HBr Uptake on Ice:  Uptake Coefficient, H₂O/HBr Hydrate Formation, and H₂O Desorption Kinetics

Abstract

HBr uptake on thin ice films was examined using laser-induced thermal desorption (LITD), temperatureprogrammed desorption (TPD) and Fourier transform infrared (FTIR) spectroscopy techniques. LITD was used to determine the uptake coefficient in situ during low HBr exposures as a function of HBr pressure and ice film temperature. The HBr uptake coefficient on ice was γ ) 0.61 ( 0.06 at 140 K and γ ) 0.24 ( 0.05 at 100 K at low HBr coverages for HBr pressures ranging from 3 × 10 -8 to 1.4 × 10 -7 Torr. TPD and FTIR were used to examine HBr uptake on ice at higher HBr exposures. TPD studies observed that thin ice films exposed to HBr at 140 K saturated with HBr after large exposures and formed an HBr hydrate with an H 2 O:HBr ratio of ∼3.6:1. FTIR measurements monitored the development of the H 3 O + bending vibration versus HBr exposure on ice. Saturation of the integrated absorbance for this H 3 O + bending vibration was observed after large HBr exposures. The uptake of HBr on ice also had a profound effect on H 2 O desorption from ice. H 2 O desorbed at higher temperatures in the presence of HBr. The zero-order kinetics for H 2 O desorption from pure ice were E d ) 13.4 kcal mol -1 and ν 0 ) 1.9 × 10 32 cm -2 s -1 . The zero-order kinetics for H 2 O desorption from the HBr hydrate formed after saturation HBr exposures were E d ) 10.6 kcal mol -1 and ν 0 ) 6.3 × 10 27 cm -2 s -1 .

The rate of water vapor evaporation from ice substrates in the presence of HCl and HBr: implications for the lifetime of atmospheric ice particles

“…This is assigned to the bending mode of H 2 O which is shifted to 1745 cm −1 upon HBr adsorption and is attributed to the H 3 O + bending mode. This observation suggests complete ionization of HBr and is in agreement with the literature (Barone et al,1999;Hudson et al, 2001). However, similar to the case of HCl, where no spectral change was observed (case a, Table 4) the presence of HBr was ascertained by the measurement of the decrease of J ev of H 2 O, as presented in Fig.…”

“…The study of R ev alone is therefore a necessary, but not sufficient condition to assess H subl or the binding energy of H 2 O on ice at temperatures in excess of 190 K, as has been repeatedly done in the past (Haynes et al, 1992). For HCl-doped ice H subl = 11.2±2.8 and for HBr-doped ice H subl = 11.7 ± 2.8 kcal mol −1 , the latter of which is in good agreement with Hudson et al, 2001.…”

“…Some of these studies have focused on the temperature dependence of the rates of evaporation and condensation of H 2 O on pure ice using different techniques (Haynes et al, 1992;Brown et al, 1996;Chaix et al, 1998;Fraser et al, 2001). Others have mainly revealed the impact of trace gases of atmospheric importance on the evaporation rates, such as HNO 3 (Diehl et al, 1998;Warshawsky et al, 1999) HBr (Hudson et al, 2001). The main result of these studies is the observation of the decrease of the rate of evaporation of H 2 O (R ev ) on doped ice compared to R ev on pure ice to variable extents.…”

“…Thus, the investigation of the IR absorption spectrum of pure ice has been undertaken (Schriver-Mazzuoli et al, 2000) in order to spectroscopically characterize the ice structure, as well as the impact of HNO 3 , HCl and HBr deposition (Ritzhaupt et al, 1990;Delzeit et al, 1993;Koehler et al, 1993;Horn et al, 1997;Dominé et al, 2001;Zondlo et al, 2000;Hudson et al, 2001). This led to the discovery of different surface species owing either to the formation of stoichiometric hydrates or amorphous mixtures.…”

Redistribution of trace gases by convective clouds - mixed-phase processes

“…Sommerfeld and Lamb, 1986;Clapsaddle and Lamb, 1989;Valdez et al, 1989;Mitra et al, 1990;Conklin and Bales, 1993;Diehl et al, 1995;Santachiara et al, 1995;Zondlo et al, 1997;Diehl et al, 1998;Hudson et al, 2001;Clegg and Abbatt, 2001). These studies show that the gas uptake depends on many factors, including partial pressure, temperature, surface acidity, and even whether the ice phase is growing or not (e.g.…”