the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 26 Jun 2025
Impact of stratospheric intrusion on near-surface ozone over the Sichuan Basin in China driven by terrain forcing of Tibetan Plateau
Abstract. Stratospheric ozone (O3) intrusion acts as a major natural source of tropospheric O3 affecting atmospheric environment. Targeting a stratospheric intrusion (SI) hotspot, the Tibetan Plateau (TP) with the immediately adjoining O3 pollution region of Sichuan Basin (SCB) in Southwest China, this study assesses the seasonal contribution of SI to the near-surface O3 over SCB and reveals the multi-scale coupling mechanisms of atmospheric circulations with the seasonally discrepant terrain effects of the TP. Results show that SI over the TP penetrates deep to the near-surface atmosphere in SCB with a maximum increment of 38.7 % in the O3 level, providing an extra contribution of 11.1–16.0 % to regional O3 pollution. The evolution of South Asian High with the peripheral subsidence in the warm season and the subtropical jet strtucture in the cold season trigger tropopause folding, driving stratospheric O3 injecting into the troposphere over the TP. Two primary pathways for SI-derived O3 entering the near-surface layer over the basin are identified with downslope transport along the TP’s leeward slope into the western SCB region and downwind transport to the central and eastern SCB regions associated with the seasonally discrepant effects of TP thermal and mechanical forcing.
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RC1: 'Comment on egusphere-2025-2628', Anonymous Referee #1, 18 Jul 2025
Stratospheric intrusion (SI) is a significant contributor to elevated tropospheric ozone levels. As a global hotspot for SI, the surface ozone concentration in the Tibetan Plateau region has garnered attention due to the influence of SI. However, it remains unclear whether ozone pollution in the surrounding low-altitude Sichuan Basin region is also regulated by this process. To address this issue, this study utilized the WRF-Chem model for investigation. The findings contribute to a comprehensive understanding of ozone pollution causes in the region, but I have some comments as below, and I hope this could help author to improve the paper for later submission.
1. Why were only four months selected as representatives for studying annual ozone pollution? Why not choose data from the entire year?
2. In Figure 1, there were several SI events in January. How were the specific cases selected?
3. Lines 242-244: Ozone concentration inherently exhibits diurnal variation. How can it be demonstrated that this is due to vertical mixing?
4. How was the "relative contribution" calculated?
5. Lines 273-275: The tropopause over the Tibetan Plateau is almost the highest in the global during the Northern Hemisphere summer, making it difficult for deep tropospheric convection to penetrate the tropopause. Additionally, EP1 does not occur during the summer monsoon season.
6. Lines 276-278: How does the SAH trigger SI? Is there any evidence to support this conclusion?
7. Line 280: What is shown in Figure 1?
8. Lines 285-286: Previous studies only found that the location of tropopause folding is related to the subtropical westerly jet, but there is no evidence proving that the subtropical westerly jet is the driving factor for tropopause folding.
9. Lines 285-297: Are these results derived from Figures 5c and 5d?
10. Figure 5: It is unlikely that the SAH is depicted in Figure 5a since it represents April. What do the red contours in Figures 5c and 5d indicate? It is suggested that the authors include the westerly jet in theCitation: https://doi.org/10.5194/egusphere-2025-2628-RC1 -
RC2: 'Comment on egusphere-2025-2628', Anonymous Referee #2, 29 Jul 2025
SUMMARY
The authors investigate four cases of Stratospheric Ozone Intrusion (SI) events in the Tibetan Plateau (TP) region to quantify their downstream impacts on regional O3 pollution over the Sichuan Basin (SCB) in Southwest China, based on WRF-Chem simulations and observational analysis. The dynamical mechanisms for the different SI events are also analyzed.
REVIEW AND RECOMMENDATION
This is a regional case study of few SI events over the SCB, but I believe this topic could still provide useful information for the community by archiving case studies of SI events and their remote impacts on the regional air quality over the downstream region. However, I have some concerns regarding the sensitivity of the analyses. For example, the manuscript would benefit from a more rigorous examination of the sensitivity of results, particularly regarding the representativeness of the four selected SI cases, the domain definitions, and the influence of other contributing factors. Additionally, it appears that some parts of the manuscript are difficult to follow and would benefit from further editing to improve readability for publication. Therefore, I recommend major revisions before the paper can be considered for publication. My key concerns are outlined below.
MAJOR COMMENTS
- Sensitivity of the results
Since this study aims to quantify the SI contribution to regional O3 pollution, it is important to more thoroughly discuss the representativeness of the four selected cases and the sensitivity of the results to the choice of SI events and background conditions (i.e., other contributing factors to O3 concentration).
