The southwestern United States desert and mountain areas are generally characterized by very good visibility. Until recent years, scenic vistas of natural landmarks and mountains with a visual range of over 100 miles were common. These vistas have been considered a major resource of the Southwest, and numerous national parks, forests, and monuments have been created to preserve them. The tourism resulting from the Southwest's scenery has played a major role in the economy of the region. In this paper, the authors describe the results of the fall 1977 aircraft measurements of Project VISTTA. The Southwest visibility-reducing aerosol is characterized with regard to spatial distribution, particle size distribution, and chemical composition as a function of size. The emissions from a copper smelter and a coal-fired power plant in the region, as measured in elevated plumes, are characterized with regard to chemical composition as a function of size. The contribution of various species to light scattering is also quantitatively assessed.