Source contributions to the regional distribution of secondary particulate matter in California

Abstract

Source contributions to PM2.5 nitrate, sulfate and ammonium ion concentrations in California's San Joaquin Valley (SJV) (4–6 January 1996) and South Coast Air Basin (SoCAB) surrounding Los Angeles (23–25 September 1996) were predicted using a three-dimensional source-oriented Eulerian air quality model. The air quality model tracks the formation of PM2.5 nitrate, sulfate and ammonium ion from primary particles and precursor gases emitted from different sources though a mathematical simulation of emission, chemical reaction, gas-to-particle conversion, transport and deposition. The observed PM2.5 nitrate, sulfate and ammonium ion concentrations, and the mass distribution of nitrate, sulfate and ammonium ion as a function of particle size have been successfully reproduced by the model simulation. Approximately 45–57% of the PM2.5 nitrate and 34–40% of the PM2.5 ammonium ion in the SJV is formed from precursor gaseous species released from sources upwind of the valley. In the SoCAB, approximately 83% of the PM2.5 nitrate and 82% of the PM2.5 ammonium ion is formed from precursor gaseous species released from sources within the air basin. In the SJV, transportation related sources contribute approximately 24–30% of the PM2.5 nitrate (diesel engines ∼13.5–17.0%, catalyst equipped gasoline engines ∼10.2–12.8% and non-catalyst equipped gasoline engines ∼0.3–0.4%). In the SoCAB, transportation related sources directly contribute to approximately 67% of the PM2.5 nitrate (diesel engines 34.6%, non-catalyst equipped gasoline engine 4.7% and catalyst equipped gasoline engine 28.1%). PM2.5 ammonium ion concentrations in the SJV were dominated by area (including animal) NH 3 sources (16.7–25.3%), soil (7.2–10.9%), fertilizer NH 3 sources (11.4–17.3%) and point NH 3 sources (14.3–21.7%). In the SoCAB, ammonium ion is mainly associated with animal sources (28.2%) and catalyst equipped gasoline engines (16.2%). In both regions, the majority of the relatively low PM2.5 sulfate (<5 μg m −3) is associated with upwind sources. Most of the locally generated sulfate is emitted from diesel engines and high-sulfur fuel combustion processes in both modeling domains. Emissions control programs should target the sources listed above to reduce PM2.5 concentrations.