HOx, NOx, and ClOx: Their Role in Atmospheric Photochemistry

Sources of atmospheric odd nitrogen and hydrogen are reviewed and their role m ozone photochemistry is discussed. A model, containing few adjustable parameters, gives good agreement with observed distributions of stratospheric and mesospheric ozone. Nitric oxide emitted by supersonic aircraft would lead to a significant reduction in the concentration of atmospheric ozone if the globally averaged source of NO should exceed 2 × 107molecules cm−2s−1. A traffic model projected by Broderick etal. for 1990 could lead to a reduction of about 2% in the column density of O3.Sources of atmospheric chlorine are discussed. It is argued that HCl should be the dominant form of atmospheric chlorine and that it is produced mainly from aerosols of marine origin. The atmospheric source strength is about 2 × 108tons per year according to Chesselet etal. and HCl may be removed by gas phase reaction with NH3. The role of chlorine compounds as a catalyst for recombination of odd oxygen is discussed and shown to play no major role in the normal atmosphere. Reactions of OH and HO2with O3may provide an important sink for tropospheric odd oxygen such that O3may not be a passive tracer for tropospheric motions.