Comparison of Inorganic Chlorine in the Southern Hemispheric lowermost stratosphere during Late Winter 2019

Inorganic chlorine (Cly) is the sum of the degradation products of long-lived chlorinated source gases. These include the reservoir species (HCl and ClONO2) and active chlorine species (i.e. ClOx). The active chlorine species drive catalytic cycles that deplete ozone in the polar winter stratosphere. This work presents calculations of inorganic chlorine (Cly) derived from chlorinated source gas measurements on board the High Altitude and Long Range Research Aircraft (HALO) during the Southern hemisphere Transport, Dynamic and Chemistry (SouthTRAC) campaign in late winter and early spring 2019. Results 5 are compared to Cly of the Northern Hemisphere derived from measurements of the POLSTRACC-GW-LCYCLE-SALSA (PGS) campaign in the Arctic winter of 2015/2016. A scaled correlation was used for PGS data, since not all source gases were measured. Cly from a scaled correlation was compared to directly determined Cly and agreed well. An air mass classification based on in situ N2O measurements allocates the measurements to the vortex, the vortex boundary region, and mid-latitudes. Although the Antarctic vortex was weakened in 2019 compared to previous years, Cly reached 1687±20 ppt at 385 K, therefore 10 up to around 50 % of total chlorine could be found in inorganic form inside the Antarctic vortex, whereas only 15 % of total chlorine could be found in inorganic form in the southern mid-latitudes. In contrast, only 40 % of total chlorine could be found in inorganic form in the Arctic vortex during PGS and roughly 20 % in the northern mid-latitudes. Differences inside the respective vortex reaches up to 565 ppt more Cly in the Antarctic vortex 2019 than in the Arctic vortex 2016 (at comparable distance to the local tropopause). As far as is known, this is the first comparison of inorganic chlorine within the respective 15 polar vortex. Based on the results of these two campaigns, the difference of Cly inside the respective vortex is significant and larger than reported inter annual variations.

Furthermore, as the polar night jet acts as a barrier, air composition is different inside and outside the vortex. High concentrations of reactive halogenated substances can be maintained inside the vortex because there is little mixing with the surrounding area. During the HALO flights, the aircraft encountered air masses with different characteristics due to their origin. To make systematic conclusion about the distribution of trace gases, a reliable, accurate method of separating the measurements in terms of their region is needed. For this reason, the following describes how air masses have been classified using highly resolved in 130 situ measurements.

