NOx from lightning and the calculated chemical composition of the free troposphere

In the free troposphere, injection from the stratosphere, emissions a t the surface transported upward by dynamic processes, aircraft emiss ions and production in lightning strokes are the main sources of oxid es of nitrogen (NOx). The global source is not well known and has rec ently been estimated to be between 7 and 18 MtNO2/yr in the 8-12 km a ltitude region. In this paper a parameterization of the emissions of NOx from lightning is proposed, linking it to the intensity of latent heat generation in convection calculated in a numerical weather pred iction model. The parameterization is scaled to give an annual global emission of 16 MtNO2/yr. The resulting distribution of the concentra tion of NOx, ozone, OH and non methane hydrocarbons is compared for t he free troposphere over the northern hemisphere with the results of a calculation where the total NOx emissions from lightning where abou t the same, but were fixed in time and space over the 18 days calcula tion (18 June-5 July 1995). The maximum differences in the free tropo spheric concentrations are significant: for NOx, OH and NMHC of the s ame order of magnitude as the concentrations themselves, for ozone on e order of magnitude less than the ozone concentration. This means th at NOx emissions from light ning may cause a variability in free trop ospheric composition which makes it quite difficult to distinguish th e importance of other sources of free tropospheric NOx, from that of lightning in periods when lightning occurs.