Open AccessStudy of finely divided aqueous systems as an aid to understanding the formation mechanism of polar stratospheric clouds: Case of HNO 3 /H 2 O and H 2 SO 4 /H 2 O systems
Author(s) -
Bogdan A.,
Molina M. J.,
Kulmala M.,
MacKenzie A. R.,
Laaksonen A.
Publication year - 2003
Publication title -
journal of geophysical research: atmospheres
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.67
H-Index - 298
eISSN - 2156-2202
pISSN - 0148-0227
DOI - 10.1029/2002jd002605
Subject(s) - fumed silica , ice nucleus , aqueous solution , frost (temperature) , differential scanning calorimetry , freezing point , analytical chemistry (journal) , materials science , chemistry , mineralogy , nucleation , chemical engineering , chromatography , thermodynamics , organic chemistry , physics , engineering , composite material
The study of nanometer‐scale aqueous systems (finely divided aqueous systems (FDAS)) can be achieved using the absorption of vapors on fumed silica (SiO 2 ) 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 H 2 O and binary HNO 3 /H 2 O and H 2 SO 4 /H 2 O 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 ΔH f < ΔH 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 HNO 3 /H 2 O droplets. The onset of freezing of a silica/HNO 3 /H 2 O sample with HNO 3 /H 2 O stoichiometry close to that of NAT (53 ± 5 wt % HNO 3 ) at temperatures ≈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 % HNO 3 ) supported on Al substrate at temperatures warmer than −73°C (200 K) suggests that in principle, Al 2 O 3 surface may induce freezing of HNO 3 hydrates as well. DSC measurements performed on the silica/H 2 SO 4 /H 2 O nanosystem showed that at stratospheric temperatures, silica particles cannot induce heterogeneous formation of sulfuric acid hydrates.