Build-up and decay of two ozone episodes through northern Iberia and southern France – An inter-regional transport analysis

Further analysis of the previously-reported relationship between blocking anticyclones over the British Isles and severe ozone episodes in the Basque Country (BC) has shown that an even wider area is affected by these situations. Two episodes have been studied. The analysis of ozone data from approximately 60 AirBase stations throughout the area showed that they were among the most severe episodes of ozone exceedances in all northern Iberia and south-western France. Concentrations have been found to be related to inter-regional transport in southern Europe, and the temporal length of the whole process (development and decay) has been found to range from 7 to 10 days. A temporal delay in the recording of maximum concentrations within the region has been observed and explained for the first time. A new pathway for the export of European pollutants towards the Atlantic Ocean has also been documented, along with the export of pollutants from the eastern Cantabrian Coast (CC) to Africa at the beginning of the episodes. A huge eastward transport of pollutants across the CC towards southern France, and along the Ebro Valley towards the Mediterranean Sea, has been documented during the dissipation phase, but may not be definitive. ► Blocking anticyclones are behind ozone episodes in a wide region of Southern Europe. ► The build-up and complete decay of these episodes can take from 7 to 10 days. ► Ozone concentrations throughout the area present a predictable behaviour. ► These concentrations are related with internal and inter-regional transport. ► A link between the local-to-intercontinental scales is shown in the episodes.