Diesel soot/global warming link seen Controversial

Diesel Fuel News, Jan 7, 2002 by Jack Peckham

Stanford University environmental professor Mark Jacobson claims that diesel soot, jet fuel combustion emissions, and other black carbon (BC) particles are second only to carbon dioxide (CO2) to blame for human-caused "global warming" threats.

In a paper presented to American Geophysical Union last month, Jacobson urged much tougher standards on fossil-fuel soot emissions and challenged the notion that fuelefficient diesels are really better than gasoline for total greenhouse effect.

Regulations should require industry to develop even better exhaust particle traps, because even the ultra-low-emissions of U.S. EPA's "Tier 2" standards would still allow diesel particulate matter (PM) to exceed that of gasoline, Jacobson calculates.

In any case, oil supplies supposedly will run out in a few decades anyway, so therefore, more research should be focused on developing "clean" replacement fuels such as hydrogen, he argues.

While Jacobson concedes that very few cars in the U.S. are diesel, the opposite trend is well underway in Europe, and total diesel consumption in the U.S. from trucks, buses and other transport engines is about 80% of Europe's diesel transport consumption level.

"As emissions decrease to meet proposed [regulatory] standards, climate should respond, but even the proposed standards limit the extent of warming reduction and health benefit [that] can be achieved," Jacobson told us. "In addition, populations are increasing, so even if efficiency is improving, consumption is still increasing. In addition, most black carbon emissions today are in developing countries, where emission reductions will take longer to occur."

Jacobson calculates that the diesel PM filters as envisioned for EPA 2007 standards would need to improve their efficiency by a factor of 10 to 90 to match gasoline Tier 2 (light-duty) PM emissions, but his calculations are based upon some assumptions that are controversial.

For instance, Jacobson assumes that the ratio of black carbon to organic matter emission from diesel exhaust is 1:1, yet many diesel engine experts say that the diesel engines of the most recent decade are vastly lower in elemental carbon (black carbon) emissions, compared to organic carbon. PM filters virtually eliminate elemental carbon particles, although some volatile PM (sulfates, water vapor, some hydrocarbon) still can escape.

Jacobson also assumes that gasoline direct-injection (GDI) technology would achieve about the same fuel efficiency as diesel, yet diesel engine producers point out that diesels are still very much more efficient than GDI and thus lower on total CO2.

Jacobson also claims that researchers measuring diesel particle-emissions, including world-recognized authorities such as University of Minnesota's David Kittelson, "have not examined the coagulation rates of emitted particles in the ambient air" and thus may underestimate diesel fine-particle emission, a contention that Kittelson flatly contradicts.

"That is not true," Kittelson told us. "Part of the CRC E-43 project [for the Coordinating Research Council] is to model the coagulation and dispersion of the roadway plume" from diesel exhaust.

In a separate study for Minnesota Dept. of Transportation, Kittelson's research team (using a sophisticated mobile emissions measurement lab that chases diesel vehicles on the road) "shows that the nanoparticles persist downwind of the roadway but their concentrations are greatly reduced by coagulation and, mainly, atmospheric dilution."

Whatever the problem may be with particle emissions from diesels or jet engine exhaust, the uncertainties of total human emissions impacts on global warming remain very large, according to top scientists studying the issue.

Nor is there universal scientific agreement that combustion soot control will do much to slow global warming, as Massachusetts Institute of Technology earth atmosphere scientist Richard Lindzen told us.

Example: Even if soot control would be somewhat more effective than CO2 reductions seen in the Kyoto agreement, the total impact would be negligible anyway, he said. Kyoto, if ever enforced, would only change global warming "a few tenths of a degree in 100 years," and soot control wouldn't have much more impact, Lindzen says.

Rather than high-cost, drastic actions with paltry effect on global warming, policy makers instead must be open to emerging, lower-cost strategies, according to Michael Schlesinger, University of Illinois Climate Research Group scientist.

In an upcoming book, "Adaptive Strategies for Climate Change," Schlesinger and coauthor Robert Lempert argue that government policymakers need to be flexible over time with schemes to control global warming.

"The one thing we know for sure about forecasts is that most of them are wrong," Schlesinger and Lempert say. "We cannot now, nor are we likely for the foreseeable future, answer the most basic questions, such as, is climate change a serious problem and how much would it cost to prevent it?"

Rather than knee-jerk, hasty control schemes based upon limited knowledge and today's technologies, policymakers should instead take advantage of future, advanced technologies that could reduce human-caused greenhouse emissions at lower costs.