Ten years of ozonesonde measurements at the south pole: Implications for recovery of springtime Antarctic ozone

Ten years of ozonesonde data at the south pole are used to investigate trends and search for indicators that can be used to detect Antarctic ozone recovery in the future. These data indicate that there have been no systematic winter temperature trends at altitudes of 7-25 km and thus no expected changes in stratospheric cloud particle surface area, which would affect heterogeneous chemistry. Springtime ozone depletion has been very severe since about 1992, with near-total loss of ozone in the 14- to 18-km region, but has lessened somewhat in 1994 and 1995, probably because of the decay of the sulfate aerosol from the Mount Pinatubo eruption which was present at 10-16 km. Sulfate aerosol particles from the Pinatubo eruption resulted in new ozone depletion in 1992 and 1993 in the 10- to 12-km region where it is too warm for polar stratospheric clouds (PSCs) to form. The volcanic aerosol also augmented depletion related to PSCs at 12-16 km. Although ozone depletion was not as severe in 1995 as in 1993, the depleted region remained intact longer than ever, with record low values throughout December in 1995. Since about 1992, a pseudo-equilibrium seems to have been reached in which springtime ozone depletion, as measured by the total column or the ozone in the 12- to 20-km main stratospheric cloud region, has remained relatively constant. Independent of volcanic aerosol, ozone depletion has extended into the upper altitudes at 22-24 km since about 1992. There is some indication that ozone depletion has also worsened at the bottom of the depletion region at 12-14 km. Extensions of the ozone hole in the vertical dimension are believed to be the result of increases in man-made halogens and not due to changes in particle surface area or dynamics. A quasi-biennial component in the ozone destruction rate in September, especially above 18 km, is believed to be related to variations in the transport of halogen-bearing molecules to the polar region. A number of indicators for recovery of the ozone hole have been identified. They include an end to springtime ozone depletion at 22-24 km, a 12- to 20-km mid-September column ozone loss rate of less than about 3 Dobson Units (DU) per day, and a 12- to 20-km ozone column value of more than about 70 DU on September 15. It is estimated that if the Montreal protocol and its amendments, banning and/or limiting substances that deplete the ozone layer, is adhered to, recovery of the Antarctic ozone hole may be conclusively detected from the aforementioned changes in the vertical profile of ozone as early as the year 2008. Future volcanic eruptions would affect ozone at 10-16 km, making detection more difficult, but indicators such as depletion in the 22- to 24-km region will be immune to these effects.