Melting of H 2 SO 4 ⋅4H 2 O Particles upon Cooling: Implications for Polar Stratospheric Clouds

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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) (H 2 SO 4 ⋅4H 2 O) 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 HNO 3 to form liquid HNO 3 -H 2 SO 4 -H 2 O 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.

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