J Kuttippurath

F Lefèvre

S Raj

P Kumar

K Abbhishek

Prof J Kuttippurath

The ozone hole measurements at the Indian station Maitri in Antarctica

Keywords: Antarctic ozone hole, Stratosphere, Climate Change, Maitri, Bharati

First-time analysis of chemical ozone loss at Maitri station in Antarctica Loss amounts to 170 DU or 40-50%,as for the Antarctic vortex average Largest loss in 2006 and smallest in 2002 in agreement with meteorology Maitri measurements are representative of the Antarctic vortex measurements

Introduction

Ozone is a highly reactive triatomic molecule and is abundant in the stratosphere at about 10-50 km, with its peak around 25-35 km in the tropical latitudes. The concentration of ozone in the stratosphere (i.e. the ozone layer) protects the earth from harmful ultraviolet (UV) radiations. Due to the increase in chlorofluorocarbons (CFCs) in the atmosphere after industrialisation, stratospheric ozone started to decline in the polar regions by late 1970s [START_REF] Farman | Large losses of total ozone in Antarctica reveal seasonal ClO x/NOx interaction[END_REF][START_REF] Huck | Interannual variability in Antarctic ozone depletion controlled by planetary waves and polar temperature[END_REF][START_REF] Müller | Simple measures of ozone depletion in the polar stratosphere[END_REF]WMO, 2018). Measurements from the ground and space have confirmed that the depletion of ozone layer started in the late 1970s and deepened further and reached a point of saturation in the Antarctic in the early 1980s (Kuttippurath et al., 2018a). The space-based total column measurements exposed the spatial extent of ozone hole [START_REF] Stolarski | Nimbus 7 satellite measurements of the springtimeAntarctic ozone decrease[END_REF]. This situation in Antarctica lead to the seasonal ozone loss and appearance of ozone hole over there (WMO, 2014). Since the stratospheric temperatures are lower, polar vortex is stronger and polar stratospheric clouds (PSCs) are more frequent and widespread, the ozone loss is strong in Antarctica as compared to that of the Arctic (WMO, 2018). The ozone loss is observed to be significant only in the colder Arctic winters such as 1995[START_REF] Nash | An objective determination of the polar vortex using Ertel's potential vorticity[END_REF], 2000, 2005, 2011and 2020[START_REF] Manney | Unprecedented Arctic ozone lossin 2011[END_REF], 2020;;[START_REF] Rex | Arctic ozone loss and climate change[END_REF]. In Antarctica, the ozone loss is severe in all winters and show similar loss of about 45-55% or around 175 DU [START_REF] Kuttippurath | Antarctic ozone loss in 1979-2010: first sign of ozone recovery[END_REF][START_REF] Sonkaew | Chemicalozone losses in Arctic and Antarctic polar winter/spring season derived from SCIAMACHY limb measurements 2002-2009[END_REF][START_REF] Tilmes | Chemical ozone loss in the Arctic and Antarctic stratospherebetween 1992 and 2005[END_REF]Huck et al., 2004). There were a few exceptions to this too, such as the winters with significant warming in 1988, 2002and 2019(e.g. Kanzawa and Kawaguchi, 1990;[START_REF] Kuttippurath | Variability in Antarctic ozone loss in the last decade (2004-2013): highresolution simulations compared to Aura MLS observations[END_REF].

