I am currently working on world-building a planet, and I wanted to try to diverge from Earth's atmosphere a little bit, but not extremely. I am planning on still having it be a oxygen-nitrogen rich atmosphere, but I would like to maybe add another substance into the mix to make things a little more interesting. I have thought about ammonia, hydrogen sulfide or maybe hydrogen chloride being present in the atmosphere but I'm curious on what other chemicals might be possible for my ecosystem.
I'm planning on the atmosphere consisting of: 80% N2, 17% O2, 1% CO2, 1% X, and the rest being random other inert gases. X represents the new chemical I want to add in. I would love to get some ideas for other chemicals to put into the atmosphere and how they would affect the environment and possible life! Thank you!