Trends in Chemical Composition of Wet Precipitation in Mexico City, Mexico: 1992–2007

This paper presents the results of a study of variation in the chemical composition of wet precipitation over a 16-year period. The ions SO4 2, NO3 , Cl , Ca 2+ , Mg 2+ K + , Na + , NH4 + and H + were analyzed. The correlation analysis showed a high positive correlation among these ions, indicating that anthropogenic emissions were the most important source. Trends in annual concentrations of SO4 2, NO3 , Cl , NH4 + , Mg 2+ , Ca 2+ , K + , H + and alkalinity as function of time were calculated using a linear least-square regression of the volume weighted mean concentrations of these ions in rainwater collected over a 16-year period. Sulfate (SO4 2) in wet precipitation showed a significant decreasing trend, corresponding to the reduction in SO2 emissions in the Mexico City valley due to the change from oil to gas fuel in thermoelectric power plants and heavy and medium sized industries. In spite of the fact that the number of motor vehicles has risen considerably in Mexico City, NO3 did not show a significant increasing trend. The NH4 + /Ca 2+ ratio showed a nearly significant increasing trend, due mainly to the significant decrease in Ca 2+ concentration. This decreasing trend was not as marked as that of SO4 2. The contribution of NH4 + to neutralization was relatively small, showing only a non significant increase in NH4 + over the study period, while a considerable decreasing SO4 2trend occurred, which could mean that part of the NH4 + ions reacted with other ions, forming ammonium compounds.