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 . Its vertical extent typically ranges from 14 to 18 km altitude, roughly corresponding to potential temperature levels of 360 K and 420 K, respectively. The AMA develops periodically during the Northern Hemisphere summer , covering a vertical extent from about 12 to 18 km altitude, and it has its maximum strength at 17 to 18 km, around the local tropopause . With the large variability in its horizontal extent, the AMA covers longitudes from northeastern Africa to East Asia . The dynamic processes associated with the AMA provide the setting for an effective vertical transport of trace substances from the lower troposphere, accompanied by a certain level of accumulation within the anticyclone. These processes affect the composition of trace gases, particle precursor gases, and aerosol particles at all levels of the UT–LS with varying intensities .

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 midlatitudes and an easterly jet stream in the tropics . Associated with the AMA are large convective systems during monsoon times which provide rapid vertical transport routes for trace substances, aerosol precursors, and aerosol particles from the boundary layer to the altitude levels of the AMA, the tropical tropopause layer (TTL), and the lower stratosphere . The lifted material includes anthropogenic releases from ground-level pollution such as ammonia that forms ammonium nitrate particles in the troposphere , which can impact ice cloud formation in the Asian monsoon upper troposphere . The TTL, which extends over an altitude range from about 14 to 18 km in this area, acts as a “gateway to the stratosphere” , as air from this region can be transported into the lower stratosphere via diabatic ascent . With the transport of climate-relevant natural and anthropogenic trace gases, water vapor, and aerosol particles into the stratosphere, precursor gases also enter the UT–LS, which can lead or contribute to the formation of new particles from the gas phase (new particle formation – NPF) . Asia is currently one of the regions with the highest emission of atmospheric sulfur worldwide. Therefore, the vertical transport of these sulfur-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 . This was initially suggested as cause for the ATAL ; however, demonstrated that ammonium nitrate formed from gaseous ammonia and nitric acid in the higher troposphere is an important, if not the dominant, component of ATAL aerosol. Furthermore, model analysis (with the GEOS-Chem transport model) by indicated the dominance of regional anthropogenic emissions of particle precursors like sulfate, nitrate, and ammonia but also aerosol particles (e.g., like primary organic aerosol) 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 ASM. In this paper, we examine the vertical distribution of the submicron aerosol particle mixing ratio and the aerosol particle size distributions within the AMA region. Balloon-borne in situ aerosol backscatter measurements from , , , and confirmed the enhanced aerosol signal observed by since 2006. In order to relate their observations to our in situ observations obtained during StratoClim 2017, we calculate the theoretically expected backscatter ratio based on our in situ aerosol particle size distributions. With a focus on the ATAL region, we compare these results with observations from the satellite-borne Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) , the airborne backscatter probe MAS (Sect. ), and the airborne lidar MAL (Sect. ) during StratoClim 2017.

The 2017 StratoClim (Stratospheric and upper tropospheric processes for better climate predictions) field campaign took place in July and August in Kathmandu (Nepal). The goal of StratoClim (http://www.stratoclim.org, last access: 14 September 2021) was to gain a better understanding of the key processes in the upper troposphere and stratosphere of the ASM region in order to better assess their effects on climate change and stratospheric trace gases including ozone. The StratoClim project comprised a measurement campaign with aircraft-borne and balloon-borne measurements (launched at tropical ground stations at different locations on the Indian subcontinent), satellite-based observations, and process-related regional and global model studies. The choice of Kathmandu as a base for the M55 Geophysica research aircraft allowed measurements to be carried over Nepal, India, Bangladesh, and the Bay of Bengal within the AMA. The M55 Geophysica was equipped with extensive instrumentation to measure aerosol and cloud microphysics, aerosol chemistry, trace gases, radiation, and other basic meteorological parameters. The 2017 StratoClim measurement campaign included eight mission flights outbound from Kathmandu (Nepal) Tribhuvan International Airport (TIA) with a total flight time of about 31 h (Fig. ). Out of the eight measurement flights, three (KTM2, KTM4, and KTM5) took place exclusively in Nepalese airspace. These flights were carried out along an axis parallel to the Himalayan mountains over almost the entire east–west extension of Nepal. Three further measurement flights (KTM3, KTM7, and KTM8) were over northeastern India. These flight patterns allowed for the study of the horizontal structure of the AMA over large parts of its north–south extension. showed that the first half of the StratoClim 2017 campaign period was less affected by regional convective activity compared with the second half, allowing one to observe the ATAL under “dry” conditions (flights KTM1 to KTM4) and under convective influence (flights KTM5 to KTM8).

