Interaction of Nitrous Acid with Polycrystalline Ice: Adsorption on the Surface and Diffusion into the Bulk

Abstract

The thermodynamic and kinetic constants needed to model the interactions of nitrous acid (HONO) with ice were measured. The experiments were done in a packed ice bed flow tube at temperatures between 213 and 253 K and at HONO gas phase concentrations between 0.4 and 137 ppbv. The experiments were performed using radioactively labeled HO 13 NO molecules. The use of the short-lived isotope 13 N ( t 1/2 ≈ 10 min) enabled in situ monitoring of the migration of HO 13 NO in the flow tube. The measurements showed that the interactions do not only occur through adsorption but also via diffusion into polycrystalline ice. A method is suggested to disentangle the bulk and the surface processes. Using this method, the temperature dependence of the linear partition coefficient was found to be K lin,C,HONO = 7.4 × 10 −11 exp(5.4 × 10 3 / T ) in units of m. The enthalpy and entropy of adsorption of HONO on ice were Δ H ads,HONO = −45(±20) kJ mol −1 and Δ S ads,HONO 0 = −17(±89) J mol −1 K −1, respectively for the standard conditions A std / V std = 1.7 × 10 9 m −1 . The time dependent uptake into the bulk was expressed using H cc,HONO * ( D b,HONO ) 1/2, the product of the dimensionless effective Henry’s law constant and the square root of the effective diffusion constant in the bulk of the polycrystalline ice matrix. H cc,HONO * ( D b,HONO ) 1/2 was evaluated to 1.6(±69%) × 10 −2 m s −1/2 . The results are discussed in the context of HONO partitioning in snowpacks.