A massive experiment was conducted at the General Motors Proving Ground to measure the dispersion of sulfates generated by catalyst equipped vehicles. 352 cars equipped with catalysts and air pumps were driven on a 10 kilometer test track to provide a simulated freeway situation with a traffic density of 5462 cars/hour. Experiments were conducted on 17 days in October 1975, providing sixty-six 30 min data sets. On most days, the experiment was carried out early in the morning in order to run under the most adverse meteorological conditions available. Samples of airborne sulfate and a gas tracer, sulfur hexafluoride, as well as extensive meteorological data were collected simultaneously at 20 sampling points under a wide variety of meteorological conditions. The maximum measured increase in sulfate from each 30 min data set was generally found next to the roadway at the lowest level and varied from 3 to 15 jug/m3. This increase in sulfate over background fell off rapidly with height and distance. Sulfate i...