With more than 20 million inhabitants, as well as being located in a closed basin at an altitude of 2240 meters above sea level, the Mexico City Metropolitan Zone (MCMZ) represents a great challenge in terms of air pollution.

One of the issues that has been gaining great importance in recent years is reactive nitrogen, which includes oxidized (NOx and NOy), reduced (NH₃ and NH₄⁺) and organic forms of nitrogen in gases and aerosol. To develop effective strategies for controlling secondary particulate matter (i.e., ammonium sulfate and ammonium nitrate), it is necessary to characterize both oxidized and reduced forms of nitrogen in the atmosphere of the MCMZ.

Since 1990, the air quality of criteria pollutants in the MCMZ has been evaluated through the Automatic Atmospheric Monitoring Network (RAMA), consisting of 34 air quality monitoring stations, and since 2002 wet atmospheric deposition has been monitored through the Atmospheric Deposition Network (REDDA) consisting of 16 sampling stations. Among the parameters that are determined in ambient air are nitrogen compounds in their oxidized form, such as nitrogen oxides (NOx). In wet atmospheric deposition, oxidized nitrogen is determined as nitrates (NO₃^-) and in its reduced form as ammonium (NH₄⁺). The foregoing indicates the need to determine the reduced nitrogen in ambient air through the establishment of an ammonia (NH₃) measurement network. The trends of nitrogen compounds are presented in this study, and the establishment of an NH₃ network, considering some of the stations of the RAMA and the REDDA, is discussed.

The stations proposed for the NH₃ network consist of six sites, two of them in the North MCMZ located in areas of industrial activity, one in the central zone with high population density and commercial activity, another in the eastern zone with industrial activity and two in the South zone with residential land use.

In order to identify the possible contribution of mobile sources to NH₃ emissions, mainly due to the reduction of NOx to NH₃ by three-way catalytic converters, NH₃ measurements will be co-located with carbon monoxide analyzers to determine the correlation between both pollutants.