A gas cleaning system (1) is designed for removing hydrogen fluoride from a process gas generated during the production of aluminium from alumina. The gas cleaning system (1) comprises a scrubbing chamber (8, 10, 12) for purposes of mixing the process gas with particulate alumina, and a filter device (24,26,28) which is located downstream of the scrubbing chamber (8,10,12) with respect to the direction of flow of the process gas. A sulphur dioxide measurement device (40, 42, 44, 50) is operative for measuring the amount of the concentration of sulphur dioxide that is present in the process gas downstream of the filter device (24, 26, 28). A controller (46) is operative-connected to the sulphur dioxide measurement device (40, 42, 44, 50) and is operative for utilizing the measured amount of the concentration of sulphur dioxide for purposes of evaluating the efficiency of the hydrogen fluoride removal by the gas cleaning system (1).