Significant reduction in ozone has also been observed in the mid-latitudes due to the high levels of halogen compounds in the atmosphere since the 1980s ( Steinbrecht et al., 2011;[START_REF] Nair | Ozone trends derived from the total column and vertical profiles at a northern mid-latitude station[END_REF][START_REF] Tully | Trends and Variability in TotalOzone from a Mid-Latitude Southern Hemisphere Site: The Melbourne Dobson Record 1978-2012[END_REF]WMO, 2018). Studies observe a decrease in ozone of about -0.7%/yr in the mid-latitudes and 4-5% in the subtropics prior to 1997 during which the halogens peaked in the mid-latitudes [START_REF] Nair | Subtropical and mid-latitude ozone trends: implications for recovery[END_REF][START_REF] Krzyścin | Onset of the total ozone increase based on statistical analyses of global ground-based data for the period 1964-2008[END_REF][START_REF] Steinbrecht | Long-term evolution of upper stratospheric ozone at selected stations of the Network for the Detection of Stratospheric Change (NDSC)[END_REF]WMO, 2018). However, recovery of ozone is found afterwards, about -0.2±0.08 to -1±0.07%/yr in the mid-latitudes and -0.2±0.06 to -0.7±0.05%/yr in the subtropics in the upper stratosphere (e.g. [START_REF] Nair | Subtropical and mid-latitude ozone trends: implications for recovery[END_REF]. Similar estimates were also found in other longterm ozone trend studies for the mid-latitudes (e.g. [START_REF] Weber | Total Ozone Trends from 1979 to 2016 Derived from Five Merged Observational Datasets -The Emergence into Ozone Recovery[END_REF]WMO, 2018). The increase in ozone is attributed to the dynamics related to the quasi-biennial oscillation, El Niño-Southern Oscillation. Therefore, the mid-latitude and low latitude ozone change will be decided by the dynamics and the amount of stratospheric chlorine in the regions (e.g. Randell et al., 2002). On the other hand, the ozone changes in the high latitudes were attributed to the reduction in halogen loading in the atmosphere (e.g. WMO, 2018). Also, there is a clear reduction in frequency for the occurrence of ozone loss saturation over the period 2001-2017 in the Antarctic, which underlines the impact of phasing out of ozone-depleting substances (ODSs) through Montreal protocol (Kuttippurath et al., 2018a). Although statistically significant positive trends are observed in the Antarctic (e.g. [START_REF] Chipperfield | Detecting Recovery of the Stratospheric Ozone Layer[END_REF][START_REF] Kuttippurath | The signs of Antarctic ozone hole recovery[END_REF][START_REF] Solomon | Emergence of healing in the Antarctic ozone layer[END_REF]Salby et al., 2015), no clear trends are found in the Arctic ozone due to large dynamical variability there. of India and its operational and research agencies make different types of observation at the stations. The station Maitri is located in the rocky coasts of Queen Maud Land at the edge of the Antarctic continent, which is devoid of snow throughout the year and experiences wind from both ocean and inland [START_REF] Ganguly | Comparative Study of the Influence of Air Pollution on UVI at Maitri in Antarctica and New Delhi in India 2012[END_REF]. There are semi-regular total column ozone (TCO) measurements at Maitri since the early 1990s using Brewer Spectrophotometer. There are studies using ozone measurements from Maitri, but mostly discussed the seasonal and interannual changes in ozone, and their relationship with meteorology such as temperature, pressure and winds (e.g. [START_REF] Ali | Surface ozone characterization at Larsemann Hills and Maitri[END_REF][START_REF] Ganguly | Long Term Trend, Diurnal and Seasonal Variations of Atmospheric Ozone at Indian Antarctic Station Maitri[END_REF][START_REF] Ghude | Comparative study of the total ozone column over Maitri, Antarctica during 1997, 2002 and 2003[END_REF][START_REF] Lal | Compilation of ozonesonde observation over Schirmacher oasis east Antarctic from 1999-2007[END_REF]. In a long-term study using ground-based and satellite datasets for the period 1979-2001, the seasonal variation in TCO over Maitri was found to be more pronounced than other temporal variability. The study also found a declining trend in the monthly average TOC during the period 1979-1988 and was attributed to the increased CFCs in the stratosphere [START_REF] Ganguly | Long Term Trend, Diurnal and Seasonal Variations of Atmospheric Ozone at Indian Antarctic Station Maitri[END_REF]. In another study, TOC measured at Maitri showed good agreement with stratospheric temperature anomaly [START_REF] Ghude | Year-to-year variation of ozone hole over Schirmacher region of East Antarctica: A synopsis of four-year measurement[END_REF][START_REF] Ghude | Comparative study of the total ozone column over Maitri, Antarctica during 1997, 2002 and 2003[END_REF]. Although, there are studies using Maitri ozone measurements, the chemical ozone loss was never estimated with these measurements. The primary goal of the ozone measurements in Antarctica is ozone loss estimation and monitoring the temporal changes of ozone hole. Therefore, we estimate the chemical ozone loss analyses using the available measurements (6 years) of Maitri TCO measurements.

