A kinetic model for uptake of HNO3 and HCl on ice in a coated wall flow system

Abstract

A simple model of gas flow and surface exchange with a single site Langmuir mechanism has been developed to describe effects of adsorption and desorption on trace gas concentrations at the outflow from a coated wall flow tube reactor. The model was tested by simulating experimental results for the uptake of HNO₃ and HCl on ice films at temperatures and gas concentrations corresponding to the ice stability region in the upper troposphere. The experimental time-dependent uptake profiles were best fitted with an additional process involving diffusion of the adsorbed molecules into the ice film. The model allowed true surface coverages to be distinguished from total uptake including transfer to the bulk, leading to more accurate estimates of the Langmuir constant, K_eq, for surface adsorption. A revised expression was obtained for the temperature dependence of the K_eq = −(4.43 ± 0.77) × 10⁵T + (10.72 ± 1.75) × 10⁷ hPa⁻¹. Reasonable fits to the desorption profiles observed following cessation of exposure of the film to HNO₃ or HCl were obtained at high surface coverage but at low coverage desorption was too slow. The analysis suggested that the ice surface was characterised by sites of different binding energy, some weakly bound sites from which the acid molecules desorbed rapidly, and some strong-binding sites which led to essentially irreversible uptake. Experiments involving competitive co-adsorption of HNO₃ and HCl, conducted at relatively high equilibrium surface coverage, were well simulated by the model, as were those where the same surface was repeatedly exposed to gas phase acids.