Strong variability of the Asian Tropopause Aerosol Layer (ATAL) in August 2016 at the Himalayan foothills

The South Asian summer monsoon is associated with a large-scale anticyclonic circulation in the Upper Troposphere and Lower Stratosphere (UTLS), which confines the air mass inside. During boreal summer, the confinement of this air mass leads to an accumulation of aerosol between about 13 km and 18 km (360 K and 440 K potential temperature), this accumulation of aerosol constitutes the Asian Tropopause Aerosol Layer (ATAL). We present balloon-borne aerosol backscatter measurements 5 of the ATAL performed by the Compact Optical Backscatter Aerosol Detector (COBALD) instrument in Nainital in Northern India in August 2016, and compare these with COBALD measurements in the post-monsoon time in November 2016. The measurements demonstrate a strong variability of the ATAL’s altitude, vertical extent, aerosol backscatter intensity and cirrus cloud occurrence frequency. Such a variability cannot be deduced from climatological means of the ATAL as they are derived from satellite measurements. To explain this observed variability we performed a Lagrangian back-trajectory analysis using 10 the Chemical Lagrangian Model of the Stratosphere (CLaMS). We identify the transport pathways of air parcels contributing to the ATAL over Nainital in August 2016, as well as the source regions of the air masses contributing to the composition of the ATAL. Our analysis reveals a variety of factors contributing to the observed day-to-day variability of the ATAL: continental convection, tropical cyclones (maritime convection), dynamics of the anticyclone and stratospheric intrusions. Thus, the ATAL is a mixture of air masses coming from different atmospheric height layers. In addition, contributions from the model boundary 15 layer originate in different geographic source regions. The location of strongest updraft along the backward trajectories reveal a cluster of strong upward transport at the southern edge of the Himalayan foothills. From the top of the convective outflow level (about 13 km; 360 K) the air parcels ascend slowly to ATAL altitudes within a large-scale upward spiral driven by the 1 https://doi.org/10.5194/acp-2020-552 Preprint. Discussion started: 22 June 2020 c © Author(s) 2020. CC BY 4.0 License.

