The saturation of ozone loss is further examined with its temporal and spatial evolution at different altitudes. Figure 4 shows the evolution of ozone inside the vortex (>65o equivalent latitude (EqL))18 at different vertical levels in the lower stratosphere between the potential temperatures 375 K (~12 km) and 575 K (~23 km) in October for the period 1979–2017. The measurements at different altitudes show similar features of time evolution of loss saturation. The ozone loss started in the early 1980s, intensified in the mid-1980s, the level of saturation first reached in 1985 and again in 1987, and it stayed around this saturation level to date at altitudes 375–550 K (~12–22 km). The loss saturation is conspicuous from 375 to 500 K (~20 km) in most winters and for most stations since 1987, but it seldom occurs above that altitude as found in 1998. Among the winters, however, there are two exceptions, the warm winters 1988 and 2002, as mentioned previously. The loss is unprecedented at altitudes 400–450 K, where all measurements show loss saturation at all stations since 1987. It is also interesting to note the loss saturation at higher altitudes at 500–575 K in 2013, and was a unique winter in this regard. In summary, the analyses at different vertical levels clearly show that the loss saturation first appeared in 1985, initiated the vortex-wide saturation in 1987 (e.g., all four stations show loss saturation values in 1987) and it continued to occur thereafter. The break in loss saturation in 1988 was due to a major warming, in which no measurement showed saturation threshold of ozone loss at any altitude. This is also evidenced by the lower stratospheric temperature measurements shown by Kuttippurath and Nair.3

Fig. 4 Vertical structure of ozone loss saturation. Ozone mixing ratios measured by ozonesondes at different Antarctic stations at potential temperature (nine potential temperatures) levels 375–575 K (~12–23 km) in October during the 1979–2017 period. The solid horizontal line connects the mean ozone values in each year. The horizontal dashed line represents 0.1 ppmv. The vertical dashed lines represent year 1987 and 2000 Full size image

The satellite measurements show loss saturation threshold of about 0.1 ppmv in 1985, and is evident at 375 K (Figure S2). The ozone values smaller than 0.1 ppmv again appeared in 1987 at 375–450 K and continued to occur in the succeeding winters too, except in 1988 and 2002. The Aura MLS measurements show the loss saturation in all winters from 2004 to 2017 at different altitudes in the lower stratosphere (375–525 K or ~12–21 km), and was very severe at 400 K, consistent with the ozonesonde measurements.19,20 The satellite measurements, however, show no loss saturation at 550 and 575 K, which could be due to the lower vertical resolution of the measurements at these altitudes. These results are further verified using box-whisker plots at different altitudes and are consistent with previous discussions (Figure S3), such as the loss saturation signatures (0.15 ppmv at the 1%-whisker) in 1985 at 375 K and the onset of vortex-wide loss saturation from 1987 onwards.

Ozone column characteristics

To examine the characteristics of ozone at the core of loss saturation layer, we computed the partial columns at 400–500 K from the ozonesonde measurements and compared to the total column ozone mapping spectrometer/ozone monitoring instrument (TOMS/OMI)21,22 overpass observations, and are displayed in Fig. 5. The comparison shows excellent agreement between ground-based and satellite measurements (Figure S4). As expected, the partial column ozone shows very small values at all stations after the first appearance of loss saturation in 1985. The estimated columns show about 25–80 Dobson Units (DU) in 1979–1984, 6–25 DU in 1985 and 15–60 DU in 1986. The measurements demonstrate the smallest and largest of that decade in 1987 (0.8–25 DU) and 1988 (25–40 DU), respectively. However, the columns show below 10 DU thereafter, with a gradual decrease that peaked (i.e., the smallest column) in 1998 with values of about 0.2 DU at South Pole. The coldest spring of 1997, 1998, 2001, 2006 and 2011 show partial columns below 1 DU, indicating the void of ozone in the lower stratosphere. The historic stations South Pole, Syowa, McMurdo and Neumayer exhibit ozone columns less than 50 DU in all years in 1989–2017; consistent with those observed from ozonesonde mixing ratio analyses. The South Pole soundings show smaller ozone columns (<10 DU) compared to the measurements from all other stations. There are high ozone column episodes at these stations (e.g., Marambio) due to the occasional excursions of vortex to the South American Peninsula. Additionally, most years, except the major warming years and 1990, show very small values; suggesting the saturation of ozone loss.

Fig. 5 Ozone column changes in the loss saturation layer. Total column ozone (TCO) distribution inside the vortex measured by TOMS/OMI and the partial column ozone computed from the ozonesonde measurements over the ozone loss saturation layer 400–500 K (~13–20 km) in the vortex in October during the 1979–2017 period. The solid horizontal line connects the mean ozone values in each year. The horizontal dashed lines represent 220 DU (i.e. the ozone hole criterion) in the upper panel and 10 DU in the lower panel. The vertical dashed lines represent year 1987 and 2000 Full size image

The TCO measurements from TOMS and OMI (Fig. 4, upper panel) show about 300–450 DU in 1979–1981, 210–400 DU in 1982–1983, 250–350 DU in 1984, 225–400 DU in 1985 and 80–400 DU thereafter. The measurements show below 220 DU in 1981–1982 and in all winters since 1987, and this is the TCO criterion used for defining “the ozone hole”.16 The winter followed experienced ozone values below the ozone hole TCO criterion at Neumayer and South Pole. In 1987, an unprecedented 110 DU was measured at the South Pole, and about 150 DU at Neumayer and McMurdo. Ever since, the South Pole and Neumayer soundings continued to measure around 120 DU except for the warm winters. On average, the total columns show around 220 DU in 1982, 1983 and 1985, but about 80–150 DU in other winters. Therefore, the TCO measurements attest the fact that the loss saturation has undoubtedly begun in 1987 and its healing started in 2001.

