Active controlled atmosphere system

An active controlled atmosphere (CA) system for a transport container, the transport container including a cargo storage space, the CA system comprising:

• a gas concentration sensor,
• a nitrogen source including an outlet configured for the introduction of nitrogen-containing gas into the cargo storage space,
• a fan operable to circulate the gas around the cargo storage space, and
• a controller;
• wherein the controller is configured to operate the CA system in each of a first mode and a second mode, wherein, in both the first mode and the second mode the nitrogen source is operated to introduce nitrogen-containing gas through the outlet, and wherein, in the first mode, the fan is deactivated and, in the second mode, the fan is activated;
• and
• wherein the gas concentration sensor is positioned so that:
• in the first mode, nitrogen-containing gas that has been introduced through the outlet passes over the sensor so that the sensor provides a signal indicative of a concentration of a first gas in the nitrogen containing gas introduced through the outlet, and
• in the second mode, nitrogen-containing gas that has been introduced through the outlet is displaced away from the sensor by the fan so that the sensor is configured to provide a signal indicative of the concentration of the first gas in an ambient environment of the cargo storage space.