trometer (POPS) (Gao et al., 2016) from Kunming, China, indicate that ATAL particles are dominated by particles with a size of ≈ 0.1 µm (Yu et al., 2017;Vernier et al., 2018). Based on CALIOP measurements, the summertime aerosol optical depth associated with the ATAL is ≈ 0.006 (Vernier et al., 2015), leading to a reduction in insolation of about 0.6 W/m 2 and a surface cooling of roughly 0.5 K.
The impact of the ATAL is also modulated by El Niño. During El Niño, the ATAL is thicker and broader over the Indian 5 region, resulting in a reduction of the solar flux and a surface cooling of about 1 K over North India. An elevated ATAL over South Asia exacerbates the severity of Indian droughts (Fadnavis et al., 2019b). The radiative forcing by the ATAL can further The chemical signature of air masses within the monsoon anticyclone (e.g. tropospheric pollutants, water vapour) is exported to the Northern Hemisphere during summer and fall through quasi-isentropic transport from low latitudes (e.g., Ploeger et al.,15 2013; Vogel et al., 2014Vogel et al., , 2016Vogel et al., , 2019Spang et al., 2015;Garny and Randel, 2016;Müller et al., 2016;Fadnavis et al., 2018;Rolf et al., 2018;Yan et al., 2019). Because of this export of air from the Asian monsoon circulation to the Northern Hemisphere, the ATAL particles contribute significantly (∼ 15%) to the Northern Hemisphere stratospheric column aerosol surface area on an annual basis (Yu et al., 2017).
Only limited information on the chemical composition of the ATAL particles is available from measurements so far. From 20 simulations, there is evidence that desert dust is lifted to UTLS altitudes and entrained into the ATAL (Fadnavis et al., 2013;Lau et al., 2018;Yuan et al., 2019). Aircraft in situ measurements suggest that at lower altitudes the chemical composition of ATAL particles are dominated by carbonaceous and sulphate materials, consistent with the expectation that aerosol trends in the UTLS in the past decades are under increasing influence of sulphur emissions in Asia (Martinsson et al., 2014;Vernier et al., 2015;Fadnavis et al., 2019a). However, the first offline (balloon-borne filter samples) chemical analysis of ATAL particles 25 suggested the presence of nitrate aerosol, but undetectable concentrations of sulphate ions (Vernier et al., 2018). Also, the evaluation of a set of remote sensing measurements indicates a strong contribution of solid ammonium nitrate particles to the ATAL (Höpfner et al., 2019). Moreover, Fairlie et al. (2020) found in model simulations for summer 2013 a different chemical compositions of the ATAL depending on the location within the Asian monsoon anticyclone. They found that nitrate aerosol is a dominant component of the ATAL on the southern flank of the anticyclone. 30 The Asian monsoon in the UTLS does not constitute a stable, persistent unimodal anticyclonic circulation (as it sometimes appears in climatologies), but can be bimodal (with an Iranian and a Tibetan mode) and moreover shows a strong day-to-day variability (Zhang et al., 2002;Yan et al., 2011;Vogel et al., 2015;Nützel et al., 2016). There is also a substantial inter-annual variability of the monsoon circulation with impact on the concentrations of tracers confined in the monsoon circulation (Santee et al., 2017;Yuan et al., 2019). This variability of the monsoon circulation impacts on the variability of trace-gas (Luo et al., 2018) and aerosol distributions (Lau et al., 2018;Yuan et al., 2019).
The strong convective activity in the Asian summer monsoon region also impacts the temperature in the monsoon region and the formation of cirrus clouds. Enhanced convection is linked to cold anomalies in the subtropical lower stratosphere (Park et al., 2007;Randel et al., 2015). Further, the occurrence fractions of cirrus in the middle to upper (16-18 km) tropical 5 tropopause layer (TTL) (e.g., Fueglistaler et al., 2009) exhibit a pronounced maximum over the Asian monsoon region, both in observations and in model simulations (Ueyama et al., 2018). Convection is likely the dominant driver of localised upper tropospheric H 2 O and cloud maxima in the region (Park et al., 2007;Ueyama et al., 2018). Ice clouds in the tropical deep convective regions are important as they exert a considerable net warming effect (Hong et al., 2016).
To address fundamental open questions and uncertainties regarding the anticyclone and the ATAL, a number of balloon-borne 10 campaigns have been conducted in summer in the Asian monsoon region employing the Compact Optical Backscatter Aerosol Detector (COBALD) instrument (Bian et al., 2012(Bian et al., , 2020Vernier et al., 2018;Brunamonti et al., 2018). Here we present an analysis of the COBALD measurements in Nainital in Northern India in August and November 2016. The signature of the ATAL is clearly visible in most of the soundings in August (but not in every single one), but we find a substantial variability of the ATAL, both in backscatter intensity and in altitude range of the ATAL. A Lagrangian back-trajectory analysis is performed profiles from our analysis.

Data analysis and processing
In order to identify the ATAL, it is necessary to discriminate aerosol and cirrus clouds (ice particles) in the backscatter measurements. For this purpose, we used the ice saturation (S ice ) from the CFH instrument and the CI from the COBALD measurements (Cirisan et al., 2014). We reject layers with CI > 7.0, BSR 940 ≥ 2 and S ice > 70% as cirrus clouds (Vernier et al., 2015;Li et al., 30 2018;Brunamonti et al., 2018). Other sections of the profiles measured during the August soundings, which show substantially elevated values of BSR 455 in the UTLS, are classified as ATAL. As we will see below, the classification is considerably simplified by the fact that the COBALD profiles reveal clearly the fingerprints of the ATAL, i.e. top and bottom of the aerosol 6 https://doi.org/10.5194/acp-2020-552 Preprint. layer can be identified with reasonable precision. We quantify the elevated values of BSR 455 by comparing the August profiles with the mean of the November measurements, when there is no ATAL during post-monsoon (Brunamonti et al., 2018). The enhancement of BSR 455 for the ATAL remains below 1.12 (and the CI below 7). Conditions with cirrus clouds embedded within the ATAL can also be easily identified, as the cirrus clouds have 10-100 times larger BSR 455 . As they completely mask these ATAL particles, these cannot be detected and quantified under such conditions (see Section 3 below).

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Following earlier work (Brabec et al., 2012;Brunamonti et al., 2018), we use here vertically binned data for the CFH and COBALD instrument. BSR 455 , BSR 940 , CI and S ice are binned in pressure intervals of 1 hPa for pressure greater than 300 hPa and 0.5 hPa for pressure less than 300 hPa, which yields an improved signal to noise ratio. This binning corresponds to a vertical resolution of approximately 25 m in the UTLS. All data were carefully quality checked and measurements showing evidence of anomalous instrumental behaviour were rejected, as described by Brunamonti et al. (2018). The processing of COBALD 10 includes the rejection of 'moon spikes' that may arise due to the oscillatory motion of the payload 60 m below the balloon, when the detector happens to be pointing towards the moon. Moon spikes affect only a tiny fraction of the COBALD data and care is taken not to confuse them with thin cirrus clouds (details see Appendix C).

