A TRACER CALIBRATION OF BACK TRAJECTORY ANALYSIS AT THE GRAND-CANYON

The potential of routine back trajectory analyses to locate sources of contaminants in air at Grand Canyon is investigated with data on meth ylchloroform (CH3CCl3) concentrations collected during the Subregional Cooperative Electric Utility, Department of Defense, National Park Se rvice, and Environmental Protection Agency Study(SCENES). Following a now-standard approach, the distribution of back trajectory segment end points over a latitude-longitude grid is examined as a function of mea sured concentrations at the fixed monitoring site. Grid cells in which segment endpoints are preferentially associated with high concentrati ons are then identified as candidate emissions sources. The method cor rectly identifies southern California as a source of CH3CCl3. An objec tive rationale is developed for screening out chance associations, att ributable to statistical fluctuations, and the results are evaluated f or real and hypothetical tracers with known, simple distributions.