ADSORPTION OF SOME ATMOSPHERICALLY IMPORTANT MOLECULES ONTO LARGE WATER CLUSTERS - SO2, SO3, HCL, AND CLONO2

The adsorption of a range of atmospherically important molecules (SO2, SO3, HCl, and ClONO2) on large water clusters have been studied using a supersonic molecular beam expansion to generate water clusters cont aining 50-450 water molecules. SO2 and HCl were found to stick with lo w efficiency to the water cluster, retaining their chemical identity. In contrast, both SO3 and ClONO2 undergo heterogeneous reactions on th e surface of the water cluster forming, respectively, H2SO4 and HOCl w ith HNO3. For SO3, calculations show that the large barrier that exist s in the gas-phase to the reaction of SO3 with water is removed for wa ter clusters of a sufficient size because of stabilization of the tran sition state by solvation. For ClONO2, the barrier to reaction is much larger and cannot be removed by solvation for any size cluster. In th is case, it is likely that reaction takes place on the water cluster b y the ionic dissociation of ClONO2 in a similar manner to that observe d for ClONO2 on ice films.