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Dust particles in the 2.5 mu m to 15 mu m diameter range contribute to regional haze that sometimes impairs visibility at the Grand Canyon and other National Parks in the southwestern U.S. The proportion of airborne dust that is attributable to land modification is unknown, but can be expected to increase as a consequence of the region’s rapid population growth. This note examines the upwind histories of air masses bringing high coarse-particle concentrations to the Grand Canyon over a five-year monitoring period. Although arid and semi-arid lands extend in all directions, and the fastest airflows generally have a northerly component, high dust concentrations are most common in air arriving from the southwest, where development has been concentrated. This empirical association suggests that the expansion of suburban and agricultural lands is raising dust levels at the Grand Canyon.