N2, a colorless, tasteless, odorless 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. [Crutzen, Paul J. and T.E. Graedel, Atmospheric Change. W.H. Freeman and Company, New York, ; 1993; 147.] [Journal of the Atmospheric Sciences; v37; 179–192; 1980.]