A tropospheric ozone maximum over the Middle East

The GEOS-CHEM global 3-D model of tropospheric chemistry predicts a summertime 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 consistent with the few observations from commercial aircraft in the region. Its origin in the model reflects a complex interplay of dynamical and chemical factors, and of anthropogenic and natural influences. The anticyclonic circulation in the middle and upper troposphere over the Middle East funnels northern midlatitude pollution transported in the westerly subtropical jet as well 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 significantly to the 03 maximum. Sensitivity simulations with anthropogenic or lightning emissions shut off indicate decreases of 20-30% and 10-15% respectively in the tropospheric O-3 column over the Middle East. More observations in this region are needed to confirm the presence of the O-3 maximum.