RESEARCH ARTICLE


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



A.P. Baez*, R.D. Belmont, R.M. García, H.G. Padilla, MC. B. Torres
Laboratorio de Química Atmosférica, Centro de Ciencias de la Atmósfera, Universidad Nacional Autónoma de México, Ciudad Universitaria, Circuito Exterior, Ciudad Universitaria, México D.F. 04510, México.


© 2009 Baezet al.;

open-access license: This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 International Public License (CC-BY 4.0), a copy of which is available at: https://creativecommons.org/licenses/by/4.0/legalcode. This license permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

* Address correspondence to this author at the Laboratorio de Química Atmosférica, Centro de Ciencias de la Atmósfera, Universidad Nacional Autónoma de México, Ciudad Universitaria, Circuito Exterior, Ciudad Universitaria, México D.F. 04510, México; Tel: +52 55 56224071; Fax: +5255 56224050; E-mail: barmando@atmosfera.unam.mx


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 SO42-, NO3-, Cl-, Mg2+ K+, Na+, NH+ 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 SO42-, 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 (SO42-) 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 SO42-. 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 SO trend occurred, which could mean that part of the NH4+ ions reacted with other ions, forming ammonium compounds.

Keywords: Chemical composition of rain, Trend analysis, Neutralization, Mexico.