Abstract
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.
Highlights
Surface ozone is a pollutant harmful to both human health and vegetation (Levy et al, 2001; Fuhrer, 2009)
We analyze the two springtime episodes of high surface ozone-mixing ratios over the western Mediterranean and the surrounding area using a comprehensive combination of surface observations, Infrared Atmospheric Sounding Interferometer (IASI) satellite observations, meteorological maps, back trajectories and regional air quality modeling to understand the principal mechanisms contributing to these events
The purpose of the analysis is to identify areas of high subsidence in the free troposphere, which could potentially influence the examined surface ozone measurements, considering that the afternoon (12:00–18:00) ozone-mixing ratios are quite representative of the boundary layer values
Summary
Surface ozone is a pollutant harmful to both human health and vegetation (Levy et al, 2001; Fuhrer, 2009). Data analysis based on large-scale atmospheric modeling studies (Li et al, 2001; Richards et al, 2013; Zanis et al, 2014; Safieddine et al, 2014; Tyrlis et al, 2014) shows the importance of the vertical transport of ozone in the Mediterranean area, in its eastern part. Studies based on observations and trajectory modeling indicate that transport above the boundary layer and entrainment of ozone from the free troposphere has an important impact on surface ozone-mixing ratios at several sites in the western USA (Cooper et al, 2011; Langford et al, 2015). We analyze the two springtime episodes of high surface ozone-mixing ratios over the western Mediterranean and the surrounding area using a comprehensive combination of surface observations, Infrared Atmospheric Sounding Interferometer (IASI) satellite observations, meteorological maps, back trajectories and regional air quality modeling to understand the principal mechanisms contributing to these events
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