Chlorine partitioning in the lowermost Arctic vortex during the cold winter 2015/2016

Abstract. Activated chlorine compounds in the polar winter stratosphere drive catalytic cycles that deplete ozone and methane, whose abundances are highly relevant to the evolution of global climate. The present work introduces a novel dataset of in situ measurements of relevant chlorine species in the lowermost Arctic stratosphere from the aircraft mission POLSTRACC–GW-LCYCLE–SALSA during winter 2015/2016. The major stages of chemical evolution of the lower polar vortex are presented in a consistent series of high-resolution mass spectrometric observations of HCl and ClONO2. Simultaneous measurements of CFC-12 are used to derive total inorganic chlorine (Cly) and active chlorine (ClOx). The new data highlight an altitude dependence of the pathway for chlorine deactivation in the lowermost vortex with HCl dominating below the 380 K isentropic surface and ClONO2 prevailing above. Further, we show that the Chemical Lagrangian Model of the Stratosphere (CLaMS) is generally able to reproduce the chemical evolution of the lower polar vortex chlorine budget, except for a bias in HCl concentrations. The model is used to relate local measurements to the vortex-wide evolution. The results are aimed at fostering our understanding of the climate impact of chlorine chemistry, providing new observational data to complement satellite data and assess model performance in the climate-sensitive upper troposphere and lower stratosphere region.


high vertical resolution necessary to resolve strong chemical gradients near the tropopause (e.g. Livesey et al., 2017;Mahieu et al., 2008;Wolff et al., 2008). On the other hand, there have been numerous airborne activities in the Arctic LMS, but only few airborne (Bonne et al., 2000;Wilmouth et al., 2006) or balloon-borne (e.g. Wetzel et al., 2015) measurement platforms that have yet sampled the polar LMS chlorine budget. Several studies focused on ozone depleting ClO and the dimer ClOOCl (Vogel et al., 2003;von Hobe et al., 2005;Sumińska-Ebersoldt et al., 2012) and heterogeneous chlorine chemistry (Wegner 10 et al., 2012;Wohltmann et al., 2013;summary in von Hobe et al., 2013). A comprehensive in situ sampling of chlorine species in the Arctic lower stratosphere was conducted in a dedicated mission in winter 1999/2000 using the ER-2 high altitude aircraft (Wilmouth et al., 2006). Generally, measurements from aircraft are able to combine high spatial resolution, a benefit in the manifold environment near the tropopause, with sufficient temporal coverage in a suitable deployment.
The work at hand is aimed at presenting a new high resolution and high accuracy in situ dataset of Arctic LMS chlorine chem- 15 istry that complements satellite products at lower altitudes and may be a useful reference on which to test model simulations.
Section 2 introduces the aircraft mission, instrumentation, model and vortex identification methods. Chlorine measurements are intercompared as a consistency check in unperturbed stratospheric conditions. The temporal evolution of chlorine partitioning throughout the winter is presented and discussed in Sec. 3. Section 4 concludes the study. 2 Measurements, model and methods 20

