Sulfide Emissions from Catalyst-Equipped Cars

An on-stream gas chromatographic technique was developed to analyze grab samples of diluted and undiluted vehicle exhaust for the reduced sulfur compounds which cause rotten-egg odors. The method was used to conduct a brief survey of two cars with modified carburetors, four customer cars with odor problems, and five experimental three-way catalyst cars. The cars were tested on a chassis dynamometer using a driving cycle developed for this study, in which a low temperature cold start is followed by a series of 80 km/h-idle cycles and a hot start. Hydrogen sulfide and carbonyl sulfide were frequently detected; ethyl mercaptan was also detected in a few samples. These sulfides are usually emitted during low temperature cold starts and at hot idle. More carbonyl sulfide was emitted from rich malfunctioning three-way catalyst cars during cruise than from rich malfunctioning cars with oxidation catalysts. Production of hydrogen sulfide over oxidation catalysts is favored by rich air/fuel ratios, low exhaust space velocity, and high catalyst temperature. Therefore, rotten-egg odors would be expected to be a problem primarily with malfunctioning cars. The maximum concentration of hydrogen sulfide and carbonyl sulfide in undiluted exhaust rarely exceeds 7 ppm, and occurs for only brief periods. Therefore, these emissions pose no known health problem.