On the Tropospheric Ozone Variations in a Coastal Area of Southwestern Europe under a Mesoscale Circulation

It is well established that surface ozone levels are greatly affected by orography, solar radiation intensity, meteorological conditions, and the levels of their precursors. In this work, the authors study the sea-land breeze circulation in its relation with the behavior of ozone in a coastal area, located in southwestern Europe, with high levels of solar radiation and an important industrial complex emitting air pollutants. Hourly mean data over a 7-yr period (1999-2005) have been used in the study. Two patterns of sea-land breeze have been identified after analyzing 2298 days of measurements: a "pure'' breeze (179 cases) and another one, called a "nonpure'' breeze (284 cases), which is the resulting flow of the former and northwesterlies synoptic forcing. Among other results, the highest levels of surface ozone were observed under pure sea-land breeze, with hourly values up to 100 mu g m(-3) in the mean daily evolution. In contrast, for a nonpure breeze, the 24-h average daily value was lower than the corresponding one under a pure breeze by a factor of 1.16 and could reach 1.60 in representative real cases. These findings give evidence that the formation and accumulation of ozone are favored by the conditions under a pure sea-land breeze: that is, perpendicular wind directions toward the coastline, effective recirculation of air masses, and formation of ozone residual layers above the sea. Because these atmospheric conditions occur in other coastal regions in the world, the conclusions of this study could be extended to them.