Two important caveats should be noted. The first relates to the fact that the analysis presented here is based solely on data gathered from well-maintained vehicles with properly working catalytic converters. As noted in Chapters 4 and 6, there is substantial evidence to suggest that high-emitting motor vehicles (perhaps because of malfunctioning catalytic converters or faulty evaporative controls) can contribute disproportionately to the VOC and CO emissions arising from a fleet of LDVs, and the response of high-emitting vehicles to ethanol-blended and MTBE-blended RFG has yet to be fully characterized. For example, one might speculate that oxygen in the fuel would provide a greater emissions benefit for high emitters with faulty catalytic converters than for ordinary vehicles. Because ethanol fuels often contain more oxygen than the equivalent MTBE fuel, this might tend to offset the disadvantages of ethanol-containing fuels implied in the committee's analysis. However, the few data on this subject that are currently available are inconclusive (e.g., see Knepper et al 1993; Mayotte et al. 1994). Moreover, for high-emitting vehicles with faulty evaporative controls, the use of ethanol-blended RFG with a higher RVP would most likely lead to elevated evaporative emissions. For these reasons, the committee recommends that the effect of RFG on emissions from high-emitting vehicles be studied in greater detail.
The other caveat relates to the overall effect on ozone pollution that might arise from the emissions differences projected here for MTBE-containing and ethanol-containing RFG blends. Recall from the committee's earlier analyses that the overall effect of RFGs might be an approximate 20% reduction in the reactivity of LDV emissions and a few parts-per-billion reduction in peak ozone concentrations. After combining exhaust and evaporative emissions, the use of ethanol, as opposed to MTBE, as an oxygenate would lead to a decrease in the effectiveness of RFGs but not a total cancellation. The net effect on ozone concentrations would be extremely small and almost certainly not discernable from the ambient ozone concentration data.