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Wavelength (nm)

FIGURE 3.12 Solar flux outside the atmosphere and at sea level, respectively. The emission of a blackbody at 6000 K is also shown for comparison. The species responsible for light absorption in the various regions (03, H20, etc.) are also shown (from Howard et al, 1960).

Figure 3.13 shows the altitude corresponding to maximum light absorption by atomic and molecular oxygen and nitrogen and by 03 as a function of wavelength up to À = 300 nm with the sun directly overhead (Friedman, 1960).

Because of the presence of these absorbing species in the upper atmosphere, only light of À > 290 nm is available for photochemical reactions in the troposphere. It is often expressed as the integrated radiation coming from all directions to a sphere and is referred to as actinic radiation, although in the strictest sense,

"actinic" means "capable of causing photochemical reactions."

The ultraviolet region, A < 400 nm, is often divided into what is known as the UV-A region from 315 to 400 nm, the UV-B region from 280 to 315 nm, and the UV-C region from 200 to 280 nm.

The effect of light scattering and absorption by atmospheric constituents on the intensity and wavelength distribution of sunlight at the earth's surface depends on both the nature and concentration of the gases and particles as well as the path length through

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