YESTERDAY was International Day for the Protection of the Ozone Layer, and this combined with the reappearance in recent weeks of the annual ozone hole over Antarctica, has focused attention once again on this fragile protective veil above our heads.
But how do we know what is going on up there? How do we measure increases and decreases in the amount of ozone in the stratosphere, and detect the existence of the "ozone holes" when they occur?
There are three standard ways of keeping tabs on stratospheric ozone. Data over a very large area can be obtained by satellite, using an instrument called TOMS - the total ozone mapping spectrometer. The satellite's strategic vantage point allows data for virtually a whole hemisphere to be gathered almost simultaneously. The information is displayed in the form of a brightly coloured map, with the various colours indicating the different concentrations of ozone in the atmosphere.
But in order to make sense of information from the satellite, and assign numbers to the various colours, it is necessary to have a selection of accurate readings taken from the ground.
This "ground truth", as it is called, is provided by instruments called spectrophotometers that point upwards and detect variations in the amount of ultraviolet light reaching the surface which, in turn, is a measure of the amount of ozone in the atmosphere.
This information, besides being useful for its own sake, is used to calibrate the data from the satellites.
Both these methods, however, measure what is called the "total ozone column"; they quantify the total amount of ozone in the vertical, all the way from the ground to the outer limits of the atmosphere.
They are less efficient at assessing how much ozone exists at any particular height. The best way to acquire this latter information is by means of "ozonesondes" - equipment sent aloft by hydrogen balloon which transmits the data for each level of the atmosphere back to earth by radio.
Here in Ireland, the intensity of ultraviolet radiation reaching the ground is measured continuously by UCG at the Mace Head Observatory in Galway, and also by Met Eireann.
Also at Cahirciveen, ozonesondes are launched regularly at Valentia Observatory as part of a project funded by the EU Stride programme and jointly organised by UCD's chemistry department and Met Eireann. The results from these ascents are processed and collated at the ozone research unit on the Belfield campus.