Appel et al. discuss the results of aerosol in-situ observations taken on board a high-altitude aircraft within the European Union’s project StratoClim from Kathmandu/Nepal in July and August 2017. The paper concentrates mainly on particle composition within the Asian Tropopause Aerosol Layer (ATAL) probed with an aerosol mass spectrometer. It is shown that the aerosol particles of the ATAL as observed during the flights were mainly composed of ammonium nitrate and organics. While ammonium nitrate as a major component of the ATAL has already been reported in a previous publication, the role of organics is an important new finding of this work. Moreover, the authors present convincing arguments for the importance of secondary particle formation for the majority of the aerosols observed within the ATAL.   

The manuscript provides an extremely important contribution to our understanding of an (concerning in-situ measurements) under-sampled region of the Earth’s atmosphere – also given the uncertainties connected to aerosols and their radiative direct and indirect impacts within the climate system. The analysis of the measurements and their interpretation is convincing. The paper is well written containing clear graphical illustrations. Thus, I strongly support its publication after taking into consideration some minor comments below and after making available the datasets (‘Data availability’).

Specific comments:

L69: ‘from the CRISTA instrument’

Many of the data in the quoted publication stem also from the MIPAS/Envisat instrument.

L176: ‘During the StratoClim aircraft campaign 2017 we found an average detection limit of 0.12 μgm−3’

Please add ‘(NTP)’ where applicable.

Figure 2:

It would be good to show (perhaps in the supplement) the number of data points per altitude bin, since lower altitudes are only sampled during take-off and landing while much more measurements exist at flight level.

L298: ‘in the ATAL is predominantly existent in the form of ammonium nitrate (AN)’

Shouldn’t ammonium sulfate also be present and (at the top of the ATAL) be of similar importance?

L317 and Fig. 4: ‘Nevertheless, the data show that the ATAL for the time of StratoClim was not only confined in the vertical direction but also indicates a decrease towards the edge of the AMA in the horizontal distribution.’

It is not very clear to me where to see in Fig. 4 the horizontal confinement in the altitude region of the ATAL. There might be a slight decrease in organics, however in nitrate, the data of which reaches to much lower equivalent latitudes, I cannot see a clear decrease.

L321: ‘Stratmann et al., 2021’

This paper has not been published. Is it available elsewhere?

L324, Figure 5:

It is not clear what this Figure should tell: NO is steadily increasing with altitude and nitrate has a maximum within the ATAL. Can any more information be derived from the regression lines?

L404-416:

This paragraph seem a bit detached at its current position. May to be possible to transfer it into the ‘Conclusions’?

L465: ‘Apparently volatile organic precursors are still available, while the precursor for ammonium nitrate, i.e. ammonia from ground emissions, is not available in relevant concentrations.’

This conclusion seems rather indirect. Are there any independent measurements of the precursor gases to support it?  Furthermore, what might be the reason for it?

L521: ‘which appears as exceedingly difficult considering the current geopolitical situation’

I don’t think that this statement provides any information within the scope of the manuscript.

L528: ‘The ERICA mass spectrometry data will be available in the Edmond database (Edm, 2017) and the Halo database (Hal,2017).’

Is the dataset available now?

Technical comments:

L4 (and elsewhere): ‘high altitude’ -> ‘high-altitude’

L13 (and elsewhere): ‘analyzed’ vs. ‘vapour’ (L163) : AE and BE seem to be mixed in the manuscript

L27: Comma after ‘As a consequence’ missing; (also elsewhere: commas seem to be missing on similar expression at the beginning of a sentence)

L206: ‘sonde’ -> ‘probe’

L219: ‘2021a, b, ,’ -> ‘2021a, b, ’

L316: ‘(Fig. 4a)).’ -> ‘(Fig. 4a).’

General comments:

This paper discussed the unique aerosol and gas measurements aboard a high-altitude aircraft. It provided a promising dataset for atmospheric research, especially for understanding the aerosol particle composition within the Asian Tropopause Aerosol Layer (ATAL). The authors presented a very useful tool – ERICA-AMS for the atmospheric study and shared the exciting results from July and August 2017. The paper is well written. The topic is well aligned with the journal scope and should be considered for publication after minor revision.

Specific comments:

Page 9, lines 256-262. The discussion about Fig 2 in this section can not support this statement. “Consequently, the ATAL chemical composition is largely determined by the relative contributions of new particle formation and secondary particle growth at altitude compared to the upward transport of already nucleated secondary or of primary particles from below.” Maybe include the gas phase measurements to indicate the new particle formation trend?

Figure 2 showed the sulfate concentration increased from 0.5 – 1 .5 ug/m^3 above 19 km. However, Figure 3 showed that the particle number concentrations from COPAS or UHSAS were less than 90 #/cc. What does the size distribution above 19 km? Are those particles all sulfuric acid? Even if we assume they were ammonium sulfate and larger than 110 nm, the integrated mass seemed still lower than the AMS data.

Figure 5: are those data points are from the averaged data? If so, what is the uncertainty?

Section 3.3 provided essential information about the mixing state of the aerosol particles. The abstract also mentioned, “…the majority of the particles encountered in the ATAL consisted solely of secondary substances, namely an internal mixture of nitrate, ammonium, sulfate, and organic matter. These particles are externally mixed with particles containing primary components as well.” Does the mixing state remain the same at different aerosol particle sizes? Do you see a spatial variance in the mixing state?

Figure 10, It is not clear to me what the particle fractions for each particle type are. For example, at 300 nm, primary was 15%, type 1 – 65% or 40%, type2 68% or 5%? Should they add up to 1?

Figure 14, why is there no data above 19 km?

Line 528: ‘The ERICA mass spectrometry data will be available in the Edmond database (Edm, 2017) and the Halo database (Hal,2017).’ Should data be available by now?