 and high latitudes as a response to the breaking of planetary and gravity waves (Haynes et al., 1991;Plumb, 2002;Butchart, 2014) in the upper stratosphere and mesosphere. This downwelling leads to an increasing contribution of stratospheric air masses from the overworld (defined as the region where isentropes are entirely located in the stratosphere; Hoskins, 1991). Over the course of the winter they contribute to the composition of the lower overworld ( < 420 K), where our measurements took place, and the lowermost stratosphere (LMS) (Rosenfield et al., 1994) (defined as the region bounded by the 380 K isentrope and the extratropical tropopause; Holton et al., 1995).
Air masses descending from the upper stratosphere and mesosphere differ chemically from the composition of the LMS, since they are potentially affected by ozone-depleting catalytic cycles (Solomon, 1999). Since the air inside the polar vortex is largely isolated and exhibits a strong diabatic descent due to radiative cooling and the wave-driven Brewer-Dobson circulation, this leads to an increased fraction of air masses with a high mean age of air in the UTLS of high latitudes (e.g. Engel et al., 2002;Ploeger et al., 2015).
The mean age of air is defined as the first moment of the transit time distribution (or the age spectrum) (Hall and Plumb, 1994;Waugh et al., 1997). Mean age can be determined from the observation of long-lived tracers, which ideally have no sources or sinks in the stratosphere and of which the temporal evolution of the mixing ratio at the tropical tropopause is well known. Notably, the mean age is a bad descriptor for the full age spectrum, which is highly skewed (e.g. Hall and Plumb, 1994) and sometimes even multimodal (Andrews et al., 1999;. To estimate the potential chemical impact of species, particularly with lifetimes of the order of weeks to only a few months, the mean age is insufficient and the full spectrum is needed (Schoeberl et al., 2000), which is, however, only available under very idealised conditions (Schoeberl et al., 2005;Ehhalt et al., 2007).
Observations of SF 6, N 2 O and CO 2 from the ER-2 aircraft show that the mean age at northern high latitudes at an altitude of 20 km is of the order of 4-6 years (Andrews et al., 2001;Engel et al., 2002). Satellite observations of SF 6 confirm this and show further a strong interannual variability in the mean age in northern high latitudes (Stiller et al., 2008(Stiller et al.,, 2012Haenel et al., 2015). The observations also indicate a potential transport of mesospheric air to lower altitudes (Engel et al., 2006b;Ray et al., 2017), which, however, strongly  Riese et al. (2014) and Müller et al. (2016). The thermal tropopause is denoted by the thick black solid line. The measurement region is depicted as a blue box subdivided into the extratropical tropopause layer (ExTL), the lowermost stratosphere (LMS) and the lower stratosphere (LS). LMS and LS are separated by the 380 K isentrope (green dashed line). Transport pathways of air masses are denoted by coloured, thick arrows from the tropical tropopause layer (TTL) (red) and the polar Arctic upper stratosphere (blue) with respective mean age and trace-gas volume mixing ratio χ. Quasi-horizontal mixing is represented by wavy double side arrows, indicating no net mass transport of air masses. Dotted lines are isentropes in K. The solid dark blue line indicates the 7 PVU contour, which is used to separate the regime of the ExTL from the LMS and LS (for details see text). Thin orange contour lines depict the zonal view of the jet stream. depends on the strength and persistence of the Arctic polar vortex during the individual winters.
In addition to diabatic descent inside and outside the polar vortex, quasi-isentropic mixing from lower latitudes leads to a contribution of relatively young air to the UTLS. As a result, a seasonal cycle of the chemical composition of the UTLS up to = 430 K establishes a relatively tropospheric character during northern summer/autumn and a more stratospheric characteristic in late winter/spring (Hegglin and Shepherd, 2007). The chemical composition and age structure of the extratropical UTLS (ExUTLS) are affected by the competing diabatic downwelling of aged air and rapid quasi-isentropic mixing down to the tropopause (Hoor et al., 2005;Engel et al., 2006a;Garny et al., 2014). The region between = 380 K and the bottom of the tropical pipe around = 450 K (Palazzi et al., 2011) is a key region for the transition between these transport regimes. The = 380 K isentrope coincides with the tropical tropopause and is therefore directly affected by diabatic vertical transport of tropospheric air through the tropical transition layer (TTL) (Fueglistaler et al., 2009) into the stratosphere. Above = 380 K these air masses, which ascended through the TTL are rapidly mixed quasi-horizontally by breaking planetary waves with air from high latitudes in addition to the shallow branch of the Brewer-Dobson circulation (Birner and Bönisch, 2011;Abalos et al., 2013). This rapid transport particularly modifies the abundance of water vapour and ozone in this region, which have seasonally varying isentropic gradients (Rosenlof et al., 1997;Randel et al., 2006;Hegglin and Shepherd, 2007;Pan et al., 2007;Ploeger et al., 2013).
In our study we focus on the transition of the tracer composition in the vortex-affected UTLS region up to = 410 K during winter 2015-2016. We will quantify the effects of quasi-isentropic mixing from the tropics and diabatic downwelling and its effect on the chemical composition as well as the evolution of the age spectrum and the mean age in this region.

