Effect of chemical product yield uncertainties on reactivities of VOCs and emissions from reformulated gasolines and methanol fuels

To account for differences in exhaust composition that arise with the use of fuels other than conventional gasoline, California regulations apply reactivity adjustment factors (RAFs) to emissions standards for new motor vehicles. The RAFs are based on estimates of the sensitivity of ozone formation to each of the individual organic compounds in the exhaust. In this study, uncertainties have been estimated for the incremental reactivities of individual organic compounds and for RAFs for motor vehicle exhaust, accounting for uncertainties in chemical rate parameters and product yields and for variability in exhaust composition. Uncertainties (1sigma) in incremental reactivities of individual compounds range from about 25 to 75 per cent of mean estimates and are typically about 10 per cent higher than previous estimates obtained by considering independent rate parameters as the only source of uncertainty in the chemical mechanism. The incremental reactivities of relatively rapidly reactin g compounds are sensitive to the peroxy radical yields in their primary oxidation reactions. RAF values of 0.87 ñ 0.11 (1sigma) and 0.42 ñ 0.06, respectively, are calculated for exhaust emissions from a test gasoline with low aromatics and low olefins content, and from an 85 per cent methanol, 15 per cent gasoline blend. The RAF values show little sensitivity to product yield uncertainties.