scope:

Introduction

The U.S. Environmental Protection Agency's (EPA's) September 1989 report, Analysis of the Economic and Environmental Effects of Methanol as an Automotive Fuel, is intended to support the Administration's alternative fuels program. EPA claims "that the primary benefit of substituting methanol for gasoline will be significant improvements in ozone levels in the most seriously polluted areas of the country."

In the report, EPA uses reactivity and emissions factors for methanol, formaldehyde, and hydrocarbons to estimate projected in-use "gasoline-VOC equivalent emissions" for gasoline and methanol vehicles. Based on these calculated "gasoline-equivalent emissions," EPA suggests that the substitution of M85 (15 percent gasoline, 85 percent methanol) flexible-fuel vehicles for conventional gasoline vehicles would result in a 30-43-percent reduction in volatile organic compound (VOC) emissions. Substitution of M100 (optimized neat methanol) vehicles would result in an 80-percent reduction in these mobile-source emissions. Although these figures appear impressive, the portion of the VOC inventory resulting from mobile sources is steadily decreasing so that reductions in mobile-source VOC emissions will have decreasing impacts on ozone levels. Taking this into account, EPA projects that the overall VOC reduction for nine cities with the severest ozone nonattainment problems would average only 1.5 percent by the year 2005, with a steady-state reduction averaging 3.3 percent by the year 2015. Note, however, that EPA does not make any projections of the actual effect of these VOC reductions on ozone levels in the nine cities.

This paper assesses both the appropriateness of EPA's relative reactivity factors for methanol and formaldehyde and EPA's use of the factors to imply significant ozone benefits from the use of methanol-fueled vehicles. We will show that no single reactivity factor for a particular compound or group of compounds is appropriate under all conditions. Thus, average reactivity factors, such as those derived by EPA, cannot be used accurately to assess the effects of an alternative fuels program on ozone reductions for a specific city. The quantitative effects on ozone levels of any potential alternative fuels program can best be estimated on a case-by-case basis for each city in question using multiday photochemical-grid models. Reactivity factors combined with corresponding emissions factors can only describe potential VOC reductions as a gross average and should not be used to predict ozone benefits of a particular alternative fuels program for any given city. Reactivity factors are useful only as a screening tool to indicate what approaches might be worthy of more extensive analysis using photochemical-grid models.