The present disclosure is directed to a method for monitoring a cleaning of a process gas passing through a wet scrubber. The wet scrubber comprises an inlet zone, a contact zone and an outlet zone. The contact zone is divided into at least two sections. The method comprises measuring a concentration of sulphur dioxide in at least a first measuring point corresponding to a first section of the at least two sections of the contact zone to obtain measurement information; and comparing the measurement information to a reference value to determine a local sulphur dioxide removal performance of the first section.