Abstract
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 favouring 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 suggests 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, in the Gulf of Genoa and between Naples and Palermo, respectively. The main cause of high ozone levels in the Gulf of Genoa was found to be outflow from the Po Valley and the Genoa area while such episodes along the Naples-Palermo transect were most often associated with trajectories from the Rome or Naples areas. Analysis of the relationship between measured concentrations of Black Carbon and ozone allowed to evaluate the degree of photochemical "ageing" of the air masses encountered along the route of the cruise ship.
Highlights
Ozone (O3), along with water vapour (H2O), carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O), is a greenhouse gas that changes the radiative balance of the Earth’s surface and contributes to climate change; it ranks as the fourth most important contributor to global warming (IPCC, 2007)
Data from monitoring stations as well as results of measurement campaigns show that ozone concentrations in the Mediterranean Basin appear to be relatively high: Lelieveld et al (2002) found summer O3 concentrations over the Mediterranean a factor of 2.5–3 higher than in the hemispheric background troposphere in the boundary layer and up to 4 km altitude
In an attempt to fill in the gap of observations in the Mediterranean basin and to gain more insight into the atmospheric dynamical and chemical mechanisms leading to high surface ozone levels, the Joint Research Centre of the European Commission (JRC, EC) has started regular ship borne measurements over the Mediterranean Sea
Summary
Ozone (O3), along with water vapour (H2O), carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O), is a greenhouse gas that changes the radiative balance of the Earth’s surface and contributes to climate change; it ranks as the fourth most important contributor to global warming (IPCC, 2007). Some campaigns included ozonesonde and/or flight data, and shipborne observations besides ground based measurements, they do not provide continuous observations over the open sea Such measurements are needed to assess the regional climate effects of ozone. In an attempt to fill in the gap of observations in the Mediterranean basin and to gain more insight into the atmospheric dynamical and chemical mechanisms leading to high surface ozone levels, the Joint Research Centre of the European Commission (JRC, EC) has started regular ship borne measurements over the Mediterranean Sea. In 2005 a collaboration has been established between JRC-EC and the Italian cruise line Costa Crociere. In this context a monitoring station for ozone and black carbon aerosols (BC) was installed on board of the cruise ship Costa Fortuna, which had a regular weekly route in the Western Mediterranean during spring, summer and autumn for two years (2006 and 2007) and in the Eastern Mediterranean in winter 2006
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