Lifetimes and transport characteristics of different-sized aerosols in the Asian Tropopause Aerosol Layer: a climate model study

Dong, Yuxin; Tian, Wenshou; Luo, Yuan; Luo, Jiali; Huang, Rui; Xue, Haiyang; Peng, Yifeng

doi:10.5194/egusphere-2025-6412

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https://doi.org/10.5194/egusphere-2025-6412

Preprints

26 Feb 2026

| 26 Feb 2026

Lifetimes and transport characteristics of different-sized aerosols in the Asian Tropopause Aerosol Layer: a climate model study

Yuxin Dong, Wenshou Tian, Yuan Luo, Jiali Luo, Rui Huang, Haiyang Xue, and Yifeng Peng

Abstract. Using the Community Earth System Model (CESM), a series of sensitivity experiments were conducted to investigate the lifetimes and transport characteristic of different-sized aerosol particles within the Asian tropopause aerosol layer (ATAL). The results reveal that during the Asian summer monsoon (ASM) period, small particles, represented by Aitken-mode sea salt (NCl a2, 0.015–0.052 μm in diameter), can reside stably in the upper troposphere–lower stratosphere (UTLS) region and undergo extensive horizontal transport. The mean lifetime of NCl a2 particles reaches up to 552 days, while those of fine (0.095–0.56 μm) and coarse (0.63–3.70 μm) sea salt particles have an average lifetime of approximately 28 days and 11 days, respectively. The trapping effect of the ASM circulation on particles released at various heights within the ATAL (180–80 hPa) can maintain even after 120 days. When aerosol particles are released below the ATAL, the number of particles entering the UTLS region varies significantly with the release sites, i.e., aerosols released over South Asia (an effective upward transport pathway) more readily enter the ASM anticyclone and the stratosphere and reside longer in the UTLS region than particles released at the ASM anticyclone hinterland and the East Asia (EA) site.

Received: 22 Dec 2025 – Discussion started: 26 Feb 2026

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Yuxin Dong, Wenshou Tian, Yuan Luo, Jiali Luo, Rui Huang, Haiyang Xue, and Yifeng Peng

Status: final response (author comments only)

RC1:
'Comment on egusphere-2025-6412', Anonymous Referee #1, 23 Mar 2026
This manuscript investigates the lifetimes and transport characteristics of different-sized aerosol particles within the ATAL using idealized sensitivity experiments with the CESM. The experimental design is commendable for its multidimensional approach, covering three particle sizes, three release locations, and three release altitudes, yielding physically interpretable results on the role of the ASM anticyclone in confining and prolonging aerosol residence in the UTLS. The study addresses the important question of aerosol persistence in the upper troposphere and lower stratosphere, which is a key factor for climate impact assessment. The manuscript is suitable for publication after some corrections and address several writing and presentation issues throughout the text.

Major comment:
The definition of the ATAL region appears inconsistent across the manuscript. In Figures 2 and 8, the ATAL region is defined as 20–60° N, 20–150° E, while in Figures 1 and 3, it is defined as 15–45° N, 0–160° E. Could the authors clarify whether a consistent regional definition was used throughout the analysis? If different definitions were adopted for different analyses, please explain the rationale behind these choices and specify the corresponding definitions clearly in the method section to avoid confusion in result interpretation.

Some other comments:
Line 12: “a series of sensitivity experiments were conducted” → “a series of sensitivity experiments was conducted”.

Line 120: “and MERRA-2 reanalysis data for summer 2000” → “and MERRA-2 reanalysis data for year 2000”.

Line 81: The summation subscript in Equation (1) has a typo: “” should be revised to “”.

Line 179: “We can note that the average mass concentration of the smallest NCl a2 particles...” uses colloquial expression that is not suitable for academic writing. It is recommended to revise to “Results show that” or directly describe the observed phenomenon.

Table 1: The unit “day” should be revised to the plural “days” in line with academic norms.

