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

ABSTRACT

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-, NO₄^-, Cl^-, Ca²⁺, Mg²⁺ 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⁺, Mg²⁺, Ca²⁺, 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⁺/Ca²⁺ ratio showed a nearly significant increasing trend, due mainly to the significant decrease in Ca²⁺ 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 SO₄^2- trend occurred, which could mean that part of the NH₄^- ions reacted with other ions, forming ammonium compounds.

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