The ATAL within the 2017 Asian Monsoon Anticyclone: Microphysical aerosol properties derived from aircraft-borne in situ measurements

The Asian summer monsoon is an effective pathway for aerosol particles and precursor substances from the planetary boundary layer over Central, South, and East Asia into the upper troposphere and lower stratosphere. An enhancement of aerosol particles within the Asian monsoon anticyclone (AMA) has been observed by satellites, called the Asian Tropopause Aerosol Layer (ATAL). In this paper we discuss airborne in situ and remote sensing observations of aerosol microphysical properties conducted during the 2017 StratoClim field campaign within the region of the Asian monsoon anticyclone. The 5 aerosol particle measurements aboard the high-altitude research aircraft M55 Geophysica (reached a maximum altitude of about 20.5 km) were conducted by a modified Ultra High Sensitivity Aerosol Spectrometer Airborne (UHSAS-A; particle diameter detection range from 65 nm to 1 μm), the COndensation PArticle counting System (COPAS, for detecting total aerosol densities of submicrometer sized particles), and the Cloud and Aerosol Spectrometer with Detection of POLarization (NIXE-CAS-DPOL). In the COPAS and UHSAS-A vertical particle mixing ratio profiles, the ATAL is evident as a distinct 10 layer between 15 km (≈ 370 K) and 18.5 km altitude (≈ 420 K potential temperature). Within the ATAL, the maximum detected particle mixing ratios (from the median profiles) were 700 mg−1 for diameters between 65 nm to 1 μm (UHSASA) and higher than 2500 mg−1 for diameters larger than 10 nm (COPAS). These values are up to two times higher than previously found at similar altitudes in other tropical locations. The difference between the particle mixing ratio profiles measured by the UHSAS-A and the COPAS indicate that the region below the ATAL at potential temperatures from 350 to 370 15 K is influenced by the fresh nucleation of aerosol particles (diameter < 65 nm). We provide detailed analyses of the vertical distribution of the aerosol particle size distributions and the particle mixing ratios and compare these with previous tropical 1 https://doi.org/10.5194/acp-2020-1241 Preprint. Discussion started: 6 January 2021 c © Author(s) 2021. CC BY 4.0 License.


Introduction
During the Asian Summer Monsoon (ASM) the Upper Troposphere / Lower Stratosphere (UT/LS) over the Indian subcontinent is strongly influenced by the Asian Monsoon Anticyclone (AMA). Inside the AMA, the ATAL (Asian Tropopause Aerosol Layer) was discovered from faint signals of satellite borne lidar measurements (Vernier et al. (2009) ;Vernier et al. (2011); Vernier et al. (2018)). Its vertical extent typically ranges from 14 to 18 km altitude corresponding to 360 K and 420 K potential 30 temperature levels, respectively. The AMA develops periodically during the summer of the northern hemisphere (Park et al. (2007)), covering a vertical extent from about 12 to 18 km altitude and has its maximum strength at 17 to 18 km, around the local tropopause (Ploeger et al. (2015); Brunamonti et al. (2018)). With the large variability in its horizontal extent, the AMA covers longitudes from Northeastern Africa to East Asia (Pan et al. (2016); Vogel et al. (2019)). The dynamic processes associated with the AMA provide the setting for an effective vertical transport of trace substances from the lower troposphere 35 accompanied with a certain level of accumulation within the anticyclone. These processes affect the composition of trace gases, particle precursor gases and aerosol particles in all levels of the UT/LS at different intensities (Randel and Park (2006) ;Park et al. (2009) Randel and Jensen (2013); ; Pan et al. (2016); Bucci et al. (2020)).
The AMA is a prominent feature with a closed, quasi-rotational circulation in the UT/LS, which is confined by a westerly jet stream in the mid-latitudes and an easterly jet stream in the tropics (Dunkerton (1995); Pan et al. (2016) ;Brunamonti et al. (2019)). Asia is currently one of the regions with the highest production of atmospheric sulphur worldwide. Therefore, the vertical transport of these sulphur-containing aerosol particles and particle precursor gases, through the high-reaching convection of the ASM, can influence the chemical balance of the stratosphere and the climate (Vernier et al. (2011);Kremser et al. (2016)). This was initially suggested as cause for the ATAL (Vernier et al. (2011);Neely et al. (2014); Yu et al. (2015)), however, Höpfner et al. (2019) demonstrated that ammonium nitrate formed from gaseous ammonia in the higher troposphere 55 is an important if not the dominant component of the ATAL aerosol. Furthermore, model analysis (with the GEOS-Chem transport model) by Fairlie et al. (2020) indicate the dominance of regional anthropogenic emissions from China and the Indian subcontinent to aerosol concentrations in the ATAL.
For a more detailed analysis of these processes, airborne measurements of the microphysical properties of aerosol particles are discussed in this study. These measurements were conducted during the 2017 StratoClim field campaign at the time of the 60 ASM. We took a closer look at the vertical distribution of the submicrometric aerosol particle mixing ratio and the aerosol particle size distributions within the AMA region. Balloon-borne in situ aerosol backscatter measurements from Vernier et al. (2015), Yu et al. (2017), Brunamonti et al. (2018), and Vernier et al. (2018) confirmed the enhanced aerosol signal observed by Vernier et al. (2011Vernier et al. ( ) since 2006. In order to relate their observations with our in situ observations obtained during StratoClim 2017, we calculated the theoretically expected aerosol particle scattering ratio based on our in situ measured aerosol particle Fisher Scientific) with diameters of 102, 147, 296 and 799 nm. During the calibration process the PSL particles were classified with a differential mobility analyzer (DMA, TSI 3080 with TSI 3081). During the field campaign in Nepal, before every mission flight the calibration of the UHSAS-A was validated with the same PSL calibration standards. The uncertainty of the measured number concentration was determined to be lower than 10 % based on laboratory characterization. This is valid as long as the statistical counting error is also lower than 10 %. For the 1 Hz resolved measurements, this is the case for ambient particle 120 number concentrations larger than 100 cm −3 . At ambient particle number concentrations as low as 1 cm −3 the data should be averaged over time intervals of about 100 seconds to gain sufficient counting statistics. Due to missing in-line temperature measurements and the wide ambient temperature range during StratoClim, the uncertainty of the UHSAS-A measurements is estimated to be up to 25 %.