In this study, we also included measurements from the first phase of the StratoClim project which took place in 2016 in Kalamata, Greece (37∘ N, 22∘ E). During this campaign phase, three flights between 33–41∘ N and 23–31∘ E that reached up to 20 km altitude were conducted in the Mediterranean region (30 August, 1 and 6 September) using the M55 Geophysica. The geographical extent and the location of these flights relative to the strong subtropical potential vorticity (PV) gradient () indicate that these flights took place at the edge of the extratropics and the tropics. The results from these (here referred to as) extratropical aerosol measurements are juxtaposed to the tropical data from the ASM during StratoClim 2017.

The main instrument used for the measurements discussed in this study is the UHSAS-A aerosol spectrometer (see Sect. ). Besides the UHSAS-A, in situ and remote sensing instruments aboard the M55 Geophysica were included for the analyses discussed in this study: the COPAS (Sect. ) and NIXE-CAS (Sect. ) in situ particle detectors, the MAS backscatter probe (Sect. ), the MAL airborne lidar (Sect. ), and the COLD2 carbon monoxide instrument (Sect. ). The meteorological parameters and the avionic data from the M55 Geophysica are provided by the Unit for Connection with Scientific Equipment (UCSE; ). The potential temperature (Θ) is calculated from the UCSE-based temperature and pressure data as defined by the World Meteorological Organization (). For the given vertical temperature and pressure distribution and for the Θ range covered during StratoClim 2017 (up to ∼477 K), the WMO-compliant Θ values do not deviate by more than ∼1 K from the results according to the recently reassessed Θ calculation ().

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 a shows the vertical distribution of the particle mixing ratio (given in number of particles per milligram 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 using data points with different colors. The median of the particle mixing ratio of all eight measurement flights, calculated over 5 K potential temperature intervals, is plotted in black together with the 25th and 75th percentiles as horizontal bars for each median value. To avoid artifacts due to fewer particles at high altitudes, the median profile and the 25th and 75th percentiles were calculated based on the 1 Hz data for Θ<420 K and based on the resampled 0.1 Hz data set (see red dots in Fig. a) for Θ>420 K. For this, we have to assume that the atmospheric conditions remain quasi-homogeneous above 420 K within a 10 s interval (about 1.7 km flight distance). The number of 1 Hz data points included in each 5 K potential temperature interval is indicated in Fig. b, where each data point implies the measurement of a complete size distribution.

From comparatively high particle mixing ratios (median of 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 10 000 mg-1 are measured between 310 and 330 K. Up to the Θ level of 345 K, the median particle mixing ratio remains at about 150 mg-1. The variability in the measurement results increases with Θ, which can be seen from the changes in the deviation of the 25th and 75th percentiles with respect to the median.

From 345 to 350 K potential temperature, the median value increases 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 show a high variability in the particle mixing ratio, both between the different flights and when the flights are considered individually. Particle mixing ratios between 6 mg-1 and over 10 000 mg-1 were measured here. This high variability is also visible in the percentiles. Apart from the variability in the sampled air masses inherent in the dynamics of the AMA, causes for such variability may also be the occurrence of NPF events, convective outflow, and scavenging by the large persistent convective cloud systems. In these cloud systems, many aerosol particles are activated to form condensation nuclei of cloud droplets or get washed out by scavenging, resulting in the observed very low aerosol particle mixing ratios . On the other hand, these strong convective systems can lead to vertical transport of polluted air from the boundary layer, with elevated 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 NPF from precursor gases. These precursor gases of natural and anthropogenic origin are also subject to vertical transport by deep convective cloud systems reaching the TTL and cause NPF under favorable thermodynamic conditions . The lapse rate tropopause (LRT in Fig. a) during the 2017 campaign period was located between 369 and 396 K at a mean Θ level of about 380 K based on the European Centre for Medium-Range Weather Forecasts (ECMWF) ERA-Interim reanalysis data. Above the tropopause region, starting at about 380 K potential temperature, the variability in the particle mixing ratio decreases with increasing Θ.

