hydrocarbons, as well as fine particles linked to reduced lung and cardiovascular function. Last year, less than 1 percent of all existing passenger vehicles in the United States were fueled by diesel, but there is an increasing trend toward diesel use. Supporters promote diesel vehicles as obtaining 20 to 30 percent better mileage than do equivalent gasoline vehicles. Such improved fuel efficiency should result in lower emission of compounds that lead to the production of carbon dioxide, a major greenhouse gas. But, according to Jacobson, diesel does not provide a dramatic reduction in carbon emissions. Instead only a modest 5 to 15 percent decrease results. This is because diesel contains more carbon per gallon of fuel than does gasoline.

''Modern'' diesel vehicles do not, however, emit the black, sooty exhaust characteristic of traditional diesel vehicles. This is thanks to better engines, improved fuel mixes and enhanced pollution control technologies such as particle traps and devices for controlling nitrogen oxide emissions.

To examine the results of using cleaner diesel technology, Jacobson and his colleagues at the California Institute of Technology, University of Iowa and Argonne National Laboratory used mathematical models to simulate the effects of replacing all gasoline-fueled vehicles in the United States with ''modern'' diesel-fueled vehicles. The scenario was ''best case,'' since diesel vehicles in the United States do not yet have pollution controls such as traps and filters.

The researchers programmed data from the extensive U.S. National Emission Inventory into their model. The inventory contains emissions from hundreds of thousands of sources including 1,700 types of vehicle sources, of which 870 types are gasoline. The researchers validated their model by comparing current conditions with observations from the U.S. Ambient Air Quality database. The computer model incorporated all the processes that affect pollution in the
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Contact: Dawn Levy
dawnlevy@stanford.edu
650-725-1944
Stanford University
9-Feb-2004

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