Spatial and temporal variation of acid rain in the Mexico City Metropolitan Zone

Abstract

At the end of the 1980´s particulates and sulfur dioxide (SO₂) were the main atmospheric pollutants in the Mexico City Metropolitan Zone (MCMZ). To reduce emissions, fuel oil was replaced by natural gas at power plants located inside Mexico City. Currently, SO₂ levels do not exceed its air quality standard; however, acid rain is present with a high contribution of sulfate (SO₄^2–). In this study, spatial and temporal variations in the chemical composition of rain in Mexico City between 2003 and 2014 were analyzed. Major ions (Na⁺, NH₄^+, K⁺, Mg²⁺, Ca²⁺, SO₄^2–, NO₃^– and Cl^–), pH, and electrical conductivity (EC) were analyzed weekly at 16 sampling stations located in the MCMZ. The pH decreased from north to south, with the lowest annual volume weighted mean (VWM) of 4.16 in 2006. Annual ion concentrations were, in decreasing order: NH₄⁺, SO₄^{2 –}, NO₃^– and Ca²⁺ for the entire study period at most of the sampling sites. The highest values for wet atmospheric deposition (kg/ha) were found in the Western area and were the maximum in 2007. Wet deposition had major levels for SO₄^2– and NO₃^– of 24 and 20 kg/ha, respectively, and were similar to the levels registered in the USA in 2013 and 2014. Considering that external emission sources play a decisive role in acid rain within the City, it is necessary to establish strategies for the emission reductions of acid rain precursors from upwind sources.