ATMOSPHERIC LIGHT-ABSORPTION - A REVIEW

The atmosphere interacts both with incoming as well as outgoing light. Two main processes take place: light scattering and light absorption. Whereas light scattering redistributes any light energy in the atmosphere, light absorption converts the light energy to internal energy of the absorbing molecules and eventually transfers it to the surrounding gas as heat. Atmospheric gases absorb light in distinct spectral regions usually at more of less broad bands. Best known is the broad absorption of ozone in the far u.v., being essential for the existence of the biological macromolecules on Earth. Gases known as greenhouse gases, e.g. CO2, CH4, N2O and water vapor absorb a wide range of infrared radiation, and thus are responsible for the greenhouse effects. Since the lifetime of these gases (except water vapor) is considerable, their distribution around the globe is fairly homogeneous. The atmospheric aerosol gives the major contribution to the atmospheric light absorption in the visible and near u.v. and near i.r. Graphitic (black) carbon, the main constituent of soot, is almost exclusively responsible for the light absorption of the particles. The light absorption by aerosols is continuous and covers the whole visible spectral range. It only depends slightly on wavelength. The optical properties of elemental carbon are determined by the size of the particles and their complex refractive index. A variety of refractive indices can be found in the literature for elemental carbon, most likely caused by different production and thus composition of the particles. Soot particles are very efficient in attenuating light; for the average size the particles have more than twice the mass extinction coefficient compared to transparent particles such as ammonium sulfate. The light absorption coefficient of a mixture of elemental carbon and transparent materials is higher for internal than for external mixtures. When incorporated into transparent particles, the absorption properties of elemental carbon can be multiplied and the single scattering albedo will decrease in comparison to an external mixture of the same materials. There are different methods to measure the light absorption coefficient of suspended particles. They can be separated in three groups, depending on the effect or methodology they use, but no standard procedure has been adopted so far. Soot is produced by all combustion processes. Since most fires on Earth are due to humans, then indirectly humans are the major source of light-absorbing aerosol particles. On a global scale black carbon amounts to 1.1-2.5% of the anthropogenic particles and to 0.2-1% of the total emitted particles. The emission factors for elemental carbon are highest for small sources such as diesel motors or fireplaces. The light-absorbing aerosol consists of fine particles, with most particles having diameters less than a few tenths of a micrometer. Particles in the size range of soot particles have an average lifetime of 7 days in the atmosphere, therefore they can be transported over large distances and are also found in remote regions. Since light-absorbing particles have a variety of sources and sinks and they are involved in precipitation cycles, their distribution on the globe is inhomogeneous. Light-absorption coefficients of the atmospheric aerosol reported in the literature differ by more than four orders of magnitudes at different locations, but nevertheless black carbon particles have been found even at very remote areas, such as the South Pole. Although light-absorbing particles are a minority component in the atmospheric aerosol, their effects cannot be neglected: since the mass extinction coefficient of soot is higher by a factor of two to three compared to transparent particles, light-absorbing substances in the atmosphere can cause at some locations up to half of the visibility reduction; light-absorbing substances in the atmosphere can be responsible for the brown appearance of urban hazes and the discoloration of the sky. The light absorption of the atmosphere in the visible (which mainly is due to particulate matter) has to be taken into account when considering radiative properties and climatic consequences. A small temperature increase due to absorption in the visible is to be expected. The value is around a few tenths of a Kelvin, but no general statement on its magnitude is possible, since a large spatial and temporal variation exists and other factors like surface albedo, the optical depth of the aerosol, its incorporation in clouds and humidity growth of the aerosol have to be considered.