9.3. Effects of atmospheric aerosols

Atmospheric aerosol particles play important role in radiation budget of the Earth’s as they scatter and absorb both shortwave solar radiation and longwave terrestrial radiation. They are also highly involved in the formation of clouds and precipitation since they operate as cloud condensation and ice nuclei (CCN and IN[22]). Aerosols can form the abundance and distribution of atmospheric trace gases by complex chemical reactions, and can affect significantly the cycles of nitrogen, sulphur, and atmospheric oxidants. Aerosol particles in the upper atmosphere, where the major part of atmospheric ozone forms, can modify the ozone removal (Mészáros, 2000). Additionally, particles are major elements of lower tropospheric air quality, and can influence harmfully the environment and human health.

Aerosols play important role in the balance of the Earth’s climate. Due to the increasing anthropogenic emission of aerosols since the industrial revolution, they can also effect the global climate change. However, the effects of aerosols on climate are not one-way, moreover excessively uncertain. The climate forcing by aerosols can be realized in two ways, basically: in direct and indirect radiative forcing.

9.3.1. Direct effects: direct radiative forcing due the scattering radiation.

Aerosol particles reflected a part of shortwave solar radiation back into the space, cooling the Earth’s atmosphere. This cooling effect of aerosols, especially by sulphate components may be compensated by the absorption of longwave terreastrial radiation primarily by elemental (black) carbon aerosols and dust particles. The global, annual mean radiative forcing still less certain and is estimated –0.4 ± 0.2 W m–2 for sulphate, –0.05 ± 0.05 W m–2 for fossil fuel organic carbon, +0.2 ± 0.15 W m–2 for fossil fuel black carbon, +0.03 ± 0.12 W m–2 for biomass burning, –0.1 ± 0.1 W m–2 for nitrate and –0.1 ± 0.2 W m–2 for mineral dust (IPCC, 2007). A large volcanic eruption can greatly increase the concentration of stratospheric sulphate aerosols, thereby increasing the negative radiative forcing. However, a single, large eruption can cool our atmosphere only for a few years.

9.3.2. Indirect effects: indirect radiative forcing through cloud formation effects

Aerosol particles can also affect the radiation balance by formation of cloud droplets. Cloud droplets are formed in the troposphere by condensation of water vapour onto aerosol particles (cloud condensation nuclei, or ice nuclei) when the relative humidity exceeds the saturation level. Without these particles, a very large supersaturation (about 400%) would be necessary for the homogeneous condensation of water vapour.

The properties and the number of particles can affect the formation and the characteristic of clouds and precipitation in many ways (Lohmann and Feichter, 2005). The increased number of aerosol particles, and therefore the increased cloud optical thickness decrease the net surface solar radiation. The more numerous smaller cloud particles reflect more solar radiation (called albedo effect or Twomey effect). Smaller particles decrease the precipitation efficiency, thereby prolonging cloud lifetime. The absorption of solar radiation by soot particles may cause evaporation of cloud particles (semi-direct effect). In mixed-phased clouds, smaller cloud droplets delay the beginning of freezing and decrease the riming efficiency. However, more ice nuclei increase the precipitation efficiency.

Anthropogenic aerosols effects on water clouds through the cloud albedo effect cause a negative radiative forcing of –0.3 to –1.8 W m–2 (IPCC, 2007).

References

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Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Solomon S., Qin D., Manning M., Chen Z., Marquis M., Averyt K.B., Tignor M., and Miller H.L.. (eds.). Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA. 996. ISBN 978 0521 88009-1.

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[22] Ice nuclei (IN): aerosol particles that can serve as nuclei of ice crystals.