Examination of suspended particulate concentration data from a year-long, regional air-quality study of a portion of the Great Lakes area revealed that depletion of particulates occurred during extended periods of travel over water surfaces. Based on particulate concentrations and meteorological measurements, residence times of background regional particulate matter were found to be between 5 and 16 h in the region. Using a simple model for the deposition of particulate matter into the lakes, deposition rates ranging from 0.2 to 3.9, μg m−2 s−1 were found. This results, for example, in an estimated 150 metric tons of material being deposited into Lake St. Clair, area 1270 km2, during a 24 h period.