A three-dimensional study of the relative importance of the different odd nitrogen sources in the troposphere is discussed. In order to consistently simulate the chemistry, all the sources are considered simultaneously but each nitrogen emitted is characterized by a given flavor. This allows us to follow the evolution of nitrogen from each source without affecting the overall chemical system of the atmosphere. The model results suggest that the NOx concentration in the southern hemisphere is mostly dominated by the lightning source, while the northern hemisphere odd nitrogen burden results mainly from the influence of fossil fuel combustion, aircraft, and lightning emissions. In particular, in the upper tropospheric northern hemisphere midlatitudes, approximately 25-30% of the NOx is due to aircraft, 15-20% to fossil fuel combustion sources at the surface, and 15-20% to lightning, assuming that the global fleet of aircraft releases 0.44 TgN/yr and that lightning produces 5 TgN/yr. The sensitivity of these figures to several parameters (aircraft and soil source strengths, as well as the lightning source strength and its vertical distribution) is also studied.