Abstract
Abstract. The evaporation flux Jev(H2O) of H2O 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. Jev(H2O) has been found to show complex behavior and to be a function of the average mole fraction χHCl of HCl in the ice film ranging from 6×1014 to 3×1017 molecule cm−2 s−1 at 174–210 K for initial values χHCl0 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 H2O vapor pressure was independent of χHCl within the measured range and equal to that of pure ice down to 80 nm thickness. The dependence of Jev(H2O) with increasing average χHCl was correlated with (a) the evaporation range rb∕e parameter, that is, the ratio of Jev(H2O) just before HCl doping of the pure ice film and Jev(H2O) after observable HCl desorption towards the end of film evaporation, and (b) the remaining thickness dD below which Jev(H2O) decreases to less than 85 % of pure ice. The dependence of Jev(H2O) with increasing average χHCl 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 Jev(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.
Highlights
HCl is among the mineral acids that control the acidity of the atmosphere, together with HNO3 and H2SO4
The change in χHCl owing to H2O evaporation is evaluated between t = tD and t = tHb, which corresponds to the time interval when the number of adsorbed HCl www.atmos-chem-phys.net/18/15903/2018/
We have investigated the impact of the deposition protocol on dD, which is the thickness of ice that is affected by the presence of HCl, namely the remaining thickness of ice whose Jev(H2O) value has decreased to 85 % of Jev(H2O) of pure ice
Summary
HCl is among the mineral acids that control the acidity of the atmosphere, together with HNO3 and H2SO4. The production of atmospheric HCl predominantly takes place in the middle and upper stratosphere where O3 is formed owing to photolysis of halogen-containing source gases such as CFCs (chlorofluorocarbons). There are no known sources of HCl in the upper troposphere (UT) because scavenging processes of HCl throughout the troposphere are very efficient, which leads to HCl background concentrations of less than 0.1 ppb (Graedel and Keene, 1995). Owing to the frequent occurrence of cirrus clouds in this atmospheric region it is of obvious interest to study the interaction of HCl with atmospheric ice particles at relevant temperature and pressure conditions (Jensen et al, 2001; Zerefos et al, 2003). The compact correlation between O3 and HCl has been used to monitor stratospheric–
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