The uptake of ClNO3 on film coatings of natural sea salt

Using a coated-insert flow tube reactor coupled to a high-resolution low-energy electron impact mass spectrometer, the uptake of chlorine nitrate on a salt film coating of natural sea salt (NSS) was studied under variation in the reactant concentration ([ClNO3] = (1-15) center dot 10(12) cm(-3)), humidity ([H2O] = (0.01-2.2) center dot 10(15) cm(-3)), and reactor temperatures of 295 and 423 K. The time-dependent character of the uptake coefficient gamma(t) = gamma (ini) exp(-t/tau) + gamma (ss) [1 - exp(-t/tau)] with parameters of the initial (gamma (ini) ) and steady-state (gamma (ss) ) uptake and characteristic time dependent on [ClNO3] was established. The only detected gas phase uptake product was Cl-2. The absence of the dependence of the steady-state uptake coefficent gamma (ss) on humidity in the aforementioned [H2O] range was established. Based on the model description of experimental dependences in terms of the modified Langmuir adsorption model, the adsorption heat of chlorine nitrate on NSS and the constant of unimolecular decay rate of the ClNO(3)aEuro broken vertical bar Z surface complex with the formation of the Cl-2 product were estimated. It follows from the comparison of these parameters for the initial and steady-state uptake that their adsorption heats are similar within the experimental error and the difference between the coefficients gamma (ini) and gamma (ss) is caused by the difference in activation energies of unimolecular decay. The coefficients gamma (ini) , gamma (ss) , and tau were estimated under tropospheric chlorine nitrate concentrations that are unavailable in laboratory conditions.