Activities within the POLSTRACC mission
The polar LMS during the Arctic winter 2015/2016 has been probed within the POLar STRAtosphere in a Changing Climate (POLSTRACC) aircraft campaign, using the German High Altitude and LOng range research aircraft (HALO). The mission objectives comprise studying (i) the structure, composition and dynamics of the Arctic winter LMS, (ii) chemical processes that affect ozone in the Arctic winter upper troposphere/lower stratosphere (UTLS), (iii) PSCs and de-/nitrification and (iv) 25 cirrus clouds in the Arctic UTLS. In a joint effort for the missions POLSTRACC/Gravity Wave Life Cycle Experiment (GW-LCYCLE II)/Seasonality of Air mass transport and origin in the Lowermost Stratosphere (SALSA) (combined under the acronym "PGS"), HALO performed a total of 18 science flights. The long range capability has been extensively used during the 156 flight hours. Spreading from the campaign bases in Oberpfaffenhofen (EDMO), Germany and Kiruna (ESNQ), Sweden, the flight tracks span a region between 25°N -87°N and 80°W -28°E over Europe, the North Atlantic and Greenland (map 30 in Fig. 1). Potential temperatures at flight level reached a maximum of 411 K. Chronologically, the observations cover most of the evolution of the polar vortex: two flights sampled the early winter formation of the vortex in mid December; the first major phase in January (eight flights) was dedicated to the established vortex, while the second major phase from end of February to mid March (eight flights) witnessed the late stages of vortex evolution including the major final warming (MFW) and vortex split. HALO was equipped with a set of in situ and remote sensing instruments, measuring atmospheric quantities at, below, above and alongside the aircraft position.
The Arctic polar vortex during the winter 2015/2016 has already been subject to a number of studies, most of them directly related to POLSTRACC activities: Overviews of the evolution of the polar vortex are given by Manney and Lawrence (2016), 5 Matthias et al. (2016) and Khosrawi et al. (2017) including satellite, reanalysis and model perspectives. Concurrently, the authors point out the exceptional strength of the vortex due to reduced planetary wave activity, accompanied by record low temperatures and extensive denitrification, dehydration and chlorine activation. Record ozone loss comparable to the extreme winter 2010/2011 (e.g. Sinnhuber et al., 2011) was only prevented by an early onset of sudden stratospheric warming (SSW) and vortex splitting events. The study by Khosrawi et al. (2017) also points to deficiencies in the representation of downward 10 transport and chlorine chemistry in the chemistry-climate model EMAC, that may significantly impact chlorine partitioning in the LMS. The formation of PSCs has been investigated by Voigt et al. (2018) with a focus on the pathways of ice PSC nucleation and Dörnbrack et al. (2017) studied a case of gravity wave-induced PSC formation. A novel approach of three-dimensional visualisation of gravity waves from remote tomographic sampling with the Gimballed Limb Observer for Radiance Imaging of the Atmosphere (GLORIA) has been presented by Krisch et al. (2017), while Johansson et al. (2018) examined how the 15 same instrument consistently fills the gap between high coverage spaceborne sampling and accurate high resolution in situ measurements. Krause et al. (2018) used chemical tracers to reveal the influence of mixing at the lower vortex edge on the age spectrum of the air.

Airborne in situ measurements
This section provides an overview of the measurement principles of the in situ instruments used in the present work. In 20 addition to the dedicated trace gas measurements, the BAsic HALO Measurement And Sensor system (BAHAMAS) provides meteorological parameters along the flight trajectory (Krautstrunk and Giez, 2012;Giez et al., 2017).

Chlorine measurements with AIMS
The Airborne chemical Ionization Mass Spectrometer AIMS features a pressure-controlled electrical discharge source and inflight calibration capability . Two different operational modes can be selected, enabling either the detection 25 of low water vapour concentrations  or measuring the trace gases SO 2 , HCl, HNO 3 , HONO and ClONO 2 . In the latter configuration, SF − 5 reagent ions are used for the ionization of the desired trace gas molecules (Marcy et al., 2005;Voigt et al., 2010;Jurkat et al., 2010). AIMS in trace gas configuration was deployed earlier onboard the HALO and Falcon research aircrafts for measurements in the troposphere and lower stratosphere from the northern mid-latitudes down to the Antarctic continent Jurkat et al., 2014Jurkat et al., , 2017. In this work, we present 30 measurements of HCl and ClONO 2 obtained during the combined PGS campaign. HNO 3 and SO 2 were also measured, but are not in the focus of this study. In-flight calibrations were performed for HCl and HNO 3 . Calibration uses an azeotrope solution of HCl or HNO 3 in water, each sealed in a permeation tube and held at constant temperature. Through the resulting constant vapour pressure of the liquid, a specified current of gaseous HCl or HNO 3 can leave the tube via a membrane. Together with a nitrogen carrier gas flow, it is then added to the atmospheric air sucked through the inlet during stable background conditions. This way, any interaction of trace molecules with the tubing between the inlet and the instrument is already taken into account by the calibration. ClONO 2 could not be calibrated directly due to the lack of an accurate source to stably generate the necessary amounts of calibration gas. Instead, we made use of the fact that the kinetics of the fluoride transfer reactions, carrying F − 5 from the SF − 5 reagent ions to the trace gas molecules, are similar for HNO 3 and ClONO 2 due to the molecular similarity. Thus, ClONO 2 values could be calculated via the calibration of HNO 3 , using literature values for the relative sensitivities of HNO 3 and ClONO 2 (Marcy et al., 2005) and taking into account the mass discrimination between the two species . AIMS measurements were performed at a 1.7 s time resolution, equivalent to roughly 350 m horizontal resolution. Smoothing the raw data in a 17 s (10 data points) running average window (Jurkat et al., , 2017 yields detection limits of 6-12 parts 10 per trillion (pptv) of molar mixing ratio and a 10-15 % precision for both HCl and ClONO 2 , with an accuracy of 12 % for HCl and 20 % for ClONO 2 .

