Angstrom's formula for spectral extinction has the following form:
where ra(A) is the aerosol optical depth at wavelength A, ¡3 is the extinction at 1 micrometer, a is Angstrom's exponent which is related to the size distribution of the aerosol particles. Cacharro et al. (1989) showed that Angstrom's formula provides a good spectral representation of atmospheric aerosol attenuation. The wavelength to particle diameter ratios is:
• If V = B D/8 << 1, we will have Rayleigh scattering.
• When particle diameters are between 0.1 and 10 microns we have the case of Mie scattering.
• When V >> 1, we have the case of geometric optics.
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