In situ observations of CH2Cl2 and CHCl3 show efficient transport pathways for very short-lived species into the lower stratosphere via the Asian and North American summer monsoons

Efficient transport pathways for ozone depleting very short-lived substances (VSLS) from their source regions into the stratosphere are a matter of current scientific debate, however they have yet to be fully identified on an observational basis. Understanding the increasing impact of chlorine containing VSLS (Cl-VSLS) on stratospheric ozone depletion is important in order to validate and improve model simulations and future predictions. We report on the first transport study using airborne in 5 situ measurements of the Cl-VSLS dichloromethane (CH2Cl2) and trichloromethane (chloroform, CHCl3) to derive a detailed description of the two most efficient and fast transport pathways from (sub-)tropical source regions into the extratropical lower stratosphere (Ex-LS) in northern hemisphere (NH) late summer. The Cl-VSLS measurements were obtained in the upper troposphere and lower stratosphere (UTLS) above Western Europe and the mid latitude Atlantic Ocean in the frame of the WISE (Wave-driven ISentropic Exchange) aircraft campaign in autumn 2017 and are combined with the results from a three10 dimensional simulation of a Lagrangian transport model as well as back-trajectory calculations. Compared to background measurements of similar age we find up to 150 % enhanced CH2Cl2 and up to 100 % enhanced CHCl3 mixing ratios in the Ex-LS. We link the measurements of enhanced mixing ratios to emissions in the region of southern and eastern Asia. Transport from this area to the Ex-LS at potential temperatures in the range of 370−400 K takes about 5−10 weeks via the Asian summer monsoon anticyclone (ASMA). Our measurements suggest anthropogenic sources to be the cause of these strongly elevated 15 Cl-VSLS concentrations observed at the top of the lowermost stratosphere (LMS). A faster transport pathway into the Ex-LS is derived from particularly low CH2Cl2 and CHCl3 mixing ratios in the UTLS. These low mixing ratios reflect weak emission sources and a local seasonal minimum of both species in the boundary layer of Central America and the tropical Atlantic. We show that air masses uplifted by hurricanes, the North American monsoon, and general convection above Central America into the tropical tropopause layer to potential temperatures of about 360− 370 K are transported isentropically within 1− 5 weeks 20 into the Ex-LS. This transport pathway linked to the North American monsoon mainly impacts the middle and lower part of the LMS with particularly low CH2Cl2 and CHCl3 mixing ratios. In a case study, we specifically analyze air samples directly 1 https://doi.org/10.5194/acp-2021-837 Preprint. Discussion started: 18 October 2021 c © Author(s) 2021. CC BY 4.0 License.

further state that CH 2 Cl 2 and CHCl 3 contribute with about 68 % and 19 % to this increase, respectively.
However, due to high Asian emissions and an efficient transport into the stratosphere via the Asian summer monsoon (ASM), the estimation of stratospheric chlorine from Cl-VSLS could even be underestimated by 8 − 26 % (Adcock et al., 2021).
Between June and September the ASM is a wide spread convective system located above the Indian subcontinent, East and Southeast Asia (e.g., Yihui and Chan, 2005). The ASM provides fast vertical transport of surface air into the large scale 95 anticyclone (ASMA) above, which spans from the upper troposphere at about 360 K potential temperature to the lower stratosphere at about 450 K potential temperature (e.g., Park et al., 2007Park et al., , 2009Bergman et al., 2013;Vogel et al., 2019). Within the ASMA air masses are somewhat confined and separated from the surrounding UTLS air by a strong gradient of potential vorticity (e.g., Ploeger et al., 2015). Several studies have shown that these air masses are transported further vertically into the tropical pipe or break out of the ASMA to enter the extratropical LMS quasi-horizontally by Rossby wave breaking events 100 (e.g., Popovic and Plumb, 2001;Garny and Randel, 2016;Vogel et al., 2014Vogel et al., , 2016. Thus, the ASM has a strong impact on the chemical composition of the stratosphere in boreal summer (e.g., Randel et al., 2010;Randel and Jensen, 2013;Santee et al., 2017).
The specific transport pathways of Cl-VSLS from their source regions into the stratosphere have not been identified on an observational basis. The most efficient transport pathway for Cl-VSLS into the stratosphere is suggested to be via the ASMA. 105 This is why Cl-VSLS emissions from the region of continental Asia are suggested to have the highest ozone depletion potential (ODP) compared to emissions from other source regions (Claxton et al., 2019). Their regionally dependent ODP is estimated to be in the range of 0.0097 − 0.0208 (CH 2 Cl 2 ) and 0.0143 − 0.0264 (CHCl 3 ) (for comparison here the ODPs of some other chlorocarbons: CFC-11: 1; CCl 4 : 0.87; HCFC-22: 0.034; CH 3 Cl: 0.015; Carpenter et al., 2018;Claxton et al., 2019).
Projecting different past CH 2 Cl 2 emission rates Hossaini et al. (2017) predict a possibly significant delay of the recovery 110 date of stratospheric ozone ranging from a few years up to no recovery at all compared to estimations including only long-lived chlorinated species. They also found a doubling of southern hemispheric spring time ozone loss caused by CH 2 Cl 2 between 2010 and 2016. Their study emphasizes the importance of studying Cl-VSLS on a regular observational basis. However, the estimated impact of Cl-VSLS on stratospheric ozone trends is small compared to that of long-lived chlorinated species or even the impact of meteorology or the 11-year solar cycle (Chipperfield et al., 2018). Nevertheless, with the expected decrease of 115 long-lived chlorinated trace gases during the next decades due to the Montreal Protocol and its amendments and adjustments the relative importance of Cl-VSLS on stratospheric ozone depletion will further increase.
Observational evidence for Cl-VSLS being transported into the stratosphere is extremely rare and their main transport pathways into the stratosphere have not been described on an observational basis. In the present paper we use in situ measurements of CH 2 Cl 2 and CHCl 3 to identify two efficient transport pathways for Cl-VSLS from the boundary layer into the Ex-LS. In 120 addition we provide observational evidence for different impacts on the stratospheric chemical composition depending on the transport pathway the two Cl-VSLS take to enter the Ex-LS in NH late summer.
2 Airborne observations and model simulations

The WISE campaign 2017
All measurements presented in this study were obtained in the frame of the WISE (Wave-driven Isentropic Exchange) campaign 125 (Riese et al., 2017(Riese et al., , last accessed: 2021 which took place in September and October 2017. A total of 15 scientific flights were carried out with the German HALO (High Altitude and LOng range) research aircraft mainly from Shannon (Ireland) and from Oberpfaffenhofen (Germany) probing a wide area above the Atlantic Ocean and Western Europe. Among other goals, the WISE campaign aimed at investigating transport and mixing processes in the extratropical tropopause layer and the Ex-LS, the impact of the Asian monsoon system on the chemical composition of the extratropical lowermost stratosphere (LMS), as well 130 as the role of halogenated VSLS for ozone depletion and radiative forcing in the upper troposphere/lower stratosphere (UTLS) region. In this study we present UTLS measurements up to a potential temperature of 404 K of the last ten WISE flights, i.e., from 28 September to 21 October 2017 ( Figure 1).

