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Abstract

In the stratosphere aerosol particles consisting of sulfuric acid (H2SO4) are formed and accumulate in the Junge-aerosol layer. There they reflect the incoming solar radiation and absorb the solar as well as the outgoing terrestrial radiation. This influences the radiation budget of the earth's atmosphere. The sulfur concentrations in the stratosphere are dominated by strong irregular variations due to volcanic emissions. These variations cannot be predicted in the climate development and therefore require constant observation. The last major volcanic eruption was the Pinatubo in the Philippines in 1991. Since then there have only been small to medium sized eruptions. Observations from satellites and the herein performed model simulations show, however, that these small eruptions are also relevant for the Earth's radiation budget due to their frequency. For this reason, a Volcanic Sulfur Emissions Inventory is created on the basis of various satellite data sets that includes almost 500 volcanic eruptions. During the implementation of emissions into the model, each volcanic eruption has to be treated individually. The implemented sulfur dioxide (SO2) emissions are then automatically converted into aerosol particles by the model and their influence on the earth's radiative forcing is calculated. The resulting Aerosol Optical Depth (AOD) and the global negative radiative forcing coincide relatively well with the observations of the various satellites in the new model simulations. Only in the case of large volcanic eruptions, such as the Pinatubo in 1991, will saturation effects of the satellite instruments or overestimated removal processes of aerosol particles in the model lead to slight deviations between observations and model simulations. Even during periods of low volcanic activity background concentrations of sulfur aerosols remain in the stratosphere. This is based on the conversion of various sulfur precursors gases into SO2. Here carbonyl sulfide (OCS) plays an important role in the sulfur budget of the stratosphere due to its long atmospheric lifetime. To identify missing sources of OCS, sensitivity studies are carried out. For this reason, emissions of carbon disulfide (CS2) and dimethyl disulfide (DMDS) have been added to the model and a comprehensive New Sulfur Chemistry Mechanism has been implemented. The newly implemented chemical reactions convert CS2 and dimethyl sulfide (DMS) into OCS. This increases the mixing ratio of OCS in the UTLS region, especially in tropical regions from approx. 500 pptv to 550 pptv. In return, DMS is broken down and the production of methyl sulfonic acid (MSA) is reduced. The model results are then evaluated by comparing them with aircraft measurements from the StratoClim campaign during the Asian Summer Monsoon in 2017. An improvement in the model is achieved through the newly added comprehensive Sulfur Chemistry Mechanism, which results in increases of the OCS mixing ratio. The latter was previously substantially underestimated in the region of performed measurements. In addition, the observations suggest unusually high particle number concentrations in the UTLS region. These can be verified by comparing them with the model simulations and satellite data as well as traced back to the increased SO2 emissions of the Sinabung volcano. Thus, this work shows the successful combination of model simulations, satellite observations and airborne in-situ measurements.