Study of the kinetics of condensation and evaporation of water vapor over atmospherically relevant pure and doped ice films : a multiple diagnostic approach

This thesis presents the different steps in the development of a multiple diagnostic experimental setup and the results concerning the kinetics of condensation and evaporation of water over pure ice, as well as ice films doped with a known amount of atmospheric trace gases. The goal of this project is to obtain reliable data concerning the evaporative flux, Jev [molec cm-2 s-1], and the concomitant rate of condensation, kcond [s-1], of H2O molecules in the presence of an ice film. A further aim of this work is the detailed investigation of the impact on the aforementioned kinetic processes of the deposition of atmospherically relevant trace gases such as hydrogen chloride (HCl), hydrogen bromide and nitric acid (HNO3) under well defined conditions including those that are representative of the atmosphere. The new experimental apparatus consisting of a stainless steel reactor may be used under three different regimes, namely static, stirred flow and dynamic condition which correspond to equilibrium, low rate of pumping and molecular flow, respectively. The separation of the kinetics into individual processes is principally afforded by the application of these 3 regimes. First, using a combination of the first two regimes, we have measured Jev and γ of H2O in the case of pure as well as HCl- or HBr-doped ice in the range 190-240 K. The resulting Arrhenius expressions for pure ice are: in the presence of an average mole fraction of HCl, ΧHCl, in the range 3.2·10-5 to 6.4·10-3 we have found: and for ΧHBr at a HBr mole fraction smaller than 6.4·10-3 was found, where R=1.987 cal mol-1 K-1. Owing to the rate separation, γ applies to an individual rate process and should be called an accommodation rather than an uptake coefficient. Of utmost importance is the fact that during most of the ice film evaporation the vapor pressure remained that of pure ice even in the presence of small amounts of HCl or HBr on the order of a fraction to several formal monolayers. This is consistent with the enthalpies of sublimation that may be derived following the relation ΔHsubl0 = Econd - Eev. It leads to different thermochemical parameters A (pre-exponential factor) and E values (energy of activation) for the three cases that are identical within experimental uncertainty. In addition, FTIR spectroscopy of the ice films involved in these experiments in transmission have revealed that different species such as amorphous HCl·H2O mixtures or the crystalline HCl·6H2O hexahydrate are formed depending on the doping protocol. These two species significantly affect the temporal change of Jev each in a different manner. For the sake of precision and accuracy of the kinetic results, a quartz crystal microbalance (QCM) was subsequently added to the experimental apparatus and used as a substrate for ice film growth. The development of a QCM-based device for the investigation of the kinetic processes of ice films at atmospherically relevant temperatures was a challenge we have succes