NOX FROM LIGHTNING AND THE CALCULATED CHEMICAL-COMPOSITION OF THE FREE TROPOSPHERE

In the free troposphere, injection from the stratosphere, emissions at the surface transported upward by dynamic processes, aircraft emissio ns and production in lightning strokes are the main sources of oxides of nitrogen (NOx). The global source of NOx production by lightning is not well known and estimates vary from 3 to 650 Mt NO2/yr. In this pa per the role of regional and episodic emissions of NOx from lightning is examined with a three-dimensional chemistry transport model, and a parameterization of the emissions is proposed, linking it to the inten sity of latent heat generation in convection calculated in a numerical weather prediction model. The parameterization is scaled to give an a nnual global emission of 16 MtNO(2)/yr. The resulting distribution of the concentration of NOx, ozone, OH and nonmethane hydrocarbons is com pared for the free troposphere over the northern hemisphere with the r esults of a calculation where the total NOx emissions from lightning w ere about the same but were fixed in time and space over the 18 days' calculation (June 18 to July 5, 1995). The maximum differences in the free tropospheric concentrations are significant: for NOx, OH and nonm ethane hydrocarbon of the same order of magnitude as the concentration s themselves, for ozone 1 order of magnitude less than the ozone conce ntration. This means that NOx emissions from lightning may cause a var iability in free tropospheric composition which makes it quite difficu lt to distinguish the importance of other sources of free tropospheric NOx from that of lightning in periods when lightning occurs.