The California Air Resources Board (CARB) and Arco, a U.S. subsidiary of the oil company British Petroleum, are poised to release preliminary results from two studies that appear to show that new, so-called "green" diesel buses are as clean or cleaner than buses powered by compressed natural gas. New diesel technology combines the use of specially formulated low-sulfur fuel and emissions control devices called particulate (soot) traps. This development comes as operators of transit, truck and school bus fleets are choosing between diesel and alternative fuels to meet progressively more stringent pollution standards.¹ While we support the cleanup of all diesels, the current generation of so-called "green" diesels will not resolve all of the environmental and health impacts of diesel combustion. The sooty exhaust from diesel-powered vehicles and machinery is a harmful air pollutant that threatens public health. Below, NRDC and Coalition for Clean Air separate fact from fiction in the debate over toxic soot.

The new studies are not apples to apples comparisons between diesel and natural gas buses

The CARB and Arco studies tested new diesel buses with particulate traps against a natural gas bus with no pollution controls. In a fair comparison, the natural gas bus would be equipped with the same technology. Particulate traps have not been developed yet for natural gas fuel, but another device, called an oxidation catalyst, has been used successfully to reduce precursors to particulates, as well as carbon monoxide.² CARB is now going back and retesting the same natural gas bus with the addition of an oxidation catalyst. However, until diesel and natural gas buses are tested using comparable control technologies, any conclusions about which is ultimately cleaner will be premature. Nonetheless, the studies clearly show that uncontrolled compressed natural gas buses are far cleaner than uncontrolled diesel buses.

Diesel exhaust control technology still is unproven in real-world operating conditions

Optimism for cleaner diesel must be tempered with caution. CARB has certified only two diesel particulate filters and one Green Diesel Technology™ school bus. New technologies show the potential to reduce soot emissions to very low levels, but potential and real-world results are not the same. If the new technology fails, degrades, or is disengaged, diesels will continue to pollute the air with black, toxic soot.

• Diesel buses and trucks tend to release much more pollution -- particularly soot -- during real-world use than their original certification tests indicate.³ In contrast, natural gas vehicles usually maintain their emission performance.⁴
  
  
• Today's traps are not durable over the lifetime of trucks and buses. Particulate traps are certified to last only 150,000 miles,⁵ about one-third of the useful lifetime of a transit bus. There are no guarantees that traps will be replaced at the end of their useful life, that they will be maintained properly, or that they will perform consistently under the range of real-world operating conditions. Experience with passenger vehicles demonstrates that control technologies must be durable over the vehicle's life and that inspection and maintenance programs are critical to ensure long-term performance. Yet no mandatory inspection and maintenance programs exist to ensure that the new diesels will remain as clean as they are claimed to be.
  
  
• Diesel particulate filters must be used in conjunction with specially formulated low-sulfur fuel. They are rendered useless when used with regular diesel, which contains high sulfur levels.⁶ However, the new low-sulfur fuel will not be widely available until 2006.
  
  
• Existing diesel particulate filters cannot be used on any vehicles manufactured before 1994.⁷ This means that they can do nothing to prevent pollution from the thousands of old, dirty diesel vehicles still on the road.
  
  
• Emissions controls are vulnerable to tampering. In 1998, several large diesel engine manufacturers settled a lawsuit with the U.S. Environmental Protection Agency (EPA) and CARB, rather than face trial in a case that resulted from a decade-long practice of designing engines that met EPA's and CARB's emission standards in the laboratory, yet emitted as much as three times as much smog-forming nitrogen oxides at highway speeds.⁸
  
  

Diesel particulate filters may increase smog

So-called clean diesel vehicles equipped with particulate traps are dirtier than older diesels in one respect: they produce more nitrogen dioxide.⁹ Nitrogen dioxide reacts to form ground-level ozone (smog). Most urban areas in California already exceed state standards for this pernicious air pollutant.¹⁰ The notoriously smoggy Los Angeles area tops the list of areas with ozone problems in the United States. It is the only area designated in extreme non-attainment with federal ozone standards.¹¹ A recent study by researchers at the University of Southern California found that smog can cause childhood asthma.¹²

The toxic, sooty trail of diesel exhaust is a well-known health hazard

Diesel exhaust contains at least 40 different substances recognized as toxic air contaminants by CARB and is associated with severe health effects, such as cancer and asthma.¹³ CARB estimates that diesel exhaust alone causes 70 percent of Californians' risk of cancer from air pollution.¹⁴ Diesel particulates are associated with serious health effects ranging from heart and lung diseases to early death.¹⁵

