Estimated contributions of primary and secondary organic aerosol from fossil fuel combustion during the CalNex and Cal-Mex campaigns

Observations during CalNex and Cal-Mex field campaigns at Bakersfield, Pasadena, Tijuana, and on board the R/V Atlantis show a substantial contribution of fossil fuel emissions to the ambient particle organic mass (OM). At least two fossil fuel combustion (FFC) factors with a range of contributions of oxidized organic functional groups were identified at each site and accounted for 60-88% of the total OM. Additional marine, vegetative detritus, and biomass burning or biogenic sources contribute up to 40% of the OM. Comparison of the FTIR spectra of four different unburned fossil fuels (gasoline, diesel, motor oil, and ship diesel) with PMF factors from ambient samples shows absorbance peaks from the fuels are retained in organic aerosols, with the spectra of all of the FFC factors containing at least three of the four characteristic alkane peaks observed in fuel standards at 2954, 2923, 2869 and 2855 cm−1. Based on this spectral similarity, we estimate the primary OM from FFC sources for each site to be 16-20%, with secondary FFC OM accounting for an additional 42-62%. Two other methods for estimating primary OM that use carbon monoxide (CO) and elemental carbon (EC) as tracers of primary organic mass were investigated, but both approaches were problematic for the CalNex and Cal-Mex urban sites because they were influenced by multiple emission sources that had site-specific and variable initial ratios to OM. For example, using the ΔPOM/ΔCO ratio of 0.0094 μg ppb V−1 proposed by other studies produces unrealistically high estimates of primary FFC OM of 55-100%.