N 2 O and Cl y from in situ measurements
Measurements of N 2 O were performed by the TRIHOP instrument, a three-channel quantum cascade laser infrared absorption spectrometer (Müller et al., 2016). The integration time is 1.5 s with a precision of 1.84 parts per billion (ppbv) (Krause 15 et al., 2018). After linear drift correction the total uncertainty of N 2 O during the POLSTRACC mission is estimated to be 2.5 ppbv. Total inorganic chlorine (Cl y ) is inferred from a correlation to dichlorodifluoromethane (CFC-12). The (stratospheric) correlation has been established using cryogenic whole air sampling on two balloon flights inside the Arctic polar vortex in 2009 and 2011 (Wetzel et al., 2015). It was updated to consider the tropospheric conditions at the time of the campaign, employing the method described in Plumb et al. (1999). CFC-12, in turn, was measured by the GhOST GC-MS instrument 20 (Sala et al., 2014) with a precision of 0.2 % at an average time resolution of 1 min.

Inferred ClO x
As a proxy for active (ozone depleting) chlorine derived from the above observational data, the quantity Cl y −(HCl+ClONO 2 ) is calculated and hereafter termed "ClO x from measurements". This quantity features an accuracy of 23 % or 20 pptv, whichever is larger, and a precision of 14-21 %, plus a possible systematic error from the inferred Cl y that may in fact be the main source 25 of uncertainty. Negative values of ClO x can occur if the errors of the original tracer measurements add up adversely, or if the stratospheric correlation between Cl y and CFC-12 is altered unforeseen, e.g. near the tropopause.

Instrument comparison
In order to assess the data quality, simultaneously measured quantities from the AIMS and GhOST instruments are compared in unperturbed conditions where the sum of reservoir gases equals the amount of Cl y . To this end, Fig. 2  2015. It started in Oberpfaffenhofen (EDMO) and was headed to the Atlantic Ocean north of Ireland, up to 59.8°N, in order to horizontally cut the tropopause at high altitude in north-south direction. Polar vortex air was only sparsely hit, because the vortex at this time of the year was higher than the maximum altitude of HALO (Matthias et al., 2016). At this early stage of the vortex development, no activated chlorine is expected and as a consequence, inorganic chlorine is should be partitioned entirely into the reservoir species HCl and ClONO 2 . The simultaneous measurements of AIMS and GhOST are compared in Fig.2b.

5
Gaps in the AIMS data occur during in-flight calibration and background measurements. AIMS measurements were averaged to the lower time resolution of GhOST to numerically compare the measurements of both instruments. Figure 2a shows (HCl + ClONO 2 ) -Cl y relative to Cl y as a function of time. For the most part, the data agree within ± 20 %, which is well within the measurement uncertainty. Larger deviations are only found in tropospheric sections of the flight, where the correlation of CFC-12 and Cl y is less robust since it varies with tropospheric sources. As shown in Fig. 2c, the correlation between (HCl 10 + ClONO 2 ) and Cl y for this flight is linear over the whole concentration range with R 2 = 0.989. Overall, the agreement is excellent, given that this comparison aggregates three independently measured and one derived trace gases. Similarly, but not shown here, ClONO 2 (Johansson et al., 2018) and HNO 3 data (e.g. Ungermann et al., 2015) from AIMS compare well to other in situ measurements and remote sensing products aboard HALO. 15 Simulations are performed with the Chemical Lagrangian Model of the Stratosphere (CLaMS) that is described elsewhere (Grooß et al., 2014, and references therein). Unlike most other Eulerian models, the Lagrangian chemical transport model CLaMS calculates the chemical composition along airparcels irregularly distributed over space that follow individually their trajectories. The underlying wind and temperature data are taken from ECMWF ERA-Interim data (Dee et al., 2011). Initialisation and boundary conditions of the model simulation for the winder 2015/2016 are described by Grooß et al. (2018). 20 Typically, the model output is written every day at 12:00 UTC. For interpolation to the observation locations and times, a Lagrangian mapping was used, employing back trajectories from the desired positions and times to the previous day. Interpolated from the model output, the CLaMS chemistry module is integrated forward to the observaion point. With that procedure, the chemical composition (including the simulated chlorine partitioning) at the observation location and time is determined from the model. 25

