Production of NOx in mid-latitude thunderstorms: Case studies using OMI data

We examine mid-latitude cases of NOx (NO + NO2) production by lightning (LNOx), evident in NO2 measurements from the Ozone Monitoring Instrument (OMI) on the Aura satellite. The NO2 enhancements associated with lightning (LNO2), which occur primarily in the mid-upper troposphere, are distinguished from low-level anthropogenic NO2 using model output from NASA’s Global Modeling Initiative (GMI) to identify background NO2 amounts. Measurements of LNO2 profiles from previous studies in and downwind of convective storms are employed in air mass factor calculations to convert the OMI NO2 slant columns into vertical column amounts. We employ improved off-line processing techniques developed in studies of tropical convective systems to minimize known artifacts in the OMI data. We also account for transport of LNOx, from the observed thunderstorms to the regions of interest where enhanced NO2 was observed by OMI, using the Lagrangian particle dispersion model, FLEXPART. We compare the model output with the OMI data. The LNO2 amounts from OMI are converted to LNOx amounts using GMI calculations of [NO2]/[NOx] ratios. Information on numbers of lightning flashes - primarily cloud-to-ground (CG) - observed by the National Lightning Detection Network (NLDN) and the World Wide Lightning Location Network (WWLLN) is combined with estimates of the fraction of all flashes that are CG flashes to obtain the total number of lightning flashes associated with the observed LNOx enhancements. From these results, we obtain the production of LNOx per lightning flash in these mid-latitude storms. We compare our results to previous studies of other mid-latitude storms and to studies of tropical and sub-tropical systems.