Effect of the humidity on the uptake of NO3 on coatings composed of MgCl2 · 6H2O and MgBr2 · 6H2O and mixtures thereof with NaCl

Abstract

The reactive uptake of NO3 radicals on the surface of wetted individual X salts and of wetted X-NaCl salts (X = MgCl2 · 6H2O and MgBr2 · 6H2O) at [H2O] = 2 × 1012−2 × 1015 cm−3 and NO3 (4.8 × 1012 cm−3) was studied using a reactor with a movable insert covered with a salt coating in combination with a mass spectrometer for monitoring the initial reactant and products. The probabilities of NO3 uptake γ on X-NaCl binary salts as functions of the content of doping salt were determined. A parametric approximation of the experimental data was proposed, which makes it possible to quantitatively predict the extent of surface enrichment of a wetted binary salt coating in doping salt and its dependence on the humidity and the content of this salt in the binary mixture. It was established that the relative surface density σX of X doping salt depends on its mole fraction μX in the X-NaCl binary salt as σX = aμX (a = 2.2 for MgBr2 and 13.1 for MgCl2) over the entire humidity range covered. The contributions of the X salts to the overall uptake of NO3 at NO3 concentration typical of the tropospheric conditions ([NO3] ∼ 107 cm−3 and relative humidities of RH ≤ 20%) were estimated.