Reaction Mechanism of O3 Uptake on MgCl2⋅6H2O as a Sea Salt Component

Abstract

The uptake of O₃ on a salt film coating of MgCl₂·6H₂O at T = 254 and 295 K is studied in the range ([O₃] = 2.5 × 10¹³–1.6 × 10¹⁴ cm^–3) using a flow reactor with a movable insert and mass spectrometric recording. The time dependence of the uptake coefficient of the ozone at different O₃ concentrations is obtained in the relative humidity range from zero to 24%. Using the method of mathematical modeling, based on the shape of the dependence of the uptake coefficient and its time decay on the ozone concentration, the uptake mechanism is established and the elementary kinetic parameters are assessed, based on which it is possible to extrapolate the time behavior of the uptake coefficient to tropospheric conditions at arbitrary ozone concentrations. The ozone uptake at room temperature occurs through the reaction mechanism of an adsorbed molecule on the surface of the substrate. The mechanism includes the stage of reversible adsorption, formation of an adsorbed complex, and its unimolecular decomposition with the release of molecular chlorine into the gas phase. At low temperatures, the uptake proceeds through recombination via the Eley-Rideal reaction mechanism: it includes reversible adsorption, formation of a surface complex, its reaction with an ozone molecule from the gas phase, and the release of an oxygen molecule into the gas phase. In this case, no chlorine is formed. The dependence of the uptake coefficient on relative humidity in the range of values from 0 to 24% at T = 254 K is not detected.