Laboratory and field tests were conducted to determine how changes in some gasoline compositional factors might affect atmospheric visibility and soiling caused by exhaust particulate matter emitted from late model automobiles. In the laboratory tests, measurements of light scattering, soiling index, and mass emission rates of air-suspendible particles were made on diluted exhaust from cars driven on a programmed chassis dynamometer. These tests showed that light scattering, and also soiling, were increased by increasing the aromatic content, removing lead antiknocks, or increasing the combined amounts of sulfur and phosphorus in gasoline.Field tests in a turnpike tunnel used two 4-car fleets operating on leaded and unleaded premium gasoline of high and average aromatic content. The air in the tunnel sampled during the tests with the cars using the unleaded gasolines soiled the filters 57% more than the air sampled during the tests with the cars using the leaded gasolines. The particulate matter from the unleaded fleet caused nearly twice as much decrease in light transmission as did that from the leaded fleet.These results were obtained with 1969-1971 models of production cars. Three 1970 model cars equipped with advanced emission control systems were tested with leaded and unleaded gasolines. The mass emission rates of air-suspendible particles and the light scattering were lower than with the production cars, but no significant effect of gasoline lead content was found.