Trajectory calculations
For the model analysis, we employ the trajectory module of the three-dimensional Lagrangian chemistry transport model In ERA-Interim changes are implemented to improve deep and mid-level convection compared to previous reanalysis data (Dee et al., 2011). CLaMS back trajectory calculations are very well suited to analyse the detailed transport pathway of an air parcel, although they consider only the advective transport, neglecting mixing processes entirely. Earlier, CLaMS trajectories were 20 applied to a variety of problems such as polar chlorine chemistry as well as transport in the tropics, in particularly in the region of the Asian monsoon and in tropical cyclones (e.g., Vogel et al., 2003;Ploeger et al., 2010Ploeger et al., , 2012Vogel et al., 2014Vogel et al., , 2019Li et al., 2017Li et al., , 2020. To analyse the transport pathways and the origin of air masses contributing to the ATAL in August 2016 over Nainital, CLaMS 40-day backward trajectory calculations were performed. As the starting point for backward trajectory calculation we use the pressure levels of the measurements recorded every second (i.e. no binned data).

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The same trajectory model set-up is used as in several previous publications (Vogel et al., 2014;Li et al., 2017Li et al., , 2020. In the CLaMS model, potential temperature is used as the vertical coordinate when the pressure is less than about 300 hPa, (i.e. in the upper troposphere and in the stratosphere); when the pressure is greater than about 300 hPa (more accurately, for pressure p exceeding a reference level of p/p surface = 0.3) a pressure-based orography-following hybrid coordinate ζ (in units of K) is used (Ploeger et al., 2010;Pommrich et al., 2014). When the back trajectory dives into the CLaMS model boundary layer, we 30 consider the air mass origin to be reached. The model boundary layer is defined as the hybrid pressure/potential temperature coordinate ζ < 120 K (Pommrich et al., 2014;Vogel et al., 2019), which corresponds to a layer ≈ 2-3 km above the surface of the Earth considering orography. An overview of the Nainital aerosol backscatter observations in August (blue) and November (green) 2016 with pressure as the vertical coordinate is provided in Fig. 1 (left). Like for other years and locations (Vernier et al., 2015(Vernier et al., , 2018, the ATAL is clearly visible as an enhancement in backscatter ratio (BSR 455 ) in August compared to the November measurements, with 5 the averaged backscatter (BSR 455 ) values in August reaching about 1.08 ( Fig. 1). In Brunamonti et al. (2018), the ATAL is defined between the mean potential temperature lapse-rate minimum and the top of the confined lower stratosphere (Fig. 1 with altitude is observed (Fig. 1, right). This observation is consistent with a decreasing confinement of the air mass within the Asian monsoon anticyclone with increasing potential temperature (Brunamonti et al., 2018;Vogel et al., 2019).
In Fig. 2  From the measurements alone we can not exclude that the ATAL and cirrus clouds can coexist in this region (see Sect. 2.3 and discussion in Sect. 5).
From a seasonal average perspective, the ATAL extends in the longitudinal direction from the Middle East to East Asia and 25 meridionally between 15 • -45 • N (e.g., Vernier et al., 2015;Fairlie et al., 2020). This climatological picture is also true for the Asian monsoon anticyclone itself (e.g., Park et al., 2008;Vogel et al., 2016). Likewise, for the Asian monsoon anticyclone, there is a large variability from day to day in spatial extent, strength, and location, manifesting in an oscillation between a state with one anticyclone and two separated anticyclones (two modes) (e.g., Zhang et al., 2002;Yan et al., 2011;Vogel et al., 2015;Nützel et al., 2016).

