Black carbon particles, the major constituent of soot, absorb solar radiation and thus in the air heat the atmosphere. In addition, when deposited on snow and ice they darken the surfaces and accelerate melting. Black carbon originates from incomplete combustion of fossil fuels, biofuels, and biomass.
Characteristics of black carbon
Black carbon (BC) is formed during combustion of hydrocarbon-based fuels. The strong absorption of visible light by black carbon is its most important characteristic for climate. In atmospheric aerosols there are no other constituents that absorb light with such strong light absorption per unit mass (Bond et al., 2013).
Black carbon can be identified from other forms of carbon in atmospheric aerosol by a unique combination of the following physical properties (e.g., Quinn et al., 2011; Bond et al., 2013):
- It strongly absorbs visible light – that makes it black.
- It is refractory; in other words it is stable still at very high temperatures, up to almost 4000K.
- It is insoluble in water, in many organic solvents including methanol and acetone.
- Its morphology varies: close to the sources it exists as agglomerates of small carbon spherules but the aggregate-like structure changes rapidly after emission due to collapsing into densely packed clusters and coating with some condensable low-vapour-pressure vapour (Martins et al. 1998., Adachi et al., 2010). The coating and morphological changes have important effects on optical properties affecting climate (e.g., Moosmüller et al. 2009).
The definition of black carbon is not unambiguous, the term has not been used consistently in the scientific literature. Good definitions and comprehensive explanations can be read, e.g., in Bond and Bergstrom (2006), Andreae and Gelenscér (2006), Moosmüller et al. (2009), and Bond et al. (2013).
Photo: Flickr, Zakysant
