The chemical mass balance (CMB) receptor model was applied to the chemically speciated diurnal particulate matter samples acquired at nine locations in California's South Coast Air Basin (SoCAB) during the summer and fall of 1987 as part of the Southern California Air Quality Study (SCAQS). Source profiles applicable to the Los Angeles area were used to apportion PM_2.5 and PM₁₀ (particles with aerodynamic diameters < 2.5 and 10 μm, respectively) to primary paved road dust, primary construction dust, primary motor vehicle exhaust, primary marine aerosol, secondary ammonium nitrate, and secondary ammonium sulfate. Suspended dust was the major contributor to PM₁₀ during the summer, whereas secondary ammonium nitrate and primary motor vehicle exhaust contributions were high during the fall. Secondary ammonium sulfate contributions were uniform across the SoCAB, with average contributions during the fall less than half those found during the summer. Marine aerosol contributions were lower during the fall than during the summer, and were substantially lower at the inland sites relative to the sites near the coast.