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In the following, we consider three specific days more closely, which are examples for particular cases (see also Sec. 4).
We consider the case of an established ATAL (6 August 2016), a case, where no ATAL was observed (15 August 2016) and a case influenced by a typhoon (18 August 2016). We selected the 6 August because the mean value of the backscatter intensity (BSR 455 =1.083) is the highest from all ATAL cases and is not influenced by cirrus. From the two no ATAL cases (12 and 15 August), 15 August is chosen because the cirrus cloud is thinner than on 12 August.
The measurement in Nainital on 18 August 2016 was partially influenced by air uplifted in a tropical typhoon (this case will be discussed in detail in Sec. 4.3). On 18 August the ATAL occurs in an broader altitude range compared to 6 August, however the intensity of the ATAL is in general lower (BSR 455 =1.065) than on 6 August.

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The location of the Asian monsoon anticyclone for these three days is shown in Fig. 3. single-modal anticyclone is found with pronounced eddy shedding events (outflow from the anticyclone) towards both the east and the west (e.g. Vogel et al., 2015Vogel et al., , 2016. 15

Results of back-trajectory analysis
As shown in Fig. 2, there is a strong day-to-day variability in the ATAL altitude range, vertical extent and the BSR 455 intensity.
To analyse the transport pathway and the origin of the air masses contributing to the ATAL, for each sounding in August 2016 backward trajectories were calculated over a time period of 40 days. The dependence of the results of the trajectory calculations on the trajectory length (40, 60, and 80 days) will be discussed in detail below (Sec. 4.4). 20 The backward trajectories were started every second along the measured balloon profile where an ATAL was detected during the flight (see Tab. 1  Date   Trajectories originating outside the latitude-longitude box are classified as Residual. In addition, the location of the maximum updraft within 18 hours along these 40-day backward trajectories is calculated. It is first determined where the maximum change in potential temperature occurs (∆Θ max ) along the 40-day backward trajectories (calculated as running mean over the change in potential temperature within 18 hours). The location of the maximum updraft within the 18 hours is then calculated as the mean location of the trajectory within 18 hours for ∆Θ max (Fig. 6b).
It is evident that the location of the end points (Fig. 6a) and of the strongest updraft ( Fig. 6b) differ substantially. Our   Depending on the altitude of the measurements two branches of trajectories are found (Fig. 7a, b and c); the higher ones are from eastern and the lower ones from western longitudes. Focusing on the trajectories from the BL (Fig. 8) (see Fig. 9). about 10 to 12 days; these are shorter times than found for Case 1 (Fig. 6) and are caused by convection between 2 and 5 August.  Similar as for Case 1, in Case 2 the location of the end points in the model BL (Fig. 9a) and of the strongest updraft (Fig. 9b) differ substantially. A cluster of locations with an updraft larger than 25 K 18h −1 is found at the southern edge of the Himalayas, over Myanmar, and west of the Tibetan Plateau.
In Fig. 9, the location of the end points in the model BL of the strongest updraft for trajectories started in the cirrus cloud found between 376 and 381 K potential temperature on 15 August (see Fig. 2

