Twenty-two years of ozonesonde measurements at the South Pole

Since 1986, the Earth System Research Laboratory and its predecessors have been making weekly balloon ozone soundings at the South Pole Station in Antarctica. During the springtime ozone hole period, the sounding frequency is increased to 2–3 per week. The 2007 springtime minimum total column ozone at South Pole was 125 Dobson units, with the layer between 14 and 21 km showing a typical 95% loss of ozone. In contrast, the 2006 minimum total column ozone was 93 Dobson units and showed 99% ozone destruction in the 14–21 km layer. Owing to variations in meteorology and stability of the polar vortex, year to year variations in the severity of the ozone hole of this magnitude are expected. Analysis of the ozone loss rate in September indicates large interannual variability suggesting a dynamic component. Detailed analysis of the 22-year record is used to search for early signs of the beginning of ozone hole recovery. The conclusion is that up to the year 2007, no definitive signs of the beginning of ozone hole recovery have been detected at South Pole Station.


Introduction
While satellite remote sensing provides global coverage of ozone, balloon-borne ozone sensors provide an accurate measure of the vertical profile of ozone. They are very useful in defining sources and sinks of ozone. Together, satellite and balloon measurements have been very successful in studying the Antarctic ozone hole. This paper reports on ozonesonde measurments at the South Pole.
The first 10 years of ozonesonde studies from the South Pole, which began in 1986, were presented by Hofmann et al. (1997). They found that Antarctic springtime ozone depletion had worsened during the 1986-1995 period but had reached a pseudo-equilibrium during the 1992-1995 period, reaching nearly zero ozone in the 14-18 km region. They found that ozone depletion had extended into the 22-24 km region and identified the high altitude depletion region as a possibly useful indicator of future ozone hole recovery. They investigated the ozone loss rate in September when the ozone hole formed and showed that especially above 18 km it varied with the quasi-biennial oscillation in equatorial winds with the maximum loss occurring during the spring following a descending easterly transition in the tropical winds. Using early equivalent chlorine projections they estimated that the beginning of ozone hole recovery would likely not be detected before the 2010-2020 period. It is the purpose of this paper to update the 10-year record with an additional 12 years of ozonesonde observations.

Observations
Balloon sounding of the Antarctic ozone hole is important as it indicates the altitude region where chemical ozone depletion is occurring. It allows accurate estimates of total and partial column ozone amounts during darkness when most satellite measurements cannot be made. With adequate sounding during September, when the ozone hole is formed, the ozone loss rate, which is a function of the abundance of ozone-destroying chemicals, can be determined as a function of altitude. Balloons also allow investigation of the upper altitudes where the depletion phenomenon is not complete. For the past 22 years, the Earth System Research Laboratory and its predecessors have been making weekly balloon ozone soundings at the South Pole Station in Antarctica. During the springtime ozone hole period, the sounding frequency is increased to 2-3 per week, allowing resolution of the rapid decline in ozone experienced during September. Figure 1 shows South Pole ozone profiles before ozone depletion began (generally August) and at the time of maximum ozone depletion (generally early October) for the 22 ozone holes of 1986 to 2007. In figure 2, profiles are shown for the year 2006 when the largest ozone hole both in area and severity occurred. The 14-21 km region of total ozone depletion is indicated in the figure and will be used to study the time variation of the severely depleted region. The upper (22-24 km) region, where ozone is only partially depleted, is also indicated in the figure. The latter may be a useful region to study ozone hole recovery (Hofmann et al. 1997). Figure 3 shows the 14-21 km average temperartures and indicates no trend in winter with a minimum near -90 C. There is some evidence for a trend toward colder summers however. In figure 4 the winter-spring temperatures are broken out monthly and show that while no trends exist in July and August, there may be marginally significant trends in June (warming) and September (cooling). The unusually warm September 2002 temperature was related to the early splitting of the vortex that year. Figure 5 shows the 14-21 km column ozone from 1986 to 2007. The near-zero ozone values that began in about 1993 ceased in the unusual year of 2002. Figure 6 shows the minimum 14-21 km column ozone measured each year from 1986 to 2007. This figure shows that while 2006 was the lowest at less than 2 DU, the years 1993 to 2001 were consistently low and that considerable variability set in following the major warming which split the vortex in 2002. This variablity is believed to be related to enhanced atmospheric wave activity which disturbs the polar vortex and disrupts the depletion phenomenon. Figure 7 shows the annual cycle of column ozone in the 14-21 km region for each year from 1986 to 2007. This figure clearly displays the degree of variability of ozone in the 14-21 km altitude range throughout the year. The years 1988 and 2002 stand out as years of considerable vortex variability. The September data (day 244 to 273 in figure 7) shows the precipitice drop in ozone in the 14-21 km region during formation of the ozone hole. As in Hofmann et al. (1997), this region will be studied for variations in the ozone loss rate.  Figure 8 shows ozone column data versus time for both total column ozone and 14-21 km ozone during September of 2006. The ozone loss rate (Dobson units per day) and its standard error can be determined from curves such as these for each year. Figure 9 shows the profile of ozone loss rates and standard errors for 2-km averages from 10-12 km to 22-24 km from 12 soundings during September 2006. The peak in ozone loss rate is well-resolved in the 16-19 km region. Figure 10 shows similar data for each September for the 22 years. The uncertainties are similar to those in figure 9. There appears to be no time variation in the height of the peak loss rate. Figure 11 shows the time variation of the ozone loss rates for total ozone and for the 14-21 km column ozone. The re-analysed data for the years 1986 to 1995 are essentially identical to those presented in Hofmann et al. (1997) because they are from the same raw dataset. However, the increasing severity in the ozone loss rate from 1986 to 1995, identified in Hofmann et al. (1997), surprisingly shows large variations after 1995, both for total ozone and for the 14-21 km column.

