CO2, also called carbon dioxide, is a grey gas which is transportable via pipes. It is produced when the population consume oxygen in the atmosphere.
like all resources transferable via pipes, it can be stored in tanks
CO2 is a danger to the population, high c02 causes suffocation if the crew is not in space suits, and can suffocate passengers. The atmosphere must be rich in oxygen and poor in CO2 for long term survival.
CO2 can be removed from the atmosphere using CO2 scrubbers. CO2 can be reprocessed into oxygen using an air reprocessor.