On Earth, our atmosphere contains 78.09% nitrogen, 20.95% oxygen, on average around 1% water vapor at sea level, 0.93% argon, 0.04% carbon dioxide, and small amounts of other gases. Looking through this list not everything seems necessary for a habitable atmosphere.
Nitrogen is needed by plants, but not in its gaseous form so supplying this rare resource which needs to be mined on Venus, Earth, Titan, from the depths of the giant planets or the sun vis water bound fertilizers seems more appropriate. Nitrogen can go
Oxygen is obviously required for respiration on al level between 0.16 and 0.5 atm. In order to keep respiration easy and activity comfortable, I will keep it slightly below the Earth standard. Oxygen is required
Water vapor is unavoidable in an inhabited section due to evaporation. There will be water vapor
Argon is a product of radioactive decay. Argon be-gone
Carbon dioxide is produced by animals and needed for plants. The Carbon dioxide needs to be taken care of either by scrubbers or plants.There will be Carbon Dioxide
Since any spacecraft is subject to the rocket equation reducing mass is always a good thing. While using a bare minimum atmosphere will only save minuscule amounts of mass any gram counts on a space vessel. Furthermore, lower pressure will reduce the amount of gas escaping into space.
Thus my first idea is a pure 0.2 atm oxygen atmosphere for most spacecraft. I this plausible? NASA used a pure Oxygen atmosphere for the Apollo missions and it worked fine. Sure there was the Apollo fire but that happened in a 1.14 atm pure oxygen atmosphere so this is a different environment. NASA used 0.35 atm on the actual flights, but I don´t understand why they used so much pressure. Shouldn´t the chance of getting the required oxygen out of an Earth-like atmosphere with 0.2 atm be the same as getting the oxygen in a pure 0.2 atm oxygen atmosphere? Or should I increase my pressure to the NASA approved value of 0.35 atm?
On space stations I would like to use an Oxygen/Helium atmosphere. Helium instead of Nitrogen because it is dirt cheap since it is a waste product of the extensive Metallic Hydrogen, Helium3, and Deuterium mining operations. Nitrogen is expansive because it is harder to get and used for agriculture. The other advantages of Helium are that is light, seems to have no adverse effects of the human body and transmits heat very well (six times better than regular air) allowing me to achieve thermal properties similar to regular air with a lot less Helium. Of cause, there is the "Helium Voice" due to the much higher speed of sound in Helium, but that´s something one can get used to over time. The station air mix would be 0.2 atm Oxygen and 0.15 atm Helium.
Of cause, not all station and vessels in my setting these mixtures but they are the most economical and thus the most common. Nitrogen/Oxygen does exist yet is only used on planets and near planet stations.
Are these atmospheres plausible? Did I mess up somewhere? Can this be improved?