Methods

Ground-based and satellite measurements

Since we estimate ozone loss at Maitri, we use the ground-based measurements from the station in the Antarctic. Measurements from other Antarctic stations are also considered to compare with that of Maitri (e.g. [START_REF] Kuttippurath | Antarctic ozone loss in 1979-2010: first sign of ozone recovery[END_REF]. The co-coordinates of the stations including Maitri are shown in Figure 1. The figure also shows the potential vorticity (PV) maps on 22 September 2002 as the vortex was very much disturbed with a major warming and on 24 September 2006, when the Antarctic experienced the biggest ozone hole ever in the observed history. In both occasions, the Maitri station was inside the polar vortex, suggesting that the station is pretty much inside the vortex in most dynamical conditions. However, please note that the vortex position will be different at different altitudes. We use the measurements made by the Brewer Spectrometer (MkIV, 153) at Maitri. Brewer instruments employ differential absorption of ozone in the UV region for ozone measurements [START_REF] Brewer | Nitrogen dioxide concentrations in the atmosphere[END_REF]. Individual observations are made by looking at the direct sun on a clear sky and are averaged to a daily mean and the measurements are limited to SZA (solar zenith angle) <80°.

The measurements are better than that of the Dobson measurements and accuracy of the instruments are about 0.5% of TCO or 1 DU [START_REF] Scarnato | Long-term total ozone observationsat Arosa (Switzerland) with Dobson and Brewer instruments (1988-2007[END_REF]Kuttippurath et al., 2018b).

A comprehensive study on polar ozone measurements by different instruments with a detailed analysis on their strength and limitations are presented in Kuttippurath et al. (2018b).

We have used the winter measurements in Antarctica, which are considered from May through November. On the other hand, most Maitri measurements are performed during winter and spring months as compared to other stations in the Antarctic. The measurements are available for the years 1999-2006. However, the measurements in 2004 and 2005 were during the summer, which are not considered as we estimate the ozone loss in winter and spring. There are 40-64 days of measurements in each year and are mostly during the period September-November, because the observations were primarily aimed at monitoring the changes in Antarctic ozone hole. Additionally, the Brewer measurements are limited to the availability of sunlight and depend on SZA, which is why most measurements are done in spring.

The ground-based measurements at Maitri are compared to the TOMS (Total Ozone Mapping Spectrometer) and OMI (Ozone Monitoring Instrument) measurements of the respective years. The TOMS retrievals make use of two wavelengths: 331.2 and 360 nm, but 270-500 nm for OMI. The measurements during the period 1999-2003 are compared to the version 8.5 TCO from TOMS on-board Earth Probe (Wellemeyer et al., 2002). The 2006 measurements are however, compared to the OMI measurements as the TOMS instrument was not operational from 2005 onwards (e.g. [START_REF] Mcpeters | OMI Total Column Ozone: Extending the Long-Term Data Record[END_REF]. The uncertainty of satellite measurements (both TOMS and OMI) is about ±1-3% or the mean bias is about ±4 DU, which depends on the station and satellite instrument for for SZA < 84° (Kuttippurath et al., 2018b;[START_REF] Kroon | Comparing OMI-TOMS and OMI-DOAS total ozone column data[END_REF][START_REF] Balis | Validation of Ozone Monitoring Instrument Total Ozone Column Measurements Using Brewer and Dobson Spectrophotometer Ground-Based Observations[END_REF]Leveltet al., 2006).

