N₂, a colorless, tasteless, odourless gas which makes up 78. 1% of the atmosphere. Atmospheric nitrogen is converted by nitrogen fixation and nitrification into compounds used by plants and animals. In the far upper atmosphere, N₂ is broken down when large numbers of energetic secondary electrons are produced and available to react with the N₂. This leads to the eventual production of NO in that part of the atmosphere and is not--by definition--anthropogenic in nature.