Evidence for a recurring eastern North America upper tropospheric ozone maximum during summer

Daily ozonesondes were launched from 14 North American sites during August 2006, providing the best set of free tropospheric ozone measurements ever gathered across the continent in a single season. The data reveal a distinct upper tropospheric ozone maximum above eastern North America and centered over the southeastern USA. Recurring each year, the location and strength of the ozone maximum is influenced by the summertime upper tropospheric anticyclone that traps convectively lofted ozone, ozone precursors and lightning NOx above the southeastern USA. The North American summer monsoon that flows northward along the Rocky Mountains is embedded within the western side of the anticyclone and also marks the westernmost extent of the ozone maximum. Removing the influence from stratospheric intrusions, median ozone mixing ratios (78 ppbv) in the upper troposphere (> 6 km) above Alabama, near the center of the anticyclone, were nearly twice the level above the U. S. west coast. Simulations by an atmospheric chemistry general circulation model indicate lightning NOx emissions led to the production of 25-30 ppbv of ozone at 250 hPa above the southern United States during the study period. On the regional scale the ozone enhancement above the southeastern United States produced a positive all-sky adjusted radiative forcing up to 0.50 W m(-2).