Ozone loss calculation

We apply the passive method [START_REF] Goutail | Early unusual ozoneloss during the Arctic winter 2002/2003 compared to other winters[END_REF]Kuttippurath et al., 2010) to estimate ozone loss from TCO measurements at Maitri and other stations. However, note that there are different methods for estimating the ozone loss, as discussed in [START_REF] Harris | Comparison of empirically derived ozone lossrates in the Arctic vortex[END_REF]. The method uses a passive tracer simulated by a chemical transport model (CTM) and we use the REPROBUS model simulations for the passive tracer simulations [START_REF] Lefèvre | The 1997 Arctic ozone depletion quantified from three-dimensional model simulations[END_REF]. It is a three-dimensional CTM with a comprehensive description of stratospheric chemistry. The model runs were forced by the European Centre for Medium-Range Weather Forecasts (ECMWF) operational analyses, with 60 vertical levels from 1000 to 0.1 hPa. The model uses the hybrid sigma-pressure vertical coordinate and vertical advection is computed directly from the wind fields. Chemical species are transported by a semi-Lagrangian method [START_REF] Williamson | Two-Dimensional Semi-Lagrangian Transport with Shape-Preserving Interpolation[END_REF]. Note that we have considered only the passive tracer simulations from the model results. The tracer is initialised on first of July so that the ozone loss on that day would be zero. The chemical ozone loss is then estimated by subtracting the passive tracer from TCO measured by the ground-based instrument at respective stations.

Large changes in ozone in the polar regions are very much related to the circumpolar circulation, position of polar vortex, and the special meteorology related to the vortex in winter and spring there (e.g. [START_REF] Hood | Interannual Variations of Total Ozone at Northern Midlatitudes Correlated with Stratospheric EP Flux and Potential Vorticity[END_REF]Huck et al., 2004;[START_REF] Solomon | Stratospheric ozone depletion: A review of concepts and history[END_REF]. The polar vortex acts as a containment vessel and is effectively separated from the mid-latitude air. Very low temperatures inside the vortex and high winds at the edge of the vortex are the key feature of this dynamical system. This situation facilitates the formation of PSCs and the high winds at the vortex edge prevent mixing of air masses from the adjacent middle latitude regions (e.g., [START_REF] Drdla | Temperature thresholds for chlorine activationand ozone loss in the polar stratosphere[END_REF]. This makes a unique heterogeneous chemistry inside the vortex and set conditions for chlorine activation and ozone loss. Therefore, the ozone loss has to be estimated within the vortex. We have used the ECMWF meteorological files [START_REF] Dee | The ERA-Interim reanalysis: configuration and performance of the data assimilation system[END_REF] to compute the potential vorticity (PV) and then to estimate the edge of the vortex using the method defined by [START_REF] Nash | An objective determination of the polar vortex using Ertel's potential vorticity[END_REF] to segregate the vortex measurements. To avoid the PV filaments in vortex edge calculation, we have also used additional criteria of 35 PV units (pvu, and one pvu is 10 -6 Km 2 kg -1 s -1 ) and 63° Equivalent Latitude (EqL, [START_REF] Grooß | Ozone chemistry during the 2002 Antarctic vortex split[END_REF]. That is, the measurements are considered to be inside the vortex, when they satisfy the conditions of [START_REF] Nash | An objective determination of the polar vortex using Ertel's potential vorticity[END_REF] vortex edge criterion, > 35 pvu and > 63° S EqL conditions.

Results and Discussion

Meteorology of the winters

The Maitri station is located at 70° S, which is almost inside the polar vortex in most winters.

