The top panels show simulated monthly mean CO and ozone concentrations at 300 hPa for the same time period. The lower left panel shows GEOS-CHEM monthly mean horizontal flux of North American anthropogenic CO averaged over the 400-150 hPa column for July 2000. The lower right panel shows vertical profiles of ozone concentrations at Huntsville, Alabama (86.6W, 34.7N) in July 2000. The thin solid lines are five ozonesonde soundings made during the month (July 1, 8, 15, 22, and 29) by Newchurch et al. [2003]. The thick solid line shows the model monthly mean values.The thin dotted, dashed, and dash dot lines show the simulated enhancements from North American anthropogenic, biogenic, and lightning emissions, respectively, as determined by difference with simulations where these emissions are shut off. The thick long dashes line shows the model monthly mean values from a simulation with lightning NO_x emissions over North America increased by a factor of four.

Deep convection plays an important role in ventilating the central and southeastern United States in summer. The semi-permanent anticyclone centered in the upper troposphere over the southern United States can circulate this convective outflow over the United States for several days before eventual export to the Atlantic (lower left panel). Rapid ozo ne production (up to 10 ppb day^-1 takes place in the circulating outflow. This production is driven in part by anthropogenic and lightning NO_x, and in part by HO_x radicals produced from convectively lifted CH₂O that originates from biogenic isoprene. It takes place near the transition between NO_x-limited and NO_x-saturated regimes for ozone production. We thus predict a large anthropogenic maximum in ozone (> 80 ppb) in the upper troposphere across the southern United States (top panels). Some evidence for this maximum is apparent in ozonesonde observations over Alabama (lower right panel). For a detailed account please see Li et al. [2004].