The burning of fossil fuels and biofuels due to human activities has greatly increased the amount of particular matter in the atmosphere. The major aerosol components are mineral dust, sea salt, sulfates, nitrates, black carbon (also termed soot) and particulate organic matter (POM). The natural aerosol species, mineral dust and sea salt, dominate the mass concentration in the atmosphere. On average they contribute 39 mg m-2 and 13 mg m-2 whereas the anthropogenic components, sulfate, POM and black carbon only contribute 3.9, 3.3 and 0.4 mg m-2 to the annual global average as deduced from 20 different global models (Kinne et al., 2006). So far, nitrate is not included in most models, because of its semi-volatile nature.

Optically, mineral dust and sea salt are less important because of their larger size. Thus, mineral dust and sea salt each contribute only as much to the aerosol optical depth as sulfate does (25%). Black carbon, which contributes only 3% to the optical depth, is the main aerosol type that absorbs solar radiation and can lead to a warming of the surrounding air. This warming can prevent cloud formation because the atmosphere becomes more stable or even lead to an evaporation of cloud droplets (e.g., Koren et al., 2004). This so-called semi-direct effect thus counteracts some of the negative aerosol forcings from scattering aerosols, such as sea salt and sulfate, at the top-of-the atmosphere (TOA) (e.g., Lohmann and Feichter, 2005).

Aerosols also act as condensation centers for cloud droplets and ice crystals, thereby changing cloud properties. If more aerosol particles compete for the uptake of water vapor, the resulting cloud droplets do not grow as large. More smaller cloud droplets have a larger surface area than fewer larger cloud droplets for the

Cloud albedo effect

Lifetime effect

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