Diesel fuel cleaner than natural gas, according to CARB environmental data

May 1, 2002
New comparative environmental data from the California Air Resources Board (CARB) indicates the South Coast Air Quality Management District (AQMD) in

New comparative environmental data from the California Air Resources Board (CARB) indicates the South Coast Air Quality Management District (AQMD) in Los Angeles may have jumped the gun in mandating compressed natural gas (CNG) for buses and other vehicles in its jurisdiction.

This data suggests emissions from CNG buses actually may be more toxic than its alternative, modern diesel systems, according to Allen Schaeffer, executive director of the Washington DC-based Diesel Technology Forum. “This study certainly challenges conventional thinking. It shows us that alternative fuels like CNG are not always as clean as they have been accepted to be, and more importantly, that we have not given fair consideration to the tremendous progress made in clean diesel technology,” said Schaeffer.

“It was a naive assumption that CNG emitted fewer particulates than diesel,” said toxicologist Charles Lapin, PhD of Glendale CA. He said he was surprised that toxic formaldehyde levels in CNG emissions were so much higher than diesel, but he acknowledged that only one or two studies have looked at the toxicity of CNG emissions.

The announcement by CARB that diesel engines with soot filters emit fewer and less-toxic compounds than “clean” CNG engines questions the wisdom of South Coast District's two-year-old CNG-only decision. That policy has seen taxpayers subsidize hundreds of millions of dollars for new CNG vehicles and fueling stations — in the midst of the natural gas shortage.

“Several months ago, California determined that it didn't adequately study the adverse effects of the gasoline additive MTBE before mandating its use, and Californians are now paying the price. The blind rush by some government agencies away from modern clean diesel to CNG-fueled public transit may carry the same harmful effects. Our message is that all fuels and technologies should be evaluated fairly and openly,” said Schaeffer.

Some 13 transit fleets and all school districts in the South Coast AQMD region have been required to direct new purchase dollars to the more expensive, less reliable, and potentially more toxic CNG buses, said Schaeffer.

CARB discovered in its tests with South Coast AQMD that a diesel bus with a soot trap outperformed a newer CNG bus in eight out of 11 pollution tests. Results also suggest that the CNG exhaust components may be more harmful than those from low-sulfur diesel fuel. CARB data corroborates studies done in Sweden in 2000 but rejected by the South Coast AQMD and many policymakers at CARB in 2000 and 2001. Results also parallel a much larger California study of trucks, waste haulers, and school buses conducted by BP/ARCO along with CARB, South Coast AQMD, and the United States Department of Energy.