Meteorological conditions during winter 2015-2016
The early Arctic winter of 2015-2016 (November-December) was among the coldest winters in the lower stratosphere (LS) after 1948. These extremely cold conditions existed due to a strong and cold Arctic polar vortex which developed in November 2015 due to very low planetary wave activity in the stratosphere (Matthias et al., 2016). From late December 2015 to early February 2016 the temperatures at = 490 K decreased to below 189 K. Therefore strong dehydration and denitrification were seen in low H 2 O and HNO 3 volume mixing ratios, which finally led to a strong chlorine activation in early winter. Using MLS data the chemical influence of the vortex could be observed on isentropes below = 400 K (Manney and Lawrence, 2016).
The major final warming (MFW) (Manney and Lawrence, 2016) occurred on 5 March 2016, which led to the vortex splitting 1 week later. This early final warming was unusual, as only five other MFWs since 1958 have appeared before the middle of March. Due to this, early warming air masses in the polar lower stratosphere were mixed with non-vortex air and prevented chemical ozone depletion, reaching record low values during winter 2015-2016 (Manney and Lawrence, 2016, and references therein).
The winter of 2015-2016 was characterised by an unprecedented anomaly of the quasi-biannual oscillation (QBO) with a westerly jet that formed within the easterly phase in the lower stratosphere (Newman et al., 2016;Osprey et al., 2016). Since the QBO affects the zonal wind direction in the tropical lower stratosphere (Niwano et al., 2003) its strength and phase is crucial for stratospheric transport processes (Baldwin et al., 2001) and westerly phases are related to a strong and cold polar Arctic vortex (Holton and Tan, 1980).
Further the winter of 2015-2016 was also affected by a strong warm phase of the El Niño Southern Oscillation (ENSO) (Chen et al., 2016;L'Heureux et al., 2017). Matthias et al. (2016) argue that the strong El Niño weakened the 2016 Arctic vortex, while Palmeiro et al. (2017) found a connec-tion of this ENSO event to the strong polar vortex and the easterly MFW.

Project overview and measurements
This work will address the evolution of composition, age structure and the influence of transport and mixing of air masses in the lower stratosphere. The composition of air masses inside the LS, which is affected by diabatic descent of upper stratospheric air masses irreversibly mixed with younger air from the TTL, is analysed by combining measurements of in situ data with model calculations of the Chemical Lagrangian Model of the Stratosphere (CLaMS) (McKenna et al., 2002;Grooß et al., 2014;Ploeger et al., 2015).

The POLSTRACC campaign 2015-2016
The data presented in this study were obtained during the POLSTRACC (The Polar Stratosphere in a Changing Climate) mission, which was part of the combined PGS (POLSTRACC/GW-LCYCLE/SALSA) framework. The main objectives of the POLSTRACC mission were the investigation of structure, composition and dynamics of the Arctic LMS and processes involving chemical ozone depletion and polar stratospheric clouds in the Arctic winter UTLS.