Figure 2 shows the average polar cap temperature in the Antarctic for different winters at 70 and 100 hPa. The temperature at 70 hPa (in the lower stratosphere) show about 195 K from July to mid-September in all winters, which gradually increases until the end of November. The temperatures from mid-September to November normally decide the ozone loss chemistry in the lower stratosphere and final warming of Antarctic winters. The temperatures below 195 K facilitate the formation of polar stratospheric clouds (PSCs) on which the halogens would be activated and make ozone loss when the vortex would be exposed to sunlight in spring. The evolution of lower stratospheric temperature is different in each winter [START_REF] Kuttippurath | Antarctic ozone loss in 1979-2010: first sign of ozone recovery[END_REF][START_REF] Tilmes | Chemical ozone loss in the Arctic and Antarctic stratospherebetween 1992 and 2005[END_REF][START_REF] Huck | Interannual variability in Antarctic ozone depletion controlled by planetary waves and polar temperature[END_REF]. For instance, the temperature increases from 195 K in mid-September to 206-212 K by the end of November in 1999November in , 2001November in , 2003November in , and 2006. . However, PSCs can still be present and thus, chlorine activation can drive ozone loss during the period [START_REF] Grooß | Strato-spheric ozone chemistry in the Antarctic: What controls the lowest values thatcan be reached and their recovery?[END_REF][START_REF] Müller | The maintenance of elevated active chlorine levels in theAntarctic lower stratosphere through HCl null cycles[END_REF]. There were minor warmings in early September 2000 and mid-September in 2006, about 2-5 K. Conversely, the winter 2002 was dynamically disturbed from July onwards with minor warmings and then this particular dynamical situation finally lead to a major warming with a vortex split event on 25 September 2002 (e.g. [START_REF] Hoppel | POAM III observations of the anomalous 2002 Antarctic ozone hole[END_REF]. This was the first-ever vortex split major warming event recorded in the measurement era. The temperature increased from 198 to 217 K from mid-to the end of September in that winter. Therefore, the warmest was 2002 and the coldest was 2006 among the analysed winters. The evolution of temperature is similar at 100 hPa, but about 3 K higher until mid-September and about 1-5 K smaller afterwards, depending on the winter. These dynamical conditions are also reflected in the TCO measured from satellites (TOMS-OMI) as illustrated in Figure 2.

The winters show minor warmings in mid-September and consequently resulted in distortion or elongation of the polar vortex in 2000, 2001, 2003 and 2006. However, a very quiet and calm September was evident in 1999 and therefore, an undisturbed, concentric and circular polar vortex is evident in that winter. In 2002, the first-ever vortex split event occurred in the southern polar region and is clearly captured by TCO map, as shown in Figure 2. Apart from that, we also wanted to show the position of Maitri station in different types of vortex disturbances. The maps depict that the station is always inside the vortex even during the vortex split event. This would make the station ideal for ozone loss and ozone trend studies in the southern polar region. We have also marked the Indian station Bharti in the maps to compare the position of both stations in terms of their vortex representation. It exhibits that Bharti station resides at the vortex edge on most occasions when the vortex is slightly disturbed. Note that we have shown the TCO with ozone hole 220 DU contour, not the vortex edge to demonstrate the position of polar vortex in the maps.

Temporal evolution of ozone

Figure 3 shows the ozone measurements at Maitri compared to the TOMS-OMI and the average of all ground-based station measurements (without Maitri) in Antarctica. In general, the Maitri measurements are made in September, October and November months. Therefore, the measurements show below 220 DU in September and early October. The measurements in November or even mid-October onwards show large interannual variability. This is an expected result as the minimum ozone in Antarctica usually occurs during the period [START_REF] Huck | Interannual variability in Antarctic ozone depletion controlled by planetary waves and polar temperature[END_REF][START_REF] Müller | Simple measures of ozone depletion in the polar stratosphere[END_REF][START_REF] Grooß | Strato-spheric ozone chemistry in the Antarctic: What controls the lowest values thatcan be reached and their recovery?[END_REF][START_REF] Kuttippurath | Antarctic ozone loss in 1979-2010: first sign of ozone recovery[END_REF]. Ozone increases gradually afterwards due to mixing with the mid-latitude air. Therefore, the ozone shows a steep decline in September, from about 250-200 DU to 150-110 DU, depending on winters.

