Risk Assessment of Gaseous Elemental Mercury (GEM) in Ambient Air in Southern Mexico City and its Temporal Variability 
José Alberto Rivas Fuentes1, Rodolfo Sosa Echeverría2, Gilberto Fuentes García3, Pablo SÁnchez Álvarez2, Alejandro de David Palacios Ibarra2, David Gay4, and David Grande5
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, 5 EEMS, Madison, WI
Atmospheric mercury is a pollutant of global concern due to its high toxicity, long atmospheric residence time (1-2 years), and persistence in environmental matrices, along with its adverse impact on ecosystems. In the atmosphere, the dominant species is gaseous elemental mercury (GEM, Hg0), which can persist for prolonged periods, enhancing its dispersion and increasing exposure to human populations and the environment. In urban areas, atmospheric mercury concentrations are influenced by a variety of anthropogenic and natural sources, as well as by meteorological processes and atmospheric reactivity that regulate its temporal variability.
The present study aims to assess the risk associated with the presence of GEM in ambient air in southern Mexico City by comparing reference values from different international organizations, as well as to characterize its temporal variability.
Based on concentrations recorded over a one-year period, an inhalation exposure risk assessment was conducted using reference values commonly applied in environmental risk assessment studies. Hazard quotients (HQ) were estimated using reference values from the U.S. Environmental Protection Agency (U.S. EPA), the Agency for Toxic Substances and Disease Registry (ATSDR), and the World Health Organization (WHO) (300 ng/m3), as well as the more stringent value proposed by the Office of Environmental Health Hazard Assessment (OEHHA) (30 ng/m3).
The results showed HQ values of 0.011 for U.S. EPA/ATSDR/WHO and 0.1 for OEHHA, indicating in all cases a low non-carcinogenic risk (HQ < 1) for the exposed population.
Preliminary results, along with ongoing analyses, allow the identification of temporal trends in GEM concentrations, thereby enhancing the understanding of mercury dynamics in urban environments such as Mexico City and providing scientific evidence to support the development of mitigation, control, and prevention strategies.