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

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.