CMD Annual Report ’00 185 The Mechanism of NO3 Uptake on Solid NaX (X = Cl, Br).

Uptake and product studies have been carried out to establish the kinetic mechanism of NO3 heterogeneous reactions with solid sea salts NaCl and NaBr. Quite recently four papers concerning this problem have been published (Gratpanche and Sawerysyn, 1999; Gershenzon et al., 1999; Seisel et al., 1997; Seisel et al., 1999). The uptake coefficients for NaCl (Gratpanche and Sawerysyn, 1999; Gershenzon et al., 1999) and NaBr (Gershenzon et al., 1999) were established to drop with exposure time from initial γo to final steady-state γss. The absolute values of γo(NaCl) = (4÷5)×10-2 (Gratpanche and Sawerysyn, 1999; Gershenzon et al., 1999; Seisel et al., 1997) and γo(NaBr) = 0.1÷0.2 (Gratpanche and Sawerysyn, 1999; Gershenzon et al., 1999) measured at low concentration of [NO3] ~ 1010÷1011 molecules cm-3 are in good agreement. The γss for NaCl and NaBr were studied by Gershenzon et al. (Gershenzon et al., 1999), using a wide range of [NO3]=3×109÷5×1013 molecules cm-3. The γss was found to be independent of NO3 concentration at low [NO3] ~ 3×109÷1011 molecules cm-3 and be inversely proportional to [NO3] when NO3 is higher than 1012 molecules cm-3. The data were explained, using a simple kinetic scheme of the adsorbed [NO3NaX] complex formation followed by bidirectional decomposition: back to release the reactants and forward to yield the atomic chlorine or bromine NO3 + NaX(s) ⇔ [NO3NaX(s)] → NaNO3(s) + X, X =Cl, Br. Earlier, the main products of the reactions under consideration were suggested (Seisel et al., 1999) to be Cl and Br atoms. Indeed, Br atoms and Br2 were directly detected in similar NO3 + KBr reaction. In the case of NaCl, HCl was the sole product observed. However, there was indirect evidence that Cl atoms are the primary products