Ionisation and solvation of stratospherically relevant molecules on ice films

Abstract

Heterogeneous reactions of stratospheric reservoir species such as HCl, N₂O₅ and ClONO₂ on type I (nitric acid hydrates) and type II (water-ice) polar stratospheric cloud (PSC) particles are believed to play an important role in the extremely large losses of stratospheric ozone observed during the Antarctic spring. Laboratory studies of such processes, using thin ice films as PSC particle surface mimics, can provide mechanistic information about individual elementary steps and overall reaction schemes. IR and mass spectrometry have been used to identify reaction products and intermediates and it has been shown that the primary step in all the studied reaction schemes involves the formation of ionic surface species. Furthermore, the nature and stability of the ions are found to be inextricably linked with the amount of ‘free’ water available to solvate them. The following questions are also addressed: Are adsorbed ions (e.g. nitrate, chloride and oxonium) reactive on ice surfaces? If so, are the products formed the same as those suggested when their corresponding molecular parents are involved? The answers, which have important implications for understanding stratospheric heterogeneous chemistry, are discussed in terms of ionic reaction mechanisms and novel surface intermediates.