Attenuation of UV-C Solar Radiation as a Function of Altitude (0 ≤ z ≤ 100 km): Rayleigh Diffusion and Photo Dissociation of O<sub>2</sub> Influence

Abstract

In this paper, we present an analysis of attenuation for UV-C radiation ( 290 nm λ ≤ ) as a function of the altitude z ( ) 0 100 km z ≤ ≤ by calculating the interaction ratio between the UV-C radiation and the molecular species susceptible of interact with UV-C radiation. The Rayleigh scattering spectral cross sections were calculated, the UV-C spectral cross sections of the species susceptible of interact with UV-C radiation and the UV extraterrestrial (ETR) solar spectrum were standardized with wavelength steps of 1 nm, and The International Standard Atmosphere model (ISO 1972) was adapted to calculate the molecular density. These data were utilized to calculate the photodissociation and Rayleigh scattering ratios as a function of the altitude and to determine to what measure the photodissociation and the Rayleigh diffusion were determinants of the attenuation of UV-C radiation. It became clear that the photo dissociation of O 2 is the primordial mechanism of attenuation for the UV-C radiation, but the Rayleigh diffusion appears like a mechanism that encreases the photon flux, raising the performance of the O 2 photodissociation. The attenuation capacities of N 2 O, CO 2 and water vapor (H 2 O) over the UV-C radiation are all similar, although smaller (less than 0.6%), and this is due to their low concentration. The O 3 , has the theoretical greater attenuation capacity, but it is found in mid-range altitudes ( ), where the residual UV-C photons has almost vanished by O 2 photo dissociation or Rayleigh diffusion, so the real effect over the UV-C attenuation is minimum.