Autores: Rodolfo Sosa Echeverría, Ana Luisa Alarcón Jiménez, María del Carmen Torres Barrera, Pablo Sánchez Álvarez, Elías Granados Hernández, Elizabeth Vega*, Mónica Jaimes Palomera Armando Retama David A. Gay

* Ciencias Ambientales | Contaminación Ambiental

Abstract

exico City Metropolitan Area (MCMA), one of the most populous cities in the world, has historically represented a challenge in terms of air pollution. Particulates and sulfur dioxide.

(SO₂) were identified as the main atmospheric pollutants three decades ago. In order to reduce emissions of these pollutants, replacing of fuel oil by natural gas in power plants was carried out in the late 1980s. This strategy resulted in the reduction of ambient air concentrations of both pollutants. SO₂ no longer exceeds its ambient air quality standard; however, acid rain remains a significant issue, with sulfate remaining as the principal component in the wet atmospheric deposition.

In this study, spatial and temporal variations in the chemical composition of precipitation (sulfate and nitrate ions, and pH) were obtained weekly at 16 sampling stations, as well as atmospheric SO₂ and nitrogen oxides (NOx) from an air quality monitoring network from 2003 to 2018 in the MCMA. Some indicators were applied to evaluate wet atmospheric deposition: SO₄²⁻/NO₃⁻ and NH₄⁺/NO₃⁻ ratios, fractional acidity as well as neutralization factors.

SO₂ ambient air concentrations have decreased from 2003 to 2018 by more than 70%, while NO_x has decreased by 20%. The SO₂/NOx ratio has declined about 60%, indicating more effective strategies in reducing SO₂ than for NOx emissions. The NOx emission sources are more diverse than for SO₂, and NOx has more complex mechanisms of atmospheric deposition and photochemistry.

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, where prevailing winds blow from north to south, as well as emission sources located in the north sector, both inside and outside MCMA. For SO₄²⁻/NO₃⁻ ratio, all sampling sites present current values in the same magnitude order, around 1.4, which is higher compared with urban sites in USA. The 2018 NH₄⁺/NO₃⁻ ratio was 1.98 for the southern sampling site, while all northern sampling sites were approximately 2.28. These results are on the same order that urban sites in USA. It was concluded that ammonium was a very important neutralizing agent of pH within the MCMA precipitation.

It is recommended to apply strategies for the emissions reduction of acid rain precursors in external sources to the North the MCMA, and to establish a Network for Urban Atmospheric Nitrogen Chemistry.