Abstract:Source contributions to primary airborne particulate matter calculated using the source-oriented UCD/CIT air quality model and the receptor-oriented chemical mass balance (CMB) model are compared for two air quality episodes in different parts of California. The first episode occurred in the San Joaquin Valley on 4–6 January 1996, with peak 24 h average PM2.5 concentrations exceeding 100 μg m⁻³. This episode was characterized by low photochemical activity and high particulate nitrate concentrations, with localized regions of high particulate carbon concentrations around urban centers. The second episode occurred in the South Coast Air Basin on 7–9 September 1993, with peak 4 h average PM2.5 concentrations reaching 86 μg m⁻³. This episode was characterized by high photochemical activity and high secondary organic aerosol concentrations. The results from the two independent source apportionment calculations show strong agreement for source contributions to primary PM2.5 total organic mass at 7 receptor sites across the two studies, with a correlation slope of 0.84 and a correlation coefficient (R²) of 0.70. Agreement for source contributions to primary PM2.5 total mass was similarly strong, with a correlation slope of 0.83 and a correlation coefficient (R²) of 0.55. Wood smoke was identified as the dominant source of primary PM2.5 at urban locations in the SJV by both source apportionment techniques. Transportation sources including paved road dust, gasoline engines, and diesel engines, were identified as the dominant source of primary PM2.5 at all locations in the SoCAB by both models. The amount of secondary particulate matter (organic and inorganic) was in good agreement with the measured values minus the primary material identified by the CMB calculation.

The UCD/CIT air quality model is able to predict source contributions to airborne particulate matter at all locations and times throughout the study domain. The regional plots of source contributions to primary PM2.5 mass concentrations generated by the UCD/CIT air quality model suggest that high concentrations (25 μg m⁻³) of primary PM2.5 mass released from the combustion of fuel with high sulfur content exist in areas adjacent to the Los Angeles International Airport and the port of Los Angeles. Emissions profiles for these sources should be developed so that they can be included in future CMB calculations.