Summertime free tropospheric ozone pool over the Eastern Mediterranean/Middle East

Abstract

Observations show that the Mediterranean troposphere is characterized by a marked enhancement in summertime ozone with a maximum over the Eastern Mediterranean. This has been linked to enhanced ozone photochemical production and subsidence under cloud-free anticyclonic conditions. The Eastern Mediterranean region has among the highest levels of background tropospheric ozone around the globe and it can be considered as a global air pollution hotspot. A 12 yr climatological analysis (1998–2009) of free tropospheric ozone was carried out over the region based on ECMWF (European Centre for Medium-Range Weather Forecasts) ERA-interim reanalysis data and simulations with the EMAC (ECHAM5-MESSy for Atmospheric Chemistry) atmospheric chemistry climate model. EMAC is nudged towards the ECMWF analysis data and includes a stratospheric ozone tracer. A characteristic summertime pool with high ozone concentrations is found in the middle troposphere over the Eastern Mediterranean/Middle East (EMME) by ERA-interim ozone data, which is supported by Tropospheric Emission Spectrometer (TES) satellite ozone data and simulations with EMAC. The enhanced ozone over the EMME is a robust feature, propagating down to lower free tropospheric levels. The investigation of ozone in relation to potential vorticity and water vapour and the stratospheric ozone tracer indicates that the dominant mechanism causing the free tropospheric ozone pool is downward transport from the upper troposphere and lower stratosphere associated with the enhanced subsidence and the limited outflow transport that dominates the summertime EMME circulation. The implications of these summertime high free tropospheric ozone values on the seasonal cycle of near surface ozone over the Mediterranean are discussed

The geographical distribution of meteorological parameters associated with high and low summer ozone levels in the lower troposphere and the boundary layer over the eastern Mediterranean (Cairo case)

“…1 with one slight difference: the signs of subsidence (positive omega, dry air) are shifted somewhat towards the east. This is in agreement with the observations made by Zanis et al (2014) and Tyrlis et al (2014) that the southeastward shift from the upper to lower levels might be related to nearly isentropic sloping and downwelling of the air masses towards southeast. In addition, strong northern flow over the eastern Mediterranean is observed at 850 hPa, especially on the days preceding the measurement, which, as mentioned before, is very frequently observed in the area during summertime (Repapis et al, 1977).…”

“…Similar observations of high upper troposphere ozone levels during anticyclonic conditions have been reported in central Europe by analysing vertical MOZAIC profiles (Tressol et al, 2008) but also at rural sites in western, central and eastern Mediterranean following the analysis of surface ozone data (Kalabokas et al, 2008;Sa´nchez et al, 2008;Schu¨rmann et al, 2009;Velchev et al, 2011). Especially as it regards the eastern Mediterranean region, atmospheric conditions of strong summer anticyclonic subsidence in the lower troposphere, leading to high ozone concentrations, have been reported as a result of analysis of satellite data as well as large-scale atmospheric modelling simulations, with which our observations are in agreement (Li et al, 2001;Eremenko et al, 2008;Foret et al, 2009;Liu et al, 2009;Coman et al, 2012;Richards et al, 2013;Doche et al, 2014;Safieddine et al, 2014;Zanis et al, 2014).…”

“…Summer anticyclones, covering very large areas over North Africa, the Mediterranean basin and the European continent, are associated with the downward transport of upper troposphere ozone, especially at the interface of high-and lowpressure systems as it is also observed in Zanis et al (2014) and Tyrlis et al (2014). In addition, a frequent midsummer peak of troposphere ozone is observed over northern Europe and the Atlantic (Mantis et al, 1981;Thouret et al, 2006).…”

“…The most affected region within the Mediterranean basin seems to be the eastern Mediterranean, where the 60 ppb EU standard is very frequently exceeded during the warm period of the year and especially during JulyÁAugust, when the atmospheric subsidence over the area is a quite common atmospheric feature and almost quasi-permanent as it comes out from the average climatology of the omega vertical velocity (Doche et al, 2014;Zanis et al, 2014). In conclusion, in the Mediterranean during some high ozone events a large part of the measured ozone could not be attributed to the local or even regional emissions, which, for the affected countries might be a reason of reconsideration of the procedures of compliance to air pollution standards.…”

“…Modelling studies of the large-scale dynamics also suggest a strong influence of the Indian Monsoon on the dry Mediterranean climate in summer, that is, by westward Rossby waves, excited by the monsoon convection, interacted with the southern flank of the mid-latitude westerlies producing adiabatic descent and hence anticyclonic conditions at the surface over the eastern Mediterranean. Therefore, the low frequency variability of this flow is controlled by the South Asian monsoon Hoskins, 1996, 2001;Lelieveld et al, 2002;Ziv et al, 2004;Richards et al, 2013;Doche et al, 2014;Zanis et al, 2014). As a result, the eastern Mediterranean is influenced by advection from Europe in the lower troposphere, associated with the Etesian winds and subsidence in the middle and upper troposphere, which is in turn associated with the westerly flow interacting with the South Asian monsoon.…”