Solar radiation aerosol scattering

A non-uegligible fraction of the solar radiation incident on the earth is lost by reflection from the top of the atmosphere and tops of clouds back into outer space. For the radiation penetrating the earth s atmosphere, some of the incident energy is lost due to scattering or absorption by air molecules, clouds, dust and aerosols. The radiation that reaches the earth s surface... 

On the other hand, aerosol particles from anthropogenic activities tend to be concentrated over or near industrial regions in the continents. Because both the direct and indirect effects of particles are predominantly in terms of scattering solar radiation, their effects are expected primarily during the day. 

The value of 10 is determined by molecular and particulate (cloud and aerosol) scattering, and surface reflection. A small fraction of the molecular scattering is the non-conservative Rotational Raman scattering (RRS) that partially fills the solar Fraunhofer lines in the scattered radiation, creating what is commonly known as the Ring effect [15] As a result, the ratio Iq/F, where F is the extraterrestrial solar flux, contains structure that is correlated with solar Fraunhofer lines. By separating these effects, one can write... 

Furthermore, the absorption of incoming solar radiation by BC is not only related to its concentration, but also depends on its location in the aerosol particle (Kaufman et al., 2002). Absorption can be two to three times stronger if the BC is located inside the scattering particle (Haywood and Boucher, 2000 Kaufman et al.,... 

A second characteristic of UV solar radiation is fhat, even for very clear afmospheres, if is composed in similar amounfs of both beam and diffuse radiation (Hulstrom et al., 1985). The first is defined as the radiation arriving directly from the sun, while the second is the solar radiation that has been scattered by gases and aerosols after entering the earth s atmosphere. This second t)/pe of radiafion reaches fhe ground in a more or less diffuse manner that is, with similar intensity from all directions in the sky. In this respect, the situation encountered in solar photocatalytic reactors is quite different from fhe one encounfered in solar fhermal collectors. The latter are able to use the whole solar spectrum, and in that case diffuse radiation accounts for a much smaller fraction of fhe global irradiance. 

The modifications of the radiative field induced by the presence of aerosols, if sufficiently large, may have a detectable effect on the climate system. Aerosols absorb and scatter solar radiation as a result there is an attenuation of the direct solar field, on whose basis it is generally assumed that the energetic input to the underlying system is reduced. 

Since however a large fraction of the scattering is in the forward direction, the depletion of solar radiation reaching the ground is not as large as expected merely on the basis of extinction. In addition the presence of the absorbing aerosol modifies the infrared characteristics of the atmosphere. The net effect depends on the size and refractive index of the particles. 

The role of aerosols in climate can be divided into two types direct and indirect. In the direct effect the particles absorb and scatter energy coming from the sun back to space. This tends to cool the atmosphere since solar radiation, which, in the absence of the aerosols, would warm the air, is now partially absorbed by the particles or reflected upwards out of the atmosphere. 

GU/p) represents the extinction over ail wavelengths between A. and per unit volume of aerosol in the size range between and dp + d(dp). It is independent of the particle size distribution function. For a refractive index, m = 1.5, G(dp) has been evaluated for the standard distribution of solar radiation at sea level, using Mie scattering functions. The result is shown in Fig. 5.8 as a function of particle size. 

It was mentioned in Chapter 4 that aerosol particles scatter and absorb solar radiation. These processes depend upon the concentration, size distribution, form, refractive index and absorption coefficient of the particles, as well as upon the wavelength of the radiation. In the case of water-soluble particles the extinction is also controlled by relative humidity (see Section 4.5). The energy absorbed by particles leads to an increase of temperature, while backscattering produces an energy loss for the system. Sines this energy loss may be characterized by the albedo, it is proposed to examine first the relation between albedo and temperature in surface air. 

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