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Emission rates of benzene-soluble matter (tar), benzo (a) pyrene (BaP), and benz (a) anthracene (BaA) were determined for gasoline engines and automobiles in simulated city driving, and for a diesel engine in simulated city bus operation. A study of the effects of operating conditions and fuel factors indicated that for minimum exhaust emission of polynuclear aromatic hydrocarbons, engines should be operated with minimum lubricating oil in the combustion chamber or exhaust system, maximum air/fuel ratio, and minimum aromatics (especially polynuclear aromatics) in the fuel. On a concentration basis, emissions of BaP and BaA from gasoline engines were greatest during decelerations, and on a mass emission basis were greatest during accelerations. For a group of 25 cars, the average BaP emission rates were 6.6 ug/M3 exhaust or 169 ug/gal gasoline. BaA concentrations were approximately four times higher. Tar emission was approximately 0.2 gm/mile.