Heterogeneous reactivity of chlorine atoms with sodium chloride and synthetic sea salt particles

Abstract

The uptake of chlorine atoms on sodium chloride (NaCl) and synthetic sea salt (SSS) particles was studied using a discharge flow reactor coupled to a molecular beam mass spectrometer. The reactive surfaces were prepared by coating the inner surface of the reactor using two different methods: either by depositing size-selected particles on halocarbon wax or by spray depositing thin films using a constant output atomizer. The observed uptake coefficients of Cl˙ on NaCl particles are γ^NaCl_Cl ≈ 2 × 10⁻² for size-selected particles or γ^NaCl_Cl ≈ 5 × 10⁻² for thin films and for SSS particles γ^SSS_Cl ≈ 4 × 10⁻³. Heterogeneous recombination of Cl atoms to Cl₂ molecules was observed for the two solid surfaces. The study was performed over the temperature range 258 to 353 K. The temperature dependence of the uptake was observed and the heat of adsorption of Cl˙ on NaCl particles was estimated at Q_ads = 63 kJ mol⁻¹ assuming an Eley–Rideal mechanism. The role of surface adsorbed water has also been shown. The atmospheric implications of these findings are discussed briefly.