Stanford University Home

Stanford Report Online

Study refutes notion that diesel-powered vehicles are better for the environment
Efforts to reduce air pollution should instead focus on hybrid cars and low-emission gasoline-powered vehicles, expert says


Diesel fuel is now at the center of a delicate balancing act between smog production and global warming. Some lawmakers and car manufacturers advocate widespread diesel use in passenger vehicles as a strategy for reducing the production of so-called "greenhouse gases" thought to cause global warming. But according to a new study, replacing gasoline vehicles in the United States with diesel vehicles -- equipped even with the most modern pollution controls -- may increase smog production over most of the country.

"If provisions in last year's United States energy bill pass this year, many diesel vehicles will qualify for the same tax credits as hybrid and hydrogen fuel cell vehicles," said Mark Jacobson, associate professor of civil and environmental engineering and lead author of the study published January 30 in the journal Geophysical Research Letters. He added, "It is a surprise to me that this proposal was made before its possible effect on public health was evaluated."

The main component of smog is ozone. In the upper atmosphere it absorbs intense cancer-causing ultraviolet radiation from the sun and prevents it from reaching Earth. In the lower atmosphere where we breathe, however, ozone is a respiratory irritant associated with decreases in lung function and increases in hospital visits for respiratory causes. Children, the elderly and individuals with preexisting respiratory disease have increased sensitivity to ozone exposure.

Ozone is formed in the lower atmosphere through sunlight-initiated chemical reactions between nitrogen oxides and hydrocarbons -- compounds found in vehicle exhaust. The U.S. Environmental Protection Agency has designated ozone and nitrogen dioxide, along with four other air pollutants, as "criteria pollutants" for setting concentration standards for the protection of public health and the environment.

Compared to gasoline-fueled vehicles, diesel-fueled vehicles emit more nitrogen oxides and 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 atmosphere: emission, gas chemistry, particle processes, meteorology, transport, radiation, clouds, and removal by rainfall and deposition.

The main result of the study is that when gasoline vehicles were switched with diesel, there was an increase in surface ozone over 75 percent of the United States, particularly in the Southeast. There was a slight decrease over the remaining 25 percent, but on average, surface level smog increased over the United States. In the lower atmosphere, just above the surface, ozone levels increased across the entire country.

In addition, the researchers found that pollutants other than ozone also increased, and confirmed that to reduce ozone levels over most of the United States, controlling vehicular nitrogen oxide emissions would be more effective than controlling hydrocarbon emissions. This is particularly true in the Southeast, where natural vegetation emits high levels of hydrocarbons that react with vehicular nitrogen oxide to form ozone.

Jacobson recommended low-emission gasoline-fueled vehicles and high-mileage gasoline-electric hybrid vehicles as the best strategy for reducing vehicle-related air pollution and climate problems simultaneously.  

Czerne M. Reid is a former News Service intern.

Mark Jacobson