A trajectory analysis of summertime ozone pollution in Slovenia

Annual number of days with exceeded ozone threshold values at the ground level measuring stations in Slovenia ranges from a few days in the interior of the country to up to 25 days at the Mediterranean stations. The highest number of ozone exceedances is usually recorded in the southwestern part of Slovenia, close to the Adriatic Sea, where favorable meteorological conditions enhance ozone formation. Local emission sources in this part of the country cannot explain the measured level of pollution. In addition, high ozone concentrations are occasionally measured at some Slovenian rural sites. We performed an ozone analysis with respect to air mass origin to provide an insight into processes leading to the high measured values of near-ground ozone. Three dimensional back trajectories were computed with a 3 hour-time interval for four measuring sites in Slovenia, generating eight arrival times per day starting at 00 UTC, for the warm parts (April – September) of the years 2003 and 2004. Trajectory clustering was used to determine typical pathways of air masses. Ozone and basic meteorological characteristics of trajectory clusters were further analyzed and multiple comparison tests were applied to determine which pairs of clusters differ significantly in measured ozone. In addition, concentration weighted trajectory (CWT) and number density (ND) of »polluted trajectories« were calculated to provide additional information about possible emission source regions. Results suggest that high ozone concentrations appear most often in the short trajectory clusters of slow-moving air masses originated from SW. Moreover, CWT and ND fields confirm that high near-ground ozone levels in Slovenia are usually associated with trajectories originating from the area of the northern Adriatic Sea and its coastal regions. A heavily industrialized Po River Basin does not appear to have a significant direct impact on measured ozone concentrations in Slovenia, but may considerably contribute to the overall pollution over the northern Adriatic Sea together with coastal emissions.