Zones of representation for PM10 measurements along the US/Mexico border

The 'Imperial/Mexicali Valley Cross-Border PM10 Transport Study' acquired a database of meteorological and air quality measurements to determine sourc e contributions to elevated PM10 concentrations and to estimate transport o f PM10 between the US and Mexico. The study was conducted from 13 March 199 2 to 29 August 1993, in a 80-km long by 20-km wide area spanning the US/Mex ico border approximately 200 km inland from the coast of the Pacific Ocean, with monitoring sites located in the Imperial Valley on the US side and in the Mexicali Valley on the Mexico side. Measurements of PM10 (particles wi th aerodynamic diameters less than 10 muM) mass, elements, water-soluble ca tions (i.e. sodium, potassium, ammonium) and anions (i.e. chloride. nitrate , sulfate), organic and elemental carbon and particle light absorption were acquired at two base sites on an every-sixth-day schedule supplemented by daily monitoring during winter and 4 times per day monitoring during intens ive periods. Measurements were also taken at as many as 30 neighborhood (sa tellite) sites during week-long intensive monitoring periods in spring, sum mer and winter. This paper examines the zones of representation of long-ter m PM10 monitors by comparing their measurements with those from a spatially dense network of satellite sites. PM10, concentrations at the Mexicali sit e were consistently 30 to 50% higher than those observed at the Calexico si te, even though the two sites were only 12 km apart. Distinct diurnal varia tions were found. with 6-h average PM10 concentrations often varying by a f actor of 2 throughout the day - lowest during afternoon (12.00-18.00 h PST) and highest during night time (18.00-24.00 h PST). On average, crustal mat erial accounted for 32-35% of annual-average PM10, carbonaceous aerosol for 20-30%, and ionic species for 8-10%. Levels of trace elements and sea salt were in the range of 1-4% of PM10. Significant concentration variations we re found within the study area. PM10 concentrations in Mexico were double t hose in the US, decreasing with increasing northerly distance. (C) 2001 Els evier Science BN. All rights reserved.