because of concern about environmental risks associated with MTBE leakage into drinking water. The other aspect of the controversy, which is the focus of this report, relates to the ozone pollution problem. MTBE and ethanol can affect the amounts and types of ozone precursor compounds emitted from tailpipes of motor vehicles as well as from the evaporation of unburned fuel. Questions persist over which oxygenate is preferable in terms of air-quality impact. This report addresses the potential impact of oxygenates in RFG on the ozone-forming potential of emissions from motor vehicles.

How should regulatory agencies determine if one RFG blend using a particular oxygenate is preferable to another? In attempting to mitigate ozone pollution, the U.S. Environmental Protection Agency (EPA) currently addresses such questions by estimating the mass of VOC emissions resulting from the use of an individual RFG blend. If the estimated mass of emissions exceeds a specified amount, that fuel blend is disallowed. However, a different method for assessing RFG blends has been proposed. Although certain fuel blends, such as those using ethanol, might result in greater amounts of emissions in terms of mass (because of the volatility of ethanol), it is argued that those emissions have a lower ozone-forming potential compared with emissions from other fuel blends. Therefore, the argument goes, EPA's assessment of RFG blends should be based not only upon mass of emissions, but also upon their reactivity (i.e., ozone-forming potential).

To help assess the scientific underpinning for this question, EPA asked the National Research Council to study it independently. In response, the Research Council formed the Committee on Ozone-Forming Potential of Reformulated Gasoline, which has prepared this report. The committee was charged to assess the utility and scientific rigor of evaluating the ozone-forming potential of the emissions resulting from RFG use (i.e., an approach that takes into account not only the total mass of emissions, but also the potential of the emissions to produce ozone). The committee was not charged or constituted to address the design or implementation of possible new regulations based on the ozone-forming potential of RFG blends. In addition, the committee was not charged or constituted to address relevant, but separate, issues about domestic sources versus foreign sources of fuel, relative energy and cost implications for the production of different RFG blends, relative health and global environmental impacts, or the use of renewable versus non-renewable fuels.

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