Tropospheric Ozone over the Middle East and Its Interannual Variability: An Integrated Analysis with Satellite Observations and a Global Chemical Transport Model

Tropospheric ozone is a major atmospheric pollutant and a greenhouse gas. Nevertheless, many processes influencing its spatio-temporal distribution are still poorly understood, mainly due to the lack of adequate observations. One such region is the Middle East, where ozone measurements are scant. In this study, the GEOS-Chem chemical transport model is used to interpret newly available tropospheric ozone data from the Tropospheric Emission Spectrometer (TES) satellite instrument. TES reveals elevated ozone in the mid-troposphere (500-300 hPa) over the Middle East in summer 2005. This study demonstrates that the Arabian anticyclone in the mid-troposphere over the Middle East plays a critical role in facilitating the buildup of ozone. Additionally, the South Asian High in the upper troposphere helps transport ozone from the Asian monsoon region. Transport from Asia and local production are predominantly responsible for the ozone buildup, each contributing 30-35% to the ozone abundance in the region. Ozone transported from the boundary layer accounts for about 25% of local production. TES retrievals of water vapour and deuterated water are used for the first time to provide an independent assessment of the ozone transport pathways. Using a GEOS-Chem simulation from 1987 to 2006, it is found that this ozone buildup fluctuates interannually by about ±7% (or ±6 ppbv). The major contributors, ozone transported from Asia and ozone produced locally, vary by ±30% (±7 ppbv) and ±15% (±3 ppbv), respectively. The variations of Asian and local sources are related to the strengths of the South Asian High and the Arabian anticyclone, respectively. It is found that in years when the Asian influence is weaker in the region, transport from other areas, such as North America, is enhanced. This tradeoff between transport from Asia and other regions is found to be linked to the position and strength of the subtropical westerly jet over central Asia. These results suggest that climate-related changes in the general circulation of the atmosphere will have implications for the transport of pollution into the Middle East. Such changes in pollution in the region could have feedbacks on the climate through changes in the radiative forcing associated with tropospheric ozone.Ph