AIRSHED MODEL EVALUATION OF REACTIVITY ADJUSTMENT FACTORS CALCULATED WITH THE MAXIMUM INCREMENTAL REACTIVITY SCALE FOR TRANSITIONAL-LOW-EMISSION VEHICLES

The California Air Resources Board recently adopted regulations for li ght-and medium-duty vehicles that require reductions in the ozone-form ing potential or ''reactivity,'' rather than the mass, of nonmethane o rganic gas (NMOG) emissions. The regulations allow sale of all alterna tively fueled vehicles (AFVs) that meet NMOG exhaust emission standard s equivalent in reactivity to those set for vehicles fueled with conve ntional gasoline. Reactivity adjustment factors (RAFs), the ratio of t he reactivity (per gram) of the AFV exhaust to that of the conventiona lly fueled vehicle (CFV), are used to correct the stringent exhaust em ission standards. Complete chemical speciation of the exhaust and conv ersion of each NMOG species to an appropriate mass of ozone using the maximum incremental reactivity (MIR) scale of Carter1 determines the R AF. The MIR approach defines reactivity where NMOG control is the most effective strategy in reducing ozone concentrations, and assumes it i s not important to define reactivity at other conditions, i.e., where NO(x) is the limiting precursor. This study used the Carnegie/Californ ia Institute of Technology airshed model to evaluate whether the RAF-a djusted AFV emissions result in ozone impacts equivalent to those of C FV emissions. A matrix of two ozone episodes in the South Coast Air Ba sin (SoCAB) of California, two base emission inventories, and exhaust emissions from three alternative fuels that meet the first level of th e low emission vehicle standards bounds the expected range of conditio ns. Although very good agreement was found previously for individual N MOG species,2 this study noted deviations of up to +/-15 percent from the equal ozone impacts for any vehicle/fuel combination required by t he California regulations. These deviations appear to be attributable to differences in spatial and temporal patterns of emissions between v ehicle fleets, rather than a problem with the MIR approach, The first formally adopted RAF, a value of 0.41 for 85 percent methanol/15 perce nt gasoline-fueled vehicles, includes a 1 0 percent increase based on the airshed modeling. The correction to the RAF is different for other fuels and may be different for air basins other than the SoCAB.