The uncertainties associated with the climate impacts of black carbon are due to our incomplete understanding on the mechanisms that dictate the distribution and properties of black carbon containing particles in the atmosphere and on snow and ice.

The main uncertainties are related to the following themes:

Black carbon emissions to the atmosphere

The largest uncertainties in global emissions are associated with BC emitted from Chinese coke making, residential wood combustion, industrial coal combustion, and on-road diesel emissions (Bond et al. 2004). Residential wood burning and transport emissions, specifically from diesel vehicles, have been identified as major sources of anthropogenic BC in Europe and they also are the most important sectors contributing to the emission uncertainties in the studied area.

Estimating the radiative forcing by black carbon requires information on the changes in black carbon emissions since the pre-industrial period. The largest uncertainty in this respect lies in the pre-industrial amount of biomass burning, which was by far the largest source of black carbon to the atmosphere until about the last century (Bond et al., 2007).

The major uncertainties in the radiative forcing by black carbon arise from our incomplete knowledge about:

  • the total amount of black carbon emitted to the atmosphere and its historical change
  • the size of emitted BC containing particles, and
  • the mixing of BC with co-emitted primary organic compounds. (Reddington et al., 2013).


Light absorption properties of black carbon in the atmosphere

The efficiency by which black carbon (BC) containing particles absorb solar radiation and heat the air depends on their mass absorption coefficient, MAC (unit: m2 per g of BC).

In atmospheric aerosol particle populations, the value of MAC depends both on the size of the black carbon particles and on how well they are mixed with other aerosol constituents, that is, to which extent black carbon resides in the same particles as sulphate, organic compounds and associated water. At present there is no general consensus on the mixing state of atmospheric black carbon (Cappa et al., 2012; Chung et al., 2012; Jacobson, 2012). This causes an uncertainty of up to a factor two in the value of MAC, and thereby in the direct radiative forcing of atmospheric black carbon.

From the measurement point of view, further uncertainties arise from the fact that black carbon is not the only light absorbing aerosol constituent in the atmosphere. In some locations, a significant portion of light absorption may be caused by the presence of mineral dust particles or light-absorbing organic particulate matter called brown carbon (Yang et al., 2009; Bahadur et al., 2012).


Vertical distribution of black carbon in the atmosphere

The radiative forcing by black carbon is very sensitive to its vertical distribution in the atmosphere. Uncertainties in the vertical profile of atmospheric black carbon, related to both model deficiencies and lack of measurement data, is one of the largest error sources in modeling black carbon radiative forcing (Schwarz et al., 2010; Samset et al., 2013).

The importance of black carbon vertical distribution is emphasized in Arctic areas. There, black carbon causes the strongest surface warming when being located in the lowest atmospheric layers, and a much weaker warming or even cooling when located higher up in the atmosphere (Flanner, 2013).


Black carbon removal from the atmosphere

The atmospheric concentrations and spatial distribution of black carbon depend crucially on how fast black carbon is removed from the atmosphere by dry or wet deposition. Dry deposition is the dominant removal pathway in the absence of clouds and rain, being efficient close to the ground or water surfaces. Wet deposition requires the presence of precipitating clouds, and is more efficient for liquid clouds compared with ice clouds.

From the modeling point of view, the single largest uncertainty factor is the rate at which particles containing black carbon become active cloud condensation nuclei in the atmosphere. Other related sources of uncertainty include how models treat ice nucleation of particles containing black carbon, their scavenging by precipitation and their dry deposition over different surface types (Koch et al., 2009; Vignati et al., 2010).

The uncertainties in black carbon removal processes are especially important when simulating the transport of black carbon to the Arctic and estimating the deposition of black carbon onto Arctic snow or ice (Liu et al., 2011; Browse et al., 2012; Sharma et al., 2013; Zhou et al., 2013).


Black carbon and clouds

Atmospheric black carbon influences cloud properties by acting as cloud condensation nuclei (CCN) or ice nuclei (IN), called indirect effects, and by affecting the atmospheric stability and thereby cloud cover, called the semi-direct effect. Regionally, the net radiative forcing resulting from the indirect and semi-direct effects may be positive or negative depending on the aerosol and cloud properties as well as atmospheric conditions.

The influence of black carbon on CCN causes usually a negative radiative forcing, the magnitude of which depends strongly on the compounds co-emitted with black carbon (Bauer et al., 2010; Chen et al., 2010; Koch et al., 2011). The radiative forcing via the influence of black carbon on IN has been estimated to be rather small, possibly negative, yet very uncertain (Gettelman et al., 2012). The radiative forcing by the black carbon semi-direct effect may be positive or negative depending mainly on the vertical position of black carbon with respect to clouds and cloud type (Koch and Del Genio, 2010). The semi-direct effect gives a big contribution to the total uncertainty in the black carbon radiative forcing.

The radiative forcing due to the interaction of black carbon with clouds is particularly uncertain in Arctic areas. This is partly because of the high fraction of ice and mixed-phase clouds in that region, and partly because of the general difficulties in modeling Arctic cloud properties (Vavrus et al., 2009; Shupe, 2011).


Black carbon on snow and sea ice

Black carbon deposited on snow or ice-covered surfaces causes a positive radiative forcing by decreasing the albedo of these surfaces. The uncertainty in this forcing is large, since it accumulates a big fraction of the uncertainties related to the various processes dictating black carbon concentrations in snow or ice-covered areas. These include (Flanner et al., 2009; Skeie et al., 2011; Goldenson et al., 2012):

  • black carbon emissions
  • the black carbon transportation efficiency into the Arctic or other snow-covered areas
  • the rate at which BC is deposited from the atmosphere in these areas, and
  • how the snow or ice surfaces are changing over the year


Black carbon forcing and climate response

The direct radiative forcing by black carbon aerosols has been estimated in several different ways. Many studies have taken into account fossil fuel and biofuel sources of black carbon, but left out black carbon originating from open biomass burning, while others attempt to take into account all the important black carbon sources. The observation-based estimates for the global direct black carbon forcing tend be higher than the model-based ones, with typical uncertainty ranges in both types of estimates being a factor more than five (Bond et al., 2013; Myhre et al., 2013).

The radiative forcing associated with aerosol-cloud interactions is very difficult to determine for black carbon separately from other aerosol constituents (link to Black carbon and clouds).

The radiative forcing caused by black carbon deposited on snow or ice has been estimated to be globally much smaller than the direct radiative forcing by black carbon, yet highly uncertain and potentially very important over snow-covered areas during the spring-summer period (Flanner et al., 2009; Goldenson et al., 2012; Bond et al., 2013).

Due to the localized heating by black carbon and associated feedbacks, the radiative forcing by black carbon alone does properly describe its climatic effects, including its influence on the surface air temperature. The relation between black carbon climate forcing and response is very complicated in Arctic areas, with black carbon vertical elevation playing a central role (link to Vertical distribution of black carbon in the atmosphere). For black carbon on snow or ice, a given forcing causes a surface air warming that is a factor 2–4 larger than a similar forcing by atmospheric carbon dioxide (Goldenson et al., 2012).




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