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 440 K, the particle mixing ratio decreases further to a median of about 50 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 from 10 to 100 mg-1, it is noticeable 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 poor counting statistics of the single 1 Hz data points at these concentrations in combination with the constantly regulated sample flow (of 50 cm3min-1). However, due to the high number of 1 Hz data points (about 400 to 4000) available for each 5 K interval, we were even able to resample the data set to a temporal resolution of 0.1 Hz in this Θ range and then 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, 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 Sect. 3.2) during the 2016 StratoClim field campaign in Greece (extratropics) and StratoClim 2017 in Nepal (tropics), and as airborne measurements within the tropics and the extratropics published in . One more data set from the tropics and the extratropics was digitized from Fig. 1 of the publication by . Figure a shows the median profiles of the COPAS measurement series from the 2016 and 2017 StratoClim campaigns together with the median profile from the 2017 StratoClim UHSAS-A data set. In addition to the tropical and extratropical profiles from , Fig. b compares the UHSAS-A and COPAS measured profiles from StratoClim 2017 (Nepal) with three further profiles measured by the COPAS instrument within the tropics during the SCOUT-AMMA 2006 (West Africa, red line), SCOUT-O3 2005 (northern Australia, pink dotted line), and TROCCINOX 2005 (Brazil, dark green dotted line) field campaigns, discussed by and .

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 a maximum of 6000 mg-1. A maximum (particle mixing ratio of up to about 6500 mg-1) in this Θ range was also observed by in the tropical central Pacific (orange line). The median of the UHSAS-A measurement remains almost constant in this Θ range with significantly lower particle mixing ratio values of about 250 mg-1. However, the variability in the particle mixing ratio measured by the UHSAS-A is, as discussed above, very high in this Θ range. The COPAS measurements shown here cover the particle diameter range from about 10 nm to 1 µm, the measurements of were from 8 nm to 3 µm, whereas the UHSAS-A detects aerosol particles in the diameter range from 65 nm to 1 µm. The difference in the particle mixing ratios between the median values of the COPAS and the UHSAS-A measurements of more than about 5500 mg-1 shows that very small aerosol particles between 10 and 65 nm in diameter dominate the aerosol total particle mixing ratios. This indicates that especially the Θ range between 350 and 370 K is influenced by NPF, which has also been shown by and .

During the ASM, between about 370 and 415 K potential temperature, the median profile of the UHSAS-A data (diameter of 65 nm to 1 µm) shows particle mixing ratios that are up to 2 times higher than the median profile observed by (diameter of 8 nm to 3 µm) in extratropical regions (see Fig. b). The median profile of the COPAS measurement during the StratoClim 2016 measurement campaign in Greece (blue line in Fig. a) 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 (or rather from 6 nm during TROCCINOX 2005). Thus, inside the AMA, higher particle mixing ratios were observed for the size diameter range from 65 nm to 1 µm 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; ). This enhanced aerosol mixing ratio can be associated with the ATAL discovered by , 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 b shows that such a maximum between roughly 340 and 390 K was also observed in the mixing ratios of fine-mode particles obtained from COPAS in other tropical locations (northern Australia, West Africa, and Brazil; ), albeit with significantly lower absolute values than for the AMA region. The increase in the particle mixing ratio for altitudes above the 420 K level over West Africa was explained by as the influence of the 2006 Soufrière Hills eruption in the Caribbean (also discussed by and ). Figure a shows that a decrease in the mixing ratios detected with the UHSAS-A and COPAS from about 170 to 80 mg-1 and from 470 to 170 mg-1, respectively, can be seen between the 410 and 420 K potential temperature levels. The altitude levels of this decrease roughly coincide with the top of the 2017 AMA circulation system (). 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.

Characterizing the ATAL, described by , requires knowledge about the vertical progression of the aerosol particle size distribution. Within UT–LS altitudes, this 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 concentration () and aerosol surface area, available for heterogeneous chemical conversion processes.

The results shown here are the first measurements made with an UHSAS-A in the tropical lower stratosphere (up to more than 20 km altitude over the Indian subcontinent). The performance of the modified UHSAS-A (see Sect. ) has been thoroughly tested in the laboratory (as described in Appendix ). To put the in situ measurements at altitude into context with some of the other few available observations, a comparison was made with other optical particle counter measurements of the aerosol particle size distributions from the stratosphere.

