| 24 Aug 2026
Seasonal Differences in Correlations and Contributions of Photochemical, Upwind Cloud and Aerosol Aqueous-Phase Oxidation, and Mixed Combustion in Secondary Organic Aerosol Formation in Coastal Southern California
Abstract. Organic aerosol (OA) formation in coastal environments is influenced by photochemical and aqueous-phase oxidation, but their relative contributions remain poorly constrained. During the Eastern Pacific Cloud Aerosol Precipitation Experiment (EPCAPE), Aerosol Mass Spectrometer measurements at Mt. Soledad in La Jolla, California were analyzed by positive matrix factorization to resolve four oxygenated OA (OOA) factors: sulfate-related (SR-OOA), more-oxidized (MO-OOA), less-oxidized (LO-OOA), and continental (C-OOA). SR-OOA was linked to marine biogenic sources and bimodal number distributions indicating in-cloud aqueous reactions. Multiple linear regression (MLR) associated SR-OOA with in-cloud aqueous reactions represented by upwind cloud vertical fraction (UCVF; 70 %). MO-OOA correlated with UCVF during months with increasing UCVF (R = 0.25–0.66) and ozone for 11 months (R = 0.36–0.76), indicating both in-cloud aqueous and photochemical oxidation. MLR showed contributions from in-cloud aqueous reactions in spring (48 %) and photochemical oxidation represented by ozone in summer, fall, and winter (44–77 %) to MO-OOA. LO-OOA showed an ozone correlation (R = 0.37) and midday maxima, while MLR associated LO-OOA with photochemical oxidation (64 %). C-OOA correlated with refractory black carbon (rBC; R = 0.33) and other combustion tracers. MLR associated C-OOA with combustion represented by rBC in winter (49 %), aerosol water aqueous oxidation represented by relative humidity (RH) in spring (37 %), and photochemical oxidation in summer and fall (37–43 %). O/C was explained by RH (39 %) and ozone (38 %), followed by UCVF (19 %). These results reveal distinct seasonal contributions of photochemical and aqueous-phase oxidation to biogenic and mixed combustion OA in coastal Southern California.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Atmospheric Chemistry and Physics.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.
