The influence of HCl on the evaporation rates of H2O over water ice in the range 188 to 210 K at small average concentrations

The evaporation flux J(ev)(H2O) of H2O from HCl-doped typically 1.5 mu m 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)(H2O) has been found to show complex behavior and to be a function of the average mole fraction chi HCl of HCl in the ice film ranging from 6 x 10(14) to 3 x 10(17) molecule cm(-2) s(-1) at 174-210 K for initial values 0 chi(0)(HCl) ranging from 5 x 10(-5) to 3 x 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 H2O vapor pressure was independent of chi HCl within the measured range and equal to that of pure ice down to 80 nm thickness. The dependence of J(ev)(H2O) with increasing average chi HCl was correlated with (a) the evaporation range r(b/e) parameter, that is, the ratio of J(ev)(H2O) just before HCl doping of the pure ice film and J(ev)(H2O) after observable HCl desorption towards the end of film evaporation, and (b) the remaining thickness d(D) below which J(ev)(H2O) decreases to less than 85% of pure ice. The dependence of J(ev)(H2O) with increasing average chi 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/H2O binary mixture (set B). The measured values of J(ev)(H2O) 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/H2O aggregate.