1a) Representativeness of the four selected cases
While I understand that this study builds on previous works (Shu et al., 2022; Shu et al., 2023), the extent to which the four selected cases differ from other SI events and the reason for selecting them as representative cases for each season requires further clarification. Do these cases significantly differ from other SI events, or do they reflect typical conditions within their respective seasons? What criteria were used to select these four cases?
1b) Sensitivity of the results on the definition of domain of interests
Lines 235-236: its relative contribution rate to near-surface O3 levels averaged over the SCB region.
How is the SCB region defined in this calculation? Is it the same area described in Section 2.2 as SCB (103–110°E, 28–33°N)? If so, I believe this definition may require some changes, especially given that the analysis focuses on the downstream impacts of the TP. This domain appears to include part of the edge of the TP. For instance, the high O3 concentrations in the northwestern portion of Figure 3 seem to originate from higher altitudes on the Eastern TP. While I assume this area is relatively small and may not significantly affect the overall results, for consistency, it would be better to exclude the TP edge from the SCB domain definition (one possible option could be the area inside the white lines shown in Fig. S3).
1c) Generalization of the results
Lines 19-21: Results show that SI over the TP penetrates deep to the near-surface atmosphere in SCB with a maximum increment of 38.7% in the O3 level, providing an extra contribution of 11.1–16.0% to regional O3 pollution.
The numbers presented in this study reflect only a few selected cases based on WRF-Chem simulations and are not generalizable. Therefore, I think the language used to describe the findings should be more cautious and explicit about the limits. As currently written, the results may imply that they represent a general SI contribution to O3 pollution across all seasons, rather than case-specific results.
I suggest that the authors explicitly state that the results are based on four selected cases from specific time periods and clearly specify the reference state used to calculate the relative contribution of the SI events.
It would also be helpful to clarify whether these estimates are derived from WRF-Chem simulations, observations, and if possible, to discuss any differences between the models and observations.
1d) Other factors contributing to the changes in ozone concentration
While the focus is on downstream impacts of the SI events, the role of non-stratospheric influences on near-surface ozone should also be further discussed (e.g., regional pollution sources, role of temperature, regional atmospheric chemistry, deposition, and other factors contributing to the near surface high ozone events). How does the amplitude of SI-driven ozone anomalies compare to the variability of ozone from other processes?
I assume the role of ozone chemistry in the WRF-Chem runs is limited, since the key chemical species are fixed to climatological values. But do they still contribute, or are their effects negligible?
There was a short discussion about the regional pollution events for EP2/EP3, but not explained enough to help readers understanding the regional pollution and how Fig. S4 explains that (L245-256). I think it needs more detailed discussions.
The seasonal differences in background dynamics and their relative roles in the different cases, have been discussed briefly. However, seasonality can also influence the climatological O3 burden. How does the climatological O3 burden vary by season?
2. The EXPstro3 case as a reference case with non-SI events
The EXPstro3 experiment was used as a reference case, without the UBC scheme and thus without non-SI events. However, I’m concerned that a better reference case would be simulations run with the UBC scheme, focusing on periods without SI events, rather than simulations run without the UBC scheme.
Is there any difference in cross-tropopause ozone transport and background ozone concentration between 1) the Base experiment during periods without SI events, and 2) the EXPstro3 experiment in general?
3.Comparison with the observations
Fig. S3 provides important implications by demonstrating the verification of WRF-Chem using ESAC4 data. I recommend including it in the main figure set.
I think it’s somewhat unclear how the relative ozone concentration was calculated (Lines 234-239). I assume that the most of the Delta O3 values (and relative contributions) are inferred from the WRF-Chem simulations (as difference between the Base and EXP_stro3). What period was used to calculate each number in Line 238?
How do the WRF-Chem results compare with observed values? For example, have you derived observational anomalies as near-surface ozone during SI-affected periods minus those during reference periods? EP4 in the observations appears to show higher ozone levels over the SCB region than the WRF-Chem results, despite having relatively lower O3 concentrations in the eastern TP (Figs. S3d and 3h). Maybe lines 229–231 apply only to the WRF-Chem results, and not to the observations?
I think it would be helpful to include a figure like Fig. 4, but based on the observations. While the observational data may not have a zero reference value like in the model, it should still capture the relative increases in O3 associated with the SI events.
