The potential of routine back trajectory analyses to locate sources of contaminants in air at Grand Canyon is investigated with data on methylchloroform (CH3CCl3) concentrations collected during the Subregional Cooperative Electric Utility, Department of Defense, National Park Service, and Environmental Protection Agency Study (SCENES). Following a now-standard approach, the distribution of back trajectory segment endpoints over a latitude-longitude grid is examined as a function of measured concentrations at the fixed monitoring site. Grid cells in which segment endpoints are preferentially associated with high concentrations are then identified as candidate emissions sources. The method correctly identifies southern California as a source of CH3CCl3. An objective rationale is developed for screening out chance associations, attributable to statistical fluctuations, and the results are evaluated for real and hypothetical tracers with known, simple distributions. ¿ American Geophysical Union 1996