Estimating the production of nitrogen oxides by lightning from satellite remote sensing
Final Report Abstract
Lightning is an important natural source of nitrogen oxides in the atmosphere, with strong impact on atmospheric chemistry. But the amount of NOx actually produced by lightning is still highly uncertain. Satellite measurements provide a new and independent dataset of long time-series with global coverage. Within the project, a systematic analysis of the sensitivity of satellites for freshly produced LNOx has been performed within a model study, indicating that thunderstorms with high lightning activity should produce enough LNOx to be visible from space. However, a subsequent empirical study based on SCIAMACHY observations could not confirm this expectation: only for some few events, enhanced NO2, spatially and temporally corresponding to flash occurence, could be found; but in most cases, virtually no NO2 enhancement at all could be observed. Our results are not compatible to current literature estimates of the mean LNOx production of about 15×10^25 molec/flash. We investigated different possible reasons for this discrepancy, but could not find a fully satisfying explanation. One peculiarity of the investigated lightning events, however, is that they are morning-time thunderstorms mostly over ocean, while global lightning activity is dominated by afternoon/evening flashes over the continents. It will thus be very interesting to perform a similar study for continental thunderstorms in the late afternoon/early evening, which hopefully can be done for upcoming geostationary satellite instruments like Sentinel 4 (scheduled for 2019). Lightning is obviously a highly variable phenomenon, and the production of NOx by lightning seems to be highly variable as well. Thus, one has to take care not to select just those studies showing a clear NOx signal and dismiss the others, as this would lead to an overestimation of the mean lightning NOx production. Consequently, LNOx estimates in literature might be biased high since positive findings are more likely to be published than negative ones (publication bias). In addition, there are significant indications that the production efficiency per flash shows regional differences. We find highest values around the US eastcoast and over the Mediterranean. Thus, global LNOx production estimates extrapolated from measurements over the US are probably biased high.
Publications
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Estimating the NOx produced by lightning from GOME and NLDN data: a case study in the Gulf of Mexico. Atmos. Chem. Phys., 6, 1075-1089, 2006
Beirle, S., N. Spichtinger, A. Stohl, K. L. Cummins, T. Turner, D. Boccippio, O. R. Cooper, M. Wenig, M. Grzegorski, U. Platt, and T. Wagner
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Sensitivity of satellite observations for freshly produced lightning NOx. Atmos. Chem. Phys., 9, 1077–1094, 2009
Beirle, S., M. Salzmann, M.G. Lawrence, and T. Wagner
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Direct satellite observation of lightningproduced NOx. Atmos. Chem. Phys., 10, 10965-10986, 2010
Beirle, S., Huntrieser, H., and Wagner, T.
