Long-term Background Ozone and Carbon Monoxide Measurements on the Maltese Islands

Abstract

Long-term measurements of ozone and carbon monoxide have been carried out on the Maltese islands. Particular importance has been given to the background station at Gordan lighthouse, Gozo. The evaluation of the data showed that monthly and yearly averages of background ozone mixing-ratios are amongst the highest in the world as observed at low-altitude stations in the mid latitudes, especially during winter. The seasonal development of ozone shows a maximum of monthly averages of 55.4ppb in April I May and a minimum of 44.lppb in December I January. The latter is about twice as high as that observed on the continent. During the four years of monitoring ozone showed a clear positive trend from 48.2ppb in 1997 to 52.2ppb in the year 2000 occurred. It was also observed that air masses that approach the lighthouse from the Northern Hemisphere show generally higher ozone concentrations. Correlation calculations between the two trace gases support the theory that these enhanced ozone levels are generated by photosmog chemistry. Events of exceptionally high ozone concentration of up to 100ppb have been observed in summer which could be related to photosmog episodes caused by pollution sources within the vicinity of the Maltese islands (e.g. Sicily and the marine traffic within the Strait of Sicily) and by Malta itself. The background carbon monoxide mixing-ratios observed on Gozo are slightly increased (::::JO%) compared to other maritime stations in the mid-latitudes and range between 224ppb in February and 135ppb in July as monthly averages. Highest CO concentrations (up to 400ppb) were recorded in connection with air directly transported from Malta to Gozo. The nearness to sources of primary pollutants, their position within the boundary-layer and the transport regime of local wind circulation systems determine the trace-gas distributions at the stations at Xewkija and Msida differently. In particular during the night and the rush-hour periods ozone becomes much more strongly depleted at these sites than at the lighthouse station. The analysis of wind measurements as well as vertical profiling experiments reveal that convection and the resulting circulation systems, such as the sea-breeze, prevent the accumulation of locally emitted pollutants in summer.