The History of Total Ozone Measurements; the Early Search for Signs of a Trend and an Update

The developments of a physical base for optical measurements of total ozone, starting at the end of the nineteenth century along with the pioneering work by Fabry and Buisson, who recorded in 1920 the first few weeks of daily measurements by comparing the intensity of solar light at two wavelengths, is presented here. The work by G.M.B. Dobson and collaborators in adopting a curved-prism Fery spectrograph, adding a Cl-Br filter for long wavelengths and quartz wedges, building a few such instruments, dispatching them to a dozen places around the world, and analysing the data for possible use for weather forecasting is of fundamental importance in the development of total ozone measurements. Measurements were not made regularly and data before the International Geophysical Year (IGY) are in general not reliable. Special attention is given to the quality control (QC) and the possibility of data re-evaluation for achieving homogeneous records of great importance for detection of ozone changes. The search for ozone trends had started in the 1970s considering the possible destruction of the ozone owing to a large SST fleet and then to the CFCs. Scientific studies on all aspects of the ozone issue expanded. A scrutiny of the series of International Ozone Assessments starting in 1981 provided the background for the Vienna Convention and its Montreal Protocol. However it was only in the 1980s that careful re-evaluation of ozone data permitted the NASA/WMO Ozone Trends Panel to detect statistically significant ozone decline in the Northern latitudes, although much smaller than the rapid ozone decline noted in the Antarctic springs. The ozone decline in both hemispheres accelerated toward the 1990s and a number of interesting facts are presented using objective characteristics as the ozone mass deficiencies (O3MD) from the pre-1976 averages. In mid-1990s the major decline in the lower stratosphere was better understood and the essential role of the stratospheric temperature and dynamics affecting the Arctic ozone decline wasalso confirmed. In 1993 and 1995 in the Northern latitudes the O3MD surpassed the spring-deficiencies over Antarctica. Total ozone deficiencies of 4-6% from pre-1976 averages continue to be observed over extra-polar latitudes except in the tropics (25 degrees N-25 degrees S). They are stronger in winter-spring than in the summer season and are very much influenced by the meteorological conditions of nearly equal concentrations of ozone depleting substances. The largest spring-O3MD over Antarctica was in 2006, the intensity of the loss, per unit surface covered by <220 matm-cm, increased and there are no signs of reduction of the ozone losses there.