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using environmental chambers (e.g., Kelly and Chang, 1999).

The RAF defined by Eq. (A) is calculated from the amounts of the individual compounds emitted and their MIR values as follows (Croes et al., 1992; California Air Resources Board, f992):

In Eq. (C) Fjj is the mass fraction of compound i in the exhaust from the test fuel, FB i is the mass fraction of compound i in the exhaust from the base fuel, and MIR; is the maximum incremental reactivity of VOC i. Thus, a specific vehicle/test fuel combination with an RAF of f.O is expected to have the same contribution to ozone formation (in terms of specific reactivity, grams of 03 per gram of VOC exhaust emissions) as that vehicle operating on base gasoline. An RAF < f.O means that f g of the VOC exhaust emissions will form less ozone than when the vehicle is run on the base fuel. As might be expected, different fuel compositions have a significant effect on the RAF; the amount of aromatics and oxygenates such as methyl tert-butyl ether (MTBE) in the fuel, sulfur content, and distillation temperature are important determinants of the RAF (e.g., see Ho and Winer, 1998).

Table 16.10 shows the reactivity adjustment factors determined for light-duty motor vehicles operated on reformulated gasoline, M85 (85% methanol, 15% gasoline), compressed natural gas, and liquefied petroleum gas. There is no universally accepted definition of "reformulated gasoline"; that shown in Table 16.10 is the "Phase 2" gasoline mandated in California starting in 1996. It has a lower vapor pressure (which will lower evaporative emissions) as well as a lower sulfur content, the latter improving the efficiency of catalytic converters used for hydrocarbons, CO, and NO (. Other changes include lower concentrations of toxics such as benzene and added oxygenated organics such as alcohols or ethers, which may reduce CO. For a detailed description of reformulated gasolines, see Calvert et al. (1993) and Ho and Winer (1998).

The allowed organic (NMOG) mass emissions (grams per mile) for TLEV, LEV, and ULEV are calculated as the appropriate standard given in Table 16.7 divided by the RAF. For example, for an RAF of 0.4f, typical of TLEV fueled with M85, the allowed emissions are (0.125/0.41) = 0.30 g per mile (California Air Resources Board, 1993). Table 16.11 summarizes the allowable emissions for various potential fuels. The standards allow a larger total emission of organics as long as the ozone-forming potential of the emissions does not exceed those from conventional gasoline-fueled vehicles. For example, vehicles fueled on compressed natural gas can emit more than double the mass of those on reformulated gasoline because of the relatively low reactivity of the organics emitted. Indeed, the allowed emissions would be even larger in this case if only unburned methane was emitted; however, there are small concentrations of larger organics in the exhaust (and in the fuel itself) that increase the net reactivity of this fuel.

It should be noted that while the RAFs give the relative contribution of a test fuel to 03 formation, they do not reflect changes in total mass emissions that may also occur with the change in fuel. The net impact of a fuel-vehicle combination can only be assessed in terms of the amount of 03 formed per vehicle mile traveled. For example, in California in 1996 the only gasoline allowed was the reformulated Phase 2 gasoline. The RAF for TLEV and LEV operating on Phase 2 gasoline is unity. However, the total organic mass emissions

FIGURE 16.35 Maximum incremental reactivities of some organics (grams of Os produced per gram of VOC) (data graciously provided by B. Croes, personal communication).

FIGURE 16.35 Maximum incremental reactivities of some organics (grams of Os produced per gram of VOC) (data graciously provided by B. Croes, personal communication).

are significantly lower, so that the contribution to net 03 formation is expected to be substantially reduced.

Similarly, Black et al. (1998) measured emissions from a number of vehicle-fuel combinations, including a vehicle fueled on compressed natural gas. They calculate a value for the RAF of 0.87, about twice that reported in Table 16.10. However, the RAF values appear to decrease as the emission rates increase, due

TABLE 16.10 Reactivity Adjustment Factors for Light-Duty Vehicle - Fuel Combinations"

Fuelfi

TLEV'

LEV'

ULEV'

Reformulated gasoline

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