Attenuation of Solar Radiation in the Atmosphere

A-3 of the Appendix lists the numerical data. In the visible and near-infrared spectral regions the fluxes were mainly derived from ground-based measurements at mountain stations supplemented by aircraft and balloons as platforms. The ultraviolet portion of the solar spectrum has been explored with instruments on board rocket sondes and satellites. These data are of special interest here. Measurements before 1980 have been reviewed by Ackerman (1971), Simon (1978), and Nicolet (1981). More recent data have been reported by Mount and Rottman (1981,1983,1985) and Mentall etal. (1985). 

In those spectral regions where the attenuation of incoming solar radiation is due primarily to absorption, the intensity of the solar flux having penetrated to the altitude level z follows from the generalized form of Beer s law, 

Unlike absorption, scattering does not result in a loss of radiation. While the direct flux of incoming solar radiation is attenuated, the diverted light undergoes multiple scattering and is available as a diffuse component, which must be added to the direct flux. In addition, some radiation is reflected at the Earth surface (or from clouds, although clouds are rarely considered). Thus, the total flux may be written 

A further complication of an already difficult subject is caused by the presence of clouds in the troposphere. Small clouds such as fair-weather cumuli produce much forward scattering so that the situation differs little from that for a clear sky. An extended cloud cover reduces photochemical activity considerably, however. The modification of the actinic radiation field by clouds, both inside and outside of the clouds, will be an important research subject of the future. 

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Part of the solar radiation entering the earth s atmosphere is scattered and absorbed by air and water vapor molecules, dust particles, and water droplets in the clouds, and thus the solar radiation incident on earth s surface is less than tlie solar couslanl. The extent of the attenuation of solar radiation depends on the length of the path of the rays through the atmosphere as well as the composition of the atmosphere (the cloud.s, dust, humidity, and smog) along the path. Most ultraviolet radiation is absorbed by the ozone in the upper atmosphere. At a solar altitude of 41.8°, the total energy of direct solar radiation incident at sea level on a clear day consists of about 3 percent ultraviolet, 38 percent visible, and 59 percent infrared radiation. 

From the foregoing parts of this book it is clear that solar radiation in the stratosphere is primarily attenuated by ozone (see Subsection 3.4.3) and at a lesser extent by the stratospheric sulfate aerosol layer (see Subsection 4.4.3). This means that any change in the stratospheric 03 burden or aerosol concentration involves modification of radiative transfer in this atmospheric domain. We should remember that the residence time of trace constituents above the tropopause is rather long because of the thermal structure and the absence of wet removal. Furthermore at these altitudes the density of the air is low as compared to that of lower layers. For this reason even an insignificant quantity of pollutants can produce relatively long and significant effects. 

We are now going to calculate the attenuation of direct (directional) solar radiation through scattering and absorption in the atmosphere. The atmosphere is assumed to be cloudless for details on the complicated effect of clouds, [5.34] is suggested. We will consider a bundle of rays that goes through an optically turbid, namely absorbent and scattering medium, Fig. 5.42. The reduction dLx of its spectral... 

In the solar occultation method a space-borne detector (e.g., photomultiplier tube, photolytic array detector, Fourrier transformed interferometer) points towards the Sun, and during brief periods (sunrise, sunset), when the optical path penetrates into the atmosphere (see Figure 4.19a), measures the attenuation of the solar radiation by the absorbing compounds. The intensity at frequency v detected by the spacecraft is... 

The magnitude of the solar heating is indicated by the so-called solar constant. In space, at the radius of the earth s orbit, the solar constant is about 443 Btu/(h)(ft2) (1396 W/m2). However, solar radiation is attenuated by passage through the atmosphere it is also reflected diffusely by the atmosphere, which itself varies greatly in composition. Table 7.2 provides representative values of the solar constant for use at ground level, as well as of the apparent daytime temperature of the sky for radiation purposes. 

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