Towards a robust estimate of the global lightning nitrogen oxides source rate and its error bound

A weighted least square fit method is introduced which provides best estimates for the annually mean global source strength S of lightning induced nitrogen oxides (LNOx) together with random and systematic error bounds. The method is demonstrated using data measured in-situ with airborne instruments near deep convection over a tropical continent at altitudes up to 20 km in Southern Brazil in February/March 2004 and February 2005 during the EU-project "Tropical Convection, Cirrus, and Nitrogen Oxides Experiment" (TROCCINOX). Model results for air composition along the flight paths for at least two values of S were provided by several global chemical transport models. LNOx estimates are obtained not only from NO mixing ratio data measured close to thunderstorms with fresh LNOx sources, but also from O₃ and CO data because of their LNOx sensitivity due to tropospheric photochemistry during the chemical life-time of theses species. The best estimate of the annual and global mean LNOx source value computed for the given data is S = 4.8 ± 2.5 Tg a^-1 (in mass units of nitrogen per year). Ongoing model improvements and inclusion of data from further in-situ and remote sensing measurements (including SCIAMACHY NO₂ columns) and data from other periods and latitudes may still change this result.