The sulfur cycle is one area in atmospheric chemistry where human activity has a large impact. The anthropogenic contribution is about three times larger than the natural in terms of emissions. Several studies have been performed over the last decades to investigate and quantify the processes of importance in the sulfur cycle [Rodhe and Isaksen, 1980; Langner and Rohde, 1991; Chin et al., 1996; Feichter et al., 1996; Kasibhatla et al., 1997; Restad et al., 1998; Roelofs et al., 1998; Koch et al., 1999; Barth et al., 2000; Iversen and Seland, 2002; Liao et al., 2003]. The COSAM exercise [Barrie et al., 2001; Lohmann et al., 2001; Roelofs et al., 2001] provides an excellent overview of different sulfur models, and comparison both between models and between models and observations. Earlier studies on the sulfur cycle focused on the acidification of the atmosphere, but lately there has been more concern about the sulfate particles formation and their impact on climate. Sulfate particles affect the radiative balance of Earth in two ways: The direct radiative effect, where the sulfate particles scatter solar radiation, and the indirect effects where the sulfate particles: (1) reduce the cloud droplet size and change the optical properties of clouds and (2) reduce the precipitation efficiency and increase cloud water content and lifetime of clouds. Both the direct and indirect effects of sulfate aerosols contribute to a cooling of Earth's surface [Intergovernmental Panel on Climate Change (IPCC), 2001].
 The main constituents in the tropospheric sulfur cycle are DMS (dimethylsulfide, CH3SCH3), SO2 (sulfur dioxide) and sulfate (SO42−). In addition H2S (hydrogen sulfide) and MSA (methanesulfonic acid, CH3SO3H) contribute to the sulfur cycle. Emissions of OCS are small although it is the most abundant sulfur specie in the atmosphere due to its long lifetime in the troposphere [Kjellström, 1998]. Quantitatively the most important emissions in the sulfur cycle are the anthropogenic emissions of SO2 and the natural emissions of DMS. Natural emissions of DMS are connected with large uncertainties and need to be better quantified [Boucher et al., 2003]. The changes in anthropogenic emission patterns of SO2 show large variations where anthropogenic emissions over Europe and the United States have decreased over the last decades and emissions in Asia have increased. The significance of these changes needs to be studied.
 To study the sulfur cycle, both chemical tracer models (CTMs) and general circulation models (GCMs) are used. Simple sulfur cycle parameterizations have been widely applied using off-line calculations of chemical oxidants. However, some former studies apply semiprognostic oxidants, i.e., that prescribed 5-days averages of J values and concentrations of OH and HO2 are used to calculate concentrations of H2O2 [Koch et al., 1999; Barth et al., 2000], Roelofs et al.  implemented a fully coupled scheme allowing chemical feedback and Liao et al.  investigate interactions between ozone-NOx-hydrocarbons chemistry and aerosols. In this study with the OsloCTM2 model [Sundet, 1997] an interactive tropospheric sulfur and oxidant chemistry scheme is applied. Hence the sulfur species and the oxidants (OH, O3, H2O2, HO2NO2, NO3) are calculated simultaneously. This sulfur-oxidant interaction is an improvement compared to previous off-line model studies as it includes chemical interactions important for sulfate formation not captured by models using prescribed oxidants. Models with prescribed oxidants tend to replenish the oxidants too fast after a complete depletion, e.g., complete titration of H2O2 by SO2 inside clouds. Interactive chemistry shows a stronger oxidation limitation than off-line models. In this article our main focus will be on the coupling between the sulfur chemistry and the oxidants. To test out model parameterizations and validate model performance our model results will be compared with observations. The use of detailed meteorological input data in the OsloCTM2 (liquid water, cloud distribution, precipitation) allows us to study e.g., the impact on the sulfur cycle of chemical interactions in the aqueous phase more thoroughly. Finally we focus on the changes in the anthropogenic emissions from 1985 to 1996 and how these have altered the sulfur cycle. Since meteorological input data represent 1996, emissions and observations for 1996 are chosen for comparison. Many previous studies used the GEIA 1985, therefore we compare sulfur distributions using the 1985 GEIA inventory with our 1996 simulations.