LONDON (Reuters) - Modern diesel cars may not be as clean as previously thought, say some experts as regulators try to roll out industry standards to satisfy "green" consumers.
Filter technologies have cleaned up diesel cars traditionally viewed as far more polluting than gasoline rivals, but their soot emissions are now underestimated, say some analysts.
Autos standards often focus on carbon emissions and fuel economy, where diesel vehicles perform best, omitting other effects, including soot, where they fare worst, say experts.
In Britain, for example, cars are exempt from road tax if their carbon dioxide (CO2) emissions are below a certain level.
The scheme does not account for soot, which also can contribute to global warming through a substance called black carbon. The omission is a mistake, say some analysts.
"Even with a particle trap (filter), all the diesels have an impact," said Stanford University's Mark Jacobson.
"Each effect will vary from vehicle to vehicle but the point is they are not accounting for this. The claim that barely detectable black carbon causes no warming is misinformation."
Diesel cars -- not electric, hybrid or gasoline -- dominate the UK tax rebates introduced two years ago.
The scheme excludes soot because new technologies and EU rules have dealt with the problem, said Britain's department for transport.
"Black carbon emitted by modern diesel cars is practically undetectable," it said in a statement.
Other regulators support that view -- diesel filters were a "game changing technology", said the head of monitoring at California's Air Resources Board (CARB.L).
"I would not characterise the little black carbon that remains in the emissions (as) still a problem," said CARB's Alberto Ayala.
But even reduced levels of soot could still be the equivalent of an extra one gram (g) of CO2 per kilometre, according to the estimates of two leading experts.
Just one gram of CO2 is significant as manufacturers try to shave off carbon emissions in a bid to qualify for incentives and improve their image.
Some 33 of the 51 car models which qualify for zero road tax in Britain are diesel, and half of these emit 99g of CO2 per kilometre, as close as possible to the 100g threshold, government data show.
Black carbon is produced from incomplete burning of fossil fuels and is blamed for accelerating global warming by soaking up heat from the sun. It can darken snow and ice when it lands, hastening a thaw such as in the Arctic or Himalayas.
Modern diesel cars also produce more NOX emissions than gasoline, which in turn creates a greenhouse gas and pollutant called ozone which can harm the lungs. But NOX emissions also destroy a powerful greenhouse gas called methane, further muddying the net effect of such cars on the environment.
A European Commission study in 2007 showed average emissions of 0.57 milligram (mg) of soot per kilometre, in laboratory tests of modern diesel engines equipped with filters, a figure well below the present EU legal limit.
That's equivalent to about 1g of CO2, according to calculations by Stanford's Jacobson, who estimated that 1 mg of soot, including black carbon and other particulates, is equivalent to about 1,500mg of CO2.
Another leading expert, Veerabhadran Ramanathan from the U.S. Scripps Institution of Oceanography, supported Jacobson's calculation.
Furthermore, Jacobson reckons that lab testing significantly underestimated actual soot emissions because of the harder acceleration people used on the road in real life.
Other scientists said that such estimates were at the upper end of estimates.
"The (research) trend is coming closer and closer to them," said Pam Pearson, a climate policy expert at the U.S. and Sweden-based International Cryosphere Climate Initiative ICCI.L.
Other researchers said that it was difficult to compare CO2 with black carbon as they behaved differently, staying in the atmosphere for centuries and just weeks respectively.