Air
You may be looking for Terrestrial Air.
On Luna oxygen for air can be chemically removed from the rocks, but nitrogen is scarce if available at all. It will need to be imported for air for colonists to breathe. Argon or helium can be substituted for nitrogen, but they may be more expensive than nitrogen. The argon needed to substitute for a given amount of nitrogen is heavier that the nitrogen, and helium though lighter than nitrogen is difficult to ship. An advantage for the colony is that the expensive Nitrogen is not used up. It is just continually recirculated by the air conditioning equipment.
There is no absolute need for healthy human beings to have carbon dioxide in the air that they inhale, but a small amount of carbon dioxide is sometimes included in supplemental oxygen to stimulate patients to breath more. Plants need carbon dioxide in the atmosphere that they grow in. A concentration of 0.032 percent is sufficient for plants although a concentration of 0.055 percent has been shown to increase the rate of growth of some plants.[1] People may be able to tolerate up to 1 percent of carbon dioxide in the air they breathe without ill effects, but since it is an expensive component to add to air for a lunar colony, no more than 0.03 percent should be left in the air with the carbon dioxide that is scrubbed out going into greenhouses for recycling.
