title: Melting of H2SO4&middot;4H2O Particles upon Cooling: Implications for Polar Stratospheric Clouds
abstract: Polar stratospheric clouds (PSCs) are important for the chemical activation of chlorine compounds and subsequent ozone depletion. Solid PSCs can form on sulfuric acid tetrahydrate (SAT) (H2SO4&middot;4H2O) nuclei, but recent laboratory experiments have shown that PSC nucleation on SAT is strongly hindered. A PSC formation mechanism is proposed in which SAT particles melt upon cooling in the presence of HNO3 to form liquid HNO3-H2SO4-H2O droplets 2 to 3 kelvin above the ice frost point. This mechanism offers a PSC formation temperature that is defined by the ambient conditions and sets a temperature limit below which PSCs should form.
authors.affiliation: Max Planck Institute for Chemistry, Postfach 3060, 55020 Mainz, Germany.
authors.initials: T
authors.lastname: Koop
authors.initials: KS
authors.lastname: Carslaw