The Kinetic Mechanisms of the Trapping of Atmospheric Gases by Marine Salt Surfaces: IX. The Heat of Adsorption of Cl Atoms on the Surface of NaCl

The heterogeneous trapping of chlorine atoms on the surface of NaCl was studied using two coaxial stream reactors connected to an EPR cavity or a mass spectrometer. The kinetics of trapping was measured by the EPR method over a wide range of chlorine atom concentrations (10¹⁰–10¹³ cm^?3) at temperatures of 250–330 K. At Cl] ≥ 10¹² cm^?3, chlorine atoms were recorded by the EPR method in the gas phase. At lower concentrations (~10¹⁰?3 × 10¹¹ cm^?3), Cl atoms were replaced with RO₂ radicals by adding hexane RH and O₂ at the entrance of the EPR cavity. This was followed by the matrix isolation of RO² in the cavity at liquid nitrogen temperature. The probability of the trapping of chlorine atoms on the chemically inactive surface of NaCl was found to increase as the concentration of Cl grew. The temperature dependence of the trapping coefficient γ was pronounced at a concentration of chlorine atoms of ~3 × 10¹⁰ cm^?3, whereas no such dependence was observed at a chlorine concentration of ~ 10¹³ cm^?3. The recombination of Cl atoms was well described by the Rideal-Eley mechanism, and the heat of adsorption of chlorine atoms on the inactive surface of NaCl was estimated at Q = 17 ± 0.6 kcal/mol. It was shown mass spectrometrically that the trapping coefficient γ of Cl atoms decreased with the time of measurements, like the partial coefficient of the formation of the HCl product, whereas the partial coefficient of the formation of the Cl₂ product, conversely, increased with the time. The characteristic time of the attainment of stationary values by all the γ coefficients weakly depended on the initial concentration of Cl and equaled several dozen seconds. Reactions of adsorbed Cl atoms formed in the trapping of NO₃ radicals by the surface of marine salt NaCl in coastal troposphere are discussed.