A physical adsorption model of the dependence of ClONO₂ heterogeneous reactions on relative humidity

We present a model of heterogeneous reactivity based on physical adsorption that describes the observed relative humidity dependence of the ClONO 2 reaction probability with H 2 O and HCl on sulfuric acid tetrahydrate ice surfaces (SAT) and with H 2 O on nitric acid trihydrate (NAT). The laboratory data are modeled using only two parameters for a given system, the measured reaction probability on a neat H 2 O ice surface, and a constant from the BET theory which describes the fraction of an acid hydrate surface covered by H 2 O as a function of relative humidity. The model indicates that ClONO 2 reactivity with both HCl and H 2 O on SAT and with H 2 O on NAT is controlled by the surface coverage of H 2 O. In contrast, the reaction of ClONO 2 +HCl on NAT is better described by an alternative model based on reactivity in solutions formed within a porous ice by capillary liquid absorption.