Mercury (Hg) in the atmosphere is distributed globally, but global-scale modeling of atmospheric Hg, as with any other reactive constituent, presents challenges.  Coarse spatial discretization is often used to achieve global coverage without incurring excessive computational cost.  However, certain forms of mercury deposit rapidly from the atmosphere and fine-scale modeling is required to address this.  Regional models covering a limited area have been applied within global models, but getting them to work together requires interpolation and/or averaging in time and space.  The use of different treatments between the global and regional models for physicochemical processes affecting Hg has been shown to induce unrealistic artifacts.  Another shortcoming is that feedback from the regional scale to the global scale (upscaling) is often neglected.

The U.S. Environmental Protection Agency (EPA) recently began development of a new generation of air quality model based on the Model for Prediction Across Scales (MPAS).  This new modeling system is intended to provide a global complement to the Community Multiscale Air Quality (CMAQ) model.  MPAS is comprised of separate interoperable programming components with each designed to simulate atmospheric, oceanic and other earth-system media in concert and on a global scale.  MPAS uses an irregular horizontal grid based on Spherical Centroidal Voronoi Tesselations (SCVTs) producing a mesh of polygonal elements covering the globe without the polar singularities intrinsic to orthogonal grids.  The mesh elements can vary in size to provide fine resolution only where needed.  Gradual mesh refinement can be used to avoid the abrupt changes in resolution that have been difficult to treat with traditional grid nesting approaches.  In addition, the same chemical and physical treatments can be used in all grid elements and upscale effects are intrinsically simulated.  Initially, this next-generation model will simulate the pollutants currently treated in the CMAQ model, including Hg.  CMAQ also treats the effect of bromine (Br) emission on tropospheric ozone chemistry.  Future work to add Hg-Br chemistry could provide additional benefit to mercury simulations.  Besides atmospheric Hg modeling, MPAS could eventually provide a consistent and seamless approach to including the oceans and other earth systems in simulations of the entire mercury cycle.