Solar energy distribution

Diurnal and seasonal variations in solar intensity are, of course, of utmost importance to ecosystems. In the extreme polar regions there is no direct solar radiation at all for more than four months of the year, whereas near the equator the overall intensity of sunlight fluctuates less than 10% annually. The spectral energy distribution also varies with the season. For example, in July in the middle latitudes (ca. 40 ), the fraction of shorter-wave UV (290-315 nm) in the total solar radiation is more than three times higher than it is in December, due to the shorter path these easily scattered wavelengths have to traverse through the atmosphere. For similar reasons, shortwave UV is more intense at high elevations, particularly in the tropics where stratospheric ozone is less concentrated (Caldwell et al., 1980). 

Because several constituents of the atmosphere (O3, O2, CO2, and water) have 

Tropical deforestation also has been implicated in global CO2 increases, since when trees are cut down, the succeeding grassy or weedy plant community can remove only a small fraction of atmospheric CO2, compared to the forest. Furthermore, much of the wood obtained is burned, adding to the CO2 burden directly. 

In addition, stratospheric ozone concentrations appear to be declining, due to photolysis by shortwave UV of volatile chlorine-containing compounds such as CFCI3 and its relatives (Reaction 1.48) the Cl atoms produced in this reaction scavenge O atoms (Reaction 1.49) and 

These compounds reach the stratosphere because they are so unreactive in all tropospheric processes, including reactions with HO- to which they are virtually inert. Measurements of atmospheric ozone in Antarctica (Stolarski et al., 1986), in the Arctic (Zurer, 1990), and even in the temperate latitudes (Watson et al., 1988) all point to a decrease in stratospheric ozone concentrations. Recent attempts to diminish the use of ozone-depleting compounds on a global basis appear to have been successful however, only minor effects on the rate of ozone destruction are likely to be observed for many years to come. 

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The solar energy distribution (-------) is also shown. Photoresponses and solar... 

Fig. 9. Absorption spectra of chlorophylls a and b, p-carotene, phycocyanin and phycoerythrin the overlaid spectrum (thick-line) is that of solar-energy distribution at earth s surface.

Figure 4-12. Appearance of a characteristic layer due to absorption of solar energy. Distribution obtained by Chapman theory for different values of the solar zenith angle.

Fig. 1. Solar energy distribution at noon in midsummer in Washington D.C. [reproduced with permission from Ref. 1 ].

Fig. 2. The sea surface solar energy distribution and typical energies for bond dissociation.

M. P. Thekaekara, "Survey of Quantitative Data on the Solar Energy and its Spectral Distribution," Conference of COMPEES, Dahran, Saudi Arabia, Nov. 1975. 

The spectral distribution of this radiation is given in Table 4.3, from which we can easily see that radiation with wavelengths below 150nm represents only a tiny fraction of the total. The energy distribution of the solar radiation corresponds to that from a black body with a temperature of around 5,000 K. 

Fig. 40 Energy distribution of solar radiation (according to CIE, No. 20) and filtered xenon arc light (Xenotest 1200) [102].

The sun s total radiation output is approximately equivalent to that of a blackbody at 10,350°R (5750 K). However, its maximum intensity occurs at a wavelength that corresponds to a temperature of 11,070°R (6150 K) as given hy Wien s displacement law. A figure plotting solar irradiance versus spectral distribution of solar energy is given in Fig. 9. See also Solar Energy. 

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