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 eq uilibrium and unimolecular decomposition of the adsorbed complex: NO3(g) NaX(s) <----> (NO3-NaX)(s); (NO3-NaX)(s) --> NaNO3 + X(s) Two techniques we re used: the matrix isolation ESR and mass spectrometry. The uptake coeffic ient (gamma) is found to be dependent on exposure time of salt to NO3 for r aw coating. The initial gamma(t --> 0) is higher than the observable steady -state gamma(obs) At room temperature gamma(obs) is independent of [NO3] at low [NO3] = 3 x 10(9) - 10(11) cm(-3), but it is inversely proportional to [NO3] at concentrations higher than 10(12) cm(-3). At temperatures above 1 00 degrees C, gamma(obs) 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 wi th the ionic lattice near dislocations, the part of surface complexes has l ower binding energy and "burns" more rapidly. That burning determines the t ransition from gamma(t --> 0) down to gamma(obs). The kinetic parameters and elementary rate coefficients are obtained. The r ecommended gamma for low atmospheric NO3 concentration are in the range of 0.002 +/- 0.04 for NaCl and 0.1-0.3 for NaBr depending on a mechanism of th e gamma(t) relaxation.