Line 352: “Given that fact that aerosol mass concentration within the ATAL exhibits a persistent increasing trend...” → “Given the fact that…”.

Line 354: “Estimates of the ATAL’s radiative effects in previous studies have based on bulk aerosol properties” → “…have been based on…”.
Citation: https://doi.org/10.5194/egusphere-2025-6412-RC1
RC2: 'Comment on egusphere-2025-6412', Hongwei Sun, 14 May 2026

The manuscript uses CESM model to simulate the transport and lifetime of passive sea salt aerosol in the Asian tropopause aerosol layer. Several sensitive tests with different aerosol size, injection regions, and injection altitudes have been considered in the manuscript, which could help increase our understanding of how the trapping effect of the Asian summer monsoon circulation influence the particle transport. It’s good to see that the manuscript shows some interesting findings. However, several caveats need to be addressed or clarified before the manuscript can be considered for publication, as outlined below.
Major comments
1. This study focus on the passive particle transport, the most straightforward way should be directly adding a passive aerosol in CESM, which can only interact with transport and gravity settling. However, this study takes sea salt as the passive aerosol, with “their radiative and chemical effects being disabled in the model” (Line 370). But more clarification is needed for doing so:
(1) Should sea salt be NaCl, rather than NCl?
(2) Line 132: For sea salt, are there aerosol microphysical growth (e.g., coagulation, condensation)?
(3) Sea salt aerosols are hygroscopic, which means water vapor influences the sea salt aerosol size and concentration. Have the authors turn off this influence in CESM?
(4) Would sea salt aerosols serve as CCN to interact with clouds in CESM? If so, the authors may need to turn off this aerosol-cloud interactions, which can influence the loss rate of sea salt aerosols.
(5) Are there sea salt emissions in CESM? What is the background sea salt aerosol size/concentration, in addition to the NCl injection in the Asian tropopause aerosol layer?
(6) Line 138: what is a1 particles? Define their size.
2. I may need help to understand the lifetime calculation that the authors used here. It seems all the calculation is based on the two-month-mean time series of aerosol mass concentration (Line 81-85):
(1) How does the author calculate the C(t), the global aerosol mass burden, based on the two-month-mean time series of aerosol mass concentration? Is the global aerosol mass burden the total column aerosol mass burden or the mass burden in the upper troposphere lower stratosphere (UTLS)? If UTLS, defining based on 200-50 hPa (Line 283) may not accurate as we know the tropopause height is very different between tropics and higher latitudes.
(2) How does the author calculate the D(t) denoting the aerosol mass loss rate during a timestep Δt, which represents the total mass removed by deposition processes during that interval (Line 84)?
(3) If I understand correctly, based on the lifetime calculation equation (Line 83), the author should get a time series of the 2-month mean aerosol lifetime based on the two-month-mean time series of aerosol mass concentration. If so, I would like to see this time series of the 2-month mean aerosol lifetime.
(4) There is a typo in Equation (1): “t-0” should be “t=0” in the numerator.
3. One important question worth checking/clarifying is if the ASM anticyclone is mainly good at trap the aerosols in the upper troposphere (near and below tropopause), or the ASM anticyclone is also good at allowing aerosols to transport upward and cross the tropopause (from the upper troposphere to the lower stratosphere). I think the author touch this point somewhere in the manuscript, but may worth a more detailed discussion.
Minor comments
Line 223-229: this paragraph’s explanation is dependent on the injection regions, as mentioned in the next paragraph. If injections happen in the EA and SA regions at 16-17 km, the lifetime is also short.
Figure 5 needs clarification: does Figure 5a show injection for all three regions? Does Figure 5b show injection at all injection altitude? But Line 234 mentions injection at 16-17 km, which may indicate Figure 5b only shows results for injection at 16-17 km? This can be related to my last comment.
Figure 6:
(1) add “probability density distributions” to the figure caption.
(2) Explain what is Day 10 to Day 120 in the figure caption.
(3) I think x axis should be using aerosol concentration, which may make more sense than using probability density, especially if you want to compare the left and right columns.
Figure 7:
(1) why use probability, instead of the particle number concentration, which can also show the change of total particle number between the two columns.
(2) Would be good to add boxes (like Figure 2) to indicate the injection regions.
(3) This is total column or only some vertical levels (UTLS)?
Figure 7 only shows the injection at 14-15 km, results from injection at 16-17 km may also be worth showing, at least in the supporting information.
Figure 8:
(1) one issue in this paper is using both pressure level and height level, which may make readers have difficulty to compare. I can see that the authors have made an effort to clarify this, but I may be a bit picky and would encourage the authors to improve it further.
(2) The red line in Figure 8c shows very low decay, any physical explanation?
(3) The ratio (in the subplots) in the end will reach a steady state of 20%, which confused me. If you look at the lines in the main plot, shouldn’t the steady-state ratio in the subplot be around 50%?
Line 285-286: In addition to the particle entering the ASM anticyclone center. Would the ratio also reflects the number of particle got scavenged/removed (due to the loss/deposition rate) after injection but before reaching North America?
Table 1: why the duration of anticyclonic confinement at 11-12 injection altitude is much larger for injections in SA than injections in ASMA?
Line 315: Can the authors explain why “The proportion of NCl a2 particles entering the ASM anticyclone center is largest when particles are released at SA and smallest when particles are released at EA”? This may be related to my last comment.
Line 331: reside stably within the “stratosphere”, or within the “UTLS”?
Line 342: change “minimal diffusion” to “minimal horizontal diffusion”.
Line 358: remove “that fact”.
Finally, based on my own research experience, I would suggest including a discussion at the end of the manuscript noting that “further studies using a Lagrangian trajectory tracking model [Sun et al., 2023; 2024] may provide a more detailed understanding of the trapping effect of the ASM circulation. In such models, each injected particle can be treated as tagged, allowing greater flexibility to evaluate different injection locations and times, and to better characterize the three-dimensional structure of the ASM anticyclone region regarding particle transport and lifetime.” For example, as shown in Figure 1 of Sun et al. (2023), each particle’s lifetime can be linked to its specific injection location.
Sun, H., Bourguet, S., Luan, L. et al. Stratospheric transport and tropospheric sink of solar geoengineering aerosol: a Lagrangian analysis. npj Clim Atmos Sci 7, 115 (2024). https://doi.org/10.1038/s41612-024-00664-8
Sun, H., Bourguet, S., Eastham, S., & Keith, D. (2023). Optimizing injection locations relaxes altitude-lifetime trade-off for stratospheric aerosol injection. Geophysical Research Letters, 50, e2023GL105371. https://doi.org/10.1029/2023GL105371