In situ trace gas measurements
Our analysis is mainly based on airborne in situ observations of the trace gas instruments HAGAR-V (CH 2 Cl 2 and CHCl 3 ) and UMAQS (N 2 O) (as described below). The corresponding avionic data is provided by the Basic HALO Measurement and Sensor System (BAHAMAS) (Krautstrunk and Giez, 2012;Giez et al., 2017). The different measurement frequencies of the instruments were matched to that of HAGAR-V's MS module of 1/180 Hz. Exceptions are the flights on 28 September and 1 October where the MS measurement frequency is 1/240 Hz. Each data point is the average of a time interval of 40 s, except for the flights on 28 September, 1 October, and 4 October where it is 60 s, corresponding to a spatial resolution at maximum 140 cruising speed of 10 km and 15 km along the flight path, respectively. The used time and location of a data point is the respective center of the averaged time interval.

High Altitude Gas AnalyzeR -5 channel version (HAGAR-V)
HAGAR-V is a novel airborne in situ instrument. It is a modernized and largely extended version of the airborne in situ instrument HAGAR (Werner et al., 2010) and is mounted in a HALO standard rack (R-G550SM). Similar to HAGAR, HAGAR-V 145 comprises a two-channel gas chromatograph with electron capture detection (GC/ECD) as well as a non dispersive infrared absorption module for the detection of CO 2 (LI-COR 7000). In contrast to HAGAR, HAGAR-V additionally comprises a mass spectrometer (MS) coupled to two GC channels by a two-position valve which allows to switch between the two channels. This novel MS module can thus be used either for the detection of a wide range of atmospheric trace gases (different target species on each channel) or to double the measurement frequency (same target species on both channels). However, during WISE only 150 one of the two GC/MS channels was used, measuring nine different species (CH 2 Cl 2 , CHCl 3 , CH 3 Cl, . In this study, the focus is on CH 2 Cl 2 and CHCl 3 measurements by HAGAR-V's novel MS module, thus the instrumental description is confined only to the GC/MS part of the instrument. A more detailed description of HAGAR-V is given by Lauther (2020).
The general MS sampling process during WISE was as follows: Ambient air is drawn from outside the aircraft to the 155 instrument and is further compressed to 3 bar (a) by two diaphragm pumps (KNF 813.5 and 814) connected in series. The sample passes through a preconcentration tube packed with about 70 mg Carboxen 572 (Supelco) at 20°C to adsorb the target species. At a usual adsorption time of 40 s the preconcentrated sample volume is about 130 ml. Afterwards the sample is desorbed by flash heating the trap to about 270°C and injected on to the separation columns by applying a helium carrier gas flow. The sampled species are separated within two 0.25 mm J&W Scientific Al 2 O 3 /Na 2 SO 4 PLOT capillary columns of 4 m 160 and 5 m length (pre-and main-column, respectively). Both columns are temperature controlled changing from initial 35°C to final 160°C in 20 s (pre-column) and 35 s (main-column), providing two sample refocusing steps in the process. The sample is detected by a quadrupole MS detector (5975C, Agilent Technologies) using electron ionization (EI) mode.
Fast GC/MS measurements are mandatory when operating from aboard an aircraft. To achieve a sample frequency of 1/180 Hz per MS channel particularly the heating and cooling rates of the preconcentration traps and the columns were The cooling of the preconcentration traps is realized by a Stirling cooler (Twinbird, SC-UD08) and each trap is heated by a self regulating Ni heating wire (which is also used as temperature sensor) convoluted around the trap tube. To our knowledge, HAGAR-V is the only state-of-the-art airborne GC/MS instrument using indirect trap heating and our thermodesorption design 170 provides consistent heating and cooling rates of 80°C s −1 and −25°C s −1 (from 270°C down to 20°C) inside the trap tube.
In addition, our thermodesorption concept avoids large variable currents at relatively low voltages (peak current < 7 A at 48 V for < 2 s, then < 2 A) and is thus well suited to be used aboard an aircraft with stringent constraints regarding electromagnetic compatibility.
The self built separation column ovens are conceptually comparable to the principles of regular modern Low Thermal Mass 175 capillary column systems (e.g., Luong et al., 2006). In parallel to the column a heating wire and a temperature sensor wire are coiled to a torus of 7 cm in diameter acquiring fast and homogenous heat application to the column (up to 20°C s −1 ) and temperature read-out. Deactivated capillary columns emerge from the torus functioning as connecting lines and particle traps to enhance the measurement stability. The cooling of the columns is realized by fans providing a setback from final to initial temperatures within 60 s. Following the compression by the inlet pumps, usually the air sample is dehydrated because water vapor can strongly affect the reproducibility of MS measurements. However, during WISE the dehydration system of HAGAR-V could not be used.
Consequently, the MS module measured only at low ambient water vapor levels (mainly at H 2 O < 100 ppm; median: 5.6 ppm), i.e., in the UTLS region. With this solution the MS module measured during about 90% of a typical WISE flight's duration (i.e., about 7.6 h per flight).

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HAGAR-V uses two different working standards for in-flight calibration to enhance the accuracy in case of non-linear system responses. Both working standards consist of compressed clean ambient air; one of them is additionally diluted with about 25 % synthetic air. The main bottles of the working standards were calibrated by the University of Frankfurt against a calibration gas that was calibrated in second generation against an AGAGE standard on the SIO-14 (CH 2 Cl 2 ) and SIO-98 (CHCl 3 ) scale.
Every second or third flight the in-flight calibration gas bottles were refilled from the main bottles after a calibration between 190 main and flight bottles. Considering possible differences between main and flight bottles, uncertainties of the mixing ratios within the main bottles, as well as potential influence from HAGAR-V's inlet pump system, the MS relative accuracy was estimated to be 2.0 % and 4.4 % for CH 2 Cl 2 and CHCl 3 , respectively. Measurement precisions were optimized during data processing, using a strongly adapted version of the IGOR Pro analysis package called NOAHChrom, originally developed by NOAA, USA. Exponentially modified Gaussian (EMG) functions were 195 fitted to the MS signal peaks within individual time windows. Thereby peak tailing could be accurately treated and neighboring peaks were included in the background fit. In addition, the MS data were corrected for small system contamination and an occasional systematic measurement bias of one calibration gas. The measurement precision was derived for each flight from the standard deviation of one of the two in-flight calibration gases relative to its mixing ratio. The median precisions during WISE were 1.7 % (1 ppt) and 2.7 % (0.4 ppt) for CH 2 Cl 2 and CHCl 3 , respectively. UMAQS simultaneously measures CO and N 2 O from aboard HALO. The instrument uses the principle of direct absorption spectroscopy of a continuous-wave quantum cascade laser operating at a sweep rate of 2 kHz . In this study we use UMAQS' measurements of N 2 O with a total drift-corrected uncertainty of 0.18 ppb (Kunkel et al., 2019). Note, for this study the N 2 O measurements are averaged over 40 − 60 s to fit the integration times of HAGAR-V's MS module, thereby 205 smoothing out instrumental noise which most likely further improves the N 2 O precision. The instrument is calibrated regularly in-flight using a secondary standard which is calibrated against a NOAA standard before and after the campaign. The accuracy of the used N 2 O mixing ratios is 0.39 ppb.