Case 3: Typhoon influence on 18 August
On 18 August the ATAL occurs over a broader potential temperature range from 362 K to 422 K compared to 6 August (364 K to 388 K, Case 1), however the the ATAL intensity (BSR 455 =1.065) is lower compared to Case 1 (BSR 455 =1.083) ( Fig. 2 and Tab. 1). The contributions from the BL are 45% and similar to Case 2, but much lower than for Case 1 (64%, Tab. 2). For Case 3, a stratospheric contribution of 15% is found, which is much higher than for Case 1 (0.3%) and Case 2 (5%) because of the  ( Fig. 11). Another branch of BL air from the western Pacific is transported around the outer edge of the Asian monsoon anticyclone to Nainital (Fig. 11). The location of the end points in the BL for Case 3 are shown in more detail in Fig. 12. The major fraction of the BL contribution is from the Ocean (30%) in contrast to Case 1 (4%) and Case 2 (5%). Also the location of ∆Θ max is found in the western Pacific, however at slightly different locations as the end points. The strong updraft over the Pacific is caused by tropical cyclone activity. The typhoon Nida was active between 29 July and 3 August over the western Pacific (https://www.jma.go.jp/ jma/jma-eng/jma-center/rsmc-hp-pub-eg/besttrack.html; storm ID: 1604 (last access: 26 May 2020) and https://en.wikipedia. 5 org/wiki/2016_Pacific_typhoon_season (last access: 26 May 2020)) and impacted the balloon sounding on 18 August 2016.
Thus, in Case 3 polluted air masses within the ATAL layer measured below 400 K are diluted by air from the maritime boundary layer. At higher potential temperature levels (above 400 K) mixing with air masses from the stratosphere occurs.
In Fig. 12, the location of the end points in the model BL of the strongest updraft for trajectories started in the cirrus cloud found between 370 and 373 K potential temperature on 18 August (see Fig. 2 (Fig. 13, top). The fractions from the BL are between 14% and 64% and from the LS between 0% and 33%. Thus there is a strong variability of the fractions of the different atmospheric height layers contributing to the ATAL as well as for the no ATAL cases in August and post-monsoon cases in November 10 within the corresponding pressure levels between 140 and 92 hPa. However, from the analysis of different atmospheric height layers no clear relation between these height layers and the occurrence of ATAL was found, that may explain the difference between the ATAL and no ATAL cases as well as between ATAL and post-monsoon soundings in November.
During the monsoon season air masses from the BL accumulate within the Asian monsoon anticyclone, thus in the altitude range of the ATAL. Therefore, back-trajectory calculations with a length of 60 and 80 days were also performed to consider the 15 sensitivity of our results regarding the trajectory length. In general the fractions from the BL contributing to the ATAL increase with trajectory time. The fractions from the BL range between 14% to 64% for 40 days (Fig. 13, top), from 40% to 83% for 60 days and from 56% to 90% for 80 days. There is an increase of the fractions from the BL between 14 and 36 percentage points between 40 and 60 days (Fig. 13, middle) and between 6 and 25 percentage points between 60 and 80 days (Fig. 13, bottom). Simultaneously the fractions from the UT decrease with time. They range between 17% to 58% for 40 days, from 8% 20 to 28% for 60 days and from 6% to 21% for 80 days (see Fig. 13). A decrease is found of the fractions from the UT between 5 and 38 percentage points between 40 and 60 days (Fig. 13, middle) and between 0 and 12 percentage points between 60 and 80 days (Fig. 13, bottom). The fractions from the LT do not change significantly with increasing trajectory length. They vary between −7 and 8 percentage points between 40 and 60 days (except for 10 November 2016, here the difference is −21 points) and between −11 and 1 points between 60 and 80 days. The fractions from the LS decrease by up to −11 percentage points 25 between 40 and 60 day and up to −7 points between 60 and 80 days.
To deduce a possible relation between the ATAL intensity and the air mass origin within the boundary layer, Fig. 14a shows a bar chart with the contributions of Tibet, Foothills, Land, Ocean and Residual where the individual measurements are sorted by increasing backscatter ratio (BSR 455 -1) × 100) (see Tabs. 1 and 3). The contributions are normalised by the total number of trajectories within the boundary layer for each day for better comparison. The balloon flight on 12 August is excluded because a 30 cirrus cloud was detected between 140 and 92 hPa and therefore no aerosol backscatter ratio could be measured in this pressure range. Because of the low statistics we exclude all flights (11,21,23 and 26 August) where the trajectory number is lower than 50% of the maximum number of trajectories (# 704) calculated on 18 August 2016 (these cases are included in Fig. 15).
There is a lot of variability of the air mass origin within the boundary layer contributing to ATAL cases characterised by a backscatter ratio BSR 455 larger than 1.058 (Fig. 14, top). Strong ATAL cases with BSR 455 values larger than 1.067 show fractions of air mass origin in the boundary layer from continental outflow higher than 70% and in particular higher than 30% from Tibet (Fig. 14, top). For weak ATAL cases (18 and 30 August), the BSR 455 values are lower (1.059−1.065) and the fractions from continental outflow is below 70%. For these cases high fractions from maritime boundary layer sources Case 2, the no ATAL case from 15 August 2016, has a BSR 455 value of 1.023 lower than both November cases, however an air mass origin in the boundary layer is found of around 80% from continental sources (Tibet, Foothills and Land). This BL 10 fraction is similar to fractions from the BL as for the ATAL cases. Case 2 is discussed in detail in Sect. 4.2 and it was shown that on 15 August a bi-modal structure of the anticyclone is found and Nainital is located between the eastern and western part of the anticyclone (Fig. 3). This bi-modal structure is found between 11 and 16 August (not shown here). Thus also the no ATAL case on 12 August and the balloon-flight on 11 August are impacted by this dynamical situation in the UTLS. Therefore, the dynamics in the UTLS seems to be the reason that on 15 August 2016 no ATAL was measured over Nainital, although a 15 similar air mass origin was found on 15 August as for the ATAL cases. Unfortunately, during August 2016 we have only one no ATAL measurement (not completely overlaid by a cirrus cloud), therefore we can not deduce a more general result regarding no ATAL measurements during the peak monsoon season in India in our study. During August 2016, a second period with a bi-modal structure of the anticyclone is found between 27 and 28 August, but here Nainital is located more in the centre of the eastern mode (not shown here).

