Maximum Supercooling of H₂SO₄Acid Aerosol Droplets

Heterogeneous reactions which take place on solid polar stratospheric cloud (PSC) particles and lead to ClO x activation, have been identified as crucial to polar ozone destruction. Scientific attention is currently focussed on whether similar heterogeneous reactions can also take place on sulfuric acid aerosol droplets. We present theoretical calculations of the homogeneous freezing of sulfuric acid droplets under stratospheric conditions, based on classical theory and using the best available thermodynamical data for the H 2 SO 4 –H 2 O system. For small cooling rates we find that most of the aerosol droplets will freeze as sulfuric acid tetrahydrate (SAT) at about 200 K, whereas for large cooling rates (e. g. in lee waves) large amounts of the aerosol droplets will stay liquid through temperatures as low as 190 K, at which point they freeze as water ice.