Temporal Variability of Reactive Nitrogen in Wet Atmospheric Deposition: A Comparative Analysis Across Selected Sites in Mexico and the United States 
Amelia Jiménez Alcántara1, Rodolfo Sosa Echeverría2, Ana Luisa Alarcón Jiménez2, Gilberto Fuentes García3, David Gay4, and Pablo Sánchez Álvarez2
1 Programa de Posgrado en Ingeniería, Universidad Nacional Autónoma de México, Ciudad de México, México, 2 Sección de Contaminación Ambiental, Instituto de Ciencias de la Atmósfera y Cambio Climático, Universidad Nacional Autónoma de México, Ciudad de México, México, 3 Facultad de Ingeniería, Universidad Nacional Autónoma de México, Ciudad de México, México, 4 National Atmospheric Deposition Program, Wisconsin State Lab of Hygiene, Madison, WI
Reactive nitrogen (Nr) is a key component of the nitrogen cycle, whose balance has been significantly altered by anthropogenic activities that have increased its presence in the atmosphere over the past century. Gaseous compounds such as nitrogen oxides (NOx) and ammonia (NH3) are transformed into soluble species, including nitrate (NO3-) and ammonium (NH4+), which are subsequently removed through wet deposition. Excess Nr leads to adverse environmental impacts, including acidification, eutrophication, and disruptions of biogeochemical cycles.
In North America, monitoring networks such as the Mexico City Atmospheric Deposition Network (REDDA) and the National Trends Network (NTN) in the United States have generated long-term datasets; however, integrated comparative analyses across regions with different land-use types and emission contexts remain limited.
This study evaluates the spatial and temporal variability of oxidized and reduced Nr in wet deposition across selected regions in Mexico and the United States during the most recent available period (2015-2025). Datasets of concentration, deposition, and precipitation were integrated to identify regional patterns, assess intra- and interannual variability and trends, and quantify the NH4+/ NO3- ratio as an indicator of the relative predominance of Nr forms.
Results reveal clear contrasts among cities. In the Mexico City Metropolitan Area (MCMA), the average NH4+/ NO3- ratio was 2.52, indicating a strong predominance of ammonium; a similar pattern was observed in Denver (2.16). In contrast, Los Angeles and New York showed ratios close to unity (1.09 and 1.14), suggesting a more balanced contribution of both species, with a slight dominance of NH4+.
These findings highlight the increasing relevance of reduced nitrogen in precipitation chemistry across urban regions of North America and underscore the need to strengthen NH3 emission controls, a key precursor that has historically been less regulated than NOx. This comparative approach provides a basis for developing more comprehensive mitigation strategies.