Measurements of nitrogen oxides at the tropopause: Attribution to convection and correlation with lightning

NO_x (NO and NO₂) and ozone were measured on 98 flights during August to November 1997 in the framework of the projects Pollution From Aircraft Emissions in the North Atlantic Flight Corridor (POLINAT 2) and Subsonic Assessment Ozone and Nitrogen Oxide Experiment (SONEX). The fully automated measurement system Nitrogen Oxides and Ozone Along Air Routes (NOXAR) was permanently installed aboard an inservice Swissair B-747 airliner operating in the North Atlantic Flight Corridor. Below the tropopause, predominantly over the U.S. east coast, the patchy occurrence of NO_x enhancements up to 3000 parts per trillion by volume (pptv) was observed frequently and led to a lognormal probability density function of NO_x. These plumes extend over several hundred kilometers. In three case studies the origin of such plumes was investigated using back trajectories, satellite infrared images, and lightning observations from the U.S. National Lightning Detection Network (NLDN) and the Optical Transient Detector (OTD) satellite instrument. In the case of frontal activity above the continental United States, the location of NO_x plumes was explained with maps of convective influence. In another case, NO_x seems to have been produced by lightning in a marine thunderstorm over the eastern Atlantic. Lightning activity triggered over the warm Gulf Stream is found to be an important source for the regional upper tropospheric NO_x budget, at least for the time period considered. With a method that we call "lightning tracing" we show for the first time that (in some cases) the number of lightning flashes, accumulated along back trajectories, was proportional to the NO_x concentrations observed several hundred kilometers downwind of the anvil outflows. We suggest that mixing processes in convective clouds reduce the initially highly heterogeneous NO_x field rapidly, but that following this phase, the structure of large-scale plumes remains stable over relatively long periods of time (as they decay).