Also, what are the background ozone levels in observations for non-SI periods across each season? The seasonality of background ozone concentrations in observations requires further discussion.
4. Interpretations of Figure 7
The implications of Fig. 7 are quite difficult to understand. For example, point C in panel (a) appears to suggest that low-level transport near the surface is dominated by horizontal circulation anomalies. However, the budget at point C shows that ADVZ is the key component driving the positive ozone anomaly at low levels, while ADHZ provides a smaller negative contribution. Is the main purpose of Figure 7 to highlight the dominant component between ADVZ and ADHZ in driving the ozone anomalies? Also, is it possible to remove the background climatological values from both ADVZ and ADHZ to better isolate the anomaly-driven contributions only? (currently I see large portion of ADVZ and ADHZ cancels each other)
MINOR & TECHNICAL COMMENTS
The term “environmental atmosphere” has been used multiple times, but I am not sure whether it is commonly accepted terminology in atmospheric science. Do you mean “atmospheric composition”? I suggest that the authors double-check the usage of this term.
Lines 75-76: “residual-rich O3” -> residual O3-rich air? I think it would be helpful if this sentence could be clarified (lines 74-77).
Line 136: rich o3 of SI -> the O3-rich air from SI
Line 144: I suggest rewriting the sentence to something like, “The intensity and location of SIs are influenced by the distinct seasonality of vertical motion.”
Lines 145-147: While alternating rising and sinking momentum leads to a deep SI event over the central TP during EP1
-> Why is there any contrast between the updraft and downdraft regions in EP1 in Figure 2a? The vertical gradient of ozone appears to be fairly symmetrical.
Line 180: Does the simulated tropopause in WACCM exhibit a similar structure to observations, including features such as tropopause folding? It would be helpful if the authors could comment on how well the model captures these structures.
Lines 197-198: Delta O3basin=EXPterr - EXPstro3+terr
I’m confused that the difference between EXPterr minus EXPstro3+terr is used to investigate the deep-basin terrain forcing. Aren’t both simulations include the basin-filling terrain instead of the deep-basin terrain? Did you mean the comparison between delta O3basin and delta O3 (Base minus EXPstro3) can show the role of deep-basin forcing?
Line 219: It might be helpful to remind readers that Delta O3 refers to the difference in O3 between the Base and EXPstro3 runs.
Line 225: lower levels (approximately ~10 ppb) -> It appears that the SCB region is mostly green rather than yellow, suggesting O3 levels lower than ~10 ppb, maybe around ~6 ppb. What is the actual area-averaged value over the SCB?
Lines 244-247: It should be noteworthy that the SCB is experiencing regional O3 pollution on both 14–15 April and 1–2 July in 2017 during the SI episodes periods (Fig. S4), in which daily maximum 8-hour average O3 concentrations exceed 160 μg m-3.
->How large is the relative contribution of this regional O₃ pollution to the near surface delta O3?
Lines 247-248: O3 pollution aggravation in the SCB with an extra contribution approximately of 11.1–16.0%.
Maybe worth so clarify that these numbers are estimated based on the WRF-Chem simulations, not based on the observations.
Line 275: anticyclone -> anticyclonic
Line 289-290: Westerlies are gradually strengthened over the TP region controlled by subtropical jet from EP3 to EP4.
I find this sentence unclear. Does it mean that the westerlies are stronger in EP4 than in EP3, and that EP4 (January 2017) occurs after EP3 (October 2017)?
Line 293: seasonal typical atmospheric circulation patterns -> seasonal patterns of atmospheric circulation
I think this sentence (lines 293-294) also need further clarification.
Figure 7: Could you also mark the TP boundary like for the SCB region?
Lines 318-319: Maybe remind the readers that the vertical velocity is defined positive downward?
Line 330: vertical gradient of ozone (dO3/dp) is mentioned but the unit is for delta O3.
Line 355: I think the subtitle should be changed to include something about “the role of deep-basin structure” not general SI.
Line 356: It might be helpful for readers, if there is a short summarizing sentence explaining what is the delta O3basin experiment.
Lines 358–360: I'm not entirely convinced that the smoothed-terrain experiment is the most ideal case to represent a reference state in contrast to the realistic deep-basin structure. As the current basin-filling terrain doesn't necessarily represent the opposite of the realistic topography. But it's just one of many possible comparative configurations. Therefore, I recommend rephrasing to something like: “… mitigates regional O₃ pollution compared to the idealized experiments with basin-filling terrain” to clarify this point.
Citation: https://doi.org/10.5194/egusphere-2025-2628-RC2
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