Temporal patterns of surface ozone levels in different habitats of the North Western Mediterranean basin

A systematic temporal analysis of surface ozone observations in four sites representative of the main types of habitats: coast, mountain, inland and urban in the North Western Mediterranean basin is presented here for the period between 1994 and 2001. Concentrations were relatively high, especially in the coastal site where the European human and plant protection thresholds were surpassed an average of 54 and 297 days per year, respectively. We observed a 22% decrease in the ozone concentrations along the past decade in the coastal site, a 14% increase in the mountain site, and no significant change in the inland and urban sites. The annual cycle of ozone in the coastal site (maximum concentrations in spring) was typical of areas with higher influences of background ozone, whereas in the other sites, the broad spring-summer maximum indicated dominance of local photochemical production. The site differences were also evident in their diurnal cycles, which were very evident in all sites except in the coastal one. There was thus significant variability among the monitoring sites, due to both the influence of local and background sources and the complex orographic and meteorological conditions of this region. (C) 2003 Elsevier Ltd. All rights reserved.