What is Black Carbon?
Black carbon (BC) is the most strongly light-absorbing component of particulate matter (PM), and is formed by the incomplete combustion of fossil fuels, biofuels, and biomass.BC is emitted directly into the atmosphere in the form of fine particles (PM2.5). BC is the most effective form of PM, by mass, at absorbing solar energy: per unit of mass in the atmosphere, BC can absorb a million times more energy than carbon dioxide (CO2). BC is a major component of “soot”, a complex light-absorbing mixture that also contains some organic carbon (OC).
Where Does It Come From?
In 2005, the United States is estimated to have emitted about 5.5 million tons (or about 5,000 Gigagrams, Gg) of primary PM2.5 of which about 0.64 million tons (12%) was BC and about 1.7 million tons (30%) was primary OC. The domestic emissions of 0.64 million tons represents about 8% of the world’s total BC emissions,making the United States the 7th largest global BC emitter (Lamarque et al., 2010).
Most U.S. emissions of BC come from mobile sources (52%), especially diesel engines and vehicles. In fact, 93% of all mobile source emissions came from diesels in 2005. The other major source domestically is open biomass burning (including wildfires), although residential heating and industry also contribute.
Total global BC emissions for 2000 are estimated to be about 7,600 Gg (about 8.4 million tons) for 2000. Asia, parts of Africa, and parts of Latin America (Central and South America) are among the regions emitting the largest amounts of BC. Sources in developing countries are substantially different than in the United States: mobile sources (19%) and open biomass burning (35%) represent a smaller portion of the global inventory, while emissions from residential heating and cooking (25%) and industry (19%) are larger.
BC emissions in the United States are projected to decline substantially by 2030, largely due to controls on new mobile diesel emissions. In fact, most developed nations have already made significant progress in reducing emissions of BC and other direct PM. Further reductions are expected to occur over the next several decades as existing regulations are fully implemented.
Overall BC emissions are likely to decrease globally in the next several decades, but this trend will be dominated by emissions reductions in developed countries and may be overshadowed by emissions growth in key sectors (transportation, residential) in developing countries, depending on growth patterns.