Chemical composition of precipitation at two sampling sites in Mexico: A 7-year study

This paper describes the results of a study of the chemical composition of bulk and wet precipitation, collected for 7 years in a large urban area and a forested rural site in Central Mexico. In rain samples from both sites, sulfate was the most abundant ion followed by ammonium. Seasonal patterns of monthly volume-weighted concentration show that the highest values of SO2−4 NO3−, Ca2+ and NH4+were measured in May, the beginning of the rainy season in Central Mexico. The lowest values were observed from July to September, the months when most thunderstorms occurred. Multiple regression correlation analysis was applied to all data and a hierarchical model was used to select the variables included in the final model. In Rancho Viejo the model showed that SO2−4 contributed with 41% in the H+ concentration prediction. In Mexico City, despite the high SO42− concentration in rain water, this ion only contributed 0.46% whereas Ca2+ contributed in wet and bulk precipitation, 61 and 41%, respectively. Finally no long-term trends were noticed.