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

NOx (NO and NO2) and ozone were measured on 98 flights during August to Nov ember 1997 in the framework of the projects Pollution From Aircraft Emissio ns in the North Atlantic Flight Corridor (POLINAT 2) and Subsonic Assessmen t Ozone and Nitrogen Oxide Experiment (SONEX), The fully automated measurem ent system Nitrogen Oxides and Ozone Along Air Routes (NOXAR) was permanent ly installed aboard an inservice Swissair B-747 airliner operating in the N orth Atlantic Flight Corridor. Below the tropopause, predominantly over the U.S. east coast, the patchy occurrence of NOx enhancements up to 3000 part s per trillion by volume (pptv) was observed frequently and led to a lognor mal probability density function of NOx These plumes extend over several hu ndred kilometers. In three case studies the origin of such plumes was inves tigated using back trajectories, satellite infrared images, and lightning o bservations from the U.S. National Lightning Detection Network (NLDN) and t he Optical Transient Detector (OTD) satellite instrument. In the case of fr ontal activity above the continental United States, the location of NOx plu mes was explained with maps of convective influence. In another case, NOx s eems to have been produced by lightning in a marine thunderstorm over the e astern Atlantic. Lightning activity triggered over the warm Gulf Stream is found to be an important source for the regional upper tropospheric NOx bud get, at least for the time period considered. With a method that we call "l ightning tracing" we show for the first time that (in some cases) the numbe r of lightning flashes, accumulated along back trajectories, was proportion al to the NOx concentrations observed several hundred kilometers downwind o f the anvil outflows. We suggest that mixing processes in convective clouds reduce the initially highly heterogeneous NOx field rapidly, but that foll owing this phase, the structure of large-scale plumes remains stable over r elatively long periods of time (as they decay).