Black carbon

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Learn more from the Center for Media and Democracy's research on climate change.

Black carbon, also called soot, arises from sources such as diesel engine exhaust, burning biomass, cooking fires, and coal plants. It is made up of tiny carbon particulate matter that contributes to global warming by absorbing heat in the atmosphere and reducing albedo, the reflection of sunlight, when deposited on snow and ice. It is also a big component of air pollution around the world.[1]

Climate change

In a paper published in May 2008 in Nature Geoscience, Carmichael and Ramanathan found that black carbon soot from diesel engine exhaust and cooking fires -- widely used in Asia -- may play a larger role than previously thought in global warming. They said that coal and cow dung-fueled cooking fires in China and India produce about one-third of black carbon; the rest is largely due to diesel exhaust in Europe and other regions relying on diesel transport. The largest source of black carbon is the burning of biomass, especially forests and grasslands.[1]

A 2010 USAID study identified black carbon as the second or third largest contributor to the current anthropogenic global warming, surpassed only by carbon dioxide and methane. Black carbon, however, has a much shorter average atmospheric residence time than carbon dioxide. It has been found that one kg of black carbon heats the atmosphere 500 to 680 times more than one kg of carbon dioxide over a 100-year time frame and 1,500 to 2,200 times over a 20-year time frame.[2] The report also considered black carbon an agent for faster melting of ice in the Himalayas: "Black carbon directly heats the surface on which it is deposited and accelerates the melting of the Arctic sea and land ice, glaciers and seasonal snow covers. As per 2006 data, China is a dominant emitter of black carbon from combustion, accounting for 61 percent of all Asian emissions, followed by India at 12 percent and Indonesia at 6 percent."[2]

A four-year study published in the Journal of Geophysical Research in 2013 concluded that black carbon’s impact on the climate is larger than that of methane and roughly two-thirds that of carbon dioxide, making it the second largest contributor to climate change.[3]

Particulate matter regulations

The EPA Office of Air Quality Planning and Standards (OAQPS) sets National Ambient Air Quality Standards under the Clean Air Act for six principal pollutants, which are called "criteria" pollutants: sulfur dioxide, particulate matter, nitrogen oxides, ozone, lead, and carbon monoxide. After the EPA sets or revises each standard and a timeline for implementation, the responsibility for meeting the standard falls to the states. Each state must submit an EPA-approved plan that shows how it will meet the standards and deadlines. These state plans are known as State Implementation Plans (SIPs)." [4]

Since 1997 coarse (diameter greater than 2.5 μm) and fine (diameter between 0.1 μm and 2.5 μm) particles have been regulated by the EPA, but ultrafine particles (diameter less than 0.1 μm) remain unregulated.[5] Roughly 80% of the ash falls into an ash hopper, but the rest of the ash then gets carried into the atmosphere to become fly ash.[6]

A 2009 court ruling concluded that the EPA standards for the amount of soot permissible in the air on an annual average ignored the advice of scientific advisers by maintaining the standard established in 1997 and must be rewritten. That limit was 15 micrograms per cubic meter of air.[7]

In a motion filed on December 7, 2010, the EPA asked for an extension in the current court-ordered schedule for issuing the new rules, which would cut emissions of pollutants including mercury and soot. EPA is under court order to issue final rules on January 16, 2011, but is seeking to extend the schedule to finalize the rules by April 2012.[8]

On June 15, 2012, EPA proposed to lower standards for particulate matter to between 12 and 13 micrograms per cubic meter (μg/m3). The agency is also taking “public comment on alternative annual standard levels down to 11 μg/m3.”

Office of Management and Budget weaken EPA rule

According to documents obtained by The Washington Post, the White House modified the Environmental Protection Agency's proposal to limit soot emissions, inviting public comment on a slightly weaker standard than the agency had originally sought. The EPA had originally wanted to tighten the annual exposure to fine-particle soot from 15 micrograms per cubic meter of air to 12 micrograms per cubic meter, according to an e-mail between the Office of Management and Budget and EPA officials. But the OMB directed the EPA to make the limit between 12 and 13 micrograms per cubic meter of air.[9]

Final rule issued

In December 2012 the EPA issued its final soot rules, tightening the federal soot standards by 20 percent - the most protective measure laid out in its June 2012 draft rule (12 micrograms per cubic meter of air). The agency will determine which areas are out of attainment in 2014, and the communities will then have six years to comply. The EPA estimates that 66 of the nation’s 3,033 counties will be found in violation of the new standard. It projects seven — all in California — will still be out of compliance by 2020.[10]

Court orders stronger rule

On January 4, 2013, a three-judge panel of the U.S. Court of Appeals for the District of Columbia Circuit rejected EPA's argument that it is required under the Clean Air Act to use a less stringent implementation regime for fine particulates than it is for more coarse -- and less dangerous -- particles. The ruling means EPA must look again at the implementation of the existing standards. The panel ruled that the Clean Air Act's language specifically dealing with particulate matter is stricter than the EPA interpretation and gives states less leeway and flexibility in meeting federal requirements than EPA was allowing for.[11]



  1. 1.0 1.1 "Black Carbon Implicated in Global Warming" Science daily, July 30, 2010.
  2. 2.0 2.1 Ramesh Prasad Bhushal, "Black carbon, a major culprit for climate change: Study" The Himalayan Times, May 2, 2010.
  3. Jeff Tollefson, "Soot a major contributor to climate change: Black carbon could result in twice as much global warming as previously estimated," January 15, 2013.
  4. "NAAQS" Sierra Club, accessed July 2010.
  5. Nel, A. "Air Pollution-Related Illness: Effects of Particles.: Science, 308(5723), 804-806. (2005, May 6).
  6. Schobert, H. H. Energy and Society. New York: Taylor & Francis, 241–255. (2002).
  7. Juliet Eilperin, "EPA tightens soot rules by 20 percent," Washington Post, Dec. 14, 2012.
  8. "EPA Seeks New Timetable for Reducing Pollution from Boilers and Incinerators/Agency committed to developing rules that are protective, cost effective and based on sound science" EPA, Dec. 7, 2010.
  9. Juliet Eilperin, "White House weakened EPA soot proposal, documents show," Washington Post, July 17, 2012.
  10. Juliet Eilperin, "EPA tightens soot rules by 20 percent," Washington Post, Dec. 14, 2012.
  11. Lawrence Hurley, "Court orders EPA to try again on soot standards implementation," Greenwire, January 4, 2013.

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