Twilight observations suggest unknown sources of HOx

Measurements of the concentrations of OH and HO2 (HOx) in the high-latitude lower stratosphere imply the existence of unknown photolytic sources of HO x. The strength of the additional HOx source required to match the observat ions depends only weakly on solar zenith angle (SZA) for 80 degrees < SZA < 93 degrees. The wavelengths responsible for producing this HOx must be lon ger than 650 nm because the flux at shorter wavelengths is significantly at tenuated at high SZA by scattering and absorption. Provided that the source s involve only a single photon, the strength of the bonds being broken must be <45 kcal mole(-1). We speculate that peroxynitric acid (HNO4) dissociat es after excitation to an unknown excited state with an integrated band cro ss section of 2-3 x 10(-20) cm(2) molecule(-1) nm (650 < lambda < 1250 nm).