Vernier et al. present observations of aerosol mass concentrations and other in-situ observations collected with a balloon that sampled the Asian Tropopause Aerosol Layer (ATAL). The authors conclude from the aerosol collected on filters and analyzed offline, that nitrate and nitrite are major constituents of aerosol in the ATAL. Then, they compare with satellite retrieved data to show that the balloon was within the ATAL concurrently as a CALIOP overpass. Finally, they use back trajectories and GEOS-Chem model results to investigate the sources of the aerosol collected onto the filter.

Though it may be of interest to ACP, the authors need to address the comments below for the article to be accepted. Further, as the article is currently written, and depending on how the authors respond to the comments below, the article currently seems more appropriate for Measurement Report instead of a research article for ACP (further discussed below).

Major Comments

1. Here are some examples of how this paper seems more appropriate for Measurement Report.

1.1 The authors mention that both nitrate and nitrite were important constituents of the aerosol composition; however, they only really focus on the potential sources of nitrate. Nitrite is a generally unusual component of aerosol composition and I would expect to have different physicochemical properties than nitrate, including in the ATAL and its impact on cirrus clouds and radiative properties. A discussion on nitrite, including sources and what form it is in the ATAL would improve the paper and make it more appropriate for a research article than a measurement report.

1.2 The authors mention that the GEOS-Chem predictions have much higher sulfate than the filter measurements and provide one short sentence speculation about why without further discussion or ramifications in a model having too high sulfate compared to observations. Ramifications of too high sulfate should be addressed, as that is an important finding. Also, how does the sulfate observations here compare to prior studies?

1.3 The authors use GEOS-Chem as is without any verification if the model predictions match observations (e.g., is CO at the right location / concentration). Satellite retrieved observations of CO could help or even be used instead of GEOS-Chem CO to make this point. Further, other satellite products could be used to try to make the points the authors are trying to make here. On top, it is unclear how well GEOS-Chem is performing for nitrate compared to observations. One obstacle behind this comparison is (a) was ISORROPIA used to predict the thermodynamic partitioning of nitrate with aerosol phase and (b) what nitrate was used within ISORROPIA. If, for example, GEOS-Chem only has ammonium nitrate and not nitric acid trihydrate (NAT) or more refractory aerosol (sodium or calcium nitrate), then the discussion about nitrate with the filter measurements does not work here.

1.4 The nitrate being either NAT or potentially refractory aerosol and not ammonium nitrate is an important finding, especially in regards to the physicochemical properties of aerosol in ATAL. However, this is not explored and expanded upon. Also, could this be a measurement artifact (see next comment)? The fact that prior studies observed ammonium nitrate and not the aerosol reported here needs to be further explored as the findings here are very different.

1.5 The authors make a speculation that Canadian wildfires impacted the ATAL in the abstract and the conclusion. With it being in the abstract, this would lead readers to expect more attention on this detail in the paper instead of just a brief passing sentence in the conclusion. This could be a section in of itself, especially regarding how forest fires could impact ATAL. Currently being in just abstract and conclusion further makes this paper feel more like a measurement report instead of a research article.

2. Page 8, line 26 – 31 and page 9, line 1 – 6, the authors discuss how ammonium nitrate could be lost on filters. However, there was no discussion if experiments were conducted to determine how much loss occurred. Further, the authors mention NAT (page 8, line 15) could explain the lack of charge balance in the observations they show. But, there is no discussion about how stable NAT would be on filters prior to and during the freezing of the filters and during the preparation of the filters for sampling. Further discussion / exploration of this is needed to put the paper into context of prior studies and for use in comparisons against chemical transport models.

3. Page 9, line 17 – 18, the authors quickly mention the pump was on for 16 minutes. This is significantly shorter than the 2 hours the authors said was needed to collect enough aerosol to have measurements above detection limit. This needs to be clarified.

4. The description of GEOS-Chem needs to be moved to methods, and further description of GEOS-Chem needs to be provided—was ISORROPIA used, what forms of nitrate were included, was nitrite in the model, how was SOA modeled, etc.

5. There were four flights of the payload, yet only three flights are shown. Why is one flight not included in Fig. 3 or any discussion?

