The Mechanism of Reactive NO3 Uptake on Dry NaX (X=Cl, Br)

A simple kinetic mechanism of nitrate radicals uptake on dry sea-salt NaCl, NaBr surfaces is proposed. The mechanism includes adsorption/desorption equilibrium and unimolecular decomposition of the adsorbed complex: NO3(g) + NaX(s) ↔ (NO3-NaX)(s); (NO3-NaX)(s) → NaNO3 + X(s) Two techniques were used: the matrix isolation ESR and mass spectrometry. The uptake coefficient (γ) is found to be dependent on exposure time of salt to NO3 for raw coating. The initial γ(t→0) is higher than the observable steady-state γobs. At room temperature γobs is independent of [NO3] at low [NO3] = 3 × 109 - 1011 cm-3, but it is inversely proportional to [NO3] at concentrations higher than 1012 cm-3. At temperatures above 100 °C, γobs becomes independent of [NO3] in a wider range of [NO3]. An increased number of dislocations is supposed to exist in the case of raw coating. Due to a wide spread of the surface sites binding energy with the ionic lattice near dislocations, the part of surface complexes has lower binding energy and "burns" more rapidly. That burning determines the transition from γ(t→0) down to γobs.