The scattering and absorption processes that take place within the atmosphere not only reduce the intensity, but also change the spectral distribution of the direct solar beam. The lowest curve in Fig. 2.1 shows the spectral distribution of solar irradiance at sea level for a zenith Sun and a clear sky. The shaded areas represent absorption, and so the curve forming the upper boundary of these shaded areas corresponds to the spectral distribution as it would be if there were scattering but no absorption. It is clear that the diminution of solar flux in the ultraviolet band (0.2-0.4 mm) is largely due to scattering, with a contribution from absorption by ozone. In the visible/ photosynthetic band (0.4-0.7 mm), attenuation is mainly due to scattering, but with absorption contributions from ozone, oxygen and, at the red end of the spectrum, water vapour. In the long, infrared tail of the distribution, scattering becomes of minor importance and the various absorption bands of water vapour are mainly responsible for the diminution in radiant flux.
The proportion of infrared radiation removed from the solar beam by absorption during its passage through the atmosphere is variable since the amount of water vapour in the atmosphere is variable. Nevertheless it is generally true that a higher proportion of the infrared is removed than of the photosynthetic waveband. As a consequence, photosynthetically available radiation (0.4-0.7 mm) is a higher proportion of the solar radiation that reaches the Earth's surface than of the radiation above the atmosphere. Photosynthetically available radiation constitutes about 45% of the energy in the direct solar beam at the Earth's surface when the solar elevation is more than 30 °.930 Skylight, consisting as it does of scattered and therefore mainly short-wave radiation, is predominantly in the visible/photosynthetic range.
Using the best available data for the spectral distribution of the extraterrestrial solar flux, Baker and Frouin (1987) have carried out calculations of atmospheric radiation transfer to estimate the PAR (in this case taken to be 350 to 700 nm) as a proportion of total insolation at the ocean surface, under clear skies, but with various types of atmosphere, and varying Sun angle. They found Ed(350-700 nm)/Ed(total) for all atmospheres to lie between 45 and 50% at solar altitudes greater than 40 The ratio increased with increasing atmospheric water vapour, because of a decrease in Ed(total). It was essentially unaffected by variation in ozone content, or in the aerosol content provided the aerosol was of a maritime, not continental, type. It was little affected by solar altitude between 40 ° and 90 °, but decreased 1 to 3% as solar elevation was lowered from 40 ° to 10
Since the efficiency with which the photosynthetic apparatus captures light energy varies with wavelength, the usefulness of a given sample of solar radiation for primary production depends on the proportion of different wavelengths of light present, i.e. on its spectral distribution. Figure 2.3 shows data for the spectral distribution of solar irradiance at the Earth's surface under clear sunny skies, within a few hours of solar noon at three different locations. As the solar elevation diminishes, the ratio of short- (blue) to long- (red) wavelength light in the direct solar beam decreases because of the intensified removal of the more readily scattered, short-wavelength light in the longer atmospheric path. On the other hand, as solar elevation diminishes, the relative contribution of skylight to total irradiance increases, and skylight is particularly rich in the shorter wavelengths: there is therefore no simple relation between solar elevation and the spectral distribution of total irradiance.
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