The ozone loss process in the stratosphere is relatively well understood and much of the loss is driven by the chemical cycles involving chlorine and bromine compounds, whose abundance peaked around 2000 in the polar stratosphere. With the abatement of atmospheric loading of ozone depleting substances (ODSs) due to the implementation of Montreal Protocol, the extent of ozone loss is expected to have decreased since 2000. Early signs of the rebound of Antarctic ozone have already been reported. However, one of the main difficulties in determining accurate trends in ozone in the southern polar stratosphere is the complete loss of ozone there. The saturation of ozone loss, i.e., the total or near-zero destruction of ozone in the lower stratospheric layers, mostly at 13–21 km, has reportedly begun in 1991, although there are references for the near-complete loss of ozone at some lower stratospheric altitudes in McMurdo observations in 1987. Nevertheless, those studies did not mention “saturation” of the ozone loss there.
Both chemistry-climate and chemical transport modelling studies on the ozone loss and projection of ozone changes often struggle to simulate the features of loss saturation such as the altitude range of saturation and ozone values below 0.1 ppmv. Henceforth, precise information on the saturation of ozone loss in the Antarctic vortex would allow a better assessment of the evolution of ozone in that region towards its recovery detection. In addition, although recent studies indicate a significant positive change and a healing in the Antarctic ozone hole, details of ozone change in the loss saturation altitudes in the polar vortex is not clearly known yet, albeit there are studies on ozone loss for individual or few years together. This situation, thus, warrants a careful and detailed examination of ozone at these loss saturation altitudes to make a clear statement on ozone recovery and its robustness at this sensitive vertical region. Here, we present, to our knowledge, the first detailed long-term (four decades) analysis of Antarctic ozone loss saturation in terms of its first occurrence, timing, spatial differences, vertical spread, inter-annual changes and temporal evolution using high-resolution ozonesondes and satellite measurements inside the vortex for the 1979–2017 period.
Results and Discussion
The Antarctic ozone loss saturation is a unique phenomenon, which is more frequent and very severe at latitudes near the South Pole and hence, the balloon-borne vertical ozone profiles at the South Pole, Neumayer and Syowa research stations show near complete ozone destruction as the values are about 1 mPa (~0.1 ppmv) or smaller. In general, the ozone profiles illustrate that the complete loss of ozone begins at the altitude of about 350 K (~12 km) and vertically spreads up to 550 K (~22 km) with frequent loss saturation occurrences at 400–450 K (~13–16 km), where the total loss of ozone occurs at most stations.
The amount of ozone prior to the onset of ozone loss in spring at these altitudes is about 3–5 ppmv and ozonesonde measurements have an uncertainty of about 5% in the lower stratosphere. Therefore, we define 0.1 ppmv as the ozone loss saturation threshold, which represents about 95–99% of ozone loss in the lower stratosphere, depending on altitude. Additionally, this would represent the loss incurred due to ozone loss catalytic cycles alone. Note that the loss saturation threshold is different from the detection limit of ozonesondes.
The analyses with ozonesonde measurements reveal that the ozone loss saturation first occurred in October 1987 and continued to occur in all winters thereafter, except in the warm winters of 1988 and 2002.14,15 Note that, the first appearance of ozone loss saturation was in 1985 at Neumayer (there were no other station measurements and no references too), but there was no loss saturation in 1986. Hence, it can be argued that widespread saturation of Antarctic ozone loss began in the cold winter of 1987 and is found in all then available measurements. The near-complete loss of ozone in the lower stratosphere in 1987 was also found in some previous analyses, although never mentioned about the “saturation” of loss. Therefore, to date, the early 1990s have been regarded as the beginning of loss saturation in the Antarctic stratospheric ozone. The role of Pinatubo volcanic aerosols is undeniable in the large loss of ozone in 1992, when all measurements show values below the saturation threshold. Similarly, the very cold winters 2001, 2003, 2006 and 2015 show most measurements down to 0.1–0.01 ppmv. The saturation was very severe, in terms of its vertical and spatial expansion, from 1991 through 2000, but signature of a positive change with comparatively higher values in ozone is observed for the period 2001–2017. This could be partly due to the dynamics, as there were a few warm winters in 2010s (e.g., 2010, 2012, 2013 and 2017). Yet, as mentioned earlier, the profiles having ozone depletion up to 0.1 ppmv still indicate the loss of about 95–99% ozone at these altitudes.
Ozone Loss | Vertical, Spatial and Temporal Features
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.
This paper had been originally published in Nature Partner Journals. To cite the original work
Kuttippurath, J., P. Kumar, P. J. Nair and P. C. Pandey, 2018, Emergence of ozone recovery
evidenced by reduction in the occurrence of Antarctic ozone loss saturation. Climate and
Atmospheric Science (2018) 1:42; doi:10.1038/s41612-018-0052-6, DOI 10.1038/s41612-018-