Source: Stanford University
Date:
2004-02-13
U.S. Push For Diesel Poses Risk To Public Health, Scientists Say
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.
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Source: American Geophysical Union
Date:
2002-10-22
Despite Lower CO2 Emissions, Diesel Cars May Promote More Global Warming Than Gasoline Cars
WASHINGTON - Laws that favor the use of diesel, rather than gasoline, engines in cars may actually encourage global warming, according to a new study. Although diesel cars obtain 25 to 35 percent better mileage and emit less carbon dioxide than similar gasoline cars, they can emit 25 to 400 times more mass of particulate black carbon and associated organic matter ("soot") per kilometer [mile]. The warming due to soot may more than offset the cooling due to reduced carbon dioxide emissions over several decades, according to Mark Z. Jacobson, Associate Professor of Civil and Environmental Engineering at Stanford University. Writing in the Journal of Geophysical Research-Atmospheres, Jacobson describes computer simulations leading to the conclusion that control of fossil-fuel black carbon and organic matter may be the most effective method of slowing global warming, in terms of the speed and magnitude of its effect on climate. Not only does soot warm the air to a much greater extent than does carbon dioxide per unit mass, but the lifetime of soot in the air (weeks to months) is much less than is that of carbon dioxide (50 to 200 years). As such, removing soot emissions may have a faster effect on slowing global warming than removing carbon dioxide emissions.
The model Jacobson used tested 12 identifiable effects of airborne particles, known as aerosols, on climate, eight of which had not previously been described in scientific literature. Jacobson notes that it is not currently possible to quantify each of these effects individually, only the net effect of all of them operating simultaneously.
"Since 1896, when Svante Arrhenius first postulated the theory of global warming due to carbon dioxide, control of carbon dioxide has been considered the most effective method of slowing warming," Jacobson says in an interview. "Whereas carbon dioxide clearly causes most global warming, control of shorter-lived warming constituents, such as black carbon, should have a faster effect on slowing warming, which is the conclusion I have drawn from this study. The Kyoto Protocol of 1997 does not even consider black carbon as a pollutant to control with respect to global warming."
The reason the issue of diesel versus gasoline is important, says Jacobson, is that, in Europe, one of the major strategies for satisfying the Kyoto Protocol is to promote further the use of diesel vehicles and specifically to provide a greater tax advantage for diesel. Tax laws in all European Union countries, except the United Kingdom, currently favor diesel, thereby inadvertently promoting global warming, Jacobson says. Further, some countries, including Sweden, Finland, Norway, and the Netherlands, also tax fuels based on their carbon content. These taxes also favor diesel, he notes, since diesel releases less carbon per kilometer [mile] than does gasoline. Nevertheless, the small amount of black carbon and organic matter emitted by diesel may warm the atmosphere more over 100 years than the additional carbon dioxide emitted by gasoline.
In Europe and the U.S., particulate emissions from vehicles are expected to decline over the next decade. For example, by 2005, the European Union will introduce more stringent standards for particulate emissions from light duty vehicles of 0.025 grams per kilometer [0.04 grams per mile]. Even under these standards, diesel powered cars may still warm the climate more over the next 100 years than may gasoline powered cars, according to the study.
The state of California is implementing an even more restrictive standard in 2004, allowing only 0.006 grams per kilometer [0.01 grams per mile] of particulate emissions. Even if the California standard were introduced worldwide, says Jacobson, diesel cars may still warm the climate more than gasoline cars over 13 to 54 years.
In an interview, Jacobson said that new particle traps being introduced by some European automobile manufacturers in their diesel cars appear to reduce black carbon emissions to 0.003 grams per kilometer [0.005 grams per mile], even below the California standard. "I think this is great, and it is an indication that tough environmental laws encourage industry to change. But," he said, "diesel vehicles emitting at this level may still warm the climate more than gasoline over a 10 to 50 year period, not only because of black carbon emissions, but also because the traps themselves require addition fuel use. Gasoline/battery hybrid vehicles now available not only get better mileage than the newest diesels but also emit less black carbon."
In practice, less than 0.1 percent of light vehicles in the United States run on diesel fuel, whereas more than 25 percent do in Europe. (Almost a third of new European cars in 2000 were diesel powered.) In both the United States and Europe, virtually all heavy trucks and buses are diesel powered, and American diesel consumption rates for all modes of ground transportation combined are about 75 to 80 percent of those in Europe.
Control of fossil fuel black carbon and organic matter will not by itself eliminate long term global warming, says Jacobson. This would require reductions in emissions of carbon dioxide and other greenhouse gases, in addition to reduction of particles. Other strategies to be considered for reducing black carbon and organic matter from the atmosphere could include the phasing out of indoor biomass and coal burning and improved particle collection from jet fuel and coal burning, he says. This reduction would provide the additional benefit of reducing the 2.7 million people who die annually from air pollution, as estimated by the World Health Organization. The health costs of particulate pollution range, in industrial countries, from $200,000 to $2.75 million per ton, Jacobson notes.
The research was supported by NASA, the Environmental Protection Agency, the National Science Foundation, the David and Lucile Packard Foundation, and the Hewlett-Packard Company.
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Source: American Chemical Society
Date:
2000-03-16
"Biodiesel" Fuel Could Reduce Truck Pollution
Fuel from renewable resources emits less carbon monoxide, particulate matter
Diesel fuel made from natural renewable sources such as vegetable oils or animal fats lowered air-polluting emissions of heavy trucks in a trial study, according to researchers. They say the so-called "biodiesel" fuel can be used in regular diesel engines without modifying them.
The finding is reported in the March 1 print edition of the journal Environmental Science & Technology by researchers from West Virginia University (WVU) in Morgantown, W. Va., and the National Renewable Energy Laboratory in Golden, Colo. It was initially published on the journal's web site on Feb. 9. The peer-reviewed journal is published by the American Chemical Society, the world's largest scientific society.
"Biodiesel fuel may have the potential to reduce our nation's reliance on imported oil and to improve air quality," said Mridul Gautam, Ph.D., a professor in the university's Department of Mechanical and Aerospace Engineering and one of the authors of the study. The scientists studied a blend of 35 percent biodiesel fuel and 65 percent conventional diesel fuel. They found that this emitted significantly less carbon monoxide and moderately less hydrocarbons and particulate matter, compared to 100 percent petroleum diesel.
"The potential of biodiesel to reduce emissions is quite significant," says Gautam. "There is a 25 percent reduction in particulate emissions alone." Emissions of carbon monoxide declined by 12 to 14 percent, and hydrocarbons by 10 percent, he added.
Biodiesel fuel is made by a reaction of vegetable oils with methanol or ethanol. The result is a less viscous, more volatile fuel. The truck engines ran just as efficiently on the biodiesel mix as on conventional diesel fuel (i.e., the average miles per gallon were essentially the same).
The research team found slightly elevated levels of nitrous oxide (NOx) emissions with the biodiesel blend. Changing the ignition timing of the engines reduced NOx emissions.
The reduction in carbon monoxide emissions is probably due to the higher oxygen content of the biodiesel fuel, the researchers say. More oxygen means the fuel is burned more completely. More complete burning also helps reduce hydrocarbon emissions.
The researchers attributed the 25 percent reduction in particulate emissions to the lower aromatic and sulfur content of biodiesel fuel, and its greater oxygen content.
The biodiesel study at West Virginia University was funded by the United States Department of Energy's Office of Transportation Technologies. The research team also included WVU scientists Donald W. Lyons, Nigel N. Clark and Wen-Guang Wang (now at Ford Motor Company in Dearborn, Mich.) and Paul Norton from the National Renewable Energy Laboratory. |
