Because of the minute quantity of sulfur in gasoline, all gasoline-powered vehicles emit sulfur oxides. Automotive emissions of sulfur oxides, however, are insignificant since, according to a 1970 Environmental Protection Agency estimate, gasoline-powered vehicles are responsible for only 0.6% of sulfur oxide emissions nationwide. Furthermore, all of the sulfur oxides in the atmosphere are gradually oxidized to sulfuric acid and sulfate by natural processes.
Catalytic converters of the type installed on 1975 model automobiles reduce hydrocarbon and carbon monoxide emissions by oxidizing them to carbon dioxide and water. In the process some of the sulfur dioxide in the exhaust gas is also oxidized to sulfur trioxide, which combines with water to form sulfuric acid and sulfates.
Sulfate emissions from 19 catalyst cars, 4 noncatalyst cars, and one diesel passenger car were measured with the 1972 Federal Test Procedure (FTP). Some vehicles were also tested with the 1975 FTP and at 30, 40, and 60 mph. A large constant-volume-type sampling system was used to dilute and cool the exhaust, and approximately 1.5% of the exhaust was sampled on filters.