Trends in ozone loss saturation occurrences

To find the changes in ozone loss saturation with time, the saturation threshold reached at each altitude (from 375 to 575 K, nine altitudes) for each station from 1979 to 2017 is analysed. We divided the number of saturation occurrences by total number of observations in the respective years to find the proportion (in %) of loss saturation measurements. Figure 6 shows the percentage of saturation occurrences at different vertical levels in each year during the ozone hole period, from September through November, to include all saturation episodes in spring. These analyses, therefore, show the history of ozone loss saturation in Antarctica, as they show the onset, progress, and peak of loss saturation, and emergence of ozone hole recovery. For instance, at 375 K, the loss saturation occurrences show about 5% until 1990, rose to 45% by 1992, and stayed around 15–50% in 1992–2001. The loss saturation gradually decreased to 1% by 2014. Similarly, the loss saturation frequency increased from about 2% in 1985 to 60% in 2001 at 400 K, and from about 15% in 1987 to 52% in 2000 at 425 K (~15 km). However, the loss saturation occurrences show a steep decrease, such as from 60% to 1% (~4.9%/year) at 400 K and from 52% to 2% (~4.1%/year) at 425 K from 2001 to 2017. The temporal evolution of loss saturation frequency is very similar at other altitudes and in the satellite measurements (Figure S5). At 400 K, for example, the satellite measurements show about 6% in 1987 and that increased to 25% by 2001. It shows a clear negative trend from 2001 onwards at all altitudes, such as from 20% in 2001 to 2% in 2017 at 400 K. Although the trend in loss saturation frequency is no different at other altitudes, the highest frequency was observed in 2015 in the satellite measurements.

Fig. 6 Frequency of loss saturation occurrences. The percentage of number of saturation occurrences (the number of loss saturation occurrences divided by the total number of measurements) at different altitudes in the lower stratosphere at 375–500 K (~12–20 km) estimated from the ozonesonde measurements in spring (September–November). The vertical dashed lines represent year 1987 and 2000. The non-saturation years are not connected with the lines, to easily identify those events Full size image

We also examined the loss saturation occurrences at all altitudes in September–November from ozonesonde and satellite measurements (Figure S6). As shown by the ozonesonde measurements, the loss saturation was about 1% in 1985, 2–3% until 1991 and then it increased to 21% by 1992. The number of saturation measurements stayed around 15–20% until 1996, rose to 29% by 1998 and again increased to 34% by 2001. The proportion of saturation was highest (36%) in the very cold winter of 2006. It is also evident that loss saturation started to subside after 2001. The satellite measurements confirm the ozonesonde observations of loss saturation, albeit with smaller amplitude due to their relatively coarser vertical and horizontal resolution.

The box plots made from ozonesonde measurements clearly depict values below 0.1 ppmv from 1987 onwards, as illustrated in Fig. 7. The 1%-whiskers show a progressive increase in the number of saturation occurrences at 400 K from 1987 onwards. The 1%-whiskers also show the largest ozone values in 1988 and 2002, and the smallest in 1992 and 1996 (very cold winter). Nevertheless, the whiskers show a gradual reduction in the number of saturation occurrences in the post-2001 period. Similarly, the satellite measurements illustrate the first appearance of loss saturation in 1987 (Figure S3, 400 K) and the healing indicated by its fewer occurrences in the 2001–2017 period. It is evident from both analyses that the loss saturation has started to decrease substantially after 2001, in agreement with the decline in ODSs.

Fig. 7 Trends in ozone loss saturation. The box-whisker plot of ozone mixing ratios at 400 K (~13 km) as measured by the ozonesondes in the Antarctic in Spring (September–November) from 1979 to 2017. The box represents 25–75% and whiskers represent 1% and 100%. The mean is shown in red and median in black. The horizontal dashed line represents 0.1 ppmv Full size image

Henceforth, this study reveals clear observational indications that the Antarctic ozone inside the loss saturation altitudes is increasing and recovering, although the saturation of ozone loss is expected to continue to occur in very cold winters due to the still high levels of ODSs in the stratosphere, as the reduction processes of atmospheric burden of halogens are comparatively slow.23,24,25 The disappearance of near-zero ozone concentrations between 12 and 21 km is a key milestone in the recovery of Antarctic ozone and it would also mark the return of ozone to 1980 levels, as 1987 is the start year of occurrence of loss saturation.26 Since there are already significant changes in the southern hemispheric climate owing to the Antarctic ozone loss,27 the recovery from loss saturation is very likely to affect that. Although not discussed the policy aspect in detail, the recovery indicated in the loss saturation layer robustly suggests that the Montreal Protocol has definitely saved the ozone layer and climate of the southern hemisphere.