Chemical Characterization and Preliminary Source Contribution of Fine Particulate Matter in the Mexicali/Imperial Valley Border Area

Abstract

The airshed shared by the Valley of Mexicali in Mexico and Imperial Valley in the United States frequently experiences high levels of particulate matter (PM) less than 10 µm (PM10). However, very little information exists on the concentration and composition of fine PM (PM <2.5 µm, or PM2.5) in the region, particularly on the Mexican side of the border. This paper presents the results of two monitoring campaigns (November 23–December 17, 2004 and April 12–22, 2005) that were conducted to obtain, for the first time, detailed composition of PM2.5 in the city of Mexicali. In both campaigns, 24-hr average PM2.5 samples were collected using low-volume equipment. Samples collected on Teflon filters were analyzed for mass and 38 metals (Na to Pb), whereas samples collected on quartz filters were analyzed for anions (sodium, potassium, ammonium), cations (chloride, nitrate, sulfate), and carbon (organic [OC] and elemental [EC]). During the fall campaign, the monitored concentration exceeded 65 µg/m3 (i.e., Mexico’s PM2.5 air quality standard for 24-hr average samples) on four different days at one or more Mexicali sites. No exceedances were reported during the spring campaign. The contribution of main chemical species to the total mass indicated that organic material was the principal component (62% in fall and 56% in spring). Average secondary OC contributions to the total observed OC for the fall data was estimated at 14% (no day exceeding 30% contribution) and 47% for the spring data (with the minimum being 33% and the maximum 68%). Ammonium sulfate contributions exceeded ammonium nitrate contributions during the spring (16% vs. 7%), whereas the reverse occurred during the fall (5% vs. 10%). Geological material contributed 6% of the mass in the fall versus 10% in the spring. Overall the results indicate that PM2.5 in Mexicali is heavily influenced by anthropogenic sources, rather than from soil resuspension. Spatial and meteorological analysis of the data gives an indication that during the fall campaign local sources were the principal contributor to high-PM events, whereas during the spring campaign regional sources from southern California could have also contributed to the observed PM2.5 levels.