Study of finely divided aqueous systems as an aid to understanding the surface chemistry of polar stratospheric clouds: Case of HCl/H2O and HNO3/HCl/H2O systems

Abstract

The conditions under which a liquid (quasi-liquid) or glassy overlayer could form around solid polar stratospheric cloud (PSC) particles have been studied with differential scanning calorimetry (DSC) on nanometer-size binary HCl/H2O and ternary HNO3/HCl/H2O systems obtained on voluminous powder of fumed silica (SiO2). A HCl/H2O nanosystem of 32 wt % HCl froze heterogeneously, because of the presence of the silica surface, between - 70°C and -85°C (203 and 188 K). This suggests that in the absence of any heterogeneous nuclei, a liquid or quasi-liquid overlayer can exist around solid particles if HCl concentration in the surface layer can reach about 30%. Experiments also showed that the ternary nanosystem freezes more readily (between -52 and -93°C, 221 and 180 K) than the binary HCl/H2O system of similar HCl concentration, except for ternary solutions with excess nitric acid. This finding indicates that in the stratosphere, complex HNO3/HCl/H2O hydrates could be formed within a supercooled liquid overlayer of suitable composition if heterogeneous nuclei, for instance, meteoritic smoke silica particles, are available. A relatively warm glass-transition temperature region (between -80 and -100°C) observed in one of the ternary samples suggests that in principle, a glassy overlayer could also form from a supercooled liquid overlayer of appropriate composition.