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The COndensation PArticle counting System (COPAS) consists of two separate units, each containing two separate condensation particle counters. Three of the condensation particle counters detect all aerosol particles with diameters larger than 6 nm, 10 nm, and 15 nm, respectively. The upper limit of the particle diameter detection range is determined with respect to the characteristics of the aerosol inlet system to about one micrometer in diameter. This describes the limit, up to which particles are aspirated with almost 100 % efficiency and transported through the aerosol lines to the detector. The fourth condensation 130 particle counter detects the aerosol particles with diameters larger than 10 nm, which have previously passed through a heated tube section (at 270°C) of about one meter length. Therefore, this channel detects residual particle cores which are non-volatile at this temperature. The COPAS has been characterized in Weigel et al. (2009)

NIXE-CAS
The Cloud and Aerosol Spectrometer with Detection of POLarization (NIXE-CAS-DPOL, here referred to as NIXE-CAS) is part of the New Ice eXpEriment Cloud and Aerosol Particle Spectrometer (NIXE-CAPS) underwing probe. Together with the Cloud Imaging Probe greyscale (NIXE-CIPg), the NIXE-CAPS can measure the particle size distribution for larger aerosol particles as well as cloud particles within a diameter range from 0.61 to 937 µm (Costa et al. (2017)). The overall measurement 140 uncertainties of the particle number concentrations and the particle sizing are estimated to be approximately 20 % by Costa et al. (2017), more detailed descriptions of the instruments performance and measuring principles are given by Baumgardner et al. (2017). For this study, the lowest size bins of NIXE-CAS (0.61 to 3 µm) have been used to provide additional information extending beyond the upper detection limit of the UHSAS-A, as such large aerosol particles potentially influence the derived scattering ratios. 145 5 https://doi.org/10.5194/acp-2020-1241 Preprint. Discussion started: 6 January 2021 c Author(s) 2021. CC BY 4.0 License.

MAS
The Multiwavelength Aerosol Scatterometer (MAS) is an elastic backscatter near range lidar that operates at wavelengths of 532 or 1064 nm. It measures the backscatter and the depolarization from cloud and aerosol particles like a remote sensing lidar, but in situ in a range of 3 to 30 m close to the aircraft. It is capable of measuring at a time resolution of 5 to 10 seconds, what translates to a horizontal resolution of 1 to 2 km, considering the M55 Geophysicas cruising speed of about 170 m s −1 .

