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 source contributions to elevated PM10 concentrations and to estimate transport of PM10 between the US and Mexico. The study was conducted from 13 March 1992 to 29 August 1993, in a 80-km long by 20-km wide area spanning the US/Mexico 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 PM 10 (particles with aerodynamic diameters less than 10 microm) mass, elements, water-soluble cations (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 intensive periods. Measurements were also taken at as many as 30 neighborhood (satellite) sites during week-long intensive monitoring periods in spring, summer and winter. This paper examines the zones of representation of long-term PM10 monitors by comparing their measurements with those from a spatially dense network of satellite sites. PM10 concentrations at the Mexicali site were consistently 30 to 50% higher than those observed at the Calexico site, even though the two sites were only 12 km apart. Distinct diurnal variations were found, with 6-h average PM10 concentrations often varying by a factor 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 material 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 were found within the study area. PM10 concentrations in Mexico were double those in the US, decreasing with increasing northerly distance.