The Western Balkan region is known for emitting alarmingly high sulphur dioxide amounts from coal-fired power plants. Though a number of environmental regulations have been introduced in recent years (e.g. desulphurisation installations, construction of modern power plants), the pollution burden is still much higher than recommended by the authorities. A number of different montoring systems are required to observe the growing pollution situation in the Western Balkan region, partly caused by a high energy demand from outside (e.g. Western Europe). Several of the top ten SO2 polluters in Europe are located in Bosnia-Herzegovina and Serbia. Here we present the first in situ measurements of sulphur dioxide in this region conducted with a German research aircraft in cooperation with local scientists in Bosnia-Herzegovina and Serbia. Two of the strongtest emitting coal-fired power plants were selected for the measurements in autumn 2020: Tuzla in Bosnia-Herzegovina and Nikola Tesla in Serbia (Nikola Tesla). The measurements were mainly conducted in the boundary layer (below ~1 km altitude in winter). Downwind of the power plants, extremely high SO2 mixing ratios exceeding 100 parts per billion (ppb = nmol mol-1) were measured at a distance of ~20-40 km from the sources. The SO2 plumes from the power plants were trapped in well-defined inversion layers between ~500-1000 m altitude. The airborne measurements can be used to validate synchronous spaceborne SO2 measurements from the TROPOspheric Monitoring Instrument (TROPOMI) onboard the Sentinel-5P satellite. A first i​n​t​e​r​c​o​m​p​a​r​i​s​o​