Sulfur and nitrogen compounds in wet atmospheric deposition and in ambient air as indicators of emissions reduction strategies at Mexico City

Humberto Bravo A.1, Rodolfo Sosa E.2, Ana Luisa Alarcón J.3, Maria del Carmen Torres B.4, Monica Jaimes P.5, Armando Retama H.6, Pablo Sánchez A.7 and Elías Granados H.8

The Mexico City Metropolitan Zone (MCMZ) is accepted as having critical levels of air pollution. In the 1980s, particulates and sulfur dioxide (SO2) were identified as the main atmospheric pollutants and an effort to reduce emissions in the MCMZ was made by replacing fuel oil with natural gas in power plants located inside. This resulted in the reduction in the levels of both pollutants. Actually SO2 levels do not exceed its ambient air quality standard; however acid rain is a significant issue. In this study, spatial and temporal variations in the chemical composition of rain in Mexico City between 2003 and 2014 were analyzed; sulfate and nitrate ions and pH were obtained weekly at 16 sampling stations located in the MCMZ. Furthermore, sulfur dioxide (SO2) and nitrogen oxides (NOx) were evaluated from 1990 to 2014 at the MCMZ air quality monitoring network.  In order to obtain the magnitude in the emissions reduction of sulfur compounds relative to nitrogen compounds, SO2/NOx were obtained for ambient air, as well as SO42- /NO3- in wet atmospheric deposition.

In 2003 a difference among the sampling sites was observed in the SO42- to NO3- equivalents ratio, sites located in the north presented a higher ratio of 2, while the sites located in the south the value was 1. In 2014, a homogeneous distribution was observed, the ratio was the same in all the Mexico City sampling sites: 1.5 (µeq/L SO42- to µeq/L NO3-). The pH values of the samples at the stations located in the south were more acidic than the samples for the stations in the north. This result is in line with meteorological conditions, prevailing winds blowing from the north to the south, as well as emission sources located in the north sector. SO2 levels have decreased from 1990 to 2014 by about 90%, while those of NOx by 20%. The SO2 / NOx ratio has declined about 60% indicating more effective strategies in reducing SO2 (change of fuel or sulfur reduction) than for NOx emissions. This, because the NOx emission sources are diverse and also NOx are part of complex mechanisms of atmospheric deposition and  photochemistry.


1Universidad Nacional Autonoma de México (UNAM),
5Gobierno de la Ciudad de México,
6Gobierno de la Ciudad de México,