The present invention relates to a reefer container for transporting and storing a plurality of produce at a predetermined temperature, the produce generating ethylene and other organic compounds during transport and storage, said reefer container comprising a substantially closed room having an atmosphere, a cooling device for maintaining the room at the predetermined temperature, and an atmosphere-controlling system for controlling the atmosphere in the room of the reefer container, the atmosphere-controlling system comprising a system inlet (161) and a system outlet (162), a flow direction of an airstream being defined between the system inlet and the system outlet, a fan (160) arranged at the system outlet, an ethylene reduction unit (100) for reducing a content of ethylene of the atmosphere in the room, a catalyst unit (150) arranged downstream the ethylene reduction unit, the catalyst unit being adapted to reduce a content of ozone generated during the reduction of ethylene, the catalyst unit having a catalyst inlet (153), a catalyst outlet (154) and a catalyst midsection (155) arranged substantially between the catalyst inlet and catalyst outlet, and a control unit (200), wherein a first measuring point (103) is arranged upstream the catalyst inlet and a second measuring point (151) is arranged at the catalyst midsection, the first measuring point and the second measuring point being connected with at least a first ozone sensor (170) so that via the first measuring point a first ozone concentration of the air before the air enters the catalyst unit can be measured, and via the second measuring point a second ozone concentration of the air present in the catalyst midsection of the catalyst unit can be measured, the first ozone sensor being connected with the control unit, and wherein the first ozone concentration is compared to the second ozone concentration in the control unit in order to calculate the expected ozone concentration of the air downstream the catalyst unit. Further, the present invention relates to a method for measuring an ozone concentration in an airstream in a reefer container according to the present invention.