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Its technical details and analysis of its measurement performance aboard the M55 Geophysica are discussed in detail by Cairo (2004), Cairo et al. (2011), andMolleker et al. (2014).

MAL
The Miniature Aerosol Lidar (MAL) aboard the M55 Geophysica is a combination of two identical stand-alone airborne lidar systems, one facing upwards and the other facing downwards. The two microjoule backscatter-depolarisation lidar systems 155 operate at a wavelength of 532 nm and are capable of measuring range-resolved backscatter and depolarization profiles along the aircraft flight track, 2 km above and underneath the aircraft. Previous applications of the MAL lidar are discussed in publications by Cairo (2004), Mitev et al. (2014), and Molleker et al. (2014). During the RECONCILE campaign a comparison study between the MAL lidar aboard the M55 Geophysica and the satellite-borne CALIOP lidar was conducted by Mitev et al. (2012).

COLD2
During the 2017 StratoClim field campaign, the Carbon Monoxide (CO) mixing ratio was measured in situ by COLD2 (Carbon Oxide Laser Detector 2), the newly improved version of the Cryogenically Operated Laser Diode spectrometer (COLD) aboard the M55 Geophysica. The previous version COLD, based on lead salts laser, operating around liquid nitrogen temperature, was successfully operated during several tropospheric and stratospheric measurement campaigns since 2005 and its functionality 165 is described in detail by Viciani et al. (2008). The present instrument is based on a room temperature quantum cascade laser and an updated electronics, with a substantial reduction of weight and dimensions, and no more need of cryogenic fluids. The detection principle of the COLD instruments is based on the Tunable Diode Laser Spectroscopy. During the 2017 StratoClim operation, the COLD2 instrument attained an in-flight sensitivity of 1 to 2 ppb with a time resolution of 1 Hz and an accuracy of 3 % (Viciani et al. (2018)). In this study the CO measurements are adopted as tracer for air masses affected by pollution or 170 biomass burning.
4 The vertical distribution of the aerosol particle mixing ratio within the AMA The identification of transport and nucleation processes (i.e. New Particle Formation, NPF) of aerosol particles in the UT/LS and their influence on the radiation balance and chemistry of the atmosphere requires the knowledge of the vertical distribution of the aerosol particle properties, such as particle size and number concentration. Figure 2 (a) shows the vertical distribution of 175 6 https://doi.org/10.5194/acp-2020-1241 Preprint. Discussion started: 6 January 2021 c Author(s) 2021. CC BY 4.0 License. the particle mixing ratio (given in number of particles per mg of ambient air) as measured by the UHSAS-A during all research flights of the StratoClim 2017 measurement campaign. The potential temperature (Θ) is used as the vertical coordinate. To ease the recognition of the variability between the individual flights, the measured particle mixing ratios (1 Hz temporal resolution) are marked by data points of different color. In black, the median of the particle mixing ratio of all eight measurement flights, calculated over 5 K potential temperature intervals, is plotted together with the 25% and 75% percentiles as horizontal bars to 180 each median value. The number of 1 Hz data points included in each 5 K potential temperature interval is indicated in Fig. 2 (b).
Following the median of the particle mixing ratio from very high values (about 1500 mg −1 ) close to the ground level (Θ = 310 K), the particle mixing ratio decreases by an order of magnitude to about 150 mg −1 up to a potential temperature of 330 K. As evident from the 1 Hz resolved data, particle mixing ratios of up to 10000 mg −1 are measured between 310 and 330 K.

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Up to the Θ level of 345 K, the median particle mixing ratio remains at about 150 mg −1 . The variability of the measurement results increases with altitude, which can be seen from the changes in the deviation of the 25% and 75% percentiles with respect to the median.
From 345 K to 350 K potential temperature, the median value inclines to a particle mixing ratio of 300 mg −1 until it reaches a maximum of 700 mg −1 at about 365 to 370 K potential temperature. Between 350 and 370 K the 1 Hz data shows 190 a high variability of the particle mixing ratio, both between the different flights, but also when the flights are considered individually. Particle mixing ratios between 6 mg −1 and over 10000 mg −1 were measured here. This high variability is also visible in the percentiles. Apart from the variability of the sampled airmasses inherent from the dynamics of the AMA, causes for such variability may also be the occurrence of New Particle Formation events (NPF) and scavenging by the large persistent convective cloud systems. In these cloud systems many aerosol particles are activated to form condensation nuclei of cloud 195 droplets or get washed out by scavenging, resulting in the observed very low aerosol particle mixing ratios (Croft et al. (2010); Yang et al. (2015)). On the other hand, these strong convective systems can lead to vertical transport of polluted air from the boundary layer, with high particle mixing ratios, up to high altitudes. A possible cause for the high particle mixing ratios in this altitude range, and sometimes up to 380 K potential temperature (flight KTM8), can also be the nucleation of aerosol particles from precursor gases (NPF). These precursor gases of natural and anthropogenic origin are also subject to vertical transport by Centre of Medium-Range Weather Forecasts (ECMWF) ERA-Interim reanalysis data. Above the tropopause region, starting at about 380 K potential temperature, the variability of the particle mixing ratio begins to abate.

