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A cooperative vehicle exhaust emissions test program was conducted by the California Air Resources Board and Chevron Research and Technology Company. The focus of the program was to determine the effect of aromatics content on nitrogen oxides (NOx) emissions. The program consisted of testing nine vehicles on three different fuels. The fuels ranged in aromatics content from 10% to 30%.* Other fuel properties were held as constant as possible. The tests were conducted in two different laboratories. In addition to the measurement of criteria emissions (hydrocarbons, carbon monoxide, and NOx), some of the hydrocarbon emissions were speciated and a reactivity of the exhaust was calculated. Only slight changes in the exhaust emissions and reactivity were observed for a change in aromatics content from 30% to 10%.

An analysis of data provided by-vehicle manufacturers during the California emissions certification process has been performed for 1983-1987 model-year light-duty vehicles. The major change in emission control system design was a decrease in the use of secondary air injection which was used on 75% of 1983 vehicles, but only 50% of 1986 and 1987 vehicles. Exhaust gas recirculation was used on 90% of vehicles from 1983-1987. The sales-weighted certification emission levels of gasoline-powered light-duty vehicles were 0.23 g/mile HC, 3.1 g/mile CO, and 0.5 g/mile NOx in 1983. Levels of HC and CO were approximately constant at 0.20 g/mile and 2.7 g/mile, respectively, from 1984-1987 with NOx levels decreasing to 0.4 g/mile for 1987.