Cross-border transport and spatial variability of suspended particles in Mexicali and California's Imperial Valley

The city of Mexicali, the state capitol of Baja California, Mexico, lies 18 9 kilometers (km) inland from the Pacific Coast on the California border at the lower end of California's 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 mu g m(-3), while 2 3 samples exceeded this PM10 standard at the Mexicali site. Air how through Mexicali is channeled by the Imperial Valley and is usually from the north west or southeast, with northwesterlies being most frequent. During the stu dy period from March 1992 through August 1993, hourly PM10 concentrations w ere higher in the border area during southerly than northerly flow. For win d flow patterns in both directions, PM10 initially decreased with wind spee d due to improved ventilation, then increased at high wind speeds due to in creased suspension of soil particles. Average cross-border transport of PM1 0 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 concent rations at the Mexicali site were highest when flow was over the main Calex ico-Mexicali urban area (northwesterly flow) but were lower when flow was f rom less urbanized desert areas (southeasterly flow). Although Mexicali's P M10 concentrations were almost double those measured at Calexico, annual av erage relative source contributions were similar, with 70% from fugitive du st, 10-15% from motor vehicle exhaust, 4-8% from vegetative burning or cook ing, 2-3% from marine aerosol (Gulf of California), 1.5-3% from secondary a mmonium sulfate, and 1.5% to 2.5% from secondary ammonium nitrate. Primary industrial source contributions were negligible. (C) 2000 Published by Else vier Science Ltd. All rights reserved.