Size-resolved composition of atmospheric aerosol particles during winter (19 December 2014 to 13 January 2015) in the San Joaquin Valley at Fresno and during summer (4 to 28 July 2015) in the Southern California Air Basin at Fontana were measured by aerosol mass spectrometer, Fourier transform infrared spectrometer, single particle soot photometer, and scanning electrical mobility sizer. The Fresno study had low-fog and high-fog winter conditions, and residential burning was a frequent contributor to evening emissions. Fireworks during Fourth of July celebrations characterized the start of the Fontana study; the remaining days were categorized as nonfirework days and were mostly affected by traffic emissions. Fresno had particle distributions with number mode diameters of 70–150 nm, and Fontana had 30–50-nm diameters. The nonrefractory organic mass mode diameters were also larger at Fresno (250–380 nm in dry mobility diameter) than at Fontana (130–150 nm, 280 nm in dry mobility diameter) as were refractory black carbon particles (Fresno: 80–180 nm; Fontana: 80–100 nm in dry volume equivalent diameter). The size dependence of organic contributions to particle mass indicated that condensation or other surface-limited processes contributed oxidized organic fractions to aerosol mass in Fontana but that volume-limited aqueous reactions produced organic mass on both low-fog and high-fog days in Fresno. Linear regression analysis of organic aerosol sources with size-resolved particle volume at different times of day also showed that residential burning-related particles increased from 70–160 nm in the evening (18:00 to 23:59) to 150–260 nm at night (00:00 to 05:59) on low-fog days.

Organic aerosol mass (OM) components were investigated at Fresno in winter and at Fontana in summer by positive matrix factorization of high-resolution time-of-flight aerosol mass spectra and of Fourier Transform infrared spectra, as well as by k-means clustering of light-scattering (LS) aerosol single-particle spectra. The results were comparable for all three methods at both sites, showing different contributions of primary and secondary organic aerosol sources to PM1. At Fresno biomass burning organic aerosol contributed 27% of OM on low-fog days, and nitrate-related oxidized OA (NOOA) accounted for 47% of OM on high-fog days, whereas at Fontana very oxygenated organic aerosol (VOOA) components contributed 58–69% of OM. Amine and organosulfate fragment concentrations were between 2 and 3 times higher on high-fog days than on low-fog days at Fresno, indicating increased formation from fog-related processes. NOOA and biomass burning organic aerosol components were largely on different particles than the VOOA components in Fresno, but in Fontana both NOOA and VOOA components were distributed on most particle types, consistent with a longer time for and a larger contribution from gas-phase photochemical secondary organic aerosol formation in summer Fontana than winter Fresno. Uncommon trace organic fragments, elevated inorganic, and alcohol group submicron mass concentrations persisted at Fontana for more than 5 days after 4 July fireworks. These unique aerosol chemical compositions at Fresno and Fontana show substantial and extended air-quality impacts from residential burning and fireworks.