Identification of vortex air by in situ measurements
The study of chlorine partitioning within this work is intended to focus solely on air masses that can be attributed to the Arctic polar stratospheric vortex. A common means for vortex identification is the Nash criterion (Nash et al., 1996), essentially defining the location of the vortex edge by the maximum gradient of potential vorticity (PV) on isentropic surfaces. In this 30 work, however, the method of Greenblatt et al. (2002) is applied where vortex air is identified by a tight correlation between the inert tracer N 2 O and potential temperature. Using this method, the subsequent analysis benefits from the high resolution of N 2 O and potential temperature measurements in contrast to reanalysis of PV fields. Thus, also small-scale patterns at a size of several kilometres or filamentary structures can be assigned correctly. Figure 3 aggregates all N 2 O data points from the PGS campaign versus potential temperature (θ). The diagram is constrained to values above 320 K, focusing thus on the stratosphere above the ExTL at high latitudes. The observed N 2 O:θ profile narrows towards the tropopause, whereas it widens towards higher altitudes, showcasing the wide variety of air masses sampled during the flights. The labels in the upper part of 5 Fig. 3 roughly indicate the region of the sampled air masses. Generally three regimes can be distinguished: the vortex regime is found at the inclined left edge of the profile characterised by a strong vertical gradient of N 2 O mixing ratio, which connects the tropospheric source region to the photochemical sink in the middle stratosphere (Schmeltekopf et al., 1977). The gradient is maintained by the stratification and isolation of the polar vortex, while isentropic homogeneity leads to a compact form of the profile. At the other extreme, air from mid latitudes exhibits a more variable gradient in N 2 O due to altering influence  Fig. 3). Observations include also outside-vortex air as indicated by the light blue points between 345 K and 360 K, so the vortex reference points in this range are determined by a quadratic interpolation between the adjoining sections. Beyond 395 K, the vortex reference profile is set 20 by linear extrapolation. A symmetric envelope is placed around the vortex reference profile. The width is determined for each flight individually by the maximum deviation of N 2 O data points below the vortex reference value in each bin. All data points lying within the envelope are then considered to belong to vortex air masses (dark points in Fig. 3). This procedure is chosen to sensibly include a high number of measurements in the vortex regime, accounting for instrumental noise and small atmospheric variability while the criterion is not weakened too much. Hereafter, the term "vortex air" is meant to refer to measurements that 25 fulfil the vortex criterion above 320 K. Equally, the term "extra-vortex air" is the complimentary set of measurements above 320 K.
Although the determined vortex reference correlation seems to be appropriate for the whole data set at first glance, we have to account for diabatic descent during the three month campaign phase. Satellite data of N 2 O (Manney and Lawrence, 2016) and analysis of the in situ N 2 O data (Krause et al., 2018) indicate that below θ = 450 K, diabatic descent, i.e. descent of air