Minor Comments

1. Page 10, line 1, the authors have the description of trajectories and deep convection different than in Fig. 5.
2. Page 11, line 16, make sure Chem in GEOS-Chem is capitalized.
3. Page 11, line 18 – 20, the authors make a statement about upper troposphere (UT) NOx lifetime and its sinks. However, the normal thought about UT NOx lifetime is that it is about 2 days. Where did this shorter lifetime and sinks come from?
4. Page 12, line 4 – 8, the authors use mass concentration for CO instead of volume mixing ratio; however, in the figures, CO is reported in volume mixing ratios. Please clarify which is correct.
5. Page 12, line 7, the should not be capitalized.
6. Fig. 1, a picture or diagram of the science payload would be good here.
7. Fig. 6, the white dot is barely visible and even missing, maybe, in some of the panels. Make it more prevalent to see it.
8. Fig. 7, what is the horizontal bar from the left axis to ZF2? What is the nitrate observed for a comparison point? Also, red and green (also for other figures) is generally not a good combination for colored blind readers.
9. Fig. 8, it was mentioned dilution of CO is observed. The figure makes CO look relatively flat.

Vernier et al. discuss the results of balloon flights over India in August 2017 and February 2018 analysing the chemical composition of aerosol particles at 15-18 km altitude. To determine the concentration of water-soluble inorganic species they used a particle impactor on the balloon followed by subsequent ion chromatography in the laboratory. They found mainly nitrate and nitrite in terms of mass concentration during summer while sulfate and Ca²⁺ were detected during winter as major aerosol constituents. Backward trajectories in combination with chemical transport model simulations have been applied to characterize the history of the sampled air-masses. The main conclusion derived from the observation is, that particles containing nitrogen are a large part of the Asian Tropopause Aerosol Layer (ATAL). As a main result from the model simulations, the general importance of NO_x from lightning with respect to the nitrogen budget of the upper troposphere in summer is noted.

The paper presents some of the extremely rare in-situ observations of the composition of particles sampled inside the ATAL. Since this is an active scientific area of general interest and since a novel dataset is described, the work qualifies for publication in ‘Atmospheric Chemistry and Physics’. However, in the presentation of the dataset and its interpretation there are some loose ends which are detailed below under ‘specific comments’. These should be discussed and presented in more detail before I would recommend publication.

Specific comments:

P2L7-9, ‘The sampled air masses in winter 2018 were likely affected by smoke from the Pacific Northwest fire event in Canada, which occurred 7 months prior to our campaign, leading to concentration enhancements of SO₄^2- and Ca²⁺.’:

1. The influence of fires is not detailed in the main text. Please add a paragraph where this finding is explained in more detail as well as adding references.
2. On P8L11, Ca²⁺ is referred to as an indication for mineral dust and K⁺ for biomass burning. How does this fit to the statement here, since K⁺ is not measured in the winter 2018 flight (see e.g. Fig. 2)?

P3L3, ‘World Health Organization recommendations’:

Please add a reference.

P3L19:

You may add here ‘(Wagner et al., 2020)’

P4-5, chapter ‘1.3 What is known about ATAL’s composition?’:

For the sake of completeness, the different views on mineral dust should also be mentioned, which is either predicted to be the major constituent of the ATAL (Fadnavis et al., 2013; Lau et al., 2018; Yuan et al., 2019; Ma et al., 2019; Bossolasco et al., 2020)  or of minor importance (Yu et al., 2015; Gu et al., 2016; Yu et al., 2017; Fairlie et al., 2020).

P5L22, ‘It translates into a mass concentration of 40 ng/m3 assuming that the aerosols were liquid sulfate droplets’:

1. ‘…of around 40…’
2. STP should be mentioned always, if it applies.

P6L28, Fig. 1, Fig. 2:

1. Provide exact dates of all flights in the text .
2. The flight abbreviations in Fig. 1 and Fig. 3 are mixed up: in Fig. 1 ‘ZFW’ is the winter 2018 flight while in Fig. 3, it is ‘ZF-1’. Please present those in a unique way.
3. From the four flights listed in Fig. 1, only 3 are mentioned in the text. Please explain why.

P8L10-11, ‘with traceable amounts of proxies for mineral dust (Ca²⁺) and biomass burning (K⁺).’

Please provide references for this statement and explain it a bit more.

P8L20, ‘…but did find the same in the flight samples of winter (Fig. 3).’:

Please formulate this sentence clearer. As it is written, one could think that ammonium has been found during the winter flight, which is not the case according to Fig. 3.

