where k ss,b,T and k b,ss,T are the rate coefficients which determine the surface-to-bulk and bulkto-surface transport fluxes of S 2 O 2- 3 (J ss,b,T and J b,ss,T , respectively), and δ T is thickness of quasi-static surface layer (Figure ). Based on this equilibrium, the θ ss parameter was . For binary STS at RH between 60% and 80%, Figure shows that the model fit results utilizing θ ss = 0.5 to 0.8 (50% to 80% of ions on the surface is depleted) can agree well with the kinetics data. In order to elucidate the effect of surface ion depletion,