Identification of vortex air in the model
For model data interpolated along the flight tracks, the measured N 2 O:θ vortex criterion from Sec. 2.4.1 is applied unchanged, in order to achieve an optimal comparison to the measurement data. For vortex-wide averages, all model data with equivalent latitude Φ e greater than 65°N are included. This choice results in a constant area (in square kilometres) and does not reflect the variability of the Nash criterion (Grooß and Müller, 2007), but -from experience -matches it quite well for a fully developed 5 polar vortex, whereas the vortex area is overestimated during very early and late stages. are flights with long sections entirely inside the vortex, resulting in a good sampling statistics and providing insight to intravortex variability. On the other hand, many flights exhibit only a patchy sampling of vortex air whenever other objectives of the PGS campaign were pursued. As an example, the PV maps in Figure 4 illustrate that a compact vortex signature is visible at 50 hPa (above 20 km altitude), whereas it becomes more spread-out and filamented below. On that particular day, vortex 15 air measurements could be conducted over Greenland and northern Canada near the PV maximum on 150 hPa, which is the approximate flight altitude. Split-off parts of vortex air could also be found at latitudes down to 42.7°N towards the end of the campaign in early spring. Figure 5 gives an overview of the temporal sampling of vortex air above different potential temperatures θ over the whole PGS campaign. Therefore, the number of individual (HCl) measurements during each flight is summed up and depicted in 20 columns. The colour code indicates the vortex air encounters divided in measurements above a certain potential temperature.
Extensive vortex air sampling was performed primarily at the end of the first main phase and during the second main phase, in concert with the gradual descent of higher vortex air masses to flight altitude. Specifically, vortex air encounters represent between 3 % and 95 % of flight time of individual flights. In situ sampling was focused on θ > 340 K for most flights, keeping significant distance to the tropopause. Vortex air above θ = 380 K was only sampled as of 26 February 2016 when mainly 25 adiabatic transport brought air masses with such high potential temperatures within reach of the HALO aircraft. The 400 K isentrope was only crossed during three occasions in March with a total time of about 10 min inside vortex air.

Measured evolution of chlorine gases during the winter
The evolution of inorganic chlorine partitioning in the lowermost Arctic polar vortex over a period of three months is assessed by means of daily statistics, which is performed by calculating averages, standard deviation and quantiles from all "vortex of changes in trace gas concentrations on a time scale of days. Beforehand, the data are binned into four layers of potential temperature, each spanning 20 K, to introduce a coarse quasi-vertical coordinate. The panels a-d in Fig. 6 display the mean of the measured distributions for the individual HALO flights. Colours indicate the θ layers. The layer 320-340 K is omitted for clarity as it shows almost no difference to the 340-360 K layer. The panels e-g are similar to panels a-c, but display the relative abundance with respect to Cl y instead of absolute mixing ratios.

5
In December 2015, the measurements indicate that inorganic chlorine is partitioned almost entirely into the reservoir species HCl and ClONO 2 below the 380 K isentropic surface. With almost 80 % contribution to Cl y , the photochemically stable HCl is predominant, whereas ClONO 2 ranges with less than 0.12 ppbv below or near the detection limit. This ratio is common in unperturbed conditions (e.g. Santee et al., 2008). At this early stage of the polar vortex, no ClO x has been detected.
The partitioning changes in January 2016, where the vortex is fully developed and temperatures are low enough to enable  Figure 7 illustrates the concept of differential deactivation pathways in this late winter period in a different manner. There, the partitioning of Cl y is shown in a ternary diagram of HCl, ClONO 2 and ClO x fractions. The dots mark individual measurements, the same that the daily averages in Fig. 6e-g were based on. The solid arrows run along the temporal evolution of the daily 5 averaged measurements and show the two pathways below 360 K (black) and above 380 K (blue), diverging towards HCl or ClONO 2 , as well as the 360-380 K isentropic layer (red) sandwiched in between. Using the supporting green isolines of the HCl/ClONO 2 fraction, we observe that below 360 K, the partitioning between the reservoir species is more HCl-heavy and after recovery, HCl is three times more abundant than ClONO 2 . Above 380 K, in contrast, the reservoir species evolve from a nearly 1:1 partitioning to a state with twice as much ClONO 2 relative to HCl.

