Evaluation of acid rain in urban areas of the United States of America and Mexico from 2003 to 2019

Air pollution has long posed a significant environmental challenge in the North American region, including both the USA and Mexico. Among its impacts, acid rain, which is characterized by a pH lower than 5.6, negatively affects ecosystems and biodiversity. This study evaluates and compares the physicochemical characteristics of precipitation in the urban areas of Denver, New York City, Los Angeles, and the Mexico City Metropolitan Area (MCMA) during the period 2003–2019. Long-term analysis revealed a substantial decline in sulfate concentrations in New York City after 2009, accompanied by a gradual rise in precipitation pH. In contrast, sulfate levels in the MCMA remained elevated and variable, while Los Angeles exhibited persistently acidic precipitation despite relatively low sulfate concentrations. Nitrate trends were weak or irregular across cities, and ammonium increased only in New York City. Results indicated the presence of acid rain in New York City, Los Angeles, and the MCMA, but not in Denver, where precipitation in 2017 did not exhibit acidic pH values. The highest concentrations of SO₄²⁻ and NO₃⁻ in precipitation were observed in the MCMA, likely due to SO₂ emissions from the Tula–Vito–Apasco industrial corridor and high NO_x emissions from mobile sources. The SO₄²⁻/NO₃⁻ ratio was used to determine the predominant ion influencing precipitation acidity. SO₄²⁻ was dominant in New York City and the MCMA, whereas NO₃⁻ was more prevalent in Los Angeles and Denver. Strategies implemented in the USA, such as the transition to cleaner fuels, policies for monitoring emission sources, and the expansion of atmospheric deposition networks, could inform efforts to reduce emissions of acid rain precursors in the MCMA.