Effects of Heavy Hydrocarbons in Gasoline on Exhaust Mass Emissions, Air Toxics, and Calculated Reactivity - Auto/Oil Air Quality Improvement Research Program

Emission effects of gasoline hydrocarbon components distilling above 300°F were investigated to determine whether the effect of 90% distillation temperature (T₉₀) found in an earlier Auto/Oil Program study is due to fuel distillation properties or to hydrocarbon composition, and also to determine whether the T₉₀ effect is linear. Twenty-six fuels were tested in two sets. In Matrix A, the independent variables were catalytically cracked (FCC) and reformate stocks with nominal distillation ranges of 300 to 350, 350 to 400 and 400+°F. In Matrix B, the independent variables were a reformate stock (320 to 370°F), a heavy alkylate (330 to 475°F), and a light alkylate distilling below 300°F, which was used to vary fuel T₅₀ at fixed levels of T₉₀. Exhaust mass and speciation were measured using ten 1989 vehicles of the Auto/Oil Current Fleet. Tailpipe hydrocarbon emissions were found to increase nonlinearly with progressive addition of the heavier components. The largest increases occurred with the fuels that had the highest concentrations of the heavier components. Tailpipe hydrocarbon mass correlated with distillation properties. The best regression model included the fuel volume percent distilled above 300°F as an exponential variable and the percent distilled between 200 and 300°F as a linear variable. Tailpipe NOx decreased with addition of the heavy components. There was no effect on CO. Fuel hydrocarbon composition affected toxic air pollutant emissions and calculated ozone-forming reactivity. Increasing fuel aromatic content increased benzene emissions. Increasing fuel paraffin and olefin content increased 1,3-butadiene emissions. Specific reactivity calculated on a unit mass basis increased with increasing FCC or reformate more than with heavy alkylate. Calculating reactivity-weighted emissions shows that adding the heavy components affects reactivity more through effects on HC mass than on specific reactivity. Quantitative estimates of all of the fuel effects are given.