Comparison of measured and CLaMS-modelled data
Simulations by the Lagrangian CTM CLaMS were performed to put the observations into a broader context, and to investigate sampling biases potentially caused by the coverage of the aircraft observations. To assess the accuracy of the model, first the model results were interpolated to the location and time of the observations employing the trajectory mapping described above. procedures. As for HCl, the model generally follows the trends of the measurement data. Before February, however, there is a clear high bias by 0.13-0.20 ppbv in almost all model HCl data which cannot be systematically seen after the break. Having ruled out technical changes on the simulation and on the instrumental sides during this time, it seems as if we can observe a model bias that is known from satellite comparisons at higher altitudes in the dark polar vortices, a problem that is observed by other models as well (Wohltmann et al., 2017;Grooß et al., 2018). Our intercomparison of model and in situ data supports the 20 rationale that chemistry-climate models struggle in reproducing the observed chlorine partitioning in the dark winter months, where some unknown process for HCl removal is lacking. Here we extend the previous observations by Grooß et al. (2018) to lower altitudes. Consistently, the discrepancy is absent as soon as sufficient sunlight returns towards the end of the winter. An overestimation of HCl is partly reflected in an overestimation of Cl y and underestimated ClONO 2 and ClO x concentrations.
ClONO 2 is better represented in the model below θ = 360 K, whereas there are indications of a slight underestimation on 25 higher isentropes. This may be induced in part by the HCl high bias until midwinter, or by generally underestimated Cl y mixing ratios recognizable above 380 K. The model is able to produce the observed change in recovery across the different isentropes.
The modelled vertical shift of HCl versus ClONO 2 recovery in March between 360-400 K is obviously subject to a sampling bias just like the measurements, but at the same time does not always follow the observational data. This deviation is probably caused by a lack of vertical resolution in the meteorological data fields that prevents the model from estimating the high vertical 30 gradients in the atmosphere in all detail.
Beyond this direct intercomparison, model data can help explore to what extent the observations along the aircraft's flight track are representative of the entire polar vortex. Therefore, the lines in Fig. 9 display the modelled concentration of the chlorine species in a vortex-wide average at the different isentropic levels. In general the local aircraft observations reflect the mean chlorine partitioning in the vortex LMS quite well. The phases when activated chlorine can be found are consistent, whilst enhanced local variation of ClO x in January is not projected into the vortex averages. Vortex-averaged HCl mixing ratios clearly suffer from the deviation during the dark episode. Toward the end of the winter, the vortex averages include air masses from lower latitudes at increased frequency. This is accompanied, for example, by a sudden drop in modelled mean ozone (not shown), which would explain the lower values of all chlorine species, compared to the measurements in the local remainders 5 of vortex air.

Variability of chlorine partitioning
To assess the validity of using daily averaged mixing ratios as in the previous sections, Fig. 10 displays in addition to mean values the extrema and quartiles from the statistics studied hitherto, where the 360-380 K layer is taken as an example that stretches over all measurement phases. Intermediate data spread, as seen in most January flights, reflects the small and steady 10 variation through the depth of the layer, while trends between consecutive flights are clearly visible. There, the statistics are based on a solid sample size, and these are the periods where measurements and model data match best. A very low variability, such as for the first December flight or for two flights on 10 and 13 March stems from very few vortex sampling points in this layer, rendering these values less representative. Consistently, the model data interpolated along the flight track deviates strongest from the measurements on these dates, as visible in the HCl and ClONO 2 results. Very high variability, as seen on 26 15 February and 6 March hints at enhanced differential processing or transport within the sampled air masses.
The standard deviation of the vortex-averaged statistics (grey shading in Fig. 10a-d)