P8L26-P9L6:

Might this be an explanation that no ammonium has been found, but nitrate? Please elaborate on this.

P9L9, ‘observations from the CALIOP lidar onboard the CALIPSO satellite.’:

How has the CALIOP data been averaged? Which co-incidence criteria have been applied? Has any cloud-clearing been used.

P9L14, ‘likely made of aspherical particles’:

Does this mean that during this part of the balloon flight, air has been sampled within a cirrus cloud? If so, please state this clearly and any implications this might have on the analysis.

P9L18, ‘was done for more than 1 h in a cloud-free region enhanced with aerosols above.’:

Please formulate this clearer – one could interpret it as if the aerosols have been above the balloon.

P9L21-24:

Is this a general comment on cirrus above Gadanki or does it refer to the situation during one of the flights. Please be clearer here.

P9L24-26, ‘Moreover, the increasing fraction of sub-visible cirrus clouds between 1998-2003 probably modified on the temperature and the water vapor budget in the Tropical Tropopause Layer (Pandit et al., 2015).’

There is something wrong with this sentence.

P10L1, ‘trajectories (black lines) and deep convection influence 1 (red dots).’

I cannot see this in Figure 5.

P11L4, ‘CO, nitrate, sulfate, and black carbon (BC) aerosol concentrations’:

1. To support a discussion on mineral dust and to compare with the observed Ca²⁺, it would be good to show here also the model perspective.
2. Please also provide the model maps of NH4⁺ and discuss differences between model and the lack of ammonium in the observations.

P11L11, ‘nitrate are significantly lower’:

Please provide here numbers (…% lower). From Fig. 7 it seems that these concentrations are not a much lower.

P11L20, ‘The NOx lifetime is believed to increase downwind from the outflow’:

Please provide a reference for this statement.

P11L31-P12L4, ‘Fig.5 shows that GEOS-chem could simulate convective activities reaching levels between 14-15 km …’:

Please describe more clearly where this is the case. Fig. 5 does not reach down to the ground, so one cannot judge if the transported air stems from the boundary layer. Further there are cases with indications of convection from HIMAWARI which are not captured by the model (ZF2, highest altitude at around 105 deg East).

P12L11, ‘We note that sulfate along the trajectories influenced by Chinese pollution during ZF2 increase significantly…’:

1. What means ‘significantly’ here? Please provide numbers (by …%).
2. Please provide possible explanations (including references) why sulfate is modelled too high compared to the measurements.

P12-13, chapter ‘7. Summary and Conclusions’:

1. Please discuss also the representativeness of these balloon observations lying more at the border of the AMA for the ATAL as a whole.
2. Please explain also the relevance of your findings on Ca²⁺ wrt the question of mineral dust as a major constituent of the ATAL (see comment and references above).
3. Please discuss also your finding of high concentrations of nitrite. What could be a relevant mechanism for its production?

P13L8-10, ‘parcels, the model ability to simulate convective influence at higher altitudes seem to be limited.’:

This is not mentioned in the main text – please explain it, where the influence of convection and the ability of the model to simulate convection is discussed.

P13L17, ‘where smaller nitrate particles were found which could also indicate the influence of new particle formation.’:

Also this is not discussed in the main text. Please provide an explanation where the relevant Figure is explained including possible references.

Technical comments:

P1L39, ‘STP’:

Explain abbreviation.

P2L5, ‘with particle size radius (0.05-2μm)’:

1. ‘with particle size radii from 0.05 to 2 µm’
2. Here and all over the text: there should be a space between number and unit

P2L6, ‘mass’:

-> ‘masses’

P2L37, ‘have’:

-> ‘has’

P3L25, ‘of ATAL’:

-> ‘of the ATAL’

P5L6, ‘Höpfner et al., 2016’:

-> ‘Höpfner et al., 2019’

P5L21, ‘a concentration of 20 particle/cm3’:

-> ‘… of about 20 …’

P6L26, ‘, to’:

-> ‘. To’

P8L14, ‘Ca₂⁺’:

-> ‘Ca²⁺’

P8L19+22, ‘Hopfner’:

-> ‘Höpfner’

P9L16, ‘, pressure’:

-> ‘, the pressure’

P10L8, ‘We conduct GEOS-Chem’:

-> ‘We have conducted GEOS-Chem’

P10L30, ‘2.5^o by 2^o horizontal’:

-> ‘latitude x longitude’ or ‘longitude x latitude’ ?