Discussion
In the following we will examine possible sources for the ozone loss rate variability and investigate further the evidence or non-evidence for the beginning of ozone hole recovery. The slope of the ozone versus time curves (see figure 8) can be affected by several variables. Obviously, an important variable is the amount of active ozone-destroying chemicals that are present in the vortex during September. It can also be affected by the winter temperature and the stability of the polar vortex during September. Winter temperature appears not to have  (Varotsos 2002), data prior to 22 September could be used to obtain a valid slope. The value of ozone in August, prior to the September depletion period, could have an affect on the slope if the depletion process were dependent on the ozone concentration. To test this, we have plotted the average August ozone at 14-21 km versus the ozone loss rate at that height in figure 12 for the 22 years of data. There appears to be a slight dependence (larger ozone loss rates for higher August ozone) but the dependence is rather weak and should not have a major effect on ozone loss rates. Thus it appears that the variability of September ozone loss rate from year to year is related mainly to the amount of ozone-destroying chemicals present. As indicated earlier (see figure 11), the South Pole September ozone loss rates show significant interannual variability. As in Hofmann et al. (1997) we have examined the data for a quasi-biennial oscillation (QBO) variation in the ozone loss rates. The results are shown in figure 13. Except for QBO cycles which do not have a well-defined  1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 Peak loss rate 15-20 km We will now examine the data in more detail for evidence of a beginning of ozone hole recovery. Figure 14 shows the time variation of all chlorine and bromine atoms measured in various molecules at the surface of the earth, converted to an 'equivalent' chlorine value using a factor of 60 applied to the more reactive bromine atoms (Montzka et al. 1996, Hofmann andMontzka 2009). This equivalent chlorine (ECl) parameter peaked at the surface in 1994. It is believed that it requires about 6 years (Newman et al. 2006) for gases emitted at the surface in the northern hemisphere (the predominant source) to reach high southern latitudes, enter the vortex at high altitudes and descend into the ozone depletion region. Thus, the maximum in chemical ozone depleting potential should have occurred in the Antarctic stratosphere about the year 2000. Figure 15 shows the time variation of the 14-21 km ozone loss rate, the measured ECl parameter, the World Meteorological Organization (WMO) scenario for future levels of ECl (WMO 2007), and equivalent effective stratospheric chlorine (EESC) (Newman et al. 2006). It is clear from the figure that there is only a general relation between the ozone loss rate in the heart of the ozone hole and EESC, becoming, on average, greater as EESC increased in the early 1990s. While full recovery of the ozone loss rate to pre-1980 values is not expected until the decade of the 2080s, a reduction in ozone loss rate should be observable before 2050, but not as early as suggested from the pre-1996 data (Hofmann et al. 1997) owing to the increased variability in ozone loss rates after 1995.
We can explore other possible indicators of the beginning of ozone hole recovery. Figure 16 shows the September ozone loss rate at the top of the ozone hole at 22-24 km. In this region, ozone depletion was not present before 1993. Similar to the 14-21 km region, the ozone loss rate displays significant variability in the 22-24 km region, again likely related to interannually varying dynamic effects. It is expected that ozone depletion in the 22-24 km region will cease as the ozone hole begins its expected recovery in the future. As suggested by the variability in the loss rates in the figure, five or more consecutive years of no ozone loss at 22-24 km would be required to confirm this expected event. The temperature in the ozone depletion region is a useful parameter to compare ozone values from different time periods as it partially compensates for dynamic effects (Solomon et al. 2005). Figure 17 shows the ozone mixing ratio versus temperature at two levels (70 and 30 hPa) and for four different time periods from 1966 to 2007. The early data (1966)(1967)(1968)(1969)(1970)(1971)(1972), obtained from ozonesonde measurements made at the South Pole over 40 years ago, show ozone mixing ratios above 1 ppm for typical stratospheric temperatures. The 1986-1989 period

Summary and conclusions
In total, 22 years of ozonesonde measurements (approximately 1400 soundings) at the South Pole from 1986 to 2007 have been analysed in order to characterize the time variation of the springtime Antarctic ozone depletion process. Following a developmental stage from 1986 to 1992, the ozone hole minimum, which occurs in late September to early October, reached a low plateau averaging about 5 DU in the 14-21 km region from 1993 to about 2001. A sizable degree of variability followed; however, the lowest ozone minimum observed, about 2 DU, occurred in 2006. This observation, and comparisons of ozone mixing ratios versus temperature in the ozone hole, lead us to conclude that the beginning of the recovery of the ozone hole has not yet been observed at South Pole Station. It was also observed that the September ozone loss rate, when the ozone hole is forming, has become highly variable since about 1996. This variability correlates with the quasi-biennial oscillation (QBO) in tropical winds, suggesting a dynamic cause of the variability. The measurements of ozone by balloon at the South Pole have contributed considerably to our knowledge of the Antarctic ozone hole and the success of the Montreal Protocol in lessening the severity of ozone depletion in Antarctica. Adherence to the Protocol will assure recovery of the Antarctic ozone hole in the future and continued monitoring of the ozone hole will verify this recovery.  1966-1972 1986-1989 1990-2004 2005-2007 1966-1972 1986-1989 1990-2004 2005-2007 70 hPa (~17 km)