Reactivity Background

This page last reviewed March 30, 2011

The ARB was the first regulatory agency to enact a regulation that uses reactivity in a more complex manner than the simple two bin exemption type of regulation. In the late 1980's, a method was needed to compare the emissions of alternatively fueled vehicles (i.e., ones that use a fuel other than gasoline, such as compressed natural gas or methanol) to the emissions from gasoline fueled vehicles. An advisory board was formed, and recommended the use of reactivity to adjust the weight of emissions so that the limits reflect the ozone-forming potential of the emissions rather than the simple mass. The Low Emission Vehicle/Clean Fuels regulation, which was adopted in 1990, uses Reactivity Adjustment Factors (RAF) to set the limits on vehicle emissions. A RAF2 is the ratio of the exhaust reactivity of the alternative fueled vehicle to the exhaust reactivity of the conventionally fueled vehicle. The exhaust reactivity is calculated by taking the sum of the mass fraction of each compound times the reactivity of the compound summed over all the compounds in the exhaust.

In designing this regulation, several important scientific issues needed to be addressed. The first was the selection of a reactivity scale to be used in the regulation. Dr. William P. L. Carter’s Maximum Incremental Reactivity (MIR) scale was chosen by ARB because it was determined to be the most appropriate reactivity scale to complement California's NOx control program. The MIR scale is defined in terms of environmental conditions in which ozone production is most sensitive to changes in hydrocarbon emissions, and therefore, represents conditions where hydrocarbon controls are most effective. As such, it complements ARB's NOx control program, which is designed to reduce ozone under conditions that are sensitive to NOx reductions. Another issue was the degree of uncertainty in the RAFs. Because RAFs are ratios of reactivities, they are similar to relative reactivities. A number of studies have found that relative reactivities have much smaller uncertainties than absolute reactivities. Work done by Yang and Milford found that uncertainties in RAFs are on the order of 15%, while uncertainties in the associated MIRs values were closer to 30 to 70%.

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For questions concerning ARB's Reactivity Program, please contact Ralph Propper at (916) 323-1535