The smallest ozone measurements in the winter are found in October, about around 100-120 DU in the coldest winters of 1999, 2003 and 2006. However, the ozone values show a slight increase (10-20 DU) from mid-October onwards such as in 2000. The November measurements exhibit ozone values between 150 and 300 DU in different winters. The situation in warm winters is slightly different as shown by the ozone distribution in 2002. As demonstrated by temperature, the winter was highly disturbed with minor warmings from July through September and then ended up with a major warming in late September with a vortex split. Therefore, excursion of vortex to mid-latitudes, and distortion, elongation and split of the vortex would cause such large and episodic changes in ozone distribution. There was an unusual chlorine deactivation, which is also one of the reasons of the high ozone values in 2002 [START_REF] Grooß | Ozone chemistry during the 2002 Antarctic vortex split[END_REF]. The intermittent high values of ozone throughout the winter months show the intense dynamical activity in the winter. Also, the high ozone values associated with the minor warmings in other winters (e.g. late September in 1999 and mid-September in 2006) and final warming (e.g. end of November in 2001November in , 2002November in and 2003) ) are clearly captured by the Maitri measurements. A similar distribution of ozone can be also found in the measurements at the Sejong station (62° S, 58° W) in the western Antarctica [START_REF] Koo | Total ozone characteristics associated with regional meteorology in West Antarctica[END_REF]. Note that the total ozone changes inside the vortex are largely decided by diabatic descent inside the vortex, the adiabatic motions related to planetary wave activity, and convergence or divergence related to the changes of tropospheric height. Therefore, the variations of ozone values are expected in daily measurements due to the dynamics. The TOMS-OMI measurements start by mid-August over the Antarctica and the overpass at Maitri shows good agreement with the ground-based observations throughout the winters.

The differences are mostly within 5%, which is also the uncertainty of the satellite overpass measurements [START_REF] Mcpeters | Validation of the Aura Ozone Monitoring Instrument total column ozone product[END_REF][START_REF] Kroon | Comparing OMI-TOMS and OMI-DOAS total ozone column data[END_REF]Kuttippurath et al., 2018b).

Nevertheless, the difference between the Brewer and satellite measurements at Maitri is slightly larger during warming periods as shown by the measurements in November 2003 and 2006. Previous analyses have also found similar results when ground-based measurements are compared to the satellite measurements. For example, [START_REF] Balis | Validation of Ozone Monitoring Instrument Total Ozone Column Measurements Using Brewer and Dobson Spectrophotometer Ground-Based Observations[END_REF] found an overestimation (about 2%) of TOMS measurement with ground-based measurements in the southern hemisphere. Similar results were also found when satellite measurements were compared to that of ground-based measurements in the southern latitudes for the period 1996-2000, where the estimation was about 3.3% [START_REF] Bramstedt | Comparison of total ozone from the satellite instruments GOME and TOMS with measurements from the Dobson network 1996-2000[END_REF]). An analogous result was observed when Zhongshan measurements were compared to EP-TOMS measurements for which the satellite measurements were 2.55% higher than that of ground-based measurements. The OMI measurements were also found to be overestimated (2-7%) the ground-based measurements at Marambio and Belgrano [START_REF] Balis | Validation of Ozone Monitoring Instrument Total Ozone Column Measurements Using Brewer and Dobson Spectrophotometer Ground-Based Observations[END_REF]Kuttippurath et al., 2018b). The satellite overpass measurements are the area-averaged value of about 100 km, which would be slightly different from the point-measurements made by the Brewer or ground-based instrument at the station. In addition, when the temperature increases in spring, the ozone loss stops and ozone hole reduces its size by November. This vortex erosion would be another reason of the differences between ground-based and space-based observations in spring and during warm winters.

