A method of operating an activated carbon scrubber apparatus (300, 300') of a transport refrigeration system for carbon dioxide (CO 2) removal from a controlled environment in an enclosed space; wherein the scrubber apparatus (300, 300') is configured to switch between: an adsorption configuration in which it is configured to provide CO 2 -rich gas from the controlled environment to a sorbent bed comprising activated carbon for CO 2 adsorption, and to return the treated gas to the controlled environment; a regeneration configuration in which it is configured to provide a regenerating gas from outside of the controlled environment to the sorbent bed to desorb CO 2 and regenerate the activated carbon, and to discharge CO 2 -rich gas outside of the controlled environment; the method comprising: alternately operating the scrubber apparatus (300, 300') in the adsorption configuration and the regeneration configuration over a plurality of cycles, each cycle comprising a single period in the adsorption configuration and a single period in the regeneration configuration; characterized in that the scrubber apparatus (300, 300') is operated at a cycle frequency of between 4 and 30 cycles per hour; controlling a heater (303, 303') to heat the sorbent bed in the regeneration configuration. wherein the scrubber apparatus (300, 300') is configured to switch between: an adsorption configuration in which it is configured to provide CO 2 -rich gas from the controlled environment to a sorbent bed comprising activated carbon for CO 2 adsorption, and to return the treated gas to the controlled environment; a regeneration configuration in which it is configured to provide a regenerating gas from outside of the controlled environment to the sorbent bed to desorb CO 2 and regenerate the activated carbon, and to discharge CO 2 -rich gas outside of the controlled environment; an adsorption configuration in which it is configured to provide CO 2 -rich gas from the controlled environment to a sorbent bed comprising activated carbon for CO 2 adsorption, and to return the treated gas to the controlled environment; a regeneration configuration in which it is configured to provide a regenerating gas from outside of the controlled environment to the sorbent bed to desorb CO 2 and regenerate the activated carbon, and to discharge CO 2 -rich gas outside of the controlled environment; the method comprising: alternately operating the scrubber apparatus (300, 300') in the adsorption configuration and the regeneration configuration over a plurality of cycles, each cycle comprising a single period in the adsorption configuration and a single period in the regeneration configuration; characterized in that the scrubber apparatus (300, 300') is operated at a cycle frequency of between 4 and 30 cycles per hour; controlling a heater (303, 303') to heat the sorbent bed in the regeneration configuration. alternately operating the scrubber apparatus (300, 300') in the adsorption configuration and the regeneration configuration over a plurality of cycles, each cycle comprising a single period in the adsorption configuration and a single period in the regeneration configuration; characterized in that the scrubber apparatus (300, 300') is operated at a cycle frequency of between 4 and 30 cycles per hour; controlling a heater (303, 303') to heat the sorbent bed in the regeneration configuration.