Visibility in Grand Canyon National Park has shown degradation during the last decade. The Penn State University/National Center for Atmospheric Research (PSU/NCAR) regional atmospheric model is applied to the diagnosis of atmospheric transport in the region in order to better understand how air-pollution transport, and thus visibility, may be influenced by the complex terrain. Both idealized and real-data model simulations are used to examine the local topographic forcing. Significant transport of air pollutants is produced by the differenlial thermal forcing and channeling associated with the orography. It is also demonstrated that the combination of a meteorological model and observations via four-dimensional data assimilation provides the most accurate and complete wind-field information for air-quality applications in regions of complex terrain.