On the other hand, the all station averaged data show the complete seasonal evolution of TOC in Antarctica. The ozone starts to decrease in early July, the reduction in ozone gradually progresses in August and the rate of decrease peaks in September. The lowest ozone values are observed during the end of September and early October, and the ozone picks up due to mixing with mid-latitude air afterwards until the end of November, as also found in the Maitri ground-based measurements. The station average and Maitri data show very good agreement up to the minimum ozone period (early October). The average data show slightly higher values as these data include measurements from the edge stations such as Marambio, Rothera and San Martin, where the ozone values are relatively higher than other stations (e.g. [START_REF] Kuttippurath | Antarctic ozone loss in 1979-2010: first sign of ozone recovery[END_REF][START_REF] Zhang | The relationship between polar vortex and ozone depletion in the Antarctic stratosphere during the period 1979-2016[END_REF]. As stated previously, the planetary wave activity can disturb the polar vortex at times, which would make some stations outside the vortex (e.g. Marambio and Rothera) and thus show slight differences in ozone measurements between satellite and ground-based measurements during warming periods and at the end of spring, as found in the figure.

Chemical ozone loss in the winters 1999-2003 and 2006

We have analysed the ozone loss in 6 winters; 1999-2003 and 2006, and have averaged the ozone loss values for 7 days to suppress the daily variations in ozone to make a more robust ozone loss estimate. In 1999, the winter was not very cold as compared to other winters. The measurements are mostly restricted to October and November, and there is a constant ozone loss of about 150 DU during the period. In 2000, the ozone loss was 60 DU in August and it increased to 130 DU in early September and peaked to 160 DU by end of September. The ozone loss in 2001 was very severe and the loss was about 60 DU in mid-August, 80 DU in early September and the loss peaked by early October to 160 DU. The ozone loss in the warm winter also show a similar loss of about 170 DU in October. The ozone loss evolution shows similar features and loss values in 2003 and2006, about 80 DU in September to 150-160 DU in October. The loss in peak period in October was relatively higher in 2006 and 2003, about 170 DU. The maximum ozone loss is generally found in early October as the PSCs terminates with relatively higher temperatures during the period.

We have also estimated the peak ozone loss in relative units for the period 15 September -15 October (ozone loss/passive tracer multiplied by 100). The maximum loss estimated in 1999 is about 43% in 1999, 57% in 2000, 56% in 2001, 46% in 2002, 57% in 2003 and 59% in 2006. The ozone loss in 2002 was slightly different from other winters, as the winter was warmer than all other winters. The year witnessed the first major warming with a vortex split event in the observational record, which occurred on 25 September 2002. The ozone loss shows a sharp decline afterwards (e.g. [START_REF] Feng | Three-dimensional model studyof the Arctic ozone loss in 2002/2003 and comparison with 1999/2000 and 2003/2004[END_REF][START_REF] Kuttippurath | Antarctic ozone loss in 1979-2010: first sign of ozone recovery[END_REF]. Therefore, the ozone loss was similar to that of other winters and the peak ozone loss in DU observed by mid-September was lower (10-20 DU) than that of other winters, about 46%.

The ozone loss estimated at Maitri compares very well with TOMS-OMI satellite measurements (mostly within 2 DU) in all winters, as also shown in Kuttippurath et al. (2010[START_REF] Kuttippurath | Antarctic ozone loss in 1979-2010: first sign of ozone recovery[END_REF]. It is interesting to note that the ozone loss estimated at Maitri is very similar to that of the all station average of other ground-based measurements in Antarctica. The differences are mostly about 1-2 DU throughout the winter, except for one or two measurements in some winters such as the loss estimated at Maitri for November 2003. This suggests that the Maitri measurements are representative of the entire Antarctic vortex and the measurements are highly valuable for monitoring the temporal changes in the Antarctic ozone hole. The ozone loss estimated for the winters show comparable values to that estimated by previous studies.

