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Soiling can cause large reductions in solar energy system production. To study the spatial variability of soiling, transmission loss and mass accumulation of particulates on photovoltaic (PV) cover plates were measured at five sites across the continental United States. Three sites were in the Front Range of Colorado in rural, suburban, and urban areas representing a semi-arid environment. One site was in Cocoa Florida in a hot and humid environment, and the final site was in Albuquerque, New Mexico, in a hot and arid environment. Total suspended particulate concentrations (TSP) were measured simultaneously at each site. Comparisons between transmission loss and mass accumulation measurements are made. Both mass accumulation and ambient TSP are shown to have some predictive power for transmission loss. Mean deposition velocities of 1.5 cm/s were observed. For every g/m(2) of PM deposited on the PV cover plate, a 2.8% reduction in transmission was observed independent of site. These results provide a method for estimating soiling rates at sites across the United States.