the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 14 Jul 2026
Contribution of stratospheric intrusion to tropospheric hydroxyl radical
Abstract. The hydroxyl radical (OH) is a key species that participates in many oxidation processes in the troposphere. Previous studies have reveal significant impact of lightning and air pollution on interannual variabilities and long-term trends of tropospheric OH. The abundance of OH in the troposphere is influenced by a multitude of factors, including ozone, water vapour, and ultraviolet radiation, among others. The tropospheric ozone is significantly influenced by the presence of stratospheric intrusion. It is hypothesized that the intrusion process will also exert an influence on tropospheric OH. In this study, the contribution of stratospheric intrusion to tropospheric OH is estimated through the use of a climate chemistry model. The findings suggest that the contribution of the stratosphere to tropospheric OH is most significant over the subtropical ocean regions (reaching 2×10⁻² ppt) and over the Tibetan Plateau in the summer (reaching 3×10⁻² ppt). The relative contribution of stratospheric intrusion can reach 12 % over the Tibetan Plateau, northern South America and South Africa, and 20 % over the subtropical oceans. It is expected that stratospheric contribution will increase in the future due to enhanced stratosphere troposphere exchange and stratospheric ozone recovery.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2026-3817', Anonymous Referee #1, 04 Aug 2026
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RC2: 'Comment on egusphere-2026-3817', Anonymous Referee #2, 24 Aug 2026
The manuscript investigates the influence of stratospheric processes on tropospheric OH. The authors apply a chemistry-climate model with an ozone tagging approach and a perturbation experiment to quantify the contribution of stratospheric ozone to tropospheric OH. The manuscript presents an interesting and potentially valuable approach. However, revisions are needed to strengthen the quantitative interpretation and provide more detailed chemical mechanism analyses. I recommend publication after major revision.
Major comments
(1) The current approach quantifies the impact from stratospheric ozone intrusion, rather than the full influence of stratospheric intrusion processes. Other chemically relevant stratospheric transported species (e.g., NOy) are not considered. I suggest revising the manuscript title and relevant descriptions accordingly. For example, “stratospheric ozone intrusion contribution” would more accurately describe the calculated quantity than “stratospheric intrusion contribution.”
(2) The manuscript would benefit from a more quantitative analysis of OH chemical budgets, including the relative contributions of OH production and loss pathways. At present, many interpretations are based on qualitative comparisons among StrOH, ozone, humidity, and radiation. However, these relationships alone are insufficient to establish the dominant controlling mechanisms. Additional diagnostics, such as OH production/loss budgets would strengthen the conclusions.
Specific comments
(1) Line 214: The authors calculate mass-weighted tropospheric OH concentrations, while Figure 2 presents volume-weighted OH concentrations. I recommend applying the same weighting method for both model evaluation and subsequent analyses.
(2) Line 233: The authors attribute the secondary maximum of OH below the tropical tropopause in EAC4 to lightning NOx emissions. The authors should provide the corresponding vertical distribution of lightning NOx emissions or NOx concentrations to support this explanation.
(3) Before discussing seasonal variations of StrOH, the authors should first quantify the overall contribution of stratospheric ozone to global tropospheric OH. This information is essential for evaluating the importance of this process for atmospheric oxidation capacity and the lifetime of short-lived greenhouse gases.
(4) Line 237: Why was 400 hPa selected for the analysis? Besides the analyses at 400 hPa and the surface, the impact on tropospheric mean OH would be more relevant for atmospheric oxidation studies. In addition, the authors present StrOH in ppt, which makes it difficult to evaluate its relative importance compared with background OH concentrations. I suggest presenting StrOH in units of molecules cm-3 and the relative contribution (%) of StrOH to total OH, which are more commonly used quantities in OH studies.
(5) Line 245: The spatial and temporal distributions of StrO3 should also be presented. This would help clarify the pathway through which stratospheric ozone transport influences tropospheric OH.
(6) Line 246: The discussion of factors controlling StrOH variations is currently insufficiently supported. For example, the authors discuss the roles of solar radiation and humidity, but quantitative evidence is lacking. Additional model diagnostics, such as OH chemical budgets, should be provided to support these conclusions.
(7) The negative StrOH contribution is interesting. The authors attribute this effect mainly to the OH loss reaction of OH +O3. However, more evidence is needed to support this conclusion. For example, the authors should compare OH production from ozone photolysis with OH loss through O3 reactions. A comparison of OH chemical budgets between the original and perturbed simulations would also strengthen this discussion.
(8) Figure 3: The meaning of the black contour lines is not clearly described. Please clarify.
(9) Section 3.3: The selection criteria for the four analyzed regions (Tibetan Plateau, subtropical ocean, northern South America, and South Africa) should be explained. Currently, this section mainly presents seasonal cycles for individual regions, but the broader implications remain unclear. For example, what general conclusions can be obtained from these regional analyses? Are different regions controlled by different chemical regimes? What are the implications for regional oxidation capacity? Additional quantitative analyses are needed to identify the dominant controlling factors.
(10) Line 305: The authors state that the relative contribution of stratospheric intrusion could be doubled if EAC4 and CESM2-WACCM OH values are used as references. I do not think this inference is justified. The higher OH concentrations in these datasets may result from differences in other factors, such as surface emissions, chemical mechanisms, and model parameterizations. Since the authors did not calculate StrOH using these models, they cannot directly infer that the stratospheric contribution would be doubled. This statement should be removed or revised.
Citation: https://doi.org/10.5194/egusphere-2026-3817-RC2
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