Summertime buildup and decay of lightning NO_x and aged thunderstorm outflow above North America

This study explores the upper tropospheric anticyclone above eastern North America and its influence on the summertime buildup and decay of lightning NO_x(LNO_x) and thunderstorm outflow. LNO_x transport is simulated with a particle dispersion model that releases a LNO_x tracer from the locations of millions of cloud-to-ground lightning flashes during May-September 2004 and 2006. On average, upper tropospheric zonal flow in May transitions to a closed anticyclone above northern Mexico and the southern United States in July that strengthens in August and rapidly decays in September. Concentrations of the LNO_x tracer reach a maximum above the southern United States and Gulf of Mexico in July and August. Fourteen study sites across North America exhibit high day-to-day variability of the LNO_x tracer in the upper troposphere during summer, with the sites most heavily influenced by the North American summer monsoon having the greatest background concentrations. During late spring and September the western sites have low concentrations with little variability. In general, the west coast sites plus Barbados have the most aged thunderstorm outflow, while the east coast sites have the least aged outflow. More than 80% of summertime upper tropospheric NO_x above the eastern United States is produced by lightning. To produce the best available observation-based view of upper troposphere NO_x above North America, measurements from six aircraft campaigns are combined in a single composite plot. The modeled upper tropospheric NO_x matches the general continental-scale distribution of NO_x in the composite plot, supporting the dominant role of LNO_x in the simulations.