 Ehrenfels Melissa A. https://orcid.org/0000-0003-3634-2195 1 2 Schulze Benjamin C. 3 7 Jensen Andrew R. https://orcid.org/0000-0003-0238-2367 1 2 8 Tasnia Afsara 4 9 Day Douglas A. https://orcid.org/0000-0003-3213-4233 1 2 Campuzano-Jost Pedro https://orcid.org/0000-0003-3930-010X 1 2 Handschy Anne V. 1 2 Schueneman Melinda K. 1 2 10 Yun Seonsik 1 2 Kim Dongwook https://orcid.org/0000-0002-2172-3588 1 2 Sueper Donna 1 5 Pye Havala O. T. https://orcid.org/0000-0002-2014-2140 6 10 Murphy Benjamin N. https://orcid.org/0000-0003-3542-5378 6 11 Skipper T. Nash 6 12 Barsanti Kelley C. https://orcid.org/0000-0002-6065-8643 4 13 de Gouw Joost A. https://orcid.org/0000-0002-0385-1826 1 2 Jimenez Jose L. https://orcid.org/0000-0001-6203-1847 1 2 Department of Chemistry, University of Colorado Boulder, Boulder, CO 80309, USA Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado, Boulder, CO 80309, USA Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA Department of Chemical & Environmental Engineering, Center for Environmental Research & Technology, University of California-Riverside, Riverside, CA 92521, USA Aerodyne Research Inc., Billerica, MA 01821, USA Office of Research and Development, U.S. Environmental Protection Agency, Research Triangle Park, NC 27709, USA currently at: Bain & Company, Denver, CO 80202, USA currently at: Department of Chemistry and Biochemistry, Kalamazoo College, Kalamazoo, MI 49006, USA currently at: South Coast Air Quality Management District, Diamond Bar, CA 91765, USA currently at: Office of Applied Science and Environmental Solutions, U.S. Environmental Protection Agency, Research Triangle Park, NC 27709, USA currently at: Norwegian Meteorological Institute, 0371 Oslo, Norway currently at: Office of Air and Radiation, U.S. Environmental Protection Agency, Research Triangle Park, NC 27709, USA currently at: NSF National Center for Atmospheric Research, Boulder, CO 80305, USA 20 03 2026 2026 1 45 Copyright: © 2026 Melissa A. Ehrenfels et al. 2026 This work is licensed under the Creative Commons Attribution 4.0 International License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/4.0/ This article is available from https://ar.copernicus.org/preprints/ar-2026-12/ The full text article is available as a PDF file from https://ar.copernicus.org/preprints/ar-2026-12/ar-2026-12.pdf

Urban secondary organic aerosol (SOA) contributes to degraded air quality which can affect human health. Improvements in Los Angeles (LA), CA air quality have mainly plateaued since 2010. In summer 2022, measurements were made to quantify the SOA formation potential (SOA-FP) in ambient LA air. Two oxidation flow reactors (OFRs) ingested ambient air: one was equipped with an electrically conductive polymer inlet that denuded lower volatility species, and the other was run without an inlet. This allowed the separate quantification of SOA-FP from higher vs. lower volatility precursors. To our knowledge these are the first direct measurements of these fractions. Measured ambient SOA was similar and total SOA-FP was lower in 2022 vs. 2010, consistent with higher ambient OH causing greater consumptions of SOA precursors in 2022. The dual-OFR measurements suggest ~31 % of the total SOA-FP is due to compounds with volatilities in the SVOC and lower IVOC ranges. Results are compared to two box models: one based on CRACMM and the other adapted from a recent Caltech publication. CRACMM predicted ambient OA well but underpredicted SOA-FP by about a factor of 2, while the Caltech model underestimated OA and overpredicted SOA-FP by a factor of 2.5. Our study finds terpenoids contribute to, but do not dominate, SOA-FP.

 National Science Foundation AGS-2206655 Cooperative Institute for Research in Environmental Sciences Innovative Research Program (IRP) Seed Grant National Oceanic and Atmospheric Administration AC4 NA21OAR4310222