974398 - Advances in the Evaluation of Reactive Nitrogen in Mexico City Metropolitan Area

Mexico City Metropolitan Area (MCMA) is a Megalopolis that represents a challenge in terms of air pollution. For more than 30 years, the air quality of criteria pollutants has been measured by the Automatic Atmospheric Monitoring Network (RAMA), and since 2003 wet atmospheric deposition has been monitored through the Atmospheric Deposition Network (REDDA). Nitrogen compounds in oxidized forms, such as nitrogen oxides (NOx) are measured in ambient air. In wet atmospheric deposition, nitrogen is measured as nitrates (NO3-) in oxidized form and as ammonium (NH4+) in its reduced form.
Trends of nitrogen compounds in ambient air (NOx) and in wet atmospheric deposition (NH4+ and NO3-) at MCMA are evaluated in this study. NOx levels have been decreasing since 2003 at all monitoring stations; however, photochemical pollution and acidic precipitation are important issues.
The REDDA of the Secretary of the Environment of Mexico City has been working continuously with the University of Mexico (UNAM), collecting the wet atmospheric deposition at 16 sampling sites located throughout the city, while the UNAM is responsible for the chemical analysis of the samples. Over time, the collaboration with the National Atmospheric Deposition Program (NADP) and the U.S. Environmental Protection Agency (USEPA) has strengthened field sampling protocols, and participation in United States Geological Survey (USGS) and World Meteorological Organization (WMO) laboratory inter-comparison programs has strengthened chemical analysis performance.
Among the obtained results in wet atmospheric deposition, the presence of NH4+ stands out as the major ion in the chemical composition of precipitation and as the major nitrogen ion compared with NO3⁻ , and the NH4⁺/NO3⁻ ratio interestingly varies among regions of the city. The average ratio NH4⁺/NO3⁻ was 2.5, which is similar to other urban areas in North America, such as the Denver/Boulder, Colorado metropolitan area. The highest values are in the region of high urban and industrial activities, as well as landfill sources.
To develop effective strategies for controlling secondary particulate matter, it is necessary to characterize both oxidized and reduced forms of nitrogen in the atmosphere of Mexico City. Establishment of an ammonia (NH3) measurement network is needed to determine the concentration of reduced nitrogen in ambient air. It is recommended to collaborate with other urban areas in North America and establish monitoring approaches like the Network for Urban Atmospheric Nitrogen Chemistry in Denver and the RAMA and REDDA in MCMA.