Air mass classification by in situ measurements
The maximum gradient of potential vorticity (PV) is a commonly used indicator to define the location of the vortex edge, also known as the Nash criterion (Nash et al., 1996). Unfortunately, the usage of PV has a major drawback as it is a model-derived quantity with a rather coarse resolution. Hence, small-scale features like vortex filaments with different chemical compositions 135 may not be taken into account. In this work, an extended version of the vortex definition by Greenblatt et al. (2002a) is used instead. The technique by Greenblatt et al. (2002a) uses the tight correlation between N 2 O and potential temperature (Θ) to determine the inner edge of the vortex boundary. A tracer like N 2 O exhibits a horizontal gradient across the vortex edge in the stratosphere with lower mixing ratios inside the vortex and higher mixing ratios outside the vortex. It can be measured in situ with a sufficiently high time resolution to reveal small structures in the atmosphere. Air inside the vortex is isolated 140 from the surroundings leading to a strong vertical gradient due to strong diabatic descent inside the vortex. Additionally, the isolation inside the vortex benefits mixing on isentropic surfaces and therefore a small variability on isentropes (variability of about 6 ppb (Greenblatt et al., 2002a)). The low mixing ratios inside the vortex are a result of diabatic descend of high altitude air with less N 2 O. In contrast, the mid-latitudes vertical gradient is weak and more variable as it is influenced by tropical and polar air (Krause et al., 2018;Marsing et al., 2019). In between there is a transition region (vortex boundary region), which is 145 influenced by the vortex as well as by mid-latitudes. Towards tropopause altitudes, the N 2 O profiles of vortex and mid-latitudes merge and differentiation becomes difficult.
Based on the method of Greenblatt et al. (2002a), one flight is chosen to generate a vortex reference profile. This flight should at best be completely in the vortex. However, during the SouthTRAC campaign there was no flight that only sampled vortex air. In addition, there is an interest in not only distinguishing between vortex and non-vortex air, but also in assigning the 150 campaign measurements to the vortex, vortex boundary region, and mid-latitudes. For this reason, several flights were used to create reference profiles for the vortex and mid-latitudes. Stratospheric transport and mixing is related to the isentropic surfaces whereas mixing at the extratropical tropopause affects the lowest 25 K relative to the local tropopause. Therefore, classification was done with two vertical coordinates, potential temperature (Θ) and potential temperature above the local tropopause (∆Θ).
The vortex reference profile (see Fig. 2) was generated by using all flights, which had contact to the vortex core. Data from 155 these flights were pre-filtered by taking only the measurements polewards of 60 • S equivalent latitude and 20 K above the local tropopause. The lowest levels of potential temperature above the local tropopause are strongly influenced by extra-tropical tropospheric air, i. e. the tropopause mixing layer in the lowermost stratosphere (Hoor et al., 2005). With an iterative filter procedure (see supporting information) the lower envelope of the remaining measurements is obtained and is used to generate the vortex profile function (Werner, 2006). For the mid-latitudes profile (see Fig. 2), all flights were taken into account, focusing 160 only on measurements between -40 • and -60 • equivalent latitude and again 20 K above the local tropopause. This time, the upper envelope of the measurements was evaluated by the iteration procedure to build a reference profile function for the midlatitudes. A more detailed description of the creation of reference profiles can be found in the supporting information. The two reference profiles in Θ-coordinates are displayed in Figure 2a, the two reference profiles in ∆Θ-coordinates in Figure 2b.
In general, it cannot be assumed that a single N 2 O vortex profile can be representative for the entire winter. Subsidence of 165 the vortex air by several kilometers due to radiative cooling (Schoeberl and Hartmann, 1991) leads to a changing N 2 O profile throughout the polar winter. The diabatic descent, however, starts very early in late fall and maximum decent rates occur in the late fall/early winter phase (Greenblatt et al., 2002b). The campaign took place in late winter and a further diabatic descent is not expected. Therefore, only one reference vortex profile was generated for the campaign. Looking at flights from the first and second phase of the SouthTRAC campaign, the vortex profile fits for both phases.

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A vortex and mid-latitudes reference N 2 O data set (N 2 O vor and N 2 O mid ) can be calculated by using the fit functions for the vortex and mid-latitudes profile and the N 2 O measurements of the UMAQS instrument of all flights. The following then applies for each N 2 O measurement: if the mixing ratio is below the respective N 2 O vor with a prescribed cutoff value, then it is assigned to the vortex. Otherwise, if the mixing ratio is above the respective N 2 O mid with an associated variability, than it is assigned to mid-latitudes. Mixing ratios above the respective N 2 O vor with a prescribed cutoff value and below the respective N 2 O mid 175 with an associated variability are assigned to the boundary region. For the mixing ratios, where N 2 O vor with a prescribed cutoff value and N 2 O mid with an associated variability overlap, these measurements can not be fully assigned to one region.

Overview of the sample regions
In 2019, extraordinary meteorological conditions led to a sudden rise in stratospheric temperatures over Antarctica. This minor sudden stratospheric warming (minor SSW) event affected the shape, location and strength of the polar vortex. From mid-180 August to early September 2019, the polar vortex has been displaced and weakened towards the eastern South Pacific and South America (Safieddine et al., 2020;Wargan et al., 2020). The SouthTRAC campaign flights took place from early September to early October and in the first half of November and were thus timed close to the minor SSW event and captured the late evolution of the Antarctic polar vortex.

