Transport of tropospheric ozone over the Bay of Biscay and the Eastern Cantabrian Coast of Spain

During the 1989 field campaigns of the European Commission's Mesometeorolog ical Cycles of Air Pollution in the Iberian Peninsula (MECAPIP) project (19 88-91), airborne data were obtained under typical summer synoptic weather c onditions, that is, a ridge of the Azores high pressure system extending ov er the north coast of the Iberian Peninsula combined with a thermal low in the interior of the peninsula. This paper presents a detailed analysis of t he flights over the coastal area of Bilbao and a high-resolution simulation of the trajectories of the polluted air masses to determine the mechanisms leading to the type of stratification observed. Over the Bay of Biscay, on the eastern Cantabrian coast, the data showed the presence of two distinct groups of atmospheric strata with high ozone concentrations. The higher la yers contained older pollutants, trapped between an inversion at about 1200 m and a near-isothermal layer at about 2000 m, and were moving under persi stent north to northeasterly winds. The lowest layers, close to the surface in the Bilbao area, contained recently emitted pollutants. Back trajectori es computed by using the 3D wind field simulated by the Regional Atmospheri c Modeling System (RAMS) have shown that the source of the pollutants in th e upper layers, 36 h before, was within a region near the English Channel.