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Black Carbon May be Second-Most Significant Global Warming Pollutant After Carbon Dioxide; Alters Picture of Diesel Engine Benefits

Primary contribution to observed global warming since 1750. Click to enlarge. Source: Testimony of Dr. Jacobson

Black carbon—contained in soot from the combustion of biomass and fossil fuels—may be responsible for around 16% of the gross warming the planet is currently experiencing and may be the second-most significant global warming pollutant after carbon dioxide and ahead of methane, according to testimony provided by five scientists before the US House Committee on Oversight and Government Reform in October.

Because of their increased fuel efficiency relative to gasoline-engined vehicles, diesels are seen as an improvement overs gasoline vehicles with respect to global warming issues. However, once soot warming is factored in, the difference between the two platforms is greatly reduced, as diesel emits more soot than gasoline. 

Testifying before the committee were:

  • Dr. Mark Z. Jacobson, Prof. of Civil and Environmental Engineering, Atmosphere/Energy Program, Stanford University

  • Dr. Tami C. Bond, Asst. Prof. of Civil and Environmental Engineering, University of Illinois at Urbana-Champaign

  • Dr. V. Ramanathan, Prof. of Climate and Atmospheric Sciences, Scripps Institute of Oceanography, University of San Diego

  • Dr. Charles Zender, Assoc. Prof. of Earth System Science, University of California at Irvine.

  • Dr. Joel Schwartz, Professor of Environmental Epidemiology, Harvard University

The black carbon in soot performs its warming by absorbing sunlight, converting it into infrared (heat) radiation, and emitting that heat radiation to the air around it.  Soot on the surface of snow and sea ice contribute to both the melting of those surfaces as well as the warming of the air.

Because of the relatively short lifetime of soot in the atmosphere compared to greenhouse gases, control of soot may be the fastest method of slowing warming for a specific period, according to Dr. Jacobson. 

Black carbon, noted Dr. Bond, adds 2-3 order of magnitude more energy to the climate system than an equivalent mass of CO2 because black carbon is an extremely good absorber of visible light. While carbon dioxide stays in the atmosphere for decades, it absorbs just a small amount of infrared radiation.

Particles from burning biomass (which differ from biofuel particles) are less oily and contain a much lower black carbon fraction than fossil fuel soot particles, according to Dr. Jacobson. Biomass-burning particles thus tend to cool climate on a global scale (although the biomass-burning gas warming exceeds its global cooling due to permanent deforestation. The panel thus focused on soot particles resulting from the combustion of fossil fuels and biofuels.

...fossil fuel plus biofuel soot may contribute to about 16% of gross global warming (warming due to all greenhouse gases plus soot plus the heat island effect), but its control in isolation could reduce 40% of net global warming.

—Dr. Jacobson

Methods proposed to control fuel soot include improving engines; switching fuels; adding particle traps; and changing vehicle technologies.

In sum, there is not an advantage and a potential disadvantage of diesel versus gasoline in terms of climate and air pollution impact. However, neither type of vehicle is satisfactory or useful for solving climate and health problems as the emissions from both are very high. Even modest improvements in mileage standards for all vehicles are beneficial, but will only delay the eventual increase in emissions due to a larger population.

A more certain method of reducing global arming caused by both fossil-fuel soot and carbon dioxide is to convert vehicles from fossil fuels to electric, plug-in hybrid or hydrogen fuel cell vehicles, where the electricity or hydrogen is produced by a renewable energy sources [sic], such as wind, solar geothermal, hydroelectric, wave, or tidal power.

—Dr. Jacobson

(A hat-tip to Green Car Congress reader MR!)



Rafael Seidl

@ Harvey D -

engine-out emissions from small diesels won't be clean enough, though treating the exhaust gas would be cheaper than for large units. Given EPA and CARB emissions regs plus still-low gasoline prices throughout the US, manufacturers will surely stick with spark ignition powerplants for at least the first wave of PHEVs sold there. Btw, there is little value in running a PHEV genset at a constant speed, because it doesn't have to feed a grid with a constant AC frequency. Better to let it run at whatever speed it can most efficiently deliver the required power level.

Fwiw, I think the best genset for a series hybrid PHEV would be a hermetically sealed stirling engine with four phase-shifted free pistons and integrated linear alternators. The package could be flat enough to fit in the vehicle underbody, cp. GM's Hy-Wire skateboard car minus the hydrogen nonsense. A well-designed Stirling will run cleanly and efficiently on many different fuels. The intrinsically poor dynamic response is irrelevant in a series hybrid PHEV setup. The poor specific power would be more of an issue, unless the engine becomes part of the load-bearing structure.

For fairly large and heavy vehicles (e.g. over 3750lbs) whose owners need to cover a lot of freeway/rural highway miles each year, straight T2B5 diesels are probably a better choice than PHEV drivetrains. To keep diesel-related emissions low in urban/suburban traffic, a beefy start-stop system (cp. Valeo STaRS-X) could be added.


@ ananomous

With DPF – yes, lower PM emissions than from gasoline engines. Did you bother to look at the links I provided in my first post?

Here are a few more:

“…Some vehicles with SI engines approach diesel emission levels at relatively moderate specific torque….”

