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Total ozone

Ozone plays an important role in the atmospheric chemical and radiative processes. It is formed from the photodissociation of molecular oxygen mainly in the stratosphere at low latitudes, absorbs a sinificant fraction of the solar ultraviolet radiation dangerous for living organisms (producing, as a consequence, the thermal structure of the stratosphere), is a greenhouse gas, and intervenes in many chemical reactions in the atmosphere. Observations from the surface, from balloons, and from space show that the global-average total column ozone amount for the period 1997-2001 was approximately 3% below the pre-1980 average values. This decay is extremely evident in the polar spring (“ozone hole”), and relatively strong at mid latitudes in winter.


Deseasonalized, area-weighted seasonal (3-month average) total ozone deviations, estimated from five different global datasets. Each dataset was deseasonalized with respect to the period 1979-1987, and deviations are expressed as percentages of the ground-based time average for the period 1964-1980. Results are shown for the region 60°S-60°N (top) and the entire globe (90°S-90°N) (bottom). The different satellite datasets cover 1979-2001, and the ground-based data extend back to 1964. TOMS, Total Ozone Mapping Spectrometer; SBUV, Solar Backscatter Ultraviolet; NIWA, National Institute of Water and Atmospheric Research (New Zealand). The figure is taken from WMO , Scientific Assessment of Ozone Depletion, WMO Report 47, 2002.

No significant change is detected in the tropical stratosphere. Conversely, an increase of tropospheric ozone is observed, possibly related to photochemical production in polluted conditions. Changes of the ozone distribution are expected to affect the ultraviolet radiation amounts that reach the surface, the atmospheric radiative budget (and temperature) through the greenhouse effect, and possibly influence many chemical processes and the atmospheric thermal structure.
 
Measurements of total ozone are carried out from the surface since 1920s, and routinely from space since 1978. Ground-based observations are useful to understand dynamical and radiative phenomena, and necessary to determine trends and verify the satellite stability, and are fundamental to understand the behaviour of the ultraviolet radiation.
 
Total ozone is measured at Lampedusa since 1998 with a Brewer spectrophotometer. These observations have shown that, in an unexpected way, a negative correlation exists between total ozone and aerosol optical depth. The correlation is due to the influence of synoptic circulation, that modulates total ozone and, at Lampedusa, aerosol transport from Sahara.


Figure 1: evolution of direct-Sun total ozone measurements, carried out with the Brewer spectrophotometer (black circles), and daily TOMS observations (red circles).The ticks indicate the beginning of the year