baseline level of 339 ppm to about 140 ppm. EPA also has imposed new NO-,, control standards on vehi cles themselves, beginning with 1994 models. California, meanwhile, has made ΝΟ χ reduction a centerpiece of its own RFC program, which be gins in March 1996. These highly prescriptive regu lations call for, among other things, a maximum av erage sulfur content of 30 ppm, which will decrease NO^ emissions by an estimated 15%.
The value of oxygenates The requirement to add oxygenates to RFG has been the most controversial part of the new regulations. Adding oxygen to gasoline is an accepted way to re duce carbon monoxide (CO) emissions, which are known to have detrimental health effects. The most common fuel oxygenates are ethanol (derived mostly from corn), methanol (a petroleum product), and two ethers: ETBE (ethyl-iert-butyl ether) and MTBE (methyl-rerf-butyl ether). Unlike urban ozone, which is primarily a sum mertime problem, CO pollution is more severe dur ing the winter. So EPA has instituted a separate win tertime oxygenated fuel program, mandated by a different section of the Clean Air Act. The winter time "oxy fuels" program began in 36 U.S. cities in 1992. This program requires the addition of 2.7% ox ygen by weight in wintertime gasoline. The new RFG rules, however, require 2% oxygen by weight be cause higher levels of some oxygenates can drive up evaporative VOC emissions in the summer. Beyond limiting CO emissions, oxygenates have little advantage. In a comprehensive July 1993 pa per in Science on controlling vehicle emissions (5), J. G. Calvert, John Seinfeld (chair of the 1991 Na tional Research Council study on urban ozone), and their co-authors stated that oxygenates "appear to of fer negligible benefits in terms of decreasing atmo spheric ozone formation. No convincing argument based on combustion or atmospheric chemistry can be made for the addition of ethanol to gasoline." The authors went on to say that congressional require ments for oxygenates in the Clean Air Act Amend ments of 1990 were "arbitrary, in a scientific sense," considering that there was little quantitative infor mation at the time on the relation between fuel com position and emissions. Why, then, was a requirement for oxygenates writ ten into the RFG section of the Clean Air Act amend ments, which are primarily targeted at summer time ozone, not wintertime CO? Politics played some role. According to the Sierra Club's Early, it was part of a strategy to build a coalition that would ensure passage of the Act. "We had the opportunity, by put ting the oxygenate requirement in [the RFG sec tion] , to get the support of those members who were responsive to the corn and ethanol industry, as well as members who might be influenced by the MTBE manufacturers," he says. There continues, however, to be a debate as to whether oxygenates could actually raise NO x emis sions, which would put one provision of the RFG rules in conflict with another. When EPA was preparing its "simple model," the agency acknowledged that add ing oxygenates to gasoline might drive up NO^ emis
sions (2). But according to the EPA rule on RFG pub lished last February, "Under the final complex m o d e l . . . oxygen has been found to result in no NOx increase, in fact, it results in a very slight decrease." The rule went on to say, though, that as a result of other changes that occur to the fuel when oxy genates are added, "there is no assurance under the simple model that oxygenate addition will not in crease NO^ emissions. The more oxygenate add ed . . . the greater the possibility for a NO x in crease. For this reason EPA believes it is still appropriate to cap the maximum oxygen content un der the simple model at 2.7%. Any higher oxygen con centrations will require use of the complex model." Refiners are required to use the complex model in 1998. The Auto/Oil study also found that oxygenates could pose a problem, driving up NO x by as much as 5% in low-aromatic fuels (which reduce air tox ics). In addition, "splash b l e n d i n g " ethanol— mixing it in tanks during truck transport—was seen to increase light-duty vehicle contribution to ozone because of increased vapor pressure and evapora tive emissions.
California's RFG Perhaps the biggest challenge to EPAs assessment of the NO x risk from oxygenates has come from Cali fornia. According to the California Air Resources Board predictive model, which is similar in intent to EPA's complex model, oxygen levels above 2% by weight result in NOx increases (6). Why the discrepancy between California's assess-
Oxygenates on trial Originally, EPA's rule for RFG was designed to be "fuel neutral" with regard to oxygenates—it did not specify what kind of oxygenate a refiner would have to use as long as the gasoline included 2% oxy gen by weight. Most refiners would probably choose in that case to use MTBE, a petroleum product, instead of ethanol products de rived from corn. But in December 1993, in a move widely considered to be moti vated by the Clinton administration's desire to appease farm belt politicians, EPA revealed that it was mandating a 30% market share for "renewable" oxygenates such as ethanol and ETBE in RFG. In the face of subsequent criticism from the oil industry and environ mental groups, EPA agreed last summer to a phase-in in which only 15% of the market share would go to renewables in 1995, with 30% required in the following year. EPA officials during congressional hearings last year admitted that there are no environmental benefits to favoring ethanol over methanol. They spoke instead of the importance of developing re newable fuel sources and of decreasing U.S. dependence on im ported oil. Privately, some EPA officials say they don't agree with the ethanol mandate and wouldn't mind if the American Petroleum Institute wins its lawsuit challenging the rule. That challenge already has had some success: In September the U.S. Court of Appeals for the District of Columbia ruled that EPA would have to stay the 15% ethanol requirement until after the law suit is decided. The 30% mandate may never take effect if, as many involved in the debate expect, the government loses its case in court. —TONY REICHHARDT
VOL. 29, NO. 1, 1995 / ENVIRONMENTAL SCIENCE & TECHNOLOGY • 3 9 A