For instance, our results are comparable to the values estimated by Huck et al. (2007) and are about 105-120±10 DU for the winters discussed. The ozone loss at Maitri for the winters 1999 and 2000 are also comparable to that found from the POAM satellite measurements [START_REF] Hoppel | POAM III observations of the anomalous 2002 Antarctic ozone hole[END_REF] and SLIMCAT model simulations [START_REF] Feng | Three-dimensional model studyof the Arctic ozone loss in 2002/2003 and comparison with 1999/2000 and 2003/2004[END_REF], as they also estimate similar ozone loss values. Our analyses are in good agreement with the partial column loss estimated (130-145±10) by [START_REF] Tilmes | Chemical ozone loss in the Arctic and Antarctic stratospherebetween 1992 and 2005[END_REF] for 1996 and 2003. The Maitri results are in very good agreement with that of [START_REF] Kuttippurath | Antarctic ozone loss in 1979-2010: first sign of ozone recovery[END_REF] as they also estimate a loss of about 150-170±10 DU for the winters [START_REF] Solomon | Stratospheric ozone depletion: A review of concepts and history[END_REF]-2003and 2006. However, our results for 2000-2001 are also comparable to the total column ozone loss estimated by Sinnhuber et al. (2002) and the partial column ozone loss found in Strahan and Douglas (2018).

Conclusions

India has a well-planned polar research programme and has ozone measurements since 1990s in Antarctica. We have considered TCO measurements from the Maitri station of Antarctica.

The winter TCO measurements by the Brewer instrument are used for the winters [START_REF] Solomon | Stratospheric ozone depletion: A review of concepts and history[END_REF]-2003and 2006. The winters 2000, 2001, 2003and 2006 were colder winters of Antarctica, and 2006 was coldest among them, in which the largest ozone hole was observed. Therefore, ozone loss amounted to 170 DU or 57-59% in 2006. The winters 1999 and 2002 were relatively warm and thus ozone loss was restricted, about 43-46%, and the winter 2002 experienced the first vortex split event in the southern polar region in the observational era. This dynamical situation led to the smallest ozone hole with lower ozone loss (43% in September) in that winter. The observed ozone loss matches very well with the all station average that includes measurements from nine stations in the Antarctic continent; implying that the Maitri measurements very much represent the ozone loss of the entire Antarctic region. This suggest that the station is inside the polar vortex in most winters. However, the differences in spring measurements and during warming periods indicate that the erosion of polar vortex. The analyses presented suggest the continued monitoring of ozone in the region to monitor one of the important trace gases on the earth that influences public health and changes in climate.

FIGURE CAPTIONS

  India has three research bases in Antarctica: the first station was established in 1980 as Dakshin Gangotri (70.4° S, 11.4° E), Maitri (70.4° S, 11.4° E) in 1989 and Bharati (69.2° S, 76.2° E) in 2012, and the latter two are only currently active. The Ministry of Earth Science (MoES)

Figure 3

 3 Figure 3 also shows the comparison between Maitri measurements and TOMS-OMI overpass

Fig. 1 :

 1 Fig. 1: The ozone measurement stations in Antarctica. The Maitri station together with other

Fig. 2 :

 2 Fig. 2: Temporal evolution of wintertime polar meteorology. Evolution of polar cap (60°-90° S)

Fig. 3 :

 3 Fig. 3: Ozone measurements at Maitri and other Antarctic stations. The ozone measurements

Fig. 4 :

 4 Fig. 4: Ozone loss at Maitri and the Antarctic ozone hole. The chemical ozone loss estimated

Figure 1 Figure 2 Figure 3

 123 Figure 1

Figure 4

 4 Figure 4

Acknowledgements

We thank the Department of Science and Technology (DST), Ministry of Human Resource Development (MHRD), Indian Institute of Technology Kharagpur (IIT KGP), Ocean Environment Panel of Naval Research Board (OEP-NRB), Defence Research Development Organisation (DRDO), National Centre for Ocean Information Services (INCOIS), Hyderabad and Ministry of

Earth Science (MoES) for facilitating and funding the study. We also thank all the data managers and the scientists who made available those data for this study. This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Data availability

The data used in this study are publicly available (https://woudc.org/). The analyses codes can be provided on request. The OMI data are available on https://earthdata.nasa.gov/earthobservation-data/near-real-time/download-nrt-data/omi-nrt. Since the satellite data used are already freely available on public domains, the analysed data can also be provided for any scientific study.

Code availability

The analyses codes are available on request.

Competing Interests

The authors confirm that there are no known conflicts of interest and no competing interests.