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Up to a potential temperature of 420 K, the median of the particle mixing ratio decreases to about 80 mg −1 , except for a local maximum at a potential temperature of 390 K. From there, up to a potential temperature of 445 K, the particle mixing ratio decreases further to a median of about 40 mg −1 . Up to about 475 to 480 K, the highest Θ levels reached during the StratoClim 2017 measurement campaign, the median of the particle mixing ratio remains between 40 and 50 mg −1 . Especially for potential temperatures larger than 420 K and low particle mixing ratios in the range of 10 to 100 mg −1 it is noticeable, 210 that the 1 Hz data points form a vertical and slightly inclined discrete stripe pattern towards larger particle mixing ratios. This is due to the weak counting statistics of the single 1 Hz data points at these concentrations in combination with the constantly regulated sample flow (of 50 cm 3 min −1 ). However, due to the high number of 1 Hz data points (about 400 to 4000) available for each 5 K interval, even in this Θ range we are able to calculate robust median values with the given Θ resolution.
For relating these UHSAS-A results to the particle mixing ratios observed in other tropical and subtropical UT/LS regions, 215 two data sets from the tropics as well as two sets from the extratropics were selected. These measurements were conducted with the COPAS instrument (described in Sec. 3.2) during the 2016 StratoClim field campaign in Greece (extratropics) and StratoClim 2017 in Nepal (tropics), as well as air-borne measurements within the tropics and the extratropics, published in Borrmann et al. (2010) and read out of Fig.1 from the publication by Brock et al. (1995).

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In the region of the upper troposphere with potential temperatures between about 350 and 370 K, the median of the particle mixing ratio measured by COPAS during StratoClim 2017 (green line) reaches its maximum of 6000 mg −1 . A maximum (particle mixing ratio up to about 6500 mg −1 ) that was also observed by Brock et al. (1995) in the tropical Central Pacific (orange line). The median of the UHSAS-A measurement remains almost constant in this Θ range with significantly lower values of the particle mixing ratio of about 250 mg −1 . However, the variability of the particle mixing ratio measured by the 230 UHSAS-A is, as discussed above, very high in this Θ range. The COPAS measurements shown here cover the particle diameter range from 10 to about 1000 nm, while the UHSAS-A detects aerosol particles in the diameter range from 65 to 1000 nm. The difference in the particle mixing ratios between the median values of the COPAS and the UHSAS-A measurements of about 5750 mg −1 shows, that very small aerosol particles between 10 and 65 nm in diameter dominate the aerosol total number densities. This indicates that especially the Θ range between 350 and 370 K is influenced by the fresh nucleation of aerosol 235 particles (NPF), what has also been shown by Weigel et al. (2020a) and Weigel et al. (2020b).
During the ASM, between about 370 and 415 K potential temperature, the median profile of the UHSAS-A data shows particle mixing ratios that are up to two times higher than the median profile observed by Brock et al. (1995) in extratropical regions (see Fig. 3 (b)). This is particularly noticeable since the particle mixing ratios of Brock et al. (1995) cover particle diameters from 8 to 3000 nm and the UHSAS-A only detects aerosol particles in the diameter range from 65 to 1000 nm.

