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Air composition varies horizontally and. vertically. The air over deserts has different proportions of gasses than over jungles. But generally at sea level, Nitrogen is about 78%, Oxygen 20%, Argon 0.9%, CO2 about 0.4%, However, nitrogen and argon do not dissolve well in water. When you heat water, dissolved O2 & N2 escape with water vapour. So ocean warming increases atmospheric CO2, but you never hear that discussed.