Black Carbon Soot Pollution May Offset Diesel Engine Benefits
WASHINGTON - According to testimony provided by five scientists before the US House Committee on Oversight and Government Reform in late October, black carbon, contained in soot from the combustion of biomass and fossil fuels, may be the second-most significant global warming pollutant after carbon dioxide, just ahead of methane. 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.
"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 percent of net global warming," said Dr. Mark Z. Jacobson, Prof. of Civil and Environmental Engineering, Atmosphere/Energy Program, Stanford 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.
In sum, there is not an advantage and a potential disadvantage of diesel versus gasoline in terms of climate and air pollution impact, Jacobsen concluded. 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, such as wind, solar geothermal, hydroelectric, wave, or tidal power.
Full testimony video and transcripts are available at
http://oversight.house.gov/story.asp?ID=1550
(Source: Committee on Oversight and Government ReForm)