CLaMS simulations
To support the interpretation of airborne measurements we use global three-dimensional simulations of the Chemical La-210 grangian Model of the Stratosphere (CLaMS; McKenna, 2002a,b;Pommrich et al., 2014) as well as pure CLaMS backtrajectory calculations. Pure CLaMS trajectory calculations consider only the advective (reversible) transport, neglecting (irreversible) mixing processes entirely (e.g., Vogel et al., 2019;Hanumanthu et al., 2020). However, in this study back-trajectories are useful to trace back the detailed transport pathway and transport time of an air parcel in the UTLS to possible source regions in the boundary layer and therefore provide added value compared to three-dimensional CLaMS simulations. Both 215 three-dimensional CLaMS simulations and back-trajectory calculations are driven by ECMWF ERA-Interim reanalysis data with a horizontal resolution of 1°× 1° (Dee et al., 2011). The irreversible part of transport was set to discrete mixing steps every 24 h in the three-dimensional CLaMS simulation. CLaMS employs a hybrid vertical coordinate (ζ) which, in this study, transforms from a strictly isentropic coordinate (Θ) to a pressure-based orography-following coordinate system (σ coordinates) below a threshold of 300 hPa. More detailed information about CLaMS is given by Pommrich et al. (2014) and references 220 therein. Equivalent latitudes and the location of the thermal tropopause (lapsrate, according to WMO) along the flight path was calculated from ERA-Interim reanalysis data.

Artificial tracers of air mass origin
In this study CLaMS simulations of artificial tracers of air mass origin (also referred to as surface origin tracers, Ω i ; Vogel et al., , 2016Vogel et al., , 2019 are used to identify the location of the origin of air masses whose impact can be seen in the concentration 225 data gathered during WISE. The surface origin tracers are released within 24 defined regions in the boundary layer (ζ = 120 K ∼ 2 − 3 km above ground, including orography) as shown in Figure 2

Back-trajectory calculations
In order to investigate the transport pathways corresponding to the WISE measurements analyzed here, the trajectory module of CLaMS was used to calculate back-trajectories. Other than in the three-dimensional CLaMS simulation, trajectories are calculated using only the advective part of transport without mixing. The advantage is that a single pathway for each air parcel 240 can be calculated and analyzed with the additional information of its transport time. The back-trajectories are initialized at the time and location of the center of the respective MS sample integration time window and end at the first contact with the model boundary layer (below 2 − 3 km above surface). In general, the maximum length of a trajectory is confined to 120 days.
The spatial uncertainty of calculated back-trajectories increases with time because mixing processes occurring during transport are neglected. However, the back-trajectory analysis is used here in a statistical way (ensembles of about 100 to 200 245 trajectories) and not to consider single trajectories. In addition, the maximum trajectory length of 120 days was chosen to match a large part of the time frame of the three-dimensional CLaMS simulation but the average length of the used backtrajectories is 50 days. We will show (in sections 3.1.2 and 3.1.3) that the results of the three-dimensional CLaMS simulation in which mixing of air parcels is included agrees very well with the results of the back-trajectory analysis. The analysis presented in this paper is mainly based on the CH 2 Cl 2 -N 2 O correlation observed during WISE (Figure 3). With a photochemical lifetime of 123 years (Ko et al., 2013) N 2 O has a much longer lifetime than CH 2 Cl 2 . As expected, the correlation is relatively compact for data points with low N 2 O mixing ratios (i.e., N 2 O < 325 ppb, relatively old, mixed and processed air). Towards younger air masses (N 2 O > 325 ppb) there is a distinct split of the correlation into two branches. In the 255 stratosphere, the upper branch of the CH 2 Cl 2 -N 2 O correlation shows up to 150 % enhanced CH 2 Cl 2 mixing ratios compared to data of the lower branch at the same N 2 O mixing ratios. For N 2 O > 328.5 ppb, data points with low CH 2 Cl 2 mixing ratios even anticorrelate with N 2 O (Figure 3, inlay). In general, the majority of measurements was obtained in the stratosphere above the thermal tropopause (TP) with an increasing number of observations below the thermal TP for increasing N 2 O mixing ratios.
Thereby mainly air parcels of the lower CH 2 Cl 2 -N 2 O correlation branch are from below the thermal TP.   The most frequent convection to potential temperatures on the order of 360 K is expected to originate in the tropics. Therefore, tropical monthly averaged ground-based CH 2 Cl 2 measurements of AGAGE at Ragged Point, Barbados ( The observed decrease of low CH 2 Cl 2 mixing ratios for increasing N 2 O mixing ratios (from older to younger air) agrees well with the decreasing tropical monthly averaged CH 2 Cl 2 mixing ratios from about July to September 2017, as observed by AGAGE. Extratropical NH ground-based AGAGE observations yield significantly higher CH 2 Cl 2 mixing ratios than those in the tropics. It is thus very likely that the lower branch of the CH 2 Cl 2 -N 2 O correlation is caused by the tropical Atlantic CH 2 Cl 2 270 surface seasonality.  matches the surface CH2Cl2 minimum at Barbados in September 2017. Further, the axis of the time series is adjusted so that 1 month corresponds to a change of ≈ 2 ppb N2O, which is the typical N2O change per month of age in the UTLS (Andrews et al., 2001). The gradient of the CH2Cl2-N2O correlation's lower branch qualitatively fits the temporal variation of the ground-based CH2Cl2 measurements.
The graph illustrates the congruence of ground-based tropical AGAGE CH2Cl2 measurements and airborne extratropical WISE CH2Cl2 measurements when assuming that the variation of stratospheric CH2Cl2 with age (here expressed in terms of N2O mixing ratio) arises from the temporal variation of CH2Cl2 at the ground ("tape recorder effect"). Although this simplified view ignores the impact of mixing processes and chemical reduction of CH2Cl2 it qualitatively explains the lower branch of the correlation curve for air parcels younger than a few months.

Correlation filter
In order to separately analyze the CH 2 Cl 2 -N 2 O correlation's distinct features, the measurements are filtered relative to a "mean correlation". The "mean correlation" is derived from a quadratic fit applied to the CH 2 Cl 2 -N 2 O correlation for N 2 O < 325 ppb, i.e., where there is no visible split of the correlation ( Figure 5, left). In order to identify chemically contrasting air masses 275 of potentially different origin, we focus on the most extreme differences in the chemical composition: Measurements more Filter relevant for data above 325 ppb N 2 O CH 2 Cl 2 -rich (filter = 5 ppt) CH 2 Cl 2 -poor (filter = -10 ppt) Residual data "mean correlation" Figure 5. Left panel: CH2Cl2-N2O correlation color coded with the used definition of the data filter. Red data points are considered measurements of CH2Cl2-rich air, consisting of measurements more than 5 ppt higher than the "mean correlation" and N2O ≥ 325 ppb. Blue data points are considered measurements of CH2Cl2-poor air, consisting of measurements more than 10 ppt lower than the "mean correlation" and N2O ≥ 325 ppb. The "mean correlation" is derived from a quadratic fit to the correlation for N2O < 325 ppb. Right panel: Scatter plot of CH2Cl2 as a function of the potential temperature and color coded to highlight CH2Cl2-rich (red) and CH2Cl2-poor (blue) air. On average the CH2Cl2-rich air is found at higher potential temperatures than the CH2Cl2-poor air.
than 5 ppt higher than the "mean correlation" are considered CH 2 Cl 2 -rich air; measurements more than 10 ppt lower than the "mean correlation" are considered CH 2 Cl 2 -poor air. In addition, only measurements with N 2 O > 325 ppb (corresponding to Θ < 390 K) are considered. The choice of these filter conditions allows the CH 2 Cl 2 -rich and -poor air masses to be clearly discriminated. It will further be shown below that this filter definition yields a good correspondence with the impact of different 280 air mass origins on the CH 2 Cl 2 -N 2 O correlation.
The thus defined measurements of CH 2 Cl 2 -rich air contain on average 66 % higher mixing ratios than those of CH 2 Cl 2 -poor air ((59±8) ppt vs (36±5) ppt, respectively). In addition, the median potential temperature of measurements of CH 2 Cl 2 -rich air is 16.2 K higher than that of CH 2 Cl 2 -poor air (377.8 K vs 361.6 K, respectively). These findings not only indicate tropospheric intrusions of air from two different source regions into the stratosphere, the different levels of potential temperature also suggest 285 two different transport mechanisms. One is transporting CH 2 Cl 2 -rich air mainly to the top of the LMS (Θ ≈ 380 K) and the other is transporting CH 2 Cl 2 -poor air mainly to the middle and lower part of the LMS (Θ ≈ 350 − 370 K; Figure 5, right).

