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Black carbon is a major component of soot and is produced by incomplete combustion of fossil fuel and biomass. It is emitted from various sources including diesel cars and trucks, residential stoves, forest fires, agricultural open burning and some industrial facilities. It has a warming impact on climate 460-1500 times stronger than CO2. Its lifetime varies from a few days to a few weeks. When deposited on ice and snow, black carbon causes both atmospheric warming and an increase of melting rate. It also influences cloud formation and impacts regional circulation and rainfall patterns. In addition, black carbon impacts human health. It is a primary component of particulate matter in air pollution that is the major environmental cause of premature death globally.
Black carbon is always emitted with co-pollutant particles, such as organic carbon and sulphates, which can have a neutral or even cooling effect on the climate. The ratio of black carbon to its co-pollutants varies depending upon the emission source and fuel-type, and impacts whether the source has a net-positive or negative warming effect. For example, emissions from diesel engines have a high proportion of black carbon to cooling co-pollutants, whereas emissions from wildfires and the open-burning of biomass contain a more balanced ratio. It is important to take the net climate effect into account when assessing black carbon emission reduction measures.
Black carbon and co-pollutants make up the majority of PM2.5 air pollution, which consists of particles 2.5 micrometres or smaller in diametre (approximately 40 times smaller than a grain of table salt), and is the leading environmental cause of poor health and premature death. In 2010, household PM2.5 air pollution and ambient outdoor PM2.5 air pollution were estimated to have caused over 3.5 and 3.2 million premature deaths, respectively (Lim S. et al. 2012).
The main sources of black carbon include residential and commercial combustion and transport, which accounted for 80% of anthropogenic emissions in 2005 (UNEP & WMO, 2011). Other important sources include industrial processes and the burning of agricultural waste. There are also small sources such as fossil fuel extraction, large scale combustion (including power plants and industrial boilers) and open burning of garbage. New data also shows that kerosene lamps may be a significant source of black carbon (Jacobson A. et al. 2013). Important regional variations in emissions are expected in the coming decades, with decreases of up to half in North America and Europe due to mitigation measures in the transport sector and significant increases in Asia and Africa.