Fig. 4.

Comparisons of OA vs. CO show the behavior of primary and secondary OA in the atmosphere and are used to examine vehicular OA and total OA in the San Joaquin Valley (Bakersfield, CA) and numerous other urban sites (6, 18, 21, 25–29). Photochemical aging increases ΔOA/ΔCO ratios and is represented by increased O:C ratios shaded in each panel. (A) Best estimates for ΔOA/ΔCO ratios expected for pure gasoline and diesel emissions are added to a ΔPOA/ΔCO value of 9.4 μg⋅m⁻³⋅ppmv⁻¹ CO to account for primary OA and shown with a range of ΔPOA/ΔCO values (21, 30). Vehicular OA is determined from AMS factor analysis and observations are well constrained at Bakersfield with the exception of the most aged air parcels, whose ΔOA/ΔCO ratios are greater than expected for the mix of gasoline and diesel use. (B) Predicted ΔOA/ΔCO slopes for a range of fuel mixtures ranging from 5% to 40% diesel agree with observations of relatively young aerosol in urban areas and vehicular OA at Bakersfield. Observed ΔOA/ΔCO ratios increase with degree of aging and/or the influence of other SOA precursor sources that do not emit CO, which are prominent at Bakersfield and sites a–c. (C) Weekday and weekend diurnal averages of vehicular ΔOA/ΔCO show greater ratios in the afternoon and over the weekend as a result of increased photochemical aging. Ratios are calculated with a 90 ppbv CO background, and SDs are shown in SI Appendix, Fig. S8.