NRC study finds little benefit from oxygenates

A National Research Council panel has concluded that the two principal types of oxygen additives used in reformulated gasolines (RFG) in the U.S. contribute little to reducing ozone pollution.

The NRC study examined the differences between the additives ethanol and methyl tertiary butyl ether (MTBE) and concluded that RFG made with ethanol is less effective but that the overall effect of either oxygen additive on reducing ozone (a major component of smog) is very small.

William Chameides, study committee chairman, said, "Motor-vehicle emissions of chemicals that form ozone pollution have decreased in recent years. 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 vehicle 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."

Chameides is a professor of earth and atmospheric sciences at Georgia Institute of Technology, Atlanta.

The 1990 Clean Air Act Amendments require the use of RFG with oxygen additives in many areas of the U.S. that have substantial ozone pollution.

RFG is designed to lower the emissions of vehicular pollutants, but oxygen additives in RFG have raised environmental concerns-particulary MTBE, which has leaked into drinking water in California, leading the state to phase out use of the additive.

The U.S. Environmental Protection Agency asked the National Research Council to study methods for certifying gasoline blends with oxygen additives.

Conclusions

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 vehicular emissions to form ozone.

It said that 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.

NRC said tougher air quality regulations and improvements to vehicles over the last few decades have substantially reduced emissions that help create near-ground ozone, and they could decrease further as more new technologies are incorporated.

"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 study said that EPA certifies RFG blends by measuring the total mass of volatile compounds that are emitted, rather than by examining the potential for these compounds to form ozone.

It said that, because ethanol-blended gasoline is more volatile, it is difficult for such blends to meet 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 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 rigor 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 RFGs be certified based on reactivity. The basic results of comparing the MTBE and ethanol fuel blends were the same, regardless of whether they were evaluated by reactivity or by the mass of their emissions.

Also, the study said that an evaluation of carbon monoxide emissions should be included in assessments of the effects of RFG. It said that CO can be a significant factor in ozone formation but often is overlooked in RFG assessments.

Reactions

The American Petroleum Institute said, "The NRC report is an important contribution in providing information that could help make our next steps together-industry, legislators, and regulators-toward continued air quality improvements both effective and affordable."

Edward Murphy, API's downstream industry segment general manager, said, "The report shows that efforts to reduce ozone pollution in the U.S. have clearly had a positive impact on our nation's air quality.

"The study says that EPA data indicate that peak ozone concentrations in 41 metropolitan areas dropped significantly from 1986 to 1997, despite growing fuel usage.

"The report confirms that cleaner-burning reformulated gasoline can achieve the emissions reductions envisioned by Congress and EPA without the federal government-mandated use of oxygenates such as ethanol and MTBE. This same conclusion was reached by a 6-year, $40 million joint research program conducted by the oil and automotive industries.

"The NRC report also confirms that there is no air quality benefit of one oxygenate over another and that differences in cleaner-burning gasoline formulations have small effects on air quality.

"These findings suggest that the long-term environmental difference between the oil refining industry's proposal to reduce sulfur in gasoline and EPA's proposal would be almost immeasurable. However, the difference in cost to achieve those gains would be significant. Clearly, industry has proposed the more affordable, less economically damaging of two effective solutions."

The Oxygenated Fuels Association complained the NRC report "is completely at odds with a host of real-world findings compiled by state and federal environmental agencies over several years that saw that RFG with oxygenates is working effectively."

OFA said, "America is currently enjoying some of the best air quality this nation has seen in more than 5 decades. The majority of this improved air quality is the result of mobile source-based anti-pollution programs that focus on clean cars, clean fuels, and educated drivers as the frontline defense against vehicular pollution.

"Fuels and cars are a system. Clean cars don't work without clean fuels. Oxygenates dilute sulfur in gasoline and other components and make fuels work better in cleaner cars by protecting catalytic converters and reducing engine deposits."