Source apportionment of secondary organic aerosol during a severe photochemical smog episode

The UCD/CIT air quality model was modified to predict source contributions to secondary organic aerosol (SOA) by expanding the Caltech Atmospheric Chemistry Mechanism to separately track source apportionment information through the chemical reaction system as precursor species react to form condensable products. The model was used to predict source contributions to SOA in Los Angeles from catalyst-equipped gasoline vehicles, non-catalyst equipped gasoline vehicles, diesel vehicles, combustion of high sulfur fuel, other anthropogenic sources, biogenic sources, and initial/boundary conditions during the severe photochemical smog episode that occurred on 9 September 1993. Gasoline engines (catalyst+non-catalyst equipped) were found to be the single-largest anthropogenic source of SOA averaged over the entire model domain. The region-wide 24-h average concentration of SOA produced by gasoline engines was predicted to be 0.34 μg m−3 with a maximum 24-h average concentration of 1.81 μg m−3 downwind of central Los Angeles. The region-wide 24-h average concentration of SOA produced by diesel engines was predicted to be 0.02 μg m−3, with a maximum 24-h average concentration of 0.12 μg m−3 downwind of central Los Angeles. Biogenic sources are predicted to produce a region-wide 24-h average SOA value of 0.16 μg m−3, with a maximum 24-h average concentration of 1.37 μg m−3 in the less-heavily populated regions at the northern and southern edges of the air basin (close to the biogenic emissions sources). SOA concentrations associated with anthropogenic sources were weakly diurnal, with slightly lower concentrations during the day as mixing depth increased. SOA concentrations associated with biogenic sources were strongly diurnal, with higher concentrations of aqueous biogenic SOA at night when relative humidity (RH) peaked and little biogenic SOA formation during the day when RH decreased.