Some aspects of the stratospheric Cl-ClO-Cl cycle: Possible roles of ClO *, ClNO 3 and HOCl

Abstract

Depending on such factors as (a) the probabilities of exciting the various vibrational states in ClO formed in the reaction of Cl with O 3, (b) the radiative lifetime of ClO *, (c) Δ H ƒ(ClO 3), and (d) the rate coeffic`ient of the relevant three-body reaction, the production of ClO 3 via the reaction ClO * +O 2 +M→ClO 3 +M may be quite substantial in the stratosphere. The significance of this result lies in the subsequent elimination (from the stratosphere) of ClO 3 and its associated chlorine atom as HClO 4, in the manner recently suggested by Samonaitis and Heicklen. In the stratosphere, ClO 3 most probably photodissociates primarily into OClO and O. Upon photodissociation, OClO may also yield atomic oxygen. Thus the formation of ClO 3 from ClO * and O 2, and the above-mentioned photodissociation steps constitute an interesting, indirect mechanism of O 2 dissociation into two odd oxygen species. Other aspects of ClO * chemistry, applicable in stratospheric conditions, also deserve attention in view of Nicholl's recent interpretation of the Umkehr measurements by Brewer et al. The reactions of ClO with HO 2, and NO 2, possess the potential of significantly obstructing the completion of the C1-ClO-Cl cycle, at least in the region below 35 km. An accurate and critical study of the chemistry of oxyacids, higher oxides, and nitrates of chlorine in the stratospheric environment is needed. Obviously, this is only a partial list of the difficult problems associated with a proper understanding of stratospheric chlorine chemistry which appears to be far more complex than what is implied in the literature. (See also notes added in proof stage.)