Dry air is mostly nitrogen and oxygen. Nitrogen makes up about 78% of the air and oxygen about 21%, while argon, an inert gas, is third at roughly 0.93%.
The remaining fraction is trace gases, including carbon dioxide, neon, helium, methane, krypton, hydrogen, xenon and ozone. Air also holds a variable amount of water vapor, up to about 4% near the tropics but only traces near the poles, plus tiny solid particles called aerosols such as dust, sea salt and ash.
Source: UCAR Center for Science Education: What’s in the Air?