Impact of different air mass origin on the extratropical UTLS
In order to investigate the impact of different air mass origin on the WISE trace gas measurements, tracers of air mass origin simulated with CLaMS are analyzed. To focus on fast transport into the LMS in the range of approximately 6 months reflecting 290 Table 1. Median fractions of different surface origin tracers from CLaMS in measurements of CH2Cl2-rich and CH2Cl2-poor air parcels, and the respective ratios of the median fractions. The last row shows the median fraction of Ω. Ω is the sum of all (non-normalized) surface origin tracers of the respective air parcels (Ω = n=24 i=1 Ωi; cf. Section 2.3.1) which is the fraction of an air parcel actually considered in the tracer analysis of CH2Cl2-rich and -poor air. The fraction (100−Ω) % is the part of an air parcel that has already been in the free atmosphere on 1 May, 2017. The geographical location of each surface origin tracer is given in Figure 2. the mean tropospheric lifetime of CH 2 Cl 2 and CHCl 3 (see Section 1), only the fraction of air parcels released from the boundary layer since 1 May is considered. Therefore, in every air parcel each surface origin tracer fraction (Ω i ) is normalized to the sum of all fractions of surface origin tracers (Ω = n=24 i=1 Ω i ≤ 100 %) in the air parcel, thus neglecting the fraction of air that was in the free atmosphere at the initialization date of the CLaMS simulation at 1 May, 2017 (i.e., air older than 6 months). The start time of our simulations on 1 May, 2017, is further chosen to be before the onset of the Asian summer monsoon (pre-monsoon) 295 in order to include all transport processes into the LS impacted by the Asian monsoon circulation. In the following, all analyzed surface origin tracers are normalized as described above, if not stated otherwise.
Further, to work out differences of air mass origin between CH 2 Cl 2 -rich and -poor air, for each surface origin tracer the median fraction in CH 2 Cl 2 -rich air parcels is compared to that in CH 2 Cl 2 -poor air parcels. Surface origin tracers with particularly high relative median fractions in either CH 2 Cl 2 -rich or -poor air are combined following these two criteria: (1) considered 300 are only surface origin tracers with median fractions ≥ 1 % in CH 2 Cl 2 -rich or -poor air parcels, and (2) the median ratio of CH 2 Cl 2 -rich/CH 2 Cl 2 -poor air respectively CH 2 Cl 2 -poor/CH 2 Cl 2 -rich air must be > 1.8. With this definition, regions of air mass origin -as defined for the model simulation -of significantly enhanced influence on measurements of CH 2 Cl 2 -rich (poor) air relative to CH 2 Cl 2 -poor (-rich) air are combined. Table 1 lists the median fractions in CH 2 Cl 2 -rich and -poor air of each surface origin tracer fulfilling criterion (1).

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The surface origin tracers also fulfilling criterion (2) for CH 2 Cl 2 -rich air are all located in the region of southern and eastern Asia (SaEA) including India, China, and Southeast Asia (cf. Figure 2). The source region of this SaEA tracer is mostly landbased and located in the core region of the Asian summer monsoon (ASM) from where the highest CH 2 Cl 2 emissions globally are expected (Claxton et al., 2020). The median fraction of the SaEA surface origin tracer in CH 2 Cl 2 -rich air is about twice that in CH 2 Cl 2 -poor air (41.5 % vs 20.7 %, respectively).

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The surface origin tracers fulfilling criterion (1) and (2) for CH 2 Cl 2 -poor air are all located in the tropics along the mostly western part of the Intertropical Convergence Zone (ITCZ) from 120°W to about 45°E (W-ITCZ; cf. Figure 2, bottom). The source region of this W-ITCZ tracer includes a large maritime region and is not known for significant CH 2 Cl 2 emissions. The median fraction of the W-ITCZ surface origin tracer in CH 2 Cl 2 -poor air is about three times higher than in CH 2 Cl 2 -rich air (40.6 % vs 13.5 %, respectively) with a particularly high contribution from the region of Central America (CAM).

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The surface origin tracers fulfilling criterion (1) but not (2) are all geographically connected. To focus on NH regions of air mass origin and because its fraction in both CH 2 Cl 2 -rich and -poor air is very low (< 2.5 %), the surface origin tracer for the lower southern hemisphere (LSH, Figure 2, top) will not be considered in the following analysis. Without LSH, the third major region of air mass origin significantly influencing the WISE measurements by relatively fast transport mainly includes an extended region of the summertime ITCZ mostly in the eastern hemisphere and the Pacific Ocean (E-ITCZ), excluding the Thereby the Spearman's correlation coefficients R SaEA = 0.7 and R W-ITCZ = −0.72 indicate a significant monotone but not necessarily linear positive and negative correlation, respectively, with fractions of the SaEA tracer ranging from 8.5 % to 48 % and those of the W-ITCZ tracer ranging from 9.3 % to 70.8 %.

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On the one hand, of all measured air masses entering the LS in the course of NH summer a large fraction originated in southern and eastern Asia. In addition, these air masses are preferably composed of CH 2 Cl 2 -rich air and thus strongly contribute to steepening the CH 2 Cl 2 -N 2 O correlation slope (upper branch). On the other hand, young air from the region of the central and western part of the ITZC strongly influences the UTLS with CH 2 Cl 2 -poor air (lower branch). Further, measurements in between CH 2 Cl 2 -rich and -poor air in the CH 2 Cl 2 -N 2 O correlation contain moderate fractions (in the range of 20−40 %) from 335 both regions of air mass origin.  It has to be noted that the ground-based measurements of AGAGE (Section 3.1) were obtained in the CAM surface origin tracer region, which is included in the W-ITCZ tracer. The extraordinarily high impact of the CAM tracer (24.9 %) on the measurements of CH 2 Cl 2 -poor air strongly supports the comparison made in Section 3.1 and underlines our conclusion of CH 2 Cl 2 's tropical Atlantic surface seasonality being reflected in the measurements within the UTLS region.

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The influence of the E-ITCZ surface origin tracer on the CH 2 Cl 2 -N 2 O correlation is about equal in all air parcels with fractions of around 40 % (Figure 6, top right). This region of air mass origin is thus generally important for the composition of young air masses in the LMS without a specifically strong influence on either CH 2 Cl 2 -rich or -poor air.