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The change between the ATAL intensity and the air mass origin within the boundary layer (Tibet, Foothills, Land, Ocean and Residual) between 40 and 60 days (Fig. 14b) as well as between 60 and 80 days (Fig. 14c) is also analysed. In general, the fractions of the Residual is increasing with the trajectory length for all balloon flights, except for 11 November. Thus the longer the trajectories, the more older air masses are taken into account to contribute to the ATAL coming from outside the latitudelongitude box from 60 • E to 160 • E and from 5 • S to 45 • N. For strong ATAL cases, in general an increase of the fractions from 25 Ocean up to ∼7 percentage points are found (Fig. 14b) indicating the impact of maritime convection between 40 and 80 days before the soundings. In contrast, for weak ATAL cases, the fraction of continental convection (Tibet, Foothills, and Land) is increasing, indicating the impact of continental convection between 40 and 80 days before the soundings.

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COBALD measurements using the CI, the 940-to-455 nm ratio of the aerosol component of the BSR, can be used to discriminate aerosol and cirrus measurements (see Sect. 2.1). However, in case of a cirrus cloud, the dominant ice particle backscatter does not allow the COBALD sonde to detect, whether aerosol particles coexist with ice particles or not. Krämer et al. (2016) describe two types of cirrus: (1) in-situ origin cirrus observed at the altitudes where they are formed on soluble solution aerosol particles or on solid ice nucleating particles; (2) liquid origin, cirrus which are glaciated liquid clouds lifted from below to the 20 cirrus temperature region. The formation mechanism of the cirrus cloud will have implications for the aerosol concentration in the cirrus layers within the ATAL.
For our back-trajectory calculations thin cirrus clouds within the ATAL are included to infer the origin of air masses contributing to the ATAL. If we assume that these cirrus clouds are of cirrus type (2), the aerosol concentration within the ATAL would not be affected by the formation process of the cirrus particles, however the uplift (convection) of air masses from the 25 lower troposphere could transport enhanced concentrations of aerosol particles to ATAL altitudes (as proposed by Vernier et al., 2018). Vernier et al. (2018) found in some balloon-borne measurements of the ATAL from Hyderabad, India, in summer 2015 that the presence of cirrus is associated with a reduction or a minimum in aerosol concentration possibly caused by aerosol removal processes through the formation of cirrus particles of type (1). Therefore, for both cirrus types, it is important to include the back-trajectory calculations of thin cirrus clouds found within the ATAL to identify the air mass origin of air masses 30 contributing to the ATAL. Further, depending on the lifetime of the cirrus clouds within the ATAL, the occurrence of cirrus particles (e.g. through sedimentation or uptake on the ice surface) could have implication on the ATAL even when the cirrus particles are no longer present.  For each region a linear fit was calculated using all measurements except the one on 12 August, when the UTLS is filled by a 5 km thick cirrus cloud (Tab. 1). An increase of the fractions from Tibet, Foothills and Land is found for increasing ATAL intensity, while the contributions from the Ocean decrease for both the locations of end points and the locations of strongest updraft. We would like to emphasise that the calculation of BSR 455 (see Tab. 1) is based on binned data (with an altitude resolution of ∼25 m in the UTLS). Small cirrus clouds found within the ATAL are excluded for the calculation of the mean value of BSR 455 , because here the BSR 455 value for cirrus is much higher than for aerosol (Sect. 3.1). However, the calculation of the fraction of air masses uses the high-resolution measurement data (with a resolution of 1 sec) for back-trajectories including the altitude ranges where small cirrus clouds within the ATAL were detected. In the altitude range of small cirrus clouds, the aerosol 5 concentration could be enhanced by convection or reduced by aerosol removal processes such as in-situ cirrus formation. In both cases it is important to include the air masses in the altitude range where thin cirrus clouds were detected to identify the origin of air masses contributing to the ATAL.
However, cirrus clouds detected directly below the ATAL are excluded in our trajectory analysis, even if aerosol and cirrus coexisted here the bottom of the ATAL is highly uncertain. Therefore, in our analysis there is an uncertainty in the vertical 10 altitude range of the ATAL. To solve this uncertainty balloon-borne measurements in addition to COBALD would be required such as the measurements of the particle size distribution (e.g. using an optical particle spectrometer, POPS; Gao et al. (2016)).

