The influence of HCl on the evaporation rates of H 2 O over water ice in the range 188 to 210 K at small average concentrations

Abstract. The evaporation flux J ev ( H 2 O ) of H 2 O from HCl -doped typically 1.5 µm or so thick vapor-deposited ice films has been measured in a combined quartz crystal microbalance (QCMB)–residual gas mass spectrometry (MS) experiment. J ev ( H 2 O ) has been found to show complex behavior and to be a function of the average mole fraction χ HCl of HCl in the ice film ranging from 6×10 14 to 3×10 17 molecule cm −2 s −1 at 174–210 K for initial values χ HCl 0 ranging from 5 × 10 - 5 to 3 × 10 - 3 at the start of the evaporation. The dose of HCl on ice was in the range of 1 to 40 formal monolayers and the H 2 O vapor pressure was independent of χ HCl within the measured range and equal to that of pure ice down to 80 nm thickness. The dependence of J ev ( H 2 O ) with increasing average χ HCl was correlated with (a) the evaporation range r b∕e parameter, that is, the ratio of J ev ( H 2 O ) just before HCl doping of the pure ice film and J ev ( H 2 O ) after observable HCl desorption towards the end of film evaporation, and (b) the remaining thickness d D below which J ev ( H 2 O ) decreases to less than 85 % of pure ice. The dependence of J ev ( H 2 O ) with increasing average χ HCl from HCl -doped ice films suggests two limiting data sets, one associated with the occurrence of a two-phase pure ice/crystalline HCl hydrate binary phase (set A) and the other with a single-phase amorphous HCl∕H 2 O binary mixture (set B). The measured values of J ev ( H 2 O ) may lead to significant evaporative lifetime extensions of HCl -contaminated ice cloud particles under atmospheric conditions, regardless of whether the structure corresponds to an amorphous or crystalline state of the HCl∕H 2 O aggregate.