California deserves clean vehicle technology today

Natural gas is the cleanest fuel technology currently available. It is inherently cleaner than diesel, producing fewer smog-forming nitrogen oxides and virtually no visible soot from the tailpipe.¹⁶ Diesel vehicles must use low-sulfur fuel and sophisticated, foolproof pollution control equipment in order to come close to the low soot emissions of a natural gas vehicle. Despite these problems, new diesel technology can improve air quality, if used correctly in certain applications, such as long haul trucks. Alternative fuels remain the best choice for centrally fueled fleets, such as transit and school buses and certain truck applications. Alternative fuel vehicles can and should become even cleaner with the addition of new pollution controls.

Notes

1. In February 2000, the California Air Resources Board (CARB) passed a transit bus fleet rule requiring transit agencies to choose either a diesel or alternative fuel path to meet progressively more stringent pollution standards. http://www.arb.ca.gov/msprog/bus/bus.htm.

2. Nigel N. Clark et. al., "Diesel and CNG Transit Bus Emissions Characterization by Two Chassis Dynamometer Laboratories: Results and Issues", SAE Technical Paper Series, 1999-01-1469.

3. S.H. Turner et. al., "Comparison of In-Use Emission from Diesel and Natural Gas Trucks and Buses. SAE 2000-01-3473. Warrendale, PA.: Society of Automotive Engineers.

West Virginia University Transportable Heavy-Duty Vehicle Emissions Testing Laboratory, "Exhaust Emissions Test Results Report of Raley's distribution Center Tractors," In Gas Research Institute (GRI), 1997.

4. ibid.

5. CARB Verification Letters; http://www.arb.ca.gov/diesel/documents/verifieddevices.htm

6. P. Monahan, "Pollution Report Card: Grading America's School Bus Fleets," Union of Concerned Scientists, February 2002.

7. CARB website.

8. ibid.

9. Gragg, Kerstin. "Efficiency of a catalytic muffler and CRTTM systems for heavy duty vehicles," a report for MTC AB; report no. MTC 5111, 2001.

Presentation by Don McNerny, CARB, at the International Diesel Retrofit Advisory Committee meeting, Los Angeles, CA, October 2001.

10. The 2001 California Almanac of Emissions and Air Quality, CARB, April 2001, http://www.arb.ca.gov/aqd/almanac01/almanac01.htm

The Greenbook: Nonattainment Areas for Criteria Pollutants, EPA, 2002, http://www.epa.gov/oar/oaqps/greenbk/

11. ibid.

12. McConnell R, Berhane K, Gilliland F, London SJ, Islam T, Gauderman WJ, Avol E, Margolis HG, Peters JM. Asthma in exercising children exposed to ozone: a cohort study. Lancet 359:386-91, 2002.

13. Cal. EPA, Proposed Identification of Diesel Exhaust as a Toxic Air Contaminant Report, April 1998. Health Effects of Diesel Exhaust Fact sheet, Office of Environmental Health Hazard Assessment, Cal. EPA, (www.oehha.ca.gov/public_info/facts/dieselfacts.html).

Pandya RJ, Solomon G, Kinner A, Balmes JR. Diesel exhaust and asthma: Hypotheses and molecular mechanisms of action. Environmental Health Perspectives; 110(Suppl 1):103-112, 2002.

14. South Coast Air Quality Management District, "Multiple Air Toxics Exposure Study in the South Coast Air Basin (MATES-II)" March 2000.

15. Shprentz D, "Breathtaking: Premature Mortality Due to Particulate Air Pollution in 239 American Cities", NRDC, New York, May 1996.

Increased Particulate Air Pollution and the Triggering of Myocardial Infarction, Peters, Dockery, Muller and Mittleman, Circulation 2001 June 12; 103(23):2810-5.

Samet JM. Fine particulate air pollution and mortality in 20 U.S. cities. New England Journal of Medicine 343, 2000: 1742-1749. Dockery DW. "Epidemiologic Evidence of Cardiovascular Effects of Particulate Air Pollution." Environmental Health Perspectives, 109, 2001: 483-486.

16. Natural Gas Buses: Separating Myth from Fact; Clean Cities Alternative Fuel Information Series; U.S. Department of Energy; May 2000.