Conclusions
We presented a series of balloon-borne measurements performed by the COBALD instrument conducted in Nainital, Northern Pacific. Shorter transport times below 10 days are only found for the 12 (when the UTLS is filled by a 5 km thick cirrus cloud) and 17 August for air masses originating on the Tibetan Plateau.
Finally, CLaMS backward trajectory calculations identify the transport pathways from the Earth's surface to ATAL altitudes.
Very fast uplift in a convective range transports air masses up to the top of the convective outflow level (∼360 K) within a few days. Subsequently, the air parcels are slowly uplifted by diabatic heating within a large-scale upward spiral driven by the 15 anticyclonic flow in the UTLS over the Asian monsoon region from about 360 K up to ATAL altitudes. Over Nainital in summer 2016, a maximum ATAL altitude of 422 K (75 hPa) was measured. This slow uplift caused by diabatic heating in a large-scale upward spiral is consistent with concepts referred to as 'an upward spiralling range' (Vogel et al., 2016) or as 'a confined lower stratosphere' (Brunamonti et al., 2018).
Our study contributes to deducing the source regions of emissions of precursors of ATAL particles at the Earth's surface and 20 their transport pathways to the UTLS which is important to develop recommendations for regulations of anthropogenic surface emissions of ATAL precursors. In a recent study, Fadnavis et al. (2019b) argue that further increasing industrial emissions in Asia will lead to a wider and thicker ATAL having the potential to amplify the severity of droughts in India. Severe droughts would have fatal consequences on agriculture on the Indian subcontinent and therefore would result in strong socio-economic impacts in one of the most densely populated parts of the world.

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Code and data availability. The CLaMS code is available at the GitLab server: https://jugit.fz-juelich.de/clams/CLaMS (last access: 20 wavelength will give an indication of the presence of the ATAL. The same is true for cirrus particles, i.e., they can be detected by both the blue and the red channel, individually. But only by using the CI a clear discrimination of ice (CI > 7) and aerosol (CI < 7) is possible (see main paper), which is the advantage of measuring at two wavelengths.
Here and in previous work (Vernier et al., 2015(Vernier et al., , 2018Brunamonti et al., 2018) the ATAL analysis is based on the COBALD 455 nm BSR measurements. Indeed, the 455 nm BSR measurement is the preferred channel for the detection of the ATAL.

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The main reason is that the 455 nm COBALD BSR measurement has a better precision and has a higher signal-to-noise ratio (although BSR is lower at 455 nm than 940 nm, the raw signal is higher at 455 nm). Further, the accuracy is better for 455 nm, as the scale parameter in the data processing is taken directly from the sonde profile. The 940 nm channel builds on that and the BSR values are restricted by the assumption that the CI has to remain in a certain range.  Table 1 in the main paper) are marked by horizontal dashed-dot black lines.
(Note the different scaling for BSR 455 and BSR 940 .) In Fig. B1 we show the measured BSR 455 (blue line) and BSR 940 (red line) profiles from the COBALD measurements in Nainital in 2016 for two days in August (with no interference from cirrus layers or spike artefacts). The discussed features are clearly visible; although the ATAL can be detected using both the BSR 455 and BSR 940 measurements, the 455 nm channel is less noisy, so it constitutes the preferred channel for the detection of the ATAL in COBALD balloon measurements.
Appendix C: Removal of 'moon spikes' 5 The processing of COBALD includes the rejection of 'moon spikes' that may arise due to the oscillatory motion of the payload 60 m below the balloon, when the detector happens to be pointing towards the moon. Moon spikes affect only a tiny fraction of the COBALD data and care is taken not to confuse them with thin cirrus clouds.
To identify those anomalous 'spikes', we used a simple criterion; we consider the signal to be an anomalous spike when BSR 455 nm > 1.12 and CI < 7. These spikes are then removed from the data set, where the condition CI < 7 ensures that cirrus 10 clouds will be retained.
Finally, some specific cases with spikes in the BSR 455 measurements of a very small vertical extent still remained in the binned data, where BSR 455 < 1.12 and 7 < CI < 10. These specific cases occurred for 3, 15, 21, 23, and 30 August; the corresponding data points were also removed from the data set used here.