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The median profile of the COPAS measurement during the StratoClim 2016 measurement campaign in Greece (blue line) also shows lower particle mixing ratios than the comparative measurement within the AMA region (green). Compared with the extratropical COPAS measurements from StratoClim 2016, the UHSAS-A vertical profile shows mostly lower particle mixing ratios. However, in contrast to the UHSAS-A, the measurements with the COPAS also include very small aerosol particles starting from 10 nm in diameter (starting from 6 nm during TROCCINOX 2005). Thus, inside the AMA, higher 245 8 https://doi.org/10.5194/acp-2020-1241 Preprint. Discussion started: 6 January 2021 c Author(s) 2021. CC BY 4.0 License. particle mixing ratios were observed for the size diameter range from 65 to 1000 nm at altitudes between roughly 370 and 415 K than during similar measurements in the extratropics which include even much smaller particles (starting from 8 nm (Brock et al. (1995))). This enhanced aerosol mixing ratio can be associated with the ATAL discovered by Vernier et al. (2009), who observed the ATAL in about the same altitude range with potential temperatures between 370 and 420 K on the basis of satellite-borne lidar measurements. Figure 3 (b) shows that such a maximum between roughly 340 and 390 K was also 250 observed in the fine particle mixing ratios obtained from COPAS in other tropical locations (Northern Australia, West Africa, and Brazil; Borrmann et al. (2010)) albeit with significantly lower absolute values than for the AMA region. The increase in particle mixing ratio for altitudes above the 420 K level over West Africa was explained by Borrmann et al. (2010) 2020)). Aloft the particle mixing ratios are mainly controlled by the large scale isentropic transport in the lowermost stratosphere and are less influenced by the AMA.
5 The vertical profile of the aerosol particle size distribution within the Asian monsoon anticyclone Characterizing the ATAL, described by Vernier et al. (2011), requires knowledge about the aerosol particle size distribution's 260 vertical profile. Within UT/LS altitudes, it is also important for the analysis of NPF events, cloud formation, and transport processes as well as for the calculation of the radiative balance and parameters like aerosol volume density (Höpfner et al. (2019)) and aerosol surface area, available for heterogeneous chemical conversion processes.
The results shown here are the first measurements made with a UHSAS-A in the tropical lower stratosphere. The performance of the modified UHSAS-A (see Sec. 3.1) as evident from its technical parameters has been thoroughly tested in laboratory. To 265 further demonstrate the profoundness of the in situ measurements at altitude, a comparison was made with other optical particle counter measurements of the aerosol particle size distributions from the stratosphere.  The optical particle counters (LPC; Laser Particle Counter) used for the measurements from Hyderabad and Laramie were operated with a particle diameter detection range from 0.18 to 32 µm and 0.18 to 9 µm, respectively. In both cases total particle concentration measurements were made with a condensation nuclei counter (CNC) with a nominal detection diameter of 20 nm (Campbell and Deshler (2014)), flown in parallel with the LPCs. The resulting particle size distributions are compared in Considering the higher size resolution of the UHSAS-A and the difference in the detection range compared to the balloonborne instrumentation, a sufficient agreement of the measurement results can be seen. In particular, the comparative measurement from Hyderabad (India), which also took place during the ASM, shows very good agreement between the CNC and 280 UHSAS-A for particles < 0.18 µm and between the LPC and UHSAS-A for particles between 0.3 and 0.6 µm. Taking the temporal and spatial distance between these measurements into account, this comparison further indicates the quality of the measurements conducted with the modified UHSAS-A. Starting at a Θ level of 320 K, the profile of the aerosol particle size distribution has a pronounced maximum at the lower end 290 of the UHSAS-A detection size range (diameter between 65 and 80 nm), together with enhanced number concentrations also for the large aerosol particles, with diameters up to 1000 nm. Until about 326 K potential temperature, the size distributions maximum is located between particle diameters of 70 and 80 nm and the overall number concentration decreases. This is especially the case for particles with a diameter larger than 600 nm. Up to potential temperatures of about 350 K the overall shape of the size distribution remains mostly constant. In the Θ range between 350 and 370 K the aerosol size distribution is

Method
For this analysis, all 8 measurement flights from the StratoClim 2017 campaign are divided into segments of 100 seconds.
Segments in which cloud particles have been detected by the NIXE-CAS are removed from the data set to be consistent with the cloud filtered CALIOP, MAS, and MAL data sets. To ensure a high vertical resolution, flight segments with ascending 315 or descending rates which result in a potential temperature range higher than 5 K within the 100 seconds flight segment are removed as well.
For each remaining flight segment a UHSAS-A measured aerosol particle size distribution was averaged. The size range of this size distributions is extended from 65 -1000 nm to 10 -3000 nm using measurements conducted by the COPAS (Sec. 3.2) and NIXE-CAS (Sec. 3.3) instruments.