Emissions of Ultrafine Particles from Different Types of Light Duty Vehicles, Swedish National Road Administration

“…~200 fold reduction in soot emissions during steady state operation [with ULSD]…
…DPF effective over entire PM size range…”

Measuring motor vehicle PM emissions: Current issues and future, Matti Maricq

“…Filters are considered effective enough such that tailpipe emissions are less than ambient levels in laboratory…”

Tim Johnson (Corning), Symposium on Particulate Matter, August 12-13, 2003

“…trap-equipped diesels emit a lower particle number than even gasoline vehicles…”

Axel Friedrich, Umweltbundesamt, Symposium on Particulate Matter

Even the USEPA is projecting PM2.5 emissions from on-road diesel to drop from 100K tons/year nationally in 2001 to 10K tons/year in 2030 (reduction of 90%) just from the regulations currently promulgated, while emissions from on-road gasoline will RISE from 50K tons/year in 2001 to 57K tons/year in 2030. That’s almost six times as much PM from gasoline than from diesel, in spite of the fact that only about 3 times as much gasoline (by volume) is consumed in the U.S. than diesel fuel (i.e., gasoline is projected to produce about twice as much PM per unit volume of fuel consumed).

If you don’t think gasoline engines produce any PM, maybe you should consider taking your own advice.

If you don’t think gasoline engines produce any PM

Didn't say that. Try again.

Long-term projections based on speculative technology mean nothing in reality. Here in reality diesel puts out more PM than gasoline. Even a child knows this.

Put down your crack pipe.


Your "put down your crack pipe" cliche is getting boring. Try something else.

Didn't say that. Try again.

OK - I'll try again. If you think gasoline vehicles have less of an adverse environmental impact than diesel vehicles, especially T2B5 diesel vehicles, you need to take your own advice.

DPF is hardly "speculative". They've been used in ALL diesel vehicles (heavy-duty, medium-duty, light-duty) in the U.S. since at least January 1, 2007, and they've been used in Europe in Peugeot diesel vehicles since MY 2000, apparently with considerable success:

"…Prof. J. Czerwinski, University of Applied Science Biel, reported on minutely done evaluations of the filter system (Faurecia), installed in the Peugeot 607 Diesel passenger car. Somewhat surprising, and from the viewpoint of measuring even an unexpected challenge, was the fact that the exhaust contained less aerosols than the ambient air..."

As far as long-range projections, Toyota (not exactly a light-duty diesel proponent) graphically depicts that the effectiveness of a DPF actually increases with time/age ( - slide #26). Furthermore, 2007+ diesel heavy-duty diesel trucks have to be certified for 435,000 miles in the U.S. They wouldn't be able to do that if the DPF wasn't fully functional for at least that long.

Here are a few more for your viewing pleasure:

"…The PM conversion [with DPF] was close to 100% and was beyond the detection capability of the measurement system at ANL…."

Argonne National Laboratory

"...You are reducing particles of all sizes by magnitudes over gasoline engines. We have come to a level that is similar to what comes from air conditioners in the room...."

And of course there are those pesky empirical source apportionment studies that show that GASOLINE is responsible for more ambient PM2.5 than diesel, almost 3 times as much is some cases (Northern Front Range Air Quality Study). And most of these studies predate the use of DPF and ULSD.

Check out slide #2 of the results of a Washington, D.C., study, in this presentation:

That 2 percent of ambient PM2.5 attributed to diesel is similar to a study by the University of Tennessee of sources of ambient PM2.5 in large southeastern U.S. cities (e.g., Atlanta) being attributed to diesel as 3 percent or less (Seasonal Distribution and Modeling of Diesel Particulate Matter in the Southeast US).

Your "even a child knows this" argument is unconvincing. Children often believe in the tooth fairy too. Doesn't make it so. Your perception doesn't necessarily equal reality.

fred schumacher

The study appears to refer to biomass burning primarily from a deforestation perspective. Tree biomass is primarily above ground. Perennial grass biomass is primarily below ground. Perennial grass plants sequester carbon in their root system. That is how deep prairie soils were created. The study does have a member from the University of Illinois, but there is no reference in her testimony to the miscanthus study at UI which showed 4 tons per acre annual carbon sequestration. It doesn't appear this was taken into account in the analysis.


The anonymous poster using the reference to smoking crack has succeeded in egging some very useful documentation from you, Carl. Thank you for not stooping to their level of immaturity.


Hi! Gnu here!



Thanks for the informative posts. I only know of one GCC poster who has claimed to have "never lost an argument on the internet." I wonder if he's the anonymous poster, who now ignores your factual arguments and says you're wrong ... and the reason given is that he is right.


Hi five me Carl! Now down low! Now real... slow...


"Hi five me Carl! Now down low! Now real... slow..."

Obviously anonymous wanted to comment on my post, and did so in my name. Anonymous makes sarcastic comments all the time; I have yet to make one on GCC.

But I can't possibly get upset today. I spent yesterday test driving cars, and bought a 2008 Prius. I already love it. You can buy one now with a pretty good discount off the MSRP. That means it's becoming a mainstream car -- no more waiting lists. You gotta love that.

If my name appears again with some stupid sarcastic comment you'll all know it wasn't me. OK, anonymous, have at it!


Im very important.


This is all very confusing for an average american consumer trying to do the right thing. I was considering selling my 2003 toyota corolla and getting an early eighties mercedes diesel- converting it to run on svo/wvo/biodiesel. After reading this info, that seems like a bad idea. I can't afford a Prius, and I ride my bike to work about 3 days a week, but I'm not ready to give up the motorized transport just yet.



A 2003 Corolla is a very good car, from both environmental and economic perspectives. New cars are fun, and diesel from svo/wvo sounds interesting. But if it's your only car why not just keep the Corolla until something that's definitely better, and that you can afford, comes out. Auto technology is improving so fast now compared to previous decades that it just might pay to wait.


Soot is HEAVIER than air.. Diesels "use to" emit a lot of soot in "older" vehicles. but they emit "less" emissions in the atmosphere than a gas engine...Also..When are manufactures going to build a "smaller" diesel engine and put it into a small or midsize truck in the U.S.A...Are these engineers "sleeping"...

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