Kinetic model for reaction of ClONO₂ with H₂O and HCl and HOCl with HCl in sulfuric acid solutions

Abstract

The heterogeneous reactions ClONO 2 + H 2 O → HOCl + HNO 3 (1), ClONO 2 + HCl → Cl 2 + HNO 3 (2), and HOCl + HCl → Cl 2 + H 2 O (3) on stratospheric aerosols convert ClONO 2 and HCl to photo‐labile species, producing reactive Cl and ClO which are responsible for catalyzing ozone destruction in the lower stratosphere. The extent of the resulting ozone loss mirrors the steep negative temperature dependence of these reactions, which strongly depend on the solubility of ClONO 2 , HCl, and HOCl, and on the activity of H 2 O. Predicting the effect of these heterogeneous processes throughout the stratosphere requires detailed modeling of liquid phase solubility, diffusion, and reaction kinetics. A series of recent experiments from a number of laboratories have refined measurements of liquid diffusion coefficients, HCl and HOCl solubilities, and the reactivity of ClONO 2 + H 2 O, ClONO 2 + HCl and HCl + HOCl on liquid films, droplets, and aerosols. On the basis of those measurements we present a phenomenological uptake model in which parameterizations of ClONO 2 , HCl, and HOCl heterogeneous kinetics appropriate for stratospheric H 2 SO 4 /H 2 O aerosols are addressed. In this model we suggest that under high acid concentration conditions both HOCl and ClONO 2 are protonated before they react with HCl. Data for all three reactions in concentrated H 2 SO 4 solution indicate an acid‐catalyzed reaction channel, which had previously been inferred for ClONO 2 hydrolysis. This updated parameterization is most significant at relatively high temperatures above 205 K which produce H 2 SO 4 aerosols of >60 acid wt%, where the acid‐catalyzed reaction channels dominate. The comparisons between our new formulation and other recent formulations are presented.