Abstract

The GEOS-CHEM global 3-D model of tropospheric chemistry driven by assimilated meteorological observations predicts a pronounced summertime O₃ 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 in the region from commercial aircraft. We investigated the factors that lead to the maximum in the model with tagged O₃ tracer simulation and sensitivity simulations. 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 midlatitudes 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. Strong large-scale subsidence over the region takes place with continued net production of O₃ and little mid-level outflow. Transport from the stratosphere does not contribute significantly to the ozone maximum. Sensitivity simulations with anthropogenic or lightning emissions omitted indicate maximum effects on O₃ over the Middle East with decreases of 25-30% and 10-15% respectively in the tropospheric O₃ column. More observations in this region are needed to confirm the presence of the O₃ maximum, which is of interest both as a test of our understanding of tropospheric O₃ chemistry and because of its implications for anthropogenic climate forcing.