Figure 4.9 shows the solar irradiance at the top of the atmosphere and that at sea level. The absorption spectra are quite complex, but they do indicate that absorption is so strong in some spectral regions that no solar energy in those regions reaches the surface of the Earth. As we will see, absorption by 02 and 03 is responsible for removal of practically all the incident radiation with wavelengths shorter than 290 nm. On the other hand, atmospheric absorption is not strong from 300 to about 800 nm, forming a "window" in the spectrum. About 40% of the solar energy is concentrated in the region of 400-700 nm. Water vapor absorbs in a complicated way, and mostly in the region where the Sun's and Earth's radiation overlap. From 300 to 800 nm, the atmosphere is essentially transparent. From 800 to 2000 nm, terrestrial longwave radiation is moderately absorbed by water vapor in the atmosphere.
Why molecules absorb in particular regions of the spectrum can be determined only through quantum chemical calculations. In general, the geometry of the molecule explains, for example, why HzO, C02, and 03 interact strongly with radiation above 400 nm, but N2 and 02 do not. In H20, for instance, the center of the negative charge is shifted toward the oxygen nucleus and the center of positive charge toward the hydrogen nuclei, leading to a separation between the centers of positive and negative charge, a so-called electric dipole moment. Molecules with dipole moments interact strongly with electromagnetic radiation because the electric field of the wave causes oppositely directed forces and therefore accelerations on electrons and nuclei at one end of the molecule as compared with the other.
Absorption spectrum for 02 and 03
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