Study of finely divided aqueous systems as an aid to understanding the formation mechanism of polar stratospheric clouds:: Case of HNO3/H2O and H2SO4/H2O systems -: art. no. 4302

[1] The study of nanometer-scale aqueous systems (finely divided aqueous systems (FDAS)) can be achieved using the absorption of vapors on fumed silica (SiO2) powder. Being a product of flame synthesis technology, fumed silica particles (6-11 nm) can be considered to be analogous to the silica smoke particles of anthropogenic and extraterrestrial origin that are supposed to be widely present in the stratosphere and mesosphere. Here, we describe the freezing and melting behavior of nanometer-scale pure H2O and binary HNO3/H2O and H2SO4/H2O systems of varying acid content, using differential scanning calorimetry (DSC). Besides reductions of melting temperature, T-m, large reductions in freezing and melting enthalpies, with DeltaH(f) < Delta H-m, in comparison with bulk solutions have also been detected. Experiments showed that fumed silica can serve as a freezing nucleus for heterogeneous ice nucleation from dilute HNO3 /H2O droplets. The onset of freezing of a silica/HNO3 /H2O sample with HNO3/H2O stoichiometry close to that of NAT (53 +/- 5 wt % HNO3) at temperatures approximate to 7 K warmer than the ice frost point suggests that silica particles can promote heterogeneous freezing of nitric acid hydrates in the stratosphere. Freezing of bulk droplets (53.2 wt % HNO3) supported on Al substrate at temperatures warmer than -73 degrees C (200 K) suggests that in principle, Al2O3 surface may induce freezing of HNO3 hydrates as well. DSC measurements performed on the silica/ H2SO4 /H2O nanosystem showed that at stratospheric temperatures, silica particles cannot induce heterogeneous formation of sulfuric acid hydrates.