Rainwater chemical composition at two sites in Central Mexico

• Published in: Atmospheric research Vol. 80; no. 1; pp. 67 - 85
• Main Authors: Báez, A.P., Belmont, R.D., García, R.M., Torres, M.C.B., Padilla, H.G.
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier B.V 01.04.2006; Elsevier
• Subjects: Acidity in forests; Annual variations; Central Mexico; Earth, ocean, space; Exact sciences and technology; External geophysics; Meteorology; Rainwater chemistry; Water in the atmosphere (humidity, clouds, evaporation, precipitation); Mexico; Mexico, Distrito Federal, Mexico City; Central Mexico; Acidity in forests; Rainwater chemistry; Annual variations; time series analysis; atmospheric precipitation; rain water; urban areas; North America; trajectory; Ion composition; Interannual variation; annual variations; acidity; Air mass; acid rains; spatial variations; rural areas; chemical composition
• ISSN: 0169-8095; 1873-2895
• DOI: 10.1016/j.atmosres.2005.06.008

Chemical analyses were performed on rainwater samples collected at the National Autonomous University of Mexico (UNAM) in Mexico City and at a wooded site, Rancho Viejo (RV) in the State of Mexico, for the periods 1994–2000 and 1994–1999, respectively. At UNAM, rainwater was collected for the entire rainy season period each year, while at RV, technical considerations limited collection to weekends only. The results showed large variations in rainwater chemical composition in most years, mainly because of the variability of meteorological conditions and also because of changes in source emissions. Sulfates and NH 4 + showed higher annual volume-weighted mean concentrations (VWMC) in both sites. At UNAM, the maximum annual VWMC for SO 4 2− occurred in March and the minimum in July and August. Lower concentrations of almost all ions were found at RV; however, the H + concentration was higher at this site. The pH in Mexico City, calculated from the annual VWMC of H +, was 4.95, which is a little higher than pH values reported in some other countries. Despite the fact that sulfate and NO 3 − concentrations were lower at RV, the pH was lower. Air-mass back trajectories were calculated for individual concentrations of SO 4 2−, H +, NH 4 +, Ca 2+, and Mg 2+, observed at each sampling site for weekend data. At RV, sulfate concentrations were higher when air-mass back trajectories indicated a wind flow from Mexico City and Toluca at 1000 MAGL (meters above ground level) and 3000 MAGL. The hydrogen ion exhibited the same behavior. Calcium and Mg 2+ concentrations were also higher when the wind blew from urban areas at 1000 and 3000 MAGL. At UNAM, H + concentration was lower and Ca 2+ and Mg 2+ were higher when wind blew from the northern sector of the city at 1000 and 3000 MAGL. In UNAM, the NO 3 −/SO 4 2− and NH 4 +/SO 4 2− ratios were 0.5 and 1.09 in 1994 and 0.86 and 1.64 in 2000, respectively, indicating a decrease in SO 2 emissions resulting from the change of fuel oil to gas fuel. The SO 4 2−/Ca 2+ ratio was significantly lower at the UNAM site (1.82) compared to RV (5.36), and the SO 4 2−/H + ratio was significantly higher at the UNAM site (6.77) compared to RV (2.01). The Spearman's rho correlation between ionic concentrations indicated a positive correlation in most cases ( p < 0.05) for data from UNAM and RV. The multiple regression correlation analysis to predict H + concentration in Mexico City showed that NO 3 −, NH 4 +, SO 4 2−, and Ca 2+ contributed 23.2%, 20.9%, 8.0%, and 6.1%, respectively, to the H + prediction, while Cl − plus Na + plus K + only contributed 2.2%, and Mg 2+ did not contribute. Sea-salt contribution to rainwater chemical composition was negligible with any wind direction at both levels. Excess sulfate (non-sea-salt sulfate) represented 98.7% of the total sulfate in rainwater collected during weekends at RV and 98.6% for weekend and annual rain samples at UNAM.