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

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 semi-volatile organic compound (SVOC) and lower intermediate-volatility organic compound (IVOC) ranges. Evidence suggests these lower-volatility precursors are mainly from direct emissions, rather than oxidation products from upwind aging. 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.