Nitrogen makes up the largest share of dry air near Earth’s surface, accounting for about 78.08 percent by volume.
Earth’s atmosphere is a mixture of gases rather than a single substance. After nitrogen, oxygen is the next major component at about 20.95 percent, followed by argon at roughly 0.93 percent. Carbon dioxide is present in a much smaller concentration, though its climate importance is substantial.
The phrase dry air excludes water vapour, whose amount varies greatly from place to place and over time. Humid air therefore has slightly different proportions because water vapour replaces some of the other gases. Near the surface, water vapour can range from almost zero to several percent.
Nitrogen is relatively unreactive in the atmosphere, but it is essential to life after nitrogen fixation converts it into biologically usable compounds. Atmospheric composition also changes with altitude, pollution, and local conditions.