For this purpose, we chose two measurements conducted with a balloon-borne setup which continues the series of in situ data described in , , and . The comparison measurements were from Hyderabad (India; 17.47∘ N, 78.58∘ E) in August 2015, also above the ATAL () during the ASM period, and from Laramie (USA; 41.32∘ N, 105.58∘ W) in August 2013. 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 (), flown in parallel with the LPCs. In Fig. , the resulting particle size distributions are compared with the data from the UHSAS-A obtained at the highest flight level (56 hPa pressure level) of the StratoClim mission flight KTM4. This size distribution was combined with the COPAS data for an additional size bin between 6 and 65 nm. The overlapping size bin of the NIXE-CAS measurements is shown in blue and agrees well with UHSAS-A data within the uncertainties. In each case, the balloon data set (500 m vertical averaging interval) was chosen for which the altitude range corresponded mostly to the flight level of the M55 Geophysica during the UHSAS-A measurements.

For the comparison of these data sets, the higher size resolution of the UHSAS-A and the difference in the detection range compared with the balloon-borne instrumentation as well as the temporal and spatial distance between these measurements must be taken into account. Additionally, the size distribution measured during StratoClim 2017 is reported with the bin limits resulting from the calibration with PSL particles (refractive index m=1.59; see Appendices  and ), while the balloon-borne measurements were corrected for a refractive index of 1.45.

The ambient total number concentration for the observation from StratoClim 2017 of ∼9 cm-3 is slightly lower than the 10 cm-3 observed over Hyderabad but one-third larger than the 6 cm-3 measured at Laramie. For a better quantitative comparison of the size distributions, the integrated surface area (dS) and volume concentrations (dV) were calculated. For the measurements during StratoClim 2017, these values were calculated using the original bin sizes and the ones recalibrated for a refractive index of m=1.45. In both cases, for the StratoClim 2017 observation, the values for dS≈0.44 µm2cm-3 (0.52 µm2cm-3 for m=1.45) and dV≈0.021 µm3cm-3 (0.029 µm3cm-3 for m=1.45) are smaller than the measurements from Hyderabad and Laramie, with dS≈0.79 µm2cm-3 and dV≈0.05 µm3cm-3 observed at both locations. Compared with the balloon-borne measurements from Hyderabad, these lower values for total particle number, surface area, and volume concentration observed during StratoClim 2017 also agree with a lower backscatter ratio (BR) signal observed by CALIOP at this altitude during StratoClim 2017. During July and August 2015, CALIOP observed a BR of about 1.07 at ∼20 km altitude within ±5∘ latitude and ±30∘ longitude around Hyderabad (), while the BR measured by CALIOP during StratoClim 2017 at 20 km was below 1.05 (Sect. ).

Figure  shows the vertical profile of the aerosol number size distribution (Fig. a), surface area size distribution (Fig. b), and volume size distribution (Fig. c) measured by the UHSAS-A averaged over the full StratoClim 2017 campaign period. The profiles were averaged with a vertical resolution of 1 K potential temperature. The concentrations for the distributions' diameter bins are color coded and normalized according to the bin widths in terms of dN / dlogDp, dS / dlogDp, and dV / dlogDp. To be able to compare the measured concentrations of the number, surface area, and volume size distributions over the full altitude range, the ambient concentrations have been converted to standard temperature and pressure (STP; T0=273.15 K; p0=1013 hPa) using the ambient temperature and pressure measurements reported by the UCSE.

Starting at a Θ level of 320 K, the profile of the aerosol particle number size distribution (Fig. a) has a pronounced maximum at the lower end of the UHSAS-A detection size range (diameter between 65 and 80 nm) as well as enhanced number concentrations for the large aerosol particles with diameters of up to 1 µm. Until about 326 K potential temperature, the size distribution's 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 very variable. It also shows high number concentrations for large particles up to 1 µm in diameter, which are at the upper detection size range of the UHSAS-A, and a very pronounced Aitken mode. Above 370 K potential temperature, the main mode of the size distribution broadens and shifts its maximum to diameters of about 100 nm. This shift of the distributions main mode to larger particles is even more prominent in the surface area (Fig. b) and volume size distribution (Fig. c). This increase in aerosol surface area can have a local effect on heterogeneous chemical processes. Additionally, reported, based on model simulations, that these particles spread throughout the entire Northern hemispheric lower stratosphere, contribute about 15 % to the stratospheric column aerosol surface area in the Northern Hemisphere annually, and set a lower limit of the ASM contribution to the global stratospheric aerosol surface area of about 7 %. At about 395 K potential temperature, the concentrations, especially for larger particles, begin to decrease. Above 420 K, until the maximum ceiling during StratoClim 2017, the shape of the aerosol size distribution shows only low variability. Altogether, the measured vertical profiles of the aerosol size distributions for the 2017 AMA show the presence of an ATAL feature between approximately 370 and 420 K with the peak of the size distributions in the 80 to 300 nm diameter range.

