Enhancement of HOx at high solar zenith angles by overtone-induced dissociation of HNO3 and HNO4

At high solar zenith angles (SZA's) in the lower stratosphere there is a su dden increase in the measured [HOx] which is not well simulated by models. At high solar zenith angles, the intensity of solar UV radiation is very cl ose to zero, due to absorption by ozone. Since HOx formation via O(D-1) rea ctions requires solar UV radiation for O(D-1) production, some other, long wavelength photolytic HOx source is indicated. Significant enhancements to the photodissociation rates of HNO3 and HNO4 can result from visible wavele ngth excitation of OH overtone vibrations containing sufficient energy to c leave the O-O and N-O bonds. We have performed laboratory measurements and ab-initio calculations of the overtone absorption cross sections of these m olecules to quantify the contribution from this source. These results have been incorporated in several models, which reproduce the measured high SZA burst of HOx ;but not its magnitude. (C) 2000 Elsevier Science Ltd. All rig hts reserved.