Real-Time Kinetic Measurements of the Condensation and Evaporation of D₂O Molecules on Ice at 140 K < T < 220 K

Abstract

The kinetics of condensation of D 2 18 O water vapor on D 2 16 O ice was studied in the temperature range 140-220 K. The measurements have been performed in real time using the pulsed valve technique in a lowpressure flow reactor. Ice samples were prepared by several different methods. The uptake coefficient γ was observed to decrease with increasing surface temperature, and varied from γ ) 0.06 to 0.8. A significant dependence of γ on the method of preparation of the ice has been found. At 180 K, γ ) 0.13 for singlecrystal ice, 0.18 for ice condensed from the vapor phase, and approximately 0.25 for bulk ice obtained from freezing liquid D 2 16 O. The uptake coefficient attained a value of γ ) 0.8 at 140 K for cubic ice prepared by vapor condensation at 140 K. The activation energy for evaporation of D 2 O at low temperatures (140-190 K) has been measured as 12.2 kcal/mol, whereas it decreases to 8.3 kcal/mol at higher temperatures (190-220 K). The rate of evaporation at 200 K corresponds to the loss of approximately 70 ( 10 formal monolayers per second. The experimental results suggest the formation of loosely bound water adsorbed to the surface of ice whose bond energy is estimated to be 4.0 ( 0.4 kcal/mol, independent of the type of ice.

Collision dynamics and uptake of water on alcohol-covered ice

“…The value of the uptake coefficient of water on ice is an area of active debate with evidence suggesting that α w < 1 (Chaix et al, 1998;Marek and Straub, 2001;Pratte et al, 2006;Davidovits et al, 2006). However, for pure ice surfaces at our ms experimental timescales we observe complete trapping of water molecules, and therefore assume α w = 1 for consistency, and to calculate an upper bound for α b (T ).…”

Desorption kinetics of model polar stratospheric cloud films measured using Fourier Transform Infrared Spectroscopy and Temperature‐Programmed Desorption

“…These two are closely related by ⌬H sub ϭ E des Ϫ E ads where E ads is the energy barrier to adsorption. For ice films, E ads is very small below 200 K, and ⌬H sub Х E des [33]. The same condition is likely to hold true for the nitric acid hydrate films.…”

“…We assume that the same holds true for the other films in this study. A recent study [33] reported a negative effective activation barrier to ice adsorption of around Ϫ0.3 kcal/mol below 200 K for ice that has been condensed from vapor. At higher temperatures, they do report a significant, Ϫ4.2 Ϯ 1 kcal/mol barrier to ice adsorption with a concomitant decrease in the activation barrier to evaporation.…”

“…Table IV shows that these values agree fairly well with those reported in the literature. For vapor-deposited ice, Chaix et al [33] suggest that the energy barrier to ice evaporation should decrease at 200 K, with the formation of a negative effective activation barrier to ice adsorption. However, our evaporation runs were almost entirely below 200 K.…”

Review of the vapour pressures of ice and supercooled water for atmospheric applications

“…For many atmospheric applications, the uncertainty in the vapour pressure of ice is small compared to other uncertainties. For example, there is a range exceeding a factor of ten in estimates of the uptake coefficient of water vapour on ice at 200 K (Chaix et al 1998). This uncertainty propagates into computer models of cirrus clouds (Lin et al 2002;Gierens et al 2003).…”