Air is made mostly out of nitrogen, oxygen and argon that are kept in proportion to each other. Interestingly, the gas that's most abundant in air isn't oxygen but nitrogen. Air is made up of about 78 percent nitrogen, and oxygen makes up about 21 percent of air.
Argon and other gases such as carbon dioxide, neon and helium make up the rest of the air. Air also has variable amounts of water vapor. It also contains atmospheric particles called aerosols. These can be anything from the ash and dust produced by volcanic eruptions to pollution to pollen and spores released by plants.