Results of back-trajectory calculations
The back-trajectories calculated for CH 2 Cl 2 -rich and -poor air are analyzed in two steps. First, the location of maximum rate 345 of change in potential temperature (diabatic ascent rate) along each back-trajectory is derived and the transport time from the measurement to this location is calculated. Second, the back-trajectories are considered up to the point where they reach the model boundary layer. General transport pathways are derived for measurements of CH 2 Cl 2 -rich and -poor air. Within the maximum of 120 days the model boundary layer is reached by 59 out of 80 back-trajectories of CH 2 Cl 2 -rich air (74 %) and 170 out of 189 back-trajectories of CH 2 Cl 2 -poor air (90 %), and only these back-trajectories are analyzed in the following.

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The median time for an air parcel at the boundary layer to reach the location of measurement is 48 days; CH 2 Cl 2 -poor air in general shows shorter transport times (43 days) than CH 2 Cl 2 -rich air (64 days).
Locations of maximum diabatic ascent rate and transport times

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Of all trajectories related to CH 2 Cl 2 -poor air more than 60 % exhibit the location of max. ∆Θ 18h above the region of Central America with the rest being located above southern and eastern Asia and along the ITZC (Figure 7, left). The transport times since the ascent above Central America mainly range between 1 − 5 weeks and are much shorter than for those air parcels with the center of hurricane Maria at some point along its track (Figure 8). The 1°radius of tolerance was chosen because it corresponds to the spatial resolution of the ERA-Interim reanalysis data used for the trajectory calculation, as well as (roughly) to the hurricane's radius from its core. This analysis directly links 27 WISE measurements to the convection of hurricane Maria with transport times ranging between one week and one month. Interestingly, CH 2 Cl 2 mixing ratios of measurements linked to hurricane Maria positively correlate with transport time (R Pearson = 0.85; Figure 9, top right). Those air samples related to short transport times contain the lowest CH 2 Cl 2 mixing ratios at N 2 O > 325 ppb measured during WISE (Figure 9, top left). According to the back-trajectories, most of the air parcels lifted up by hurricane Maria left the model boundary layer above the tropical Atlantic where CH 2 Cl 2 sources are small ( Figure   9, bottom). In that region the seasonal minimum of CH 2 Cl 2 mixing ratios is in September (cf. Section 3.1). This implies that 395 when air masses lifted up by hurricane Maria mix, they can only increase their CH 2 Cl 2 mixing ratio, i.e., mixing with air of higher CH 2 Cl 2 mixing ratio. Air parcels related to longer transport times did not take a direct path to the extratropics after being lifted up by hurricane Maria and rather followed the subtropical jet stream eastwards around the globe, thereby enhancing the chances of mixing with air of higher CH 2 Cl 2 mixing ratios.
In general, despite being a significant source of convection, hurricane Maria did not contribute to the transport of enhanced 400 CH 2 Cl 2 mixing ratios into the stratosphere and rather led to the transport of CH 2 Cl 2 -poor air, a consequence of CH 2 Cl 2 's tropical Atlantic boundary layer seasonality. This result is consistent with the lack of strong CH 2 Cl 2 sources in the oceanic region of convection. Nevertheless, our analysis shows that large hurricanes can provide a fast transport into the extratropical UTLS. Below in Section 4 it is discussed how tropospheric air masses observed on 1 October are mixed into the LS a few days after the observation. For instance, this is of particular importance for brominated short-lived substances (e.g., CH 2 Br 2 and

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CHBr 3 ) that have a high ODP and some of their largest emission sources located in tropical oceans (e.g., Hepach et al., 2015;Rotermund et al., 2021).  The back-trajectories from the location of measurement to the model boundary layer are analyzed to identify the main trans-410 port pathways of CH 2 Cl 2 -rich and -poor air into the stratosphere. As representative examples, Figure 10 shows the trajectories of the WISE flights on 1 October (left) and 7 October (right) for CH 2 Cl 2 -poor and -rich air, respectively.
Almost all trajectories of CH 2 Cl 2 -rich air show the following general pathway: The air parcels are convectively lifted up above southern and eastern Asia to Θ ≈ 360 K. Further ascent of the air parcels occurs in a clockwise upward spiraling motion (Vogel et al., 2019), following the dynamics of the Asian summer monsoon anticyclone (ASMA) mainly to potential 415 temperatures in the range of 370 − 400 K. Preferably within this potential temperature range, the air parcels break out of the ASMA eastwards (e.g., Honomichl and Pan, 2020), following the subtropical jet stream at about 40°N until, eventually, they quasi-isentropically enter the extratropics above the eastern Pacific or western Atlantic Ocean (e.g., Vogel et al., 2014Vogel et al., , 2016.
All trajectories suggest that the air parcels arrived from the west to the location of measurement and most trajectories suggest a slight diabatic descent of up to −10 K in the extratropics a few days before the measurement.

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The majority (> 60 %) of trajectories of CH 2 Cl 2 -poor air show a strong uplift above the region of Central America up to potential temperatures mainly in the range of 360 − 370 K. After convection, the trajectories experience a northward drag towards an anticyclonic structure located above North America, and most of these trajectories further directly enter the extratropics above the Atlantic Ocean or the North American east coast leading to short transport times to the location of measurement. the globe before entering the extratropics. This significantly increases the transport time of an air parcel by about 3 weeks and has the potential to cause it to descend by up to about −10 K as indicated by the back-trajectory calculations. However, this concerns a minority of air parcels and has only a minor effect on the median transport time of all observed air parcels lifted up above the region of Central America (max. ∆Θ 18h between 0°− 30°N and 50°− 120°W) which is still 44 days shorter than that of the observed air parcels lifted up above southern and eastern Asia (max. ∆Θ 18h between 0°− 40°N and 60°− 160°E; 430 24 vs 68 days, respectively). Below, in Section 4, the transport pathway from Central America to the Ex-LS is discussed in more detail.
The analysis of the entire set of back-trajectories shows that for the majority of measurements there are two distinct transport pathways into the Ex-LS. CH 2 Cl 2 -rich air is transported by the ASMA and CH 2 Cl 2 -poor air mainly by convection above Central America, which includes the North American monsoon, the ITCZ and hurricanes. In general, air parcels are lifted 435 up to similar potential temperature levels by the convection of the ASM in Asia and the convection above Central America.
The key difference yielding the observed higher potential temperatures of CH 2 Cl 2 -rich air from Asia compared to those of CH 2 Cl 2 -poor air from Central America is the additional uplift by the ASMA following the convection within the ASM (e.g., Müller et al., 2016;Brunamonti et al., 2018;Vogel et al., 2019;von Hobe et al., 2021). The slow upward spiraling dynamics within the ASMA also contributes to the longer transport time from the boundary layer to the location of the measurement 440 of CH 2 Cl 2 -rich air compared to that of CH 2 Cl 2 -poor air. Another aspect adding to the different transport times is the longer transport pathway from Asia because air masses were always observed to reach the location of measurement from the west.

