NOAA’s measurements of ozone at South Pole registered a record low for the month of October. This year, the average amount of ozone measured by a Dobson instrument at South Pole in the second half of October was 114 Dobson Units (DU), or 60 percent below the pre-ozone hole conditions. The October ozone average has historically been based on 15-31 October levels, and is an indicator of the amount of ozone depletion directly over South Pole. Ozone column amounts lower than the minimum thickness of 220 DU are deemed as the threshold for onset of an “ozone hole” each austral spring. Much of this is driven by depletion at altitudes between ~13 and 21 km high, where ozone is removed almost completely.

The NOAA ozone record at the South Pole station began in 1963 following the first efforts by the Weather Bureau to measure total column ozone with the Dobson instrument in 1958. The South Pole record is one of the longest in Antarctica, and measurements prior to 1979 are used as the benchmark to define the pre-ozone hole natural ozone levels. Since the 1980s an ozone hole regularly develops over Antarctica due to high levels of ozone-depleting substances. The levels of these gases have been reduced somewhat since then by the international agreement of the Montreal Protocol and continue to decline at about 1% per year.

Overall, the stratospheric ozone layer is recovering slowly from damage done by these compounds, but year-to year variability in meteorological conditions over Antarctica can result in an unusual event, such as this year's size of the ozone hole at 28.2 million square kilometers – the third largest ozone hole on record. Large fluctuations in the 50-year record of total ozone measured at South Pole demonstrate the influence of meteorology on the October-averaged total column ozone variability. The symmetry of the vortex observed this year over the Antarctic continent allowed the low ozone to persist much later into the month.

More information: http://gml.noaa.gov/dv/spo_oz/