P11L12, ‘is’:

-> ‘are’

P12L11, ‘increase’:

-> ‘increases’

P12L13, ‘NO3’:

-> ‘NO₃^-‘

P12L20, ‘onboard’:

-> ‘aboard’ or ‘on board’

P14-20, refernces:

doi’ s are missing for some the references, please check.

P25, Fig. 4:

Why are only positive ascent rates given in the color scale?

P26, Fig. 5:

x-axis title missing in bottom right panel.

References:

Bossolasco, A., Jegou, F., Sellitto, P., Berthet, G., Kloss, C., and Legras, B.: Global modelling studies of composition and decadal trends of the Asian Tropopause Aerosol Layer, 32 pp., 2020.

Fadnavis, S., Semeniuk, K., Pozzoli, L., Schultz, M. G., Ghude, S. D., Das, S., and Kakatkar, R.: Transport of aerosols into the UTLS and their impact on the Asian monsoon region as seen in a global model simulation, Atmos. Chem. Phys., 13, 8771–8786, https://doi.org/10.5194/acp-13-8771-2013, 2013.

Fairlie, T. D., Liu, H., Vernier, J.-P., Campuzano‐Jost, P., Jimenez, J. L., Jo, D. S., Zhang, B., Natarajan, M., Avery, M. A., and Huey, G.: Estimates of Regional Source Contributions to the Asian Tropopause Aerosol Layer Using a Chemical Transport Model, J. Geophys. Res., 125, https://doi.org/10.1029/2019JD031506, 2020.

Gu, Y., Liao, H., and Bian, J.: Summertime nitrate aerosol in the upper troposphere and lower stratosphere over the Tibetan Plateau and the South Asian summer monsoon region, Atmos. Chem. Phys., 16, 6641–6663, https://doi.org/10.5194/acp-16-6641-2016, 2016.

Lau, W. K. M., Yuan, C., and Li, Z.: Origin, Maintenance and Variability of the Asian Tropopause Aerosol Layer (ATAL): The Roles of Monsoon Dynamics, Scientific reports, 8, 3960, https://doi.org/10.1038/s41598-018-22267-z, 2018.

Ma, J., Brühl, C., He, Q., Steil, B., Karydis, V. A., Klingmüller, K., Tost, H., Chen, B., Jin, Y., Liu, N., Xu, X., Yan, P., Zhou, X., Abdelrahman, K., Pozzer, A., and Lelieveld, J.: Modeling the aerosol chemical composition of the tropopause over the Tibetan Plateau during the Asian summer monsoon, Atmos. Chem. Phys., 19, 11587–11612, https://doi.org/10.5194/acp-19-11587-2019, 2019.

Wagner, R., Bertozzi, B., Höpfner, M., Höhler, K., Möhler, O., Saathoff, H., and Leisner, T.: Solid ammonium nitrate aerosols as efficient ice nucleating particles at cirrus temperatures, J. Geophys. Res., 125, e2019JD032248, https://doi.org/10.1029/2019JD032248, 2020.

Yu, P., Rosenlof, K. H., Liu, S., Telg, H., Thornberry, T. D., Rollins, A. W., Portmann, R. W., Bai, Z., Ray, E. A., Duan, Y., Pan, L. L., Toon, O. B., Bian, J., and Gao, R.-S.: Efficient transport of tropospheric aerosol into the stratosphere via the Asian summer monsoon anticyclone, Proc. Natl. Acad. Sci. U.S.A., 114, 6972–6977, https://doi.org/10.1073/pnas.1701170114, 2017.

Yu, P., Toon, O. B., Neely, R. R., Martinsson, B. G., and Brenninkmeijer, C. A. M.: Composition and physical properties of the Asian Tropopause Aerosol Layer and the North American Tropospheric Aerosol Layer, Geophys. Res. Lett., 42, 2540–2546, https://doi.org/10.1002/2015GL063181, 2015.

Yuan, C., Lau, W. K. M., Li, Z., and Cribb, M.: Relationship between Asian monsoon strength and transport of surface aerosols to the Asian Tropopause Aerosol Layer (ATAL): Interannual variability and decadal changes, Atmos. Chem. Phys., 19, 1901–1913, https://doi.org/10.5194/acp-19-1901-2019, 2019.