A tropospheric ozone maximum over the Middle East

The GEOS-CHEM global 3-D model of tropospheric chemistry predicts a summert ime O-3 maximum over the Middle East, with mean mixing ratios in the middle and upper troposphere in excess of 80 ppbv. This model feature is consiste nt with the few observations from commercial aircraft in the region. Its or igin in the model reflects a complex interplay of dynamical and chemical fa ctors, and of anthropogenic and natural influences. The anticyclonic circul ation in the middle and upper troposphere over the Middle East funnels nort hern midlatitude pollution transported in the westerly subtropical jet as w ell as lightning outflow from the Indian monsoon and pollution from eastern Asia transported in an easterly tropical jet. Large-scale subsidence over the region takes place with continued net production Of O-3 and little mid- level outflow. Transport from the stratosphere does not contribute signific antly to the 03 maximum. Sensitivity simulations with anthropogenic or ligh tning emissions shut off indicate decreases of 20-30% and 10-15% respective ly in the tropospheric O-3 column over the Middle East. More observations i n this region are needed to confirm the presence of the O-3 maximum.