High‐resolution analysis of direct and scattered radiation in the stratosphere between 175 and 210 nm

Direct and scattered components of solar radiation in the stratosphere are examined at wavelengths between 175 and 210 nm. Absorption by oxygen in the Schumann‐Runge system is treated exactly using temperature dependent cross sections at a spectral resolution of 0.5 cm −1 (≈ 0.002 nm). Consistent with previous studies employing spectrally averaged cross sections, the calculated ratio of scattered to direct flux is more than a factor of 2 smaller than observed below 210 nm. The flux ratio is constrained above a minimum value at large opacities due to a twilight effect, occurring when the attenuation of scattered light from overhead paths is not as great as over the slant path of the direct beam; however, this phenomenon alone cannot account for observations. An improved description of scattering by O 2 for wavelengths below 200 nm is presented which includes effects of incoherent scattering. Use of the revised parameters for oxygen scattering does not bring the calculated flux ratios into agreement with observations, but does lead to small changes in the scattered field and O 2 photodissociation rate below 190 nm.