Articles | Volume 17, issue 6
Research article
22 Mar 2017
Research article |  | 22 Mar 2017

An investigation on the origin of regional springtime ozone episodes in the western Mediterranean

Pavlos Kalabokas, Jens Hjorth, Gilles Foret, Gaëlle Dufour, Maxim Eremenko, Guillaume Siour, Juan Cuesta, and Matthias Beekmann

Abstract. For the identification of regional springtime ozone episodes, rural European Monitoring and Evaluation Programme (EMEP) ozone measurements from countries surrounding the western Mediterranean (Spain, France, Switzerland, Italy, Malta) have been examined with emphasis on periods of high ozone-mixing ratios, according to the variation of the daily afternoon (12:00–18:00) ozone values. For two selected high ozone episodes in April and May 2008, composite NCEP/NCAR reanalysis maps of various meteorological parameters and/or their anomalies (geopotential height, specific humidity, vertical wind velocity omega, vector wind speed and temperature) at various tropospheric pressure levels have been examined together with the corresponding satellite Infrared Atmospheric Sounding Interferometer (IASI) ozone measurements (at 3 and 10 km), CHIMERE simulations, vertical ozone soundings and HYSPLIT back trajectories. The observations show that high ozone values are detected in several countries simultaneously over several days. Also, the examined spring ozone episodes over the western Mediterranean and in central Europe are linked to synoptic meteorological conditions very similar to those recently observed in summertime ozone episodes over the eastern Mediterranean (Kalabokas et al., 2013, 2015; Doche et al., 2014), where the transport of tropospheric ozone-rich air masses through atmospheric subsidence significantly influences the boundary layer and surface ozone-mixing ratios. In particular, the geographic areas with observed tropospheric subsidence seem to be the transition regions between high-pressure and low-pressure systems. During the surface ozone episodes IASI satellite measurements show extended areas of high ozone in the lower- and upper-troposphere over the low-pressure system areas, adjacent to the anticyclones, which influence significantly the boundary layer and surface ozone-mixing ratios within the anticyclones by subsidence and advection in addition to the photochemically produced ozone there, resulting in exceedances of the 60 ppb standard.

Short summary
The main atmospheric mechanisms linked with spring surface ozone episodes over the western Mediterranean are examined. It comes out that high surface midday ozone values are usually linked with regional ozone episodes, which are strongly influenced by some specific meteorological conditions. The better understanding of the ozone variability in the lower troposphere and the boundary layer over the examined regions will help in the formulation of more effective policies in environment and climate.
Final-revised paper