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One size-bin from 10 to 65 nm in diameter was calculated by subtraction of the UHSAS-A measured total number concentration (particle diameter range: 65 -1000 nm) from the particle number concentration measured by the COPAS N 10 channel (particle diameter range: 10 to about 1000 nm) described in Sec. 3.2. The measurements conducted by the NIXE-CAS instrument (see Sec. 3.3) extend the size distribution for large aerosol particles by one size bin with diameters up to 3000 nm. This way, as composite, the largest possible aerosol particle diameter range is covered with measurements which can be achieved 325 from all of the Geophysica instruments. One example of this combined 100 s averaged aerosol size distributions is shown in Fig. 6.
The aerosol particle scattering ratio (SR) is calculated using Eq. 1 with the aerosol backscatter coefficient β ap and the molecular backscatter coefficient β mol .
For every 100 s averaged aerosol size distribution the aerosol backscatter coefficient β ap is calculated based on the Mie-theory as comprehensively described in Cairo et al. (2011). Accounting for the aerosols chemical composition (Höpfner et al. (2019) observed the presence of ammonium nitrate particles), these calculations use a refractive index of 1.5 for the size distributions measured within the ATAL altitude region up to 420 K potential temperature. At Θ levels higher than 420 K a refractive index of 1.45 is used, which better reflects the stratospheric aerosol properties.

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To be able to compute the particle scattering ratio, the molecular scattering coefficient β mol for each averaging interval is calculated. Based on Collis and Russell (1976), the simplified method from Cairo et al. (2011) is used together with the temperature and pressure measured by the UCSE system aboard the M55 Geophysica. To be able to compare the resulting SR values with the SR measured by CALIOP, MAS, and MAL, β ap and β mol are calculated using a wavelength of 532 nm.

Comparison between scattering ratios obtained from in situ and remote sensing data
For a direct comparison between these in situ aerosol size distribution based SR and the aerosol particle SR measured by the satellite-borne lidar CALIOP, a CALIOP data set is needed that was measured within a comparable time period and about the same geographical region as the StratoClim 2017 flight missions. Over the time period from 4 to 31 August 2017 a vertical profile of the aerosol particle SR at a wavelength of 532 nm measured by CALIOP was averaged between 15 to 45 degree North and 70 to 100 degree East. This temporal and horizontal averaging is needed to increase the signal to noise ratio. To be able to 345 detect the ATAL, the CALIOP data set was reanalyzed based on the CALIOP data level 1 V4.10 data set and calibrated between an altitude of 36 to 39 km as previously described by Vernier et al. (2009). During this data processing, backscattering due to ice cloud particles was removed by applying a filter for the volume depolarization ratio within a pixel greater than 5% (Vernier et al. (2011);Vernier et al. (2015)). The vertical profile was then averaged with a vertical resolution of 200 m. Additionally, vertical profiles of the aerosol particle SR at a wavelength of 532 nm measured by the MAS and MAL instruments aboard  shows that the properties derived from the in situ measured particle size distributions can be broadly reconciled with the satellite observations.
6.4 The ATALs relation to CO and the AMA-centered equivalent latitude The previous section shows that the intensity of the convective influence has an impact on the characteristics of the ATAL, here Besides the strong convective vertical transport associated with the ASM, another feature of the ATAL should be considered, namely the confinement of its air masses within the AMA. This confinement can lead to an accumulation of aerosol particles and trace gases within the AMA region. One measure to relate the geographical position of our measurements to the position 430 of the AMA core and its "border" is the AMA-centered equivalent latitude (EQLAT). The center of the AMA is defined by the lowest values of the potential vorticity (PV) on the 380 K potential temperature level. An equivalent latitude for which 90 degree North corresponds to the center of the AMA was projected for a closed PV contour according to Ploeger et al. (2015).
It has to be noted that the definition of the EQLAT is only valid for a range about 20 K above and beneath the 380 K isentrope, as outside this range PV contours in the AMA region are frequently not closed. Furthermore, Ploeger et al. (2015) found that 435 the edge of the confinement caused by the AMA can be determined from a local maximum in the gradient of PV along the 380 K isentrope, and is on average located at around 65 degree EQLAT. The EQLAT is calculated based on the ECMWF ERA-Interim reanalysis.
Figures 9 (d), (e), and (f) show the correlation between the EQLAT and the aerosol particle SR (full campaign period, first half and second half, respectively). While there is no direct correlation between the SR and the EQLAT, high values of aerosol 440 particle SR (larger 1.08) only occur for flight segments with a EQLAT larger 63 degrees, for the first half of the campaign period ( Fig. 9 (a)). During the second half of the StratoClim 2017 campaign aerosol particle SR larger 1.08 were only observed during flight segments with an EQLAT larger 66 degree. This matches well with the edge of the AMAs confinement at about 65 degree EQLAT observed by Ploeger et al. (2015). At high Θ levels above about 420 K (blueish colors) the SR values are always lower than 1.04. This shows that during the ASM typical ATAL aerosol SR values could only be observed horizontally 445 and vertically within the confinement of the AMA.