Comparison of CH 2 Cl 2 and CHCl 3
In this section the results of the CH 2 Cl 2 analysis are used to investigate CHCl 3 data measured during WISE. Figure 11 ( left) shows the CHCl 3 -N 2 O correlation color coded to highlight air parcels of CH 2 Cl 2 -rich (red) and -poor (blue) air (cf. Sec-445 tion 3.1.1). In general, the CHCl 3 -N 2 O correlation reveals similar but less clearly pronounced structures as observed for the CH 2 Cl 2 -N 2 O correlation. The CHCl 3 -N 2 O correlation similarly is less compact for higher N 2 O mixing ratios. However, a distinct split of the correlation, as observed for CH 2 Cl 2 , is not clearly visible, but a broad scatter on the CHCl 3 axis with mixing ratios in the stratosphere being enhanced by up to 100 % compared to the lowest measurements at similar N 2 O values.
Measurements of CH 2 Cl 2 -rich air also show clearly enhanced CHCl 3 mixing ratios and measurements of CH 2 Cl 2 -poor air 450 also contain the lowest CHCl 3 mixing ratios at given N 2 O values. Nevertheless, there are a few significant differences which will be analyzed in the following.
The seasonal cycle of CHCl 3 is less pronounced but in phase with that of CH 2 Cl 2 (cf. Figure B1 in Appendix B). Based on a comparison with ground-based AGAGE observations, CHCl 3 data for measurements of CH 2 Cl 2 -poor air between N 2 O values of 229.5 ppb and 331 ppb reflect CHCl 3 's tropical surface seasonality as it similarly was observed for CH 2 Cl 2 .

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In our data, high CH 2 Cl 2 concentrations coincide with high CHCl 3 concentrations in many, but not in all cases. There are examples of high CHCl 3 concentrations where CH 2 Cl 2 concentrations are relatively low. This suggests that air from regions with relatively stronger CHCl 3 than CH 2 Cl 2 sources was measured. However, air masses of CH 2 Cl 2 -rich air clearly stand out https://doi.org/10.5194/acp-2021-837 Preprint. Discussion started: 18 October 2021 c Author(s) 2021. CC BY 4.0 License. by their elevated CHCl 3 mixing ratios in the region of Θ ≈ 380 K (Figure 11, center). Based on the results of Section 3.1.3, we therefore suggest that the ASMA is the dominant factor also for the transport of enhanced CHCl 3 mixing ratios to the Ex-LS 460 at Θ ≈ 380 K. CH 2 Cl 2 -rich air f(x) = -3.2 + 0.22x, R = 0.85 Figure 11. CHCl3-N2O correlation (left) and CHCl3 as a function of potential temperature (center) color coded to highlight measurements of CH2Cl2-rich and -poor air; CHCl3-CH2Cl2 correlation of measurements of only CH2Cl2-rich air (right).
Due to their similar photochemical lifetime CH 2 Cl 2 and CHCl 3 are expected to linearly correlate in the stratosphere; however varying correlation slopes can arise due to different emission ratios in the source regions defining the respective composition of the air parcel. The measurements of CH 2 Cl 2 -rich air show a significant positive linear correlation with CHCl 3 (Figure 11, right) suggesting sources or source regions with similar emission ratios of these species. Due to the strong evidence for CH 2 Cl 2 -rich 465 air being significantly affected by anthropogenic sources, the significant positive correlation with CHCl 3 suggests that this also holds for CHCl 3 . The highest anthropogenic emissions of CHCl 3 are expected to originate from China (Fang et al., 2018), which is within the region of sources particularly impacting the air masses of CH 2 Cl 2 -rich air analyzed here (Section 3.1.2).
This suggests a significant anthropogenic impact that clearly enhances CHCl 3 concentrations in the upper LMS.
At a closer look, the CHCl 3 -CH 2 Cl 2 correlation in Figure 11 (right) reveals two correlation lines with different slopes.

470
The nature of the different slopes can be better understood when looking at the CHCl 3 -CH 2 Cl 2 correlation of all WISE measurements color coded with CLaMS's surface origin tracers (Figure 12; left and center). The CHCl 3 -CH 2 Cl 2 correlation fans out towards higher mixing ratios giving the impression of several correlation lines with different slopes. The data points forming the steepest correlation slope show the highest W-ITCZ tracer fractions and the lowest SaEA tracer fractions while for the data points forming the lowest correlation slope the opposite is the case. The CHCl 3 -CH 2 Cl 2 correlation slope thus flattens 475 with increasing entry of air masses originating from southern and eastern Asia. This suggests larger CHCl 3 :CH 2 Cl 2 emission ratios in the region of the central and western ITCZ region (with presumably mostly biogenic sources) than in southern and eastern Asia (where anthropogenic sources likely dominate).  Compared to the lowest correlation line, the wider range of surface origin tracer fractions apparent in the correlation lines with steeper slopes might be due to the different strengths of seasonality of CH 2 Cl 2 and CHCl 3 possibly affecting the linear 480 relationship between CHCl 3 and CH 2 Cl 2 . In addition, the highest mixing ratios pertain to the steeper correlation lines. However, all of those were observed at low potential temperature levels and below the thermal TP (Figure 12, right) where mixing ratios can easily exceed those in the stratosphere above. The evolution of tropospheric air masses at potential temperatures above 350 K as shown in the steeper correlation line is discussed further below in Section 4. In summary the analysis suggests clear similarities between CH 2 Cl 2 and CHCl 3 when emitted by anthropogenic sources and differences between the two species 485 mainly due to additional (presumably biogenic) CHCl 3 sources. Figure 13 shows measured and simulated WISE tracers as a function of equivalent latitude and potential temperature. It illustrates the air masses of enhanced CH 2 Cl 2 and CHCl 3 that were transported from southern and eastern Asian sources by the ASMA to potential temperatures of around 380 K and to the Ex-LS. At slightly lower potential temperatures and equivalent lat-490 itudes we observed particularly low CH 2 Cl 2 mixing ratios and partly low CHCl 3 mixing ratios. The corresponding air masses were sampled at potential temperatures mainly above 360 K but mostly below the thermal tropopause and were uplifted from Central American as well as tropical Atlantic and Northern African source regions via convection by hurricanes, by the ITCZ and the North American monsoon and transported further towards the location of measurement at higher latitudes; therefore these air masses are characterized by low values of equivalent latitude.  function of equivalent latitude and potential temperature. Note that equivalent latitudes ≤ 0°N are likely calculated artifacts due to a negative bias induced by convection (Pan et al., 2012). The open symbols indicate a measurement location below or equal to the thermal tropopause (TP) and the location of full symbol data points is above the thermal TP. The coloration of the panel at bottom right correspond to the lower branch (blue) and to the upper branch (red) of the CH2Cl2-N2O correlation (referred to as CH2Cl2-poor and -rich air, respectively; cf. Section 3.1.1). CH2Cl2-poor air measured at potential temperatures generally up to 375 K mostly originated from the Central American as well as tropical Atlantic and Northern African boundary layer (central and western ITCZ) and has mostly not (yet) entered the lower stratosphere at the time of measurement. CH2Cl2-rich air is strongly influenced by air masses from southern and eastern Asia and was measured almost exclusively in the extratropical lower stratosphere. Note that the fraction of surface origin tracers given in the graphs is not an absolute fraction of the whole air parcels but of the air masses younger than 6 months within the air parcels (cf. Section 3.1.2). The two different transport pathways from the boundary layer into the extratropical UTLS region are described in the text.

Discussion
The presented distribution of air masses from different source regions in the NH UTLS is in good agreement with a similar study by Rotermund et al. (2021) also based on WISE measurements, but using bromine observations. However, the lower stratospheric region of high bromine concentrations from Asian source regions described by Rotermund et al. (2021) is at lower potential temperatures and higher equivalent latitudes than the CH 2 Cl 2 -rich air described in the present paper. This could be due to relatively stronger (mostly biogenic) bromine emission sources in the adjacent region of the ASM compared to the 500 mostly anthropogenic CH 2 Cl 2 emission sources mainly located in the core region of the ASM. In addition, in the present paper the first five research flights in September are not analyzed in contrast to the study by Rotermund et al. (2021). Nevertheless, compared to the very short-lived bromine species analyzed by Rotermund et al. (2021), the combination of a longer lifetime, highly significant Asian emission sources, and a strong seasonal cycle clearly benefits the use of CH 2 Cl 2 observations to derive details about the different transport mechanisms and pathways from the source region into the NH summertime UTLS.