Inferred inorganic chlorine
The metric describing the combined effect of all ozone depleting substances (ODS) as an equivalent amount of inorganic chlorine in the stratosphere, related to tropospheric source gases in a simple matter, is the equivalent effective stratospheric 195 chlorine (EESC) (Newman et al., 2007;Daniel et al., 1995). Changes to the EESC are mainly due to Cl y , as Br y makes up a smaller fraction (Strahan et al., 2014). The 6 min time resolution of the MS channel makes it difficult to detect fine structures such as filaments and small scale dynamical perturbations. Therefore, an up-sampling of the MS data to the time resolution of the ECD channels was performed before calculating Cl y . Measuring CFC-12 on both the ECD and MS channel of the instrument allows to up-sample the measurements of the organic 205 source gases by using the higher resolved measurements of CFC-12 from the ECD channel. Measurements of CFC-12 on the ECD channel not only have a higher time resolution of 1 min, but also a better precision than on the MS channel. As shown in  Fig. 6 260 displays scaled correlations of three long-lived substances against CFC-12. They correspond well to the correlations measured during the SouthTRAC campaign. Therefore, CCl y based on correlations with CFC-12 can be used as a good proxy for the amount of inorganic chlorine. As already mentioned earlier, Cl total can be derived to calculate Cl y . A correlation function for the Antarctic late winter 2019 has been derived for the indirect calculation of Cl y as a function of CFC-12 mixing ratios (Eq.

Up-sampling GhOST-MS measurements
2). The coefficients for the correlation function with CFC-12 as the reference substance, based on the balloon measurements, 265 can be taken from Table 2. In addition, the fit coefficients are given if one wants to use N 2 O as the reference. CFC-12 from the GhOST-ECD channel is used to determine inorganic chlorine.
(2) Figure 7 shows semi-directly and indirectly determined inorganic chlorine as a function of mean age.

Chlorine partitioning in the Antarctic winter 2019 lower stratosphere
Since inorganic chlorine plays a major role in ozone depletion, it is worth investigating its distribution in the Antarctic stratosphere. For the analysis only measurements were taken, which are polewards of 40 • equivalent latitude. As a vertical coordinate Θ was chosen. All measurements have been binned into 5 K potential temperature bins between 270 and 420 K of Θ (see Fig. 8).

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Bins which contain less than five data points are not included in the analysis. The uncertainties represented by the errorbars are the 1 σ standard deviations of the means. Up to the potential temperature at which an air mass classification begins, Cl y is given for all measurements. From the potential temperature at which air mass classification begins, Cl y is given according to the region. Measurements within the overlap area in the classification (see Fig 2) are counted both as vortex and mid-latitudes measurements.

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The inferred Cl y throughout the troposphere is close to zero and increases in the tropopause region.  Using Θ as the vertical coordinate (Fig. 9a), vertical profiles of vortex classified Cl y of PGS and SouthTRAC show different results. Although the Cl y vortex profiles are similar until around 350 K, the SouthTRAC profile increased stronger and values become more than 435 ppt larger than during PGS within the vortex at equal potential temperatures. Differences become 330 slightly larger when using ∆Θ as the vertical coordinate (Fig 9b). Inside the vortex of the respective hemisphere, Cl y increased stronger with height above the tropopause during SouthTRAC than during PGS. Although close together between 20 and 25 K ∆Θ, the difference of Cl y increased to 565 ppt at 65 K ∆Θ. appears near the tropopause, where it was difficult to make a distinction between vortex, vortex boundary, and mid-latitudes.
The thermal WMO tropopause was used in this study, as no dynamical PV tropopause data is yet available for the SouthTRAC campaign. The dynamical tropopause seems to fit better with trace studies and has been widely used in past studies (e.g. Keber et al. (2020)). Hence, for future investigation, it is worthwhile to choose the dynamical PV tropopause instead of the thermal WMO tropopause. . At a comparable level of ∆Θ inside the vortex, only around 40% of total chlorine can be found in inorganic form, whereas roughly 20% can be found at mid-latitudes. Inside the respective vortex, the amount of Cl y 380 was higher during SouthTRAC than during PGS by up to 565 ppt (at the same ∆Θ level). Trends due to the Montreal Protocol would be negative to about -20 ppt yr -1 , which is not evident in this comparison. The difference of Cl y inside the respective vortex is significant and even larger than the inter annual variations reported by Strahan et al. (2014). For the comparison of