1. Solar Radiation and Its Transmission through the Atmosphere a. The Sun and Its Relationship to the Earth: Some Important Definitions for Atmospheric Chemistry
The sun can be considered a spherical light source of diameter 1.4 X 106 km located 1.5 X 108 km from the earth's surface. Incoming direct sunlight at the earth's surface is treated as a beam with an angle of collimation of ~ 0.5° and thus is essentially parallel to ±0.25°.
The total intensity of sunlight outside the earth's atmosphere is characterized by the solar constant, defined as the total amount of light received per unit area normal to the direction of propagation of the light; the mean value is 1368 W m"2, although variations from this mean are common (Lean, 1991).
Of more direct interest for atmospheric photochemistry is the solar flux per unit interval of wavelength. Values up to approximately 400 nm are provided by Atlas 3 (see Web site in Appendix IV) and from 400 nm on by Neckel and Labs (1984). Figure 3.12 shows the solar flux as a function of wavelength outside the atmosphere and at sea level for a solar zenith angle of 0° (Howard et al., 1960).
Outside the atmosphere, the solar flux approximates blackbody emission at ~ 5770 K. However, light absorption or scattering by atmospheric constituents modifies the spectral distribution. The attenuation due to the presence of various naturally occurring atmospheric constituents is shown by the hatched areas in Fig. 3.12.
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