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Further, elevated quantities of peroxyacetyl nitrate (PAN) were measured in the NH LMS during the WISE flight on 13 September 2017 by the GLORIA instrument with main sources in South Asia and Southeast Asia uplifted by the ASMA (Wetzel et al., 2021). Moreover, the transport pathway into the LS via the ASMA derived from back-trajectories of air originating in southern and eastern Asia was also observed for other measurements taken in the NH UTLS over Europe and the Atlantic Ocean during the HALO TACTS campaign in August and September 2012 (Vogel et al., , 2016Müller et al., 2016;510 Rolf et al., 2018). In the present study, we have for the first time directly related this transport pathway to in situ Cl-VSLS measurements in the LS and observed that air masses strongly enhanced in CH 2 Cl 2 and CHCl 3 are rather rapidly transported to the top of the NH LMS at about 380 K by this pathway. This finding supports the modeled results of Claxton et al. (2019) who show that Cl-VSLS sources located in tropical Asia have a higher potential for stratospheric ozone depletion than those from any other source region. In addition, CHCl 3 has significant biogenic sources (Engel et al., 2018). Our study suggests that not 515 only the enhanced CH 2 Cl 2 mixing ratios but also the enhanced CHCl 3 mixing ratios observed at about 380 K are significantly impacted by anthropogenic sources which are expected to be strongest in the region of southern and eastern Asia (Claxton et al., 2020) and eastern China (Fang et al., 2018), respectively.
There are several studies analyzing the transport of air into the stratosphere by convection above Central and North America and its further distribution by the North American monsoon anticyclone (NAMA) (e.g., Gettelman et al., 2004;Ray, 2004;520 Pittman et al., 2007;Weinstock et al., 2007;Herman et al., 2017;Wang et al., 2021;Clapp et al., 2021). Studies based on observational data mostly focus on the equatorward transport of air out of the NAMA (e.g., Gettelman et al., 2004;Ray, 2004;Pittman et al., 2007;Weinstock et al., 2007). Mainly model based simulations (e.g., Li, 2005;Ploeger et al., 2013;Nützel et al., 2019) and a study based on satellite observations (Clapp et al., 2021) have addressed north-and northeastward outflow of the NAMA. Here, we have described a transport pathway from the marine boundary layer in Central America and the 525 tropical Atlantic into the NH midlatitude UTLS based on in situ Cl-VSLS observations. To our knowledge in situ Cl-VSLS measurements have never been used before to study transport from the tropics south of the NAMA to the midlatitude UTLS.
The horizontal advection northwards following the convection in the tropics might be related to the NAMA as described in a recent model study by Wang et al. (2021). Further, Clapp et al. (2021) observed the main outflow (68 %) of the NAMA to be in north-eastward direction between 35°N and 60°N in July and August. In good agreement with both studies, the trajectories of WISE measurements uplifted above Central America show a northward drag towards a location of circular movement resembling the NAMA with a north-eastward escape from the circulation (cf. Figure 10, left).
Many studies have addressed the topic of tropospheric intrusions into the stratosphere above Central and North America by analyzing direct injections via overshooting convection (e.g., Smith et al., 2017;Anderson et al., 2017;Herman et al., 2017;Cooney et al., 2018;Clapp et al., 2019Clapp et al., , 2021. The results of Wang et al. (2021) suggest that for air uplifted in the region 535 of Central America (15°N to 20°N) overshooting convection is not the main transport pathway into the stratosphere during NH summer. Above this region of Central America the TP usually is at potential temperatures on the order of 380 K and most convection in this region does not uplift air higher than that. Drawn towards the NAMA, the uplifted air gets further transported horizontally to higher latitudes (Wang et al., 2021) where the TP is 1 − 2 km higher than usual due to the NAMA (Schoeberl et al., 2020). Further horizontal transport northeastward out of the anticyclone, as shown in our study, eventually causes the 540 tropical air masses (being on high potential temperatures) to isentropically enter the LS.
Particularly during the WISE flight on 1 October when we sampled the largest number of air parcels uplifted by hurricane Maria the measurements were highly impacted by air originating in the region of Central America (cf. Figure 13). With a median transport time of 18 days these measurements agree well with the fast transport pathway into the stratosphere described by Wang et al. (2021). However, despite the fact that we observed most of these air masses at latitudes around 50°N and 545 potential temperatures in the range of 350 − 370 K, the majority of measurements were below the thermal TP. Figure 14 shows the meteorological situation of this particular WISE flight on 1 October based on ERA-Interim reanalysis data (Dee et al., 2011). Obviously, the air masses breaking out of the anticyclone above the North American east coast (cf. Figure 10, left) turned into a streamer carrying a local high TP to higher latitudes. A few days after our observation, this streamer became unstable and mixed into the LS. This implies that tropical air lifted up by hurricanes and other convective systems in the region 550 of Central America can enter the Ex-LS quasi-isentropically during NH autumn even if the convection in the tropics has not transported the air above the TP. This transport pathway, described by Wang et al. (2021) as the fastest and most efficient transport of tropical tropospheric air into the North American LMS region, has been corroborated for the first time on the basis of in situ Cl-VSLS observations by the present study. We have thereby shown that tropical surface mixing ratios of VSLS from the region of Central America and the Atlantic Ocean can be efficiently transported into the Ex-LS during the late North 555 American monsoon season.
Our results further show a regional dependency of the slope of the NH UTLS CHCl 3 -CH 2 Cl 2 correlation. Observations by Say et al. (2019) in the Indian boundary layer suggest a similarly flat CHCl 3 -CH 2 Cl 2 correlation slope as observed during WISE for air masses strongly impacted by Asian sources. However, AGAGE measurements (Prinn et al., 2018) from Barbados in 2017 show seasonally varying CHCl 3 -CH 2 Cl 2 correlation slopes not necessarily matching the steep slope observed for air 560 masses strongly impacted by Central American source regions during WISE. The here presented regional dependency of the CHCl 3 -CH 2 Cl 2 correlation slope could thus be a seasonal phenomenon depending on transport efficiency and locally varying emissions. Obviously, more in situ observations of CH 2 Cl 2 and CHCl 3 in the UTLS (particularly in different seasons) and ground-based (particularly in Asia) are needed to better understand the correlation behavior of CH 2 Cl 2 and CHCl 3 in the UTLS. black diamonds indicate the second thermal TP; the white line shows the 4 PVU surface. The plots show the probing of a high TP streamer of air originating in Central America at midlatitudes on 1 October. Four days later, the streamer became unstable and a large volume mixed into the LS above the thermal TP. This figure illustrates the intrusion of tropical air into the LS: Air within the streamer has been lifted up by a hurricane into the TTL and was further transported to higher latitudes by an upper level anticyclone above North America (cf. Figure 10, left) to be finally mixed into the LS by Rossby wave breaking.

