New ozone experiment on board

The technologies used in the American satellites that deliver information on the state of the Earth's ozone layer are now fifteen years old. So it is with very good reason that ESA is planning, with the ERS-2 launch in early 1995, to open a new chapter in ozone monitoring. A completely new instrument on board ERS-2, GOME (global ozone monitoring experiment), will use the modern, precise measuring techniques familiar from stratospheric balloons but whereas balloons take isolated measurements ERS-2 will generate a complete world ozone map every three days.

The hole in the ozone layer is also fifteen years old - at least as a topic at scientific conferences. At the start of the eighties chlorofluorcarbons were still regarded above all as an irreplaceable feature of refrigerators, air conditioning systems and airsprays and only a handful of scientists working in the rarefied world of atmospheric chemistry were concerned at their damaging effect on the environment. It was not until the mid-eighties, when a dramatic fall in the ozone content of the atmosphere above the Antarctic was recorded, that CFCs became headline news and a serious political issue on the international scene.

In 1987 the major industrial countries signed a protocol - the first of its kind - in which they agreed to phase out the 'ozone killers'. But the CFC content of the atmosphere will remain a matter of concern to humanity for some fifty years more, the time it is likely to take for these chemicals to disappear entirely from the upper reaches of the atmosphere.

So the danger has not been entirely eliminated, not even in the northern hemisphere, where, for the time being at least, no real hole has yet been observed in the ozone layer, unlike over the Antarctic. All the same, the concentration of ozone in the stratosphere has already fallen by three to nine per cent in our latitudes. The uncertainty attaching to these figures is however almost as large as the measured values themselves and it is therefore high time to establish once and for all, by means of precise measurements, the extent of ozone depletion in the northern hemisphere and to determine present trends.

In what way then does GOME differ from conventional instruments carried by American and Russian spacecraft? Like these instruments, GOME will investigate characteristic peaks in the absorption spectrum of sunlight scattered by molecules in the atmosphere beneath the satellite. Those peaks point to the presence of ozone. What sets GOME apart is its DOAS (differential optical absorption spectroscopy) measuring system. Using this technique, a solar spectrum is measured simultaneously and subtracted from the various absorption spectra, thereby making the measurements independent of variations in solar intensity and in the sensitivity of GOME's sensors. This increases accuracy by three to five times compared with absolute values.

GOME offers a further advantage over conventional instruments, namely that it covers a significantly broader spectral range - the three relevant ozone absorption bands (named after their discoverers, Hartley, Huggins and Chappuis) are encompassed by its 240-790 nm range. It is this feature which extends GOME's measurement range to cover almost all seasons and latitudes. Instruments operating only in the ultraviolet cannot take measurements in the polar regions in late winter - when the processes responsible for ozone depletion are building up - for at that time of year the sun is too low for ultraviolet light to reach those regions. So it is that in Spring, when lighting conditions again allow measurements to be taken, the hole in the ozone layer is literally "discovered". And it is because conventional satellites have a blind spot for ozone in late winter that stratospheric balloons equipped with measuring instruments have to be released every year in Northern Europe.

But even if weather conditions allow these balloons to take off, the data obtained give no more than local "snapshots".

Thanks to its broad spectral range and the DOAS measurement technique, GOME can also monitor other trace gases and, in the lower levels of the atmosphere, detect air pollution caused by aerosols and microparticles.

However, there is a price to pay for stealing a march on other space nations in ozone measurement from space - ESA cannot give a 100% guarantee that the GOME experiment will meet all expectations as such a claim would have called for double the development and testing time. This is why the engineers, technicians and scientists who took up the challenge of developing the world's most effective ozone instrument in just five years will be crossing their fingers when, at about half past two on a winter's night in early 1995, ERS-2 speeds towards its orbit over the Earth's poles.

Keywords: ESA European Space Agency - Agence spatiale europeenne, observation de la terre, earth observation, satellite remote sensing, teledetection, geophysique, altimetrie, radar, chimique atmospherique, geophysics, altimetry, radar, atmospheric chemistry