The ATAL was discovered as an enhancement of the newly reanalyzed and cloud-filtered backscatter ratio (BR) signal from the CALIOP lidar aboard the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) satellite (; ). , , , and confirmed this enhancement using balloon-borne in situ backscatter and aerosol particle number concentration measurements. To undertake a comparison with these observations, we calculated the BR based on the in situ aerosol particle size distributions from the StratoClim 2017 campaign and compared it with the cloud-filtered CALIOP, MAS (Sect. ), and MAL (Sect. ) measurements for a mostly overlapping time period in 2017.

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 M55 Geophysica research aircraft inside and above the Asian monsoon anticyclone (AMA) and the Asian tropopause aerosol layer (ATAL). Here, for the first time, submicrometer-sized aerosol size distributions were measured in situ down to a 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.

The ATAL BR observations by CALIOP during the StratoClim 2017 campaign period, as discussed in studies such as , , and for previous and recent ASM seasons, were validated by calculating the BR based on the in situ aerosol size distributions as well as the BR directly measured by the MAS and MAL instruments. These four independent methods largely agree with one another and can confirm the ATAL as a layer of enhanced BR within an altitude range from 15 to 18.5 km. The maximum of the ATAL BR signal was observed at 17.5 km altitude, consistently by all four methods. The importance of the ATAL for the Earth's radiative budget was already underlined by , who reported that the regional total sky radiative forcing caused by the ATAL has been around -0.1 Wm-2 since the late 1990s. Furthermore, the in situ measurements show that the ATAL is highly variable in time and space and is not a closed, persistent layer. While there is a seasonal correlation between the CO mixing ratio and the ATAL (), no direct correlation on the smaller scale, between co-located in situ measurements of the CO mixing ratio and the size-distribution-based BR, could be found. However, values of BR that are typically elevated within the ATAL could only be observed for CO mixing ratios larger than 40 to 50 ppb. This is also consistent with the observations that enhanced BR values could only be observed within the confinement of the AMA at an equivalent latitude (EQLAT) larger than 63∘ and below its top of confinement (at about 420 K potential temperature) during the StratoClim 2017 campaign period. This regional limitation of the ATAL with respect to the dynamics of the AMA is in good agreement with the horizontal () and vertical () limitations of the AMA-caused confinement.

From an experimental perspective, the ATAL is a fairly elusive, highly variable layer situated between approximately 370 and 420 K potential temperature or about 15 to 18.5 km altitude. Its lower part is close to – if not still inside – the highest region of convective outflows, whereas its upper part can be found at tropopause levels (between 369 and 396 K during StratoClim 2017) or slightly above. Thus, the aerosol of the upper ATAL part is subject to very slow vertical ascent (with rates of about 1 K potential temperature per day, as reported by , and ). 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 new particle formation (NPF), aging by coagulation and condensational growth, and removal by scavenging 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. This goes along with an increase in aerosol surface area, which might have a local effect on heterogeneous chemical processes and additionally contributes to the global stratospheric aerosol surface area ().

Using simple box model simulations (adopting the SOCOL – SOlar Climate Ozone Links; – coagulation subroutines), 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. The short periods of time available to detect recent NPF events and the still frequent NPF encounters during StratoClim 2017 (detected by COPAS) indicate the prevalence of such events within the ASM region (). Still, these simulations suggests that the coagulation of freshly nucleated aerosol particles alone cannot cause the lower part of the aerosol size distribution as measured by the UHSAS-A inside the ATAL altitude range. Furthermore, the question of where the particles larger than roughly 500 to 800 nm in the UHSAS-A size distributions come from remains open. Condensational growth could be a major process, although the nature and amounts of the various possible condensable gases are not yet well known. Furthermore, the upward transport of already existing larger aerosol particles () can contribute to the overall size distributions as observed by the UHSAS-A. Because of this complex interaction between dynamical and microphysical processes further, much more advanced model simulations are needed to identify and quantify the importance of the various involved processes. Our data are well suited to support such model simulations, as the UHSAS-A particle size distributions extend down to 65 nm while COPAS measured simultaneously with three different lower detection limits from 6 up to 15 nm.