Abstract
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 photochemical ozone production and subsidence under cloud-free anticyclonic conditions. The eastern Mediterranean is among the regions with the highest levels of background tropospheric ozone worldwide. A 12 yr climatological analysis (1998–2009) of free-tropospheric ozone was carried out over the region based on the ECMWF (European Centre for Medium-Range Weather Forecasts) ERA-Interim reanalysis data and simulations with the EMAC (ECHAM5–MESSy) 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) in the ERA-Interim ozone data, Tropospheric Emission Spectrometer (TES) satellite ozone data and simulations with EMAC. The enhanced ozone over the EMME during summer is a robust feature, extending 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 the downward transport from the upper troposphere and lower stratosphere, in association with the enhanced subsidence and the limited horizontal divergence observed over the region. The implications of these high free-tropospheric ozone levels on the seasonal cycle of near-surface ozone over the Mediterranean are discussed.
Highlights
Ozone is central in the control of the oxidizing capacity of the troposphere (Chameides and Walker, 1973; Crutzen, 1988; Penkett, 1988)
Looking into the year-to-year relation between the free-tropospheric ozone pool and near-surface ozone, we found that the mean July–August observed O3 values at GR02 correlates positively with the mean July–August modelled ECHAM5–MESSy for Atmospheric Chemistry (EMAC) O3 and O3s values at 714 hPa (ρ = 0.53 and ρ = 0.57, respectively) over the period 1998–2008
A 12 yr climatological analysis (1998–2009) of freetropospheric ozone was carried out based on the ERAInterim reanalysis data and simulations with the EMAC atmospheric chemistry–climate model nudged towards European Centre for Medium-Range Weather Forecasts (ECMWF) analysis data
Summary
Ozone is central in the control of the oxidizing capacity of the troposphere (Chameides and Walker, 1973; Crutzen, 1988; Penkett, 1988). The EM is favourable to deep STT events because it lies southwards of the typical position of the polar front jet at the ending point of a pathway characteristic of stratospheric intrusions (Galani et al, 2003; Sprenger and Wernli, 2003) These high ozone levels can strongly impact regional air quality and radiative forcing. An important meteorological factor associated with these elevated boundary layer and surface ozone levels is the anticyclonic conditions over the central Mediterranean and the Balkans leading to enhanced photochemical ozone production and tropospheric subsidence (Kalabokas et al, 2007, 2008) Another important factor is the long-range transport of air masses from Europe, which are rich in ozone and ozone precursors, towards the sunlit EM, especially over the Aegean Sea, in addition to emissions from local sources (Zerefos et al, 2002; Kouvarakis et al, 2002).
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