Kinetic mechanisms of the uptake of atmospheric gases on the surface of sea salts. ClNO3 uptake on MgCl2 center dot 6H(2)O/NaCl

摘要

The uptake coefficients gamma of chlorine nitrate on MgCl2 center dot 6H(2)O crystallites and a MgCl2 center dot 6H(2)O-NaCl mixture deposited from an aqueous solution are measured using a flow reactor with a movable salt-substrate-coated insert equipped in combination with a mass spectrometer at 295-428 K, [ClNO3] = (0.2-12) x 10(12) cm(-3), and [H2O] = 1.0 x 10(12) - 4.3 x 10(15) cm(-3). Immediately after the exposure of the salt substrate to a ClNO3 flow, gamma(t) decreases exponentially with time, gamma(t) = gamma(0) x exp(-t/tau) + gamma (s) , to a steady-state level, gamma (s) , which depends on the temperature and the ClNO3 and H2O concentrations. The main gas-phase product is Cl-2, HOCl appears only when water vapor is admitted into the reactor. The coefficient of steady-state uptake on wetted MgCl2 center dot 6H(2)O at 295 K can be described by the approximation gamma = a + b [H2O] with a = 3.5 x 10(-3) cm(3) and b = 3.2 x 10(-18) cm(3). The mechanism of the uptake of ClNO3 on MgCl2 center dot 6H(2)O is discussed. The experimental data are treated within the framework of a steady-state uptake model to estimate the heat of adsorption of ClNO3 on MgCl2 center dot 6H(2)O (Q (ad) = 62 kJ/mol) and the activation energy of the bimolecular heterogeneous reaction ClNO3 + Z (s) = 2Cl(2) + Mg(NO3)(2) center dot 6H(2)O (E (a) = 21.8 kJ/mol; Z (s) denotes a ClNO3-MgCl2 center dot 6H(2)O surface complex). When the MgCl2 center dot 6H(2)O: NaCl is varied from 0 to similar to 3 wt %, the steady-state uptake coefficient changes from the value corresponding to uptake on pure NaCl to that characteristic of uptake on pure MgCl2 center dot 6H(2)O.