Conclusions
During the 2017 StratoClim field mission in the Asian Summer Monsoon (ASM) season, aerosol measurements were performed over Central Asia up to 20 km altitude aboard the research aircraft M55 Geophysica inside and above the Asian Monsoon Anticyclone (AMA) and the Asian Tropopause Aerosol Layer (ATAL). Here and for the first time, submicrometer sized aerosol 450 size distributions were in situ measured down to 65 nm particle diameter by a modified UHSAS-A optical particle counter.
These measurements were conducted in conjunction with condensation particle counters (COPAS), and two near-range remote sensing instruments, MAS and MAL. could be validated by calculating the SR based on the in situ measured aerosol size distributions, as well as the aerosol particle SR directly measured by the MAS and MAL instruments. All of those four independent methods largely agree with each other and can confirm the ATAL as a layer of enhanced aerosol particle SR within an altitude range from 15 to 18.5 km.
The maximum of the ATALs aerosol particle SR signal was observed at 17.5 km altitude, consistently by all four methods.
Furthermore, the in situ measurements show that the ATAL is highly variable in time and space and is not a closed, persistent 460 layer. While there is a seasonal correlation between the CO mixing ratio and the ATAL (Vernier et al. (2015)), no direct correlation on the smaller scale, between co-located in situ measurements of the CO mixing ratio and the size distribution based aerosol particle SR, could be found. But, values of SR that are typical for the ATAL could only be observed for CO mixing ratios larger than 40 to 50 ppb. This is also consistent with the observations, that during the StratoClim 2017 campaign period enhanced aerosol particle SR values could only be observed within the confinement of the AMA at a EQLAT larger 465 than 63 degree and below its top of confinement (at about 420 K potential temperature). This regional limitation of the ATAL with respect to the dynamics of the AMA is in good agreement with the horizontal (Ploeger et al. (2015)) and vertical (von Hobe et al. (2020)) limitations of the AMA-caused confinement.
From the experimental perspective the ATAL is a fairly elusive, highly variable layer situated between approximately 370 K (≈ 15 km) and 420 K (≈ 18.5 km) potential temperature. Its lower part is close to -if not still inside-the highest region 470 of convective outflows, while its upper part can be found at tropopause levels (between 369 K and 396 K during StratoClim 2017) or slightly above. Thus the aerosol of the upper ATAL part is subject to very slow -probably spiraling-vertical ascent (with rates of about 1 K potential temperature per day). At the same time, the lower part of the ATAL can be affected by rapid turbulent mixing which provides precursor gases and aerosols originating from the lower troposphere. In this complex dynamical setting microphysical processes like NPF, aging by coagulation and condensational growth, removal by scavenging 475 act on the aerosol. The vertical profile of the measured aerosol particle size distributions in combination with the vertical profiles of the particle mixing ratios from the UHSAS-A and COPAS show a pronounced Aitken-mode between the 350 and 370 K potential temperature levels, i.e. beneath the lower edge of the ATAL. With increasing altitude, the aerosol size distribution's main mode shifts towards the accumulation mode.
Our own simple box model simulations (adopting the SOCOL (SOlar Climate Ozone Links (Stenke et al. (2013)) coagulation 480 subroutines; see details in Weigel et al. (2020a)) showed that the freshly nucleated aerosol particles (as observed from COPAS) coagulate onto the background aerosol (as observed by the UHSAS-A) within a few hours. In principle this should affect or "quench" the frequently occurring NPF events, which were detected by COPAS (Weigel et al. (2020a); Weigel et al. (2020b)).
This suggests that the coagulation of freshly nucleated aerosol particles alone cannot cause the lower part of the aerosol stratosphere derived from in situ particle measurements, Atmospheric Chemistry and Physics, 5, 3053-3069, https://doi.org/10.5194/acp-