Conclusions
We have presented the first study on transport of Cl-VSLS into the Ex-LS based on tracer-tracer correlations using in situ Cl-VSLS observations. A schematic of the transport pathways we deduced in this study is shown in Figure 15. Our measurements in the LS above the midlatitude Atlantic Ocean in autumn 2017 revealed up to 150 % enhanced CH 2 Cl 2 and up to 100 % enhanced CHCl 3 mixing ratios compared to measurements with similar N 2 O mixing ratios, i.e., similarly processed air. In the 570 stratosphere, the samples of CH 2 Cl 2 -rich air also contained most of the observed CHCl 3 -rich air and the highest mixing ratios of both species detected in the stratosphere at Θ ≈ 380 K. In contrast to CHCl 3 , CH 2 Cl 2 is almost exclusively of anthropogenic origin (Engel et al., 2018) and a good correlation of CH 2 Cl 2 -rich air with CHCl 3 -rich air suggests anthropogenic sources also impacting the enhanced CHCl 3 mixing ratios observed in the region at about 380 K potential temperature. Using a global three-dimensional Lagrangian model simulation we have shown a particularly strong influence of southern and eastern Asian 575 sources in these air masses of enhanced CH 2 Cl 2 and CHCl 3 mixing ratios.
Back-trajectory calculations agree well with the global three-dimensional model simulation and reveal a distinct transport pathway via the Asian summer monsoon for the air masses of enhanced CH 2 Cl 2 and CHCl 3 mixing ratios. This pathway implies convection over southern and eastern Asia to about 360 K potential temperature 4 − 10 weeks prior to the measurement (i.e., in July and August) and a slow circular upwelling to 370 − 400 K in the ASMA. The observed air masses broke out of the 580 anticyclone eastward following the subtropical jet stream before entering the extratropics above the eastern Pacific or western Atlantic Ocean (horizontal red arrow in Figure 15). This transport pathway was also observed during the HALO TACTS campaign in 2012 (Vogel et al., , 2016Müller et al., 2016;Rolf et al., 2018).
Our results provide observational evidence to support the findings of model studies (e.g., Hossaini et al., 2015Hossaini et al., , 2019Claxton et al., 2019) which connect the recent increase in Asian CH 2 Cl 2 and CHCl 3 emissions (e.g., Leedham-Elvidge et al., 2015;585 Oram et al., 2017;Feng et al., 2018;Fang et al., 2018;Adcock et al., 2021) with an increase in the contribution to stratospheric chlorine levels by the two species. Particularly the region at about 380 K potential temperature is dominated by young air masses from southern and eastern Asia thereby strongly increasing the chlorine loading from VSLS in this layer in NH late summer. Our results emphasize that further increases in Asian CHCl 3 emissions will inevitably lead to similarly clear signatures of enhanced CHCl 3 mixing ratios in the Ex-LS as we already observe for CH 2 Cl 2 .

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A faster pathway from the (sub-)tropical boundary layer into the NH Ex-LS with transport times of 1 − 5 weeks was derived from particularly low CH 2 Cl 2 mixing ratios observed in the UTLS region. The CH 2 Cl 2 -poor air mainly originated from Central America as well as from the the tropical Atlantic Ocean and Northern Africa (central and western ITCZ) and was uplifted above Central America during the course of September. Ground-based AGAGE measurements (Prinn et al., 2018) within that region show minimum background mixing ratios of both CH 2 Cl 2 and CHCl 3 in September. This seasonal minimum is clearly 595 reflected in our UTLS measurements and allows these air masses to be distinguished from the strongly enhanced mixing ratios transported via the ASMA.
The transport pathway derived from CH 2 Cl 2 -poor air follows a general pattern: Air masses are convectively uplifted into the TTL above Central America to about 360 − 370 K potential temperature (vertical blue arrow in Figure 15). The convection is induced by the general updraft in the ITCZ region, by the North American monsoon, and by hurricanes. We could directly link 600 measurements of CH 2 Cl 2 -poor air to the uplift by the category 5 hurricane Maria (Pasch et al., 2019). After the convection, the air masses were horizontally transported to higher latitudes and drawn towards an anticyclonic structure above North America.
Resolved by back-trajectories, the anticyclone above North America was much smaller than the ASMA and was located mostly https://doi.org/10.5194/acp-2021-837 Preprint. Discussion started: 18 October 2021 c Author(s) 2021. CC BY 4.0 License. at 35°N and 80°W above Florida, likely being a remnant of the NAMA which usually declines in late September (e.g., Vera et al., 2006). Other than observed for the ASMA, the circulating air parcels above North America did not significantly increase 605 their potential temperature. Further, the air masses broke out of the anticyclone northeastward forming a streamer which carried a local high TP into higher latitudes. Eventually, these air masses mixed into the LS by Rossby wave breaking and influenced the chemical composition of the NH Ex-LS 10 − 20 K below the air masses dominated by transport via the ASMA.
Our study shows that air masses lifted by convection in the tropical region of Central America do not need to directly cross the TP or to slowly enter the tropical pipe to be transported into the stratosphere. In the TTL, fast horizontal transport north-610 ward on high potential temperature levels provides an efficient and fast pathway for air lifted up in the tropics above Central America to quasi-isentropically enter the Ex-LS during NH late summer. Air transported along this pathway was observed to be mostly CH 2 Cl 2 -poor and CHCl 3 -poor air. However, transport along this pathway may cause other ozone depleting short-lived substances with stronger sources in the region of the central and western ITCZ (such as tropical maritime and coastal sources, e.g., CH 2 Br 2 and CHBr 3 , Hepach et al., 2015) to be significantly enhanced in the middle and lower part of the LMS (e.g., 615 Rotermund et al., 2021).
Particularly the use of in situ CH 2 Cl 2 measurements as a very short-lived tracer has clearly revealed the differences between the two main transport pathways into the NH Ex-LS described in this study. In addition, we have deduced a higher CHCl 3 :CH 2 Cl 2 emission ratio in the central and western ITCZ region compared to southern and eastern Asia. The difference might be due to additional biogenic CHCl 3 sources in the ocean-rich central and western ITCZ region, while the emissions in 620 southern and eastern Asia are most likely dominated by anthropogenic continental sources. However, more UTLS observations of CH 2 Cl 2 and CHCl 3 in different seasons as well as more ground-based long-term observations of the two species in Asia are needed to complete the understanding of the seasonal and inter-annual variability of the transport pathways identified in our study. Figure 15 shows a schematic drawing of the two reported main transport pathways into the NH Ex-LS in late summer.
The scheme summarizes the main findings of this paper. Convective updraft Figure 15. Schematic meridional view of the two major transport pathways for CH2Cl2-rich and CHCl3-rich air (red) and CH2Cl2-poor and mainly CHCl3-poor air (blue) from the source region into the NH LMS. The pathway from source regions located mostly in the western part of the ITCZ starts with convection into the tropical tropopause layer (TTL) above Central America by general updraft in the ITCZ, by the North American monsoon, and by hurricanes as shown for hurricane Maria in Section 3.1.3. Quasi-isentropic transport to the north and northeast eventually transports the air into the LMS at Θ ≈ 360 K. Air masses from southern and eastern Asia are uplifted by the Asian summer monsoon (ASM) to Θ ≈ 360 K with subsequent slow upwelling within the monsoon anticyclone to Θ ≈ 380 K. These air masses break out of the anticyclone to follow the subtropical jet stream eastwards before isentropically entering the Ex-LS above the eastern Pacific or western Atlantic Ocean.

Equator
Data availability. The following measured and simulated WISE data used in this paper is available at the HALO data depository (