The western part of the United States is characterized both by major scenic attractions (Grand Canyon, Yellowstone National Park, etc.) and the excellent visibilities necessary to enjoy the vistas. Recent legislation has been enacted to protect such visibilities from degradation associated with energy and resource development, and the U.S. Environmemtal Protection Agency has been charged with establishing baseline values for both visibility and fine particles. For this purpose, a network of forty air sampling stations has been established in remote locations of the study area (representing about 25% of the contiguous forty-eight states) on a 150 km grid spacing. Each sampler collects coarse and fine (less than 2.5 μm diameter) particles through sequential filtration over two three-day periods per week. Four central stations are operated with a larger variety of meteorological and air sampling instruments, as well as visibillty probes, allowing daily samples in numerous size ranges. Many of these instruments were designed around the capabilities of PIXE, resulting in highly quantitative data at major savings in cost for such a large array. Particulate sources are being evaluated through elemental tracer and meteorological trajectory analyses, and the effects on visibility are being studied through statistical methodologies.