Heterogeneous reactivity of the nitrate radical: reactions on halogen salt at ambient temperature and on ice in the presence of HX (X=Cl, Br, I) at 190 K

Abstract

The heterogeneous reactions of NO₃ on salt (NaCl, KBr, KI and NaI) and on ice in the presence of HX (X=Cl, Br and I) were studied using a Knudsen cell reactor. The uptake of NO₃ on solid salt at ambient temperature was found to be fast with an uptake coefficient of γ=(4.6±4)×10^-2 for NaCl, γ=0.2±0.1 for KBr and γ=0.33±0.09 for KI and NaI. The solid salt substrate contains approximately 10 formal monolayers of adsorbed H₂O which enables the ionic exchange reaction. For KBr and KI the primary products Br and I were detected. We observed Br₂ and I₂ as secondary products resulting from heterogeneous recombination of Br and I, respectively. The reaction of NO₃ with NaCl resulted in HCl which was the sole observed product. The uptake of NO₃ on ice in the presence of HX was studied at 190 K. In the presence of HCl an upper limit for the uptake of NO₃ on ice of γ<1×10^-4 was determined and no products were observed. In the presence of a concurrent flow of HBr and HI the uptake coefficients of NO₃ on ice were found to be high with γ=0.35±0.08 for HBr and γ=0.55±0.1 for HI. The secondary products Br₂ and I₂, owing to the heterogeneous recombination of halogen atoms, were observed. In the case of HI we detected the primary product I as well as the secondary product IO formed in the reaction of NO₃ with I.