¹Physics Department, University of Malta, Msida, Malta MSD 06.
²Institut für Meteorologie und Klimaforschung, Forschungszentrum Karlsruhe/Universität Karlsruhe, 76021 Karlsruhe, Federal Republic of Germany.

Background Measurements:
Extensive measurements of background ozone and carbon monoxide have been carried out on the island of Gozo (36°  4.4´ N, 14°  13.2 E) which are not previously available for the central Mediterranean. Ozone concentration and several meteorological parameters have been measured continuously since 1997; carbon monoxide since 1998.
Analysis of the ozone measurements shows that the background concentrations are very high. During the four years investigated the annual average ozone mixing ratio rose steadily from 48.2 ppbv in 1997 to 52.2 ppbv in 2000. This concentration is, on average, one of the highest worldwide for low altitude stations (Monks, 2000).
A wide concentration maximum of ozone occurs in April/May followed by a variable secondary maximum in the summer months and a minimum in winter.
Anthropogenic ozone formation also appears to be relevant in winter due to the temperature and solar radiation flux being relatively high for the Mediterranean area during this time of the year.
Episodes of very high ozone levels up to 102 ppbv have been observed during summer which can be related to photosmog processes initiated on Malta itself (see case study below), surrounding Mediterranean countries and international shipping through the Strait of Sicily.
The CO mixing ratios show a rapid increase at the beginning of the year culminating in a maximum in February when the monthly average reaches 196 ppbv. This is followed by a constant decline to a broad minimum between July and October when levels drop to 127ppbv. These values are similar to other observed maritime values (Novelli et.al. 1998).

Comparison with ozone measurements from the late nineteenth century:
Historical ozone measurements made on Gozo between 1886 and 1900, using a form of the Schönbein test, were recently discovered and are presently being evaluated. The annual behaviour of ozone in the late 1800ís shows how dramatically the situation in the central Mediterranean has changed within the last century.

References:
Cartalis C., Varotsos C.; Surface ozone in Athens, Greece at the beginning and the end of the Twentieth Century, Atmos. Environ., 28 (1994), 3-8.
Monks P.S; A review of the observations and origins of the spring ozone maximum, Atmos. Environ. 34 (2000) 3545-3561.
Novelli P.C., K.A. Masarie, P.M. Lang; Distributions and recent changes of carbon monoxide in the lower troposphere, J. Geophys. Res. 103, D15, (1998) 19015-19033.