The Atmospheric Modeling Division (AMD) of EPA has been working on the development of atmospheric mercury models since the early 1990's when the Regional Lagranian Model of Air Pollution (RELMAP) was adapted to simulate mercury in support of EPA’s Mercury Study Report to Congress. As the scientific understanding of atmospheric mercury processes continued to develop in the late 1990's, it became apparent that Lagrangian-type models, also known as “puff” models, would have difficulties simulating the complex chemical and physical interactions of mercury with other pollutants that were being discovered. Thus, AMD’s focus for atmospheric mercury model development was moved to the Community Multi-scale Air Quality model (CMAQ). The CMAQ simulates atmospheric processes within a 3-dimensional array of pre-defined finite volume elements and can model complex interactions between all of the pollutants that might exist within each volume element. The CMAQ was previously developed to simulate photochemical oxidants, acidic and nutrient pollutants, and aerosol particulate matter, all of which have been shown to interact with mercury in air and in cloud water and influence its deposition to sensitive aquatic ecosystems. The “one atmosphere” approach of CMAQ where all pollutants are simulated together just as they exist in the real atmosphere has been extended to atmospheric mercury modeling at AMD.