1971: South Pole total ozone data before the ozone hole existed
1986: First year of continuous balloon soundings at the South Pole
1993: Record low total ozone related in part to the Pinatubo volcanic eruption (see below)
2003: This year is an example of contemporary ozone depletion at the South Pole while stratospheric chlorine remains near its peak. Stratospheric chlorine is not expected to increase substantially in the future and interannual variations in the magnitude of ozone loss will be determined by atmospheric dynamics and temperature variations, the latter affecting the occurrence of polar stratospheric clouds necessary for ozone depletion. However, a major volcanic eruption during the next 20-30 years, while stratospheric chlorine levels are high, will result in the additional ozone loss of about 20 DU in mid-October, as observed in 1993.
The South Pole balloon measurements of the vertical profile of ozone can be integrated to obtain the total column ozone. During most soundings, the balloon obtains altitudes of about 30 km and does not sample the atmosphere above this altitude where 10% or less of the ozone resides. The amount to add on above the highest altitude obtained by the balloons can be estimated from ozone climatology obtained by the NOAA SBUV satellite instrument. Using this procedure, total ozone can be measured at the South Pole even in darkness when instruments which use ultraviolet radiation to measure ozone are inoperative. The total ozone determined in this way is shown in the figure for several years during the July 1 to December 31 period, which includes the spring when sunlight returns to the South Pole and the ozone hole develops. On October 6,1993, the lowest total ozone value ever recorded on Earth (89 DU with an uncertainty of 5% using the SBUV extrapolation) was observed. This was mainly due to chlorine-catalyzed heterogeneous chemistry on volcanic aerosol particles from the Pinatubo eruption in the lower stratospheric 10-15 km region where it is normally not cold enough to form polar stratospheric clouds in great abundance. Antarctic total ozone is not expected to reach these extremely low values until another major volcanic eruption occurs during the period when stratospheric chlorine remains high (the next 20-30 years).