Ethanol and MTBE gasoline blends do little to reduce smog

In a study examining the differences between ethanol and MTBE (methyl tertiary-butyl ether), the committee found that reformulated gasoline made with ethanol is less efficient, but that the overall impact of either oxygen additive on reducing ozone is very small.

“Vehicle emissions of chemicals that form ozone pollution have decreased in recent years,” said committee chair William Chameides, Regents Professor of Earth and Atmospheric Sciences at the Georgia Institute of Technology, Atlanta. “But that’s largely because of better emissions control equipment and components of reformulated gasolines -other than oxygen additives – that improve air quality. Although additives do reduce some pollutants from motor vehicles emissions, the oxygenates appear to have little impact on lowering ozone levels. Moreover, it is not possible to attribute a significant portion of past reductions in smog to the use of these gasoline additives.”

1990 amendments to the Clean Air Act require the use of reformulated gasoline with oxygen additives in areas of the US that have substantial ozone pollution. These types of gasoline are sold in cities on the East Coast, in the Midwest, Texas, and California – particularly during the summer months, when low level ozone is most prevalent.

Low level ozone is formed when pollutants from many different sources, including automobiles, react chemically in the presence of sunlight. Reformulated gasolines are designed to lower the emissions of vehicle pollutants, including those that contribute to ozone formation. In addition to oxygen additives, the fuels have a number of other characteristics that lower emissions.

But the oxygen additives in reformulated gasolines have raised environmental concerns. MTBE, for example, has leaked into drinking water in California, leading the state to phase out the use of the additive. Because questions persist over which types of reformulated gasolines are preferable in improving air quality, the US EPA asked the NRC to study methods for certifying gasoline blends with oxygen additives.

The committee found that, compared with MTBE blends, ethanol blends result in more pollutants evaporating from vehicle gas tanks. Ethanol blends also increase the overall potential of emissions to form ozone. However, available data indicate that the potential for either additive to lower smog levels is small and will continue to decrease as other measures to reduce vehicle emissions take effect. Tougher air-quality regulations and improvements to vehicles over the last few decades have substantially reduced emissions that help create low-level ozone.

Projections indicate that motor vehicle emissions could decrease further in the next several years as more new technologies are incorporated, the report says. If these projections are correct, the impact that reformulated gasolines have in reducing ozone concentrations will continue to decline. Even if the contributions that motor vehicle emissions make to ozone formation have been underestimated – which has occurred in the past – oxygen additives in reformulated gasolines will have little impact on reducing smog, the committee said.

The committee noted that available data on reformulated gasolines are limited because they are mostly derived from tests on properly functioning motor vehicles. A disproportionate amount of pollutants originate from a small number of high-emitting vehicles, such as older vehicles that have malfunctioning catalytic converters. There is considerable uncertainty surrounding the effects of reformulated gasolines on these vehicles, making it difficult to quantify total overall vehicle emissions and evaluate the effects of different fuel blends.

The Clean Air Act, which mandated the reformulated gasoline program, stipulated a minimum oxygen content in the reformulated blends to help reduce emissions. The EPA must certify reformulated blends to ensure that they meet program standards. The agency currently certifies reformulated gasoline blends by measuring the total mass of volatile compounds that are emitted, rather than by examining the potential for these compounds to form ozone. Because ethanol-blended gasoline is more volatile, meaning that more pollutants are evaporated into the atmosphere, it is difficult for such blends to meet the EPA’s standards unless the ethanol is blended with a more expensive, lower-volatility gasoline that is not readily available in many markets. Ethanol proponents have argued that the EPA should evaluate the fuels by the extent to which their emissions will react in the atmosphere and contribute to ozone formation.

Reactivity has reached a level of substantial rigour as a tool for assessing the ozone-forming potential of emissions, the committee said, but there are no compelling scientific reasons at this time to recommend that reformulated fuels be certified based on reactivity.