CHEMICAL-COMPOSITION OF PRECIPITATION AT 2 SAMPLING SITES IN MEXICO -A 7-YEAR STUDY

This paper describes the results of a study of the chemical compositio n of bulk and wet precipitation, collected for 7 years in a large urba n area and a forested rural site in Central Mexico. In rain samples fr om both sites, sulfate was the most abundant ion followed by ammonium. Seasonal patterns of monthly volume-weighted concentration show that the highest values of SO42-, NO3-, Ca2+ and NH4+ were measured in May, the beginning of the rainy season in Central Mexico. The lowest value s were observed from July to September, the months when most thunderst orms occurred. Multiple regression correlation analysis was applied to all data and a hierarchical model was used to select the variables in cluded in the final model. In Rancho Viejo the model showed that SO42- contributed with 41% in the H+ concentration prediction. In Mexico Ci ty, despite the high SO42- concentration in rain water, this ion only contributed 0.46% whereas Ca2+ contributed in wet and bulk precipitati on, 61 and 41%, respectively. Finally no long-term trends were noticed . Copyright (C) 1996 Elsevier Science Ltd