Solid Polar Stratospheric Clouds Research Articles

Maximum Supercooling of H2SO4Acid Aerosol Droplets

AbstractHeterogeneous reactions which take place on solid polar stratospheric cloud (PSC) particles and lead to ClOxactivation, 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 H2SO4–H2O 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.

Reactions on sulphuric acid aerosol and on polar stratospheric clouds in the Antarctic stratosphere

Heterogeneous chemistry producing active chlorine has been identified as crucial to Antarctic ozone depletion. Most attention has focussed on reactions on solid polar stratospheric cloud (PSC) particles, although there is still no satisfactory understanding of the microchemical incorporation of HCl in PSCs. The alternative mechanism involving sulphuric acid aerosol as the reaction surface has been considered at lower latitudes, but its role in the special conditions of the polar stratosphere has been largely ignored. Recent data from the Antarctic stratosphere have suggested that HCl is present in sulphuric acid aerosol that remains liquid even at the lowest stratospheric temperatures. The available laboratory data show that cold, relatively dilute, sulphuric acid is particularly able to take up HCl that is available for reaction provided the aerosol remains liquid. Fast heterogeneous reaction rates compared to those at mid‐latitudes will produce active chlorine rapidly. Since the aerosol is present with significant surface area throughout the lower stratosphere, it should be very effective for heterogeneous reaction once temperatures drop. These surfaces, rather than PSCs, could host the initial conversion of Cl to its active form over the Antarctic.