Citation: https://doi.org/10.5194/egusphere-2025-6412-RC2

Yuxin Dong, Wenshou Tian, Yuan Luo, Jiali Luo, Rui Huang, Haiyang Xue, and Yifeng Peng

Data sets

Lifetimes and transport characteristics of different-sized aerosols in the Asian Tropopause Aerosol Layer: a climate model study Yuxin Dong https://doi.org/10.17605/OSF.IO/UCXEP

Model code and software

Lifetimes and transport characteristics of different-sized aerosols in the Asian Tropopause Aerosol Layer: a climate model study Yuxin Dong https://doi.org/10.17605/OSF.IO/UCXEP

Yuxin Dong, Wenshou Tian, Yuan Luo, Jiali Luo, Rui Huang, Haiyang Xue, and Yifeng Peng

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Short summary

The lifetime of aerosols is one of crucial factors in estimating aerosols' climate effects. Using the Community Earth System Model, a series of sensitivity experiments reveal that Aitken-mode sea salt (0.015–0.052 μm) within the Asian tropopause aerosol layer, can reside stably in the upper troposphere–lower stratosphere, with mean lifetime reaches 552 days. While those of fine (0.095–0.56 μm) and coarse (0.63–3.70 μm) sea salt have an average lifetime of 28 days and 11 days, respectively.


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