Discussion
In this section, our observed evolution of chlorine partitioning during the winter 2015/2016 is compared to previous studies using in situ and remote sensing data.
The Arctic polar vortex during winter 2015/2016 has been studied by Manney and Lawrence (2016), using measurements from the Aura MLS instrument. As for chlorine species, they retrieved HCl and ClO at a vertical resolution of 2.5-6 km down 25 to θ = 390 K. Therefore, the satellite data only have a small overlap with our in situ observations in March. Nevertheless, the depletion and recovery of HCl as well as the occurrences of active chlorine, as reported in section 3.2, consistently extend the remote sensing observations to lower altitudes, and show that chlorine activation is generally not limited to the higher altitudes inside the Arctic polar vortex.
Based on measurements on the ER-2 aircraft within the SOLVE/THESEO mission, Wilmouth et al. (2006) drew a very 30 comprehensive picture of inorganic chlorine partitioning in the Arctic polar vortex of 1999/2000, where low temperatures (Manney and Sabutis, 2000), high PSC presence and large chemical ozone loss (Rex et al., 2002) were observed. At isentropes around 440 K, ClO x /Cl y reached up to 90 % in January and HCl accounted for the remaining 10 %, whereas ClONO 2 was 11 Atmos. Chem. Phys. Discuss., https://doi.org /10.5194/acp-2019-370 Manuscript under review for journal Atmos. Chem. Phys. Discussion started: 26 April 2019 c Author(s) 2019. CC BY 4.0 License. not detectable. Thus, the higher degree of activation at higher potential temperatures, as compared to the PGS measurements, seems to come mainly at the expense of ClONO 2 . From their late winter budget analysis, Wilmouth et al. (2006) suggest that HCl recovers at a rate similar to or higher than the ClONO 2 recovery rate. They state that the evolution of inorganic chlorine partitioning in the Arctic is rather variable. Whether the chlorine deactivation into ClONO 2 or HCl is favoured, depends critically on the mixing ratios of ozone and NO x . Our late winter measurements are in line with The general year-to-year variability in the northern hemispheric polar vortex is caused by high wave activity, which impacts 15 the stability of the vortex and achievable cooling. On the process scale, the tradeoff between HCl or ClONO 2 recovery is controlled by the availability of reaction partners. NO 2 is needed for ClONO 2 formation, and may be introduced through mixing at the vortex edge (e.g. Krause et al., 2018), or through photolysis of gaseous HNO 3 . HCl recovery is favoured if low ozone leaves enough Cl radicals to react with methane, which is typically the case in the lowermost stratosphere.
The same instrumental configuration was used earlier in a probing of Antarctic polar vortex air on 13 September 2012 at 20 isentropes between 320 K and 385 K (Jurkat et al., 2017), where up to 40 % active chlorine were measured. Both the Arctic and Antarctic aircraft observations were made at (static) temperatures above 199 K, often even above 210 or 220 K. This is too warm for PSCs and for heterogeneous processes. Therefore, observable ClO x must have been activated beforehand, potentially with subsequent downward transport. This shows that, on both hemispheres, active chlorine can remain present also close to regions with temperatures below the PSC threshold. 25

Conclusions and Outlook
The present study uses high accuracy and high resolution in situ aircraft measurements in the lowermost parts and outflow regions of the Arctic polar vortex during winter 2015/2016 to investigate the evolution of inorganic chlorine species. Various different scientific targets during the campaign resulted in a comprehensive sampling of stratospheric active chlorine species over the course of the winter. 30 The observations below θ = 400 K never showe full chlorine activation. The appearance of active chlorine beyond its formation region is important as it increases the oxidation capability of the polar and high latitude UTLS and thus may impact the sensitive radiation budget through enhanced removal of ozone and methane. We highlight the difference in partitioning of Atmos. Chem. Phys. Discuss., https://doi.org/10.5194/acp-2019-370 Manuscript under review for journal Atmos. Chem. Phys. Discussion started: 26 April 2019 c Author(s) 2019. CC BY 4.0 License. the reservoir species from the lower isentropes to the higher ones during the recovery phase, where the transition from HCl recovery to mainly ClONO 2 recovery is located within the band 360-380 K. The different recovery pathways are supposed to be caused by gradients in available ozone and NO 2 .
Comparing the measurements to vortex-averaged model data, our local observations reflect the larger scale patterns of active and repartitioned chlorine. The occurrence of active chlorine is simultaneous to activation maxima at higher altitudes, as seen 5 in satellite data. While HALO with a ceiling altitude of 15 km was not anticipated to be an ideal aircraft for measurements of chlorine activation in polar vortex air, we here show the capabilities and potential of random sampling of the polar LMS. The CLaMS chemistry model, in conjunction with the underlying meteorological fields, performs very well in reproducing the in situ measurements, indicating that relevant homogeneous, heterogeneous and transport processes are realistically simulated.
One exception is the overestimation of HCl in the dark December and January vortex, which remains to be resolved (Grooß 10 et al., 2018). This study helps to reveal the finer scale variations and gradients in chlorine processing in the outflow regions of the polar vortex. It may prove useful in further checks of climate models and detailed investigations on the impact of vortex-processed air on surrounding areas, also in the light of comparable earlier and future deployments of this instrumental configuration in the polar UTLS.     values. The grey area encompasses the standard deviation from the vortex averaging (only for panels a-d).