The observations show a decline in ozone starting in mid-winter. Calculations with a numerical model have shown that this decline is due to chemical depletion by reactive chlorine and bromine compounds. By late winter, the model predicts that a ring of ozone depletion will be present around the continent. Our modern instrument was purchased in 1989 from Dr Jean-Pierre Pommereau of Service d'Aeronomie du CNRS, near Paris, and is now one of a network. The initiative for the purchase came from Dr Joe Farman, showing once again his intuition for putting the right ozone-layer instruments in the right place in Antarctica.
Background notes on the instrument.
Because the absorption bands are weak, the instrument must observe their structure over this wide range of wavelengths to discriminate absorption by ozone from scattering by air molecules in the atmosphere. However, because the light is at visible wavelengths, ozone can be measured even when the sun is at the horizon. By looking at sunlight scattered from the overhead sky, both the older and the modern instruments measure absorption by ozone even when the sky is cloudy. Modern computers allow us to relate the amount of ozone measured in the path of this scattered light to the amount of ozone in a vertical path. The calculation is more accurate for light at visible wavelengths than at UV wavelengths because there is less scattering at visible wavelengths.