Lightning-produced NOx in the tropics: Results from TROCCINOX, SCOUT-O3 and AMMA, and recommendations for future LNOx parameterizations

Lightning produces large amounts of nitrogen oxides (NOx=NO+NO2) emitted mainly into the upper troposphere. It is known that most lightning is produced in the tropics, however up to recently measurements and quantifications of NOx in the fresh outflow of tropical deep convection were rare. In the last years, the Deutsches Zentrum für Luft- und Raumfahrt (DLR) conducted (or participated in) several European airborne field experiments in the tropics focusing on lightning-produced NOx (LNOx), as TROCCINOX in Brazil in 2004 and 2005, SCOUT-O3 in northern Australia in 2005, and AMMA in West Africa in 2006. During these campaigns, the thunderstorm outflow was investigated in detail with the DLR research aircraft Falcon (<12 km) and the Russian M55 Geophysica aircraft (<20 km). In addition to the airborne trace gas measurements, stroke measurements with the Lightning Location Network (LINET), set up in the area of interest, were performed. The main focus of these measurements was 1) to analyse the trace gas composition (CO, O3, NO, NOx, NOy) in the convective outflow as a function of distance from the convective core, 2) to estimate the contribution of LNOx and boundary layer NOx to anvil-NOx in the outflow, and 3) to estimate the rate of LNOx per stroke in selected thunderstorms. The Falcon, Geophysica and LINET measurements were combined and scaled with LIS lightning measurements to estimate the LNOx production rate per LIS flash in the selected thunderstorms, and to estimate the global annual LNOx production rate based on these different thunderstorms. The method combines the estimated LNOx mass flux rate in the anvil outflow and the LINET (LIS) stroke (flash) rate. During AMMA, the LNOx mass flux in the investigated thunderstorms was in a similar range as estimated for TROCCINOX, however distinctly lower than for some SCOUT-O3 thunderstorms. During SCOUT-O3, the highest production rate of nitrogen per LIS flash was observed in thunderstorms known as Hector. If we consider these Hector thunderstorms to be typical global thunderstorms, an annual global production rate of up to ~8 Tg(N) a-1 from lightning would be estimated. However, if we consider more typical tropical and subtropical thunderstorms observed during the three field campaigns, an annual global production rate of ~2 and ~4 Tg(N) a-1, respectively, is obtained. A general tendency to higher LNOx production rates in subtropical thunderstorms was observed, which may be caused by e.g. longer flash lengths. The influence of different thermodynamical and dynamical parameters in tropical and subtropical airmasses is discussed, which may explain these differences. Recommendations for future LNOx parameterizations are given. The overall mean for the annual global LNOx production rate based on results from the AMMA, SCOUT-O3 and TROCCINOX field campaigns, is ~3 Tg(N) a-1, with a most likely range of ~1-8 Tg(N) a-1. This mean value obtained from our tropical campaigns is slightly lower than the mean value of ~5 Tg(N) a-1 given in our recent review article on LNOx (Schumann and Huntrieser, Atmos. Chem. Phys., 2007), which was based on all available literature up to publication date.