Atmos. Chem. Phys. Discuss., 10, 6129-6165, 2010
© Author(s) 2010. This work is distributed
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Ozone over the Western Mediterranean Sea – results from two years of shipborne measurements
K. Velchev, F. Cavalli, J. Hjorth, E. Marmer, E. Vignati, F. Dentener, and F. Raes
European Commission, Joint Research Centre, Institute for Environment and Sustainability, Via E. Fermi, 21020 Ispra (VA), Italy

Abstract. Ozone, along with other air pollutants, has been measured for two years from a monitoring station placed on a cruise ship that follows a regular track in the Western Mediterranean between April and October. Conditions favoring high ozone levels have been studied by analysis of weather maps and back trajectories. This analysis was focused on a transect over the open sea in the South Western Mediterranean between Tunis and Palma de Mallorca. High ozone levels were found in situations with an anticyclonic circulation over the Western Mediterranean when subsidence brings air masses down from altitudes between 1000 and 3500 m a.s.l. Analysis of composite meteorological maps suggest a relevant contribution of breeze circulation to subsidence during events with high surface ozone concentrations; this points to an important contribution from local ozone formation. A detailed back trajectory analysis of the origin of air masses with high ozone concentrations was carried out for two "hot spots" for ozone pollution, found along the coast south of Genova and between Napoli and Palermo, respectively. While it was found that the influence of plumes from areas with high pollutant levels might explain most episodes in the Northwestern transect, such "local" influences appeared to be of minor importance within the Napoli-Palermo transect.

Citation: Velchev, K., Cavalli, F., Hjorth, J., Marmer, E., Vignati, E., Dentener, F., and Raes, F.: Ozone over the Western Mediterranean Sea – results from two years of shipborne measurements, Atmos. Chem. Phys. Discuss., 10, 6129-6165, doi:10.5194/acpd-10-6129-2010, 2010.
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