The first sampling of acid rain in the Mexico City Metropolitan Area (MCMA), dating from 1980. However, from 2003 it has been properly followed by protocols of the US-EPA, as well as a strict quality assurance and quality control program. These surveys are being carried out at 26 stations distributed throughout the MCMA in collaboration with the Federal District Government (GDF) of Mexico City.

The physico-chemical parameters of pH and conductivity are evaluated in the field and in the laboratory, while the concentration of ions (Na⁺, NH₄⁺, K⁺, Mg²⁺, Ca²⁺, SO₄^2-, NO₃^- and Cl^-) is determined in the laboratory by High Performance Liquid Chromatography (HPLC).

The pH values decrease from North to South in the MCMA, the Southwest area presents the highest acidic levels. Analyzing the entire study period (2004-2012), is the last year (2012) which presented the phenomenon of acid rain (pH <5.6) throughout all the MCMA.

In relation to the concentrations of sulfate and nitrate, the highest values occur in the Northwest of the metropolitan area, showing the influence of external sources of precursors located at the North; taking into consideration the prevailing wind direction is from North to South.

In 2012, a level of sulfates 20 kg/ha, were reported in the North zone, while the South zone the levels of sulfates were 15Kg/ha. For this year the level of nitrates in the North was 20Kg/ha, while in the South was 10Kg/ha.

It is important to note that although the sources located in the MCMA have decreased emissions of acid rain precursors (SO₂ and NO_x), this has not been yet accomplished outside, and however, these emissions  also influences on the levels of SO₄^2-, NO₃^- and pH acids into MCMA.