Summertime tropospheric-ozone variability over the Mediterranean basin observed with IASI

The Mediterranean basin is one of the most sensitive regions of the world regarding climate change and air quality. This is partly due to the singular dynamical situation of the Mediterranean basin that leads to among the highest tropospheric ozone concentrations 5 over the Northern Hemisphere. Six years of summertime tropospheric ozone observed by the IASI instrument from 2007 to 2012 have been analysed to document the variability of ozone over this region. The satellite observations have been also examined in parallel with meteorological analyses (from ECMWF) to understand the processes that drive this variability. This work confirmed the presence of a steep west–east 10 ozone gradient in the lower troposphere with the highest concentrations observed over the eastern part of the Mediterranean basin. This gradient is mainly explained by the diabatic convection over the Persian Gulf during the Indian Monsoon, which induces an important subsidence of ozone rich air masses from the upper to the lower troposphere over the central and the eastern Mediterranean basin: IASI observations of 15 ozone concentrations at 3 km height show a clear summertime maximum in July that is well correlated to the maximum of downward transport of rich-ozone air masses from the upper troposphere. Even if this feature is robust over the six analyzed years, we have also investigated monthly ozone anomalies, one positive (June 2008) and one negative (June and July 2009) using daily observations of IASI. We show that the rel20 ative position and the strength of the meteorological systems (Azores anticyclone and Middle eastern depression) present over the Mediterranean are key factors to explain both the variability and the anomalies of ozone in the lower troposphere in this region.JRC.H.2-Air and Climat