Atmospheric Ultraviolet Radiation Scattering and Absorption

Abstract

Molecular scattering and absorption by atmospheric gases generally is a wavelength-dependent process. In the UV-B region of the solar spectrum the main gases of the atmosphere, composed primarily of Nitrogen, Oxygen, and some of the rare noble gases are non-absorbing. They do, however, scatter radiation in a process that is named after Lord Rayleigh. The term elastic scattering is often used to indicate that all of the incident radiation intercepted by a non-absorbing molecule is redirected into other directions i.e. there is no loss of radiation to the molecule. The process is wavelength-dependent and goes as the reciprocal of the fourth power of the wavelength. This means that the scattering becomes stronger at the shorter wavelengths. Hence, the reason for a blue sky because the visible wavelengths of the rainbow colors in the sun’s spectrum are so weakly scattered that they are obscured by the strong blue color. However, aerosols (dust) are composed of much larger particles on the order of the wavelength of light compared to the size of air molecules and these particles do not possess strong wavelength-dependent scattering characteristics. Therefore, when aerosols or clouds are present, one often sees a whitish haze or a white cloud because the rainbow colors are mixed more equally. A strongly absorbing haze, however, may take on a darker appearance and might be seen as a brown cloud.