Twilight observations suggest unknown sources of HO_x

Geophys. Res. Lett., in press, 1999.

Abstract

Measurements of the concentrations of OH and HO₂ (HO_x) in the high-latitude lower stratosphere imply the existence of unknown photolytic sources of HO_x. The strength of the additional HO_x source required to match the observations depends only weakly on solar zenith angle (SZA) for 80° < SZA < 93° . The wavelengths responsible for producing this HO_x must be longer than 650 nm because the flux at shorter wavelengths is significantly attenuated at high SZA by scattering and absorption. Provided that the sources involve only a single photon, the strength of the bonds being broken must be < 45 kcal mole^-1. We speculate that peroxynitric acid (HNO₄) dissociates after excitation to an unknown excited state with an integrated band cross section of 2- 3 ´ 10^-20 cm² molecule^-1 nm (650 < l < 1250 nm).