In October 1975, General Motors sponsored a study of sulfate exposures utilizing a fleet of catalyst equipped motor vehicles in controlled, simulated, highway driving conditions. This paper reports some EPA sponsored measurements. Sulfuric acid aerosol, in the Aitken nuclei mode, geometric mean diameter (GMD) of about 0.02 µm, is emitted in the exhaust of catalyst equipped vehicles. Measurement of sulfuric acid 20 m downwind of the roadway indicated a lack of complete neutralization by ammonia. When the wind was perpendicular to the roadway there was little coagulation of sulfuric acid into the accumulation mode, GMD of about 0.24 µm From measurement of the mass flow rate of aerosol sulfur from the simulated freeway, the aerosol sulfur emission rate per car was determined to be 3.5 ± 0.8 µg/m (5.6 ±1.3 mg/mile) corresponding to a 12 ± 3% conversion of fuel sulfur into emitted aerosol sulfur.
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The authors are grateful to General Motors Corporation for their hospitality during the experiment, and to S. Cadle and J. Heuss of the GM Research Staff for providing preliminary data. We are indebted to C. Kistler, S. Scott, W. Shepherd, J. Stikeleather, and S. Tejada for performing the wet chemical analyses, to R. Speer for assisting in the operation of the EPA van, and to Ping Auw, Kui-Chiu Kwok, V. Marple, Ruben Falldin, and R. Jordan for assistance in the assembly of the instrumented car. Comments on our data provided by R. Hammerle and W. Pierson are also gratefully acknowledged.