TERRESTRIAL SOURCES AND DISTRIBUTION OF ATMOSPHERIC SULFUR

The general circulation model ECHAM has been coupled to a chemistry an d sulphur cycle model to study the impact of terrestrial, i.e. mostly anthropogenic sulphur dioxide (SO2), sources on global distributions o f sulphur species in the atmosphere. We briefly address currently avai lable source inventories. It appears that global estimates of natural emissions are associated with uncertainties up to a factor of 2, while anthropogenic emissions have uncertainty ranges of about +/- 30 %. Fu rther, some recent improvements in the model descriptions of multiphas e chemistry and deposition processes are presented. Dry deposition is modelled consistently with meteorological processes and surface proper ties. The results indicate that surface removal of SO2 is less efficie nt than previously assumed, and that the SO2 lifetime is thus longer. Coupling of the photochemistry and sulphur chemistry schemes in the mo del improves the treatment of multiphase processes such as oxidant (hy drogen peroxide) supply in aqueous phase SO2 oxidation. The results su ggest that SO2 oxidation by ozone (O-3) in the aqueous phase is more i mportant than indicated in earlier work. However, it appears that we s till overestimate atmospheric SO2 concentrations near the surface in t he relatively polluted Northern Hemisphere. On the other hand, we some what underestimate sulphate levels in these regions, which suggests th at additional heterogeneous reaction mechanisms, e.g. on aerosols, enh ance SO2 oxidation.