Cross-border transport and spatial variability of suspended particles in Mexicali and Californiaʹs Imperial Valley

The city of Mexicali, the state capitol of Baja California, Mexico, lies 189 kilometers (km) inland from the Pacific Coast on the California border at the lower end of Californiaʹs Imperial Valley. Mexicali is adjacent to the California city of Calexico and 20 km south of the Imperial county seat of El Centro. The Mexicali and Calexico PM10 monitors were separated by only 10 km; yet, three measurements at Calexico exceeded 150 μg m−3, while 23 samples exceeded this PM10 standard at the Mexicali site. Air flow through Mexicali is channeled by the Imperial Valley and is usually from the northwest or southeast, with northwesterlies being most frequent. During the study period from March 1992 through August 1993, hourly PM10 concentrations were higher in the border area during southerly than northerly flow. For wind flow patterns in both directions, PM10 initially decreased with wind speed due to improved ventilation, then increased at high wind speeds due to increased suspension of soil particles. Average cross-border transport of PM10 was three times higher for southerly flow from Mexico than for northerly flow from the United States into Mexico. Because northerly winds were more frequent, the time-integrated cross-border transport over the study period was only about one-and-one-half times higher from Mexico. PM10 mass concentrations at the Mexicali site were highest when flow was over the main Calexico–Mexicali urban area (northwesterly flow) but were lower when flow was from less urbanized desert areas (southeasterly flow). Although Mexicaliʹs PM10 concentrations were almost double those measured at Calexico, annual average relative source contributions were similar, with 70% from fugitive dust, 10–15% from motor vehicle exhaust, 4–8% from vegetative burning or cooking, 2–3% from marine aerosol (Gulf of California), 1.5–3% from secondary ammonium sulfate, and 1.5% to 2.5% from secondary ammonium nitrate. Primary industrial source contributions were negligible.