Atmospheric dynamics and elevated ozone concentrations in the northern Adriatic

The influence of mesoscale flow on the elevated O-3 concentrations recorded during 13-19 August 2000 in the coastal town of Rijeka (45.33 degrees N, 14.45 degrees E) has been examined. Although high levels of ozone concentration are often observed there, this episode was unique due to hourly afternoon concentrations persistently higher than 180 mu g m(-3) and nocturnal levels above 100 mu g m(-3). To study this episode, available meteorological and ozone measurements were analysed, along with supplemental numerical simulations and trajectories. The measurements revealed favourable atmospheric conditions for the production of ozone, i.e., high air temperatures associated with (1) the exchange of two dominant (although relatively weak) large-scale flows: southwesterly versus northeasterly bora winds, and (2) local thermal circulation. The bora wind was relatively weak and short lasting. Although the bora transported the polluted air mass away along the western Adriatic coast, the southwesterly flow returned ozone-rich air toward the Alps, thus contributing to the overall pollution over the northern Adriatic Sea. Models revealed the superposition of the southwesterly wind and local, thermally induced winds that caused the regional transport of ozone toward the Alps and the eastern Adriatic coast. However, while the regional transport of ozone from the northern Adriatic was found to enhance somewhat the ozone concentrations in Rijeka at the end of period studied, its surrounding local emission sources, uniform local thermal circulation systems, and recirculation of pollutants were crucial factors in the formation of the large daytime and night time ozone levels observed in Rijeka.