The role of the atmosphere in nitrogen cycling

In this work an analysis of the sources, atmospheric concentration, chemical reactions and sinks of the principal atmospheric nitrogen compounds is made. Atmospheric emissions of N₂O and NH₃ are almost entirely due to biological activity on the continents and in the oceans. The combustion of fossil fuels and biomass is the principal source of NO_x. The only relevant chemical transformations in the troposphere are the oxidation of NO_x to NO₃ ^? and the formation of ammonium salts. Only 10% of the NH₃ emitted is oxidized. Washout of NH₄ ⁺ and NO₃ ^? by rainfall is the principal mechanism for removing nitrogen compounds from the atmosphere. Part of the N₂O enters the stratosphere and part must be removed in the biosphere by processes not yet established. NO_x produced in the atmosphere by the burning of fossil fuels and biomass and by lightning represents between 30 and 40% of the total nitrogen fixed. A complete nitrogen balance for the troposphere is presented. Since the photochemical oxidation of NO_x is rapid and atmospheric transport is relatively slow with respect to the cycling of water in the troposphere, nitrogen compounds return to the earth's surface close to where they were emitted. Fixed-nitrogen inputs to the continents and oceans due to biological and industrial fixation are slightly greater than those due to rain water. However, since rain falls everywhere, input from this source is only important on soils not subject to intensive agriculture.