the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 30 Dec 2025
Impact of northward tropical cyclones on ozone in Southeastern China
Abstract. Ozone pollution in Southeastern China (SEC) during autumn are usually influenced by tropical cyclones (TCs). Based on statistical analysis, WRF-CMAQ simulation, and TC vortex filtering method, this study explores the effects of the intensity and location of northward TCs on ozone in SEC. Results show that the interannual fluctuations of autumn maximum daily 8 hour average ozone from 2014 to 2024 are mainly affected by northward TCs with intensities reaching typhoon (TY) or above level. As TCs intensify to TY, the photochemical activity, horizontal transport, and vertical mixing in SEC all develop to levels favorable for pollution, which jointly lead to higher ozone compared to other TC intensities. Although ozone remains high when TC intensity exceeds TY, further enhancement is inhibited by meteorological constraints, as evidenced by simulations of two severe ozone pollution episodes with intensifying TCs. In addition, comparing with the sensitivity experiment without TCs reveal that intense solar radiation combined with peripheral northerly winds during the northward TCs period elevated ozone by over 10 ppb, with changes in biogenic emissions contributing approximately 1~3 ppb. Moreover, when TCs are located near 20°N, they primarily enhance ozone through photochemical production and horizontal transport, with impacts on the southern SEC persisting even after TCs make landfall at 30°N. Finally, it is important to recognize the non-linear interaction between TCs and large-scale circulation such as the western Pacific subtropical high, which modulates TC tracks and intensity and subsequently influences ozone levels in the SEC and even across the eastern China.
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Status: open (until 10 Feb 2026)
- RC1: 'Comment on egusphere-2025-5765', Anonymous Referee #1, 18 Jan 2026 reply
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RC2: 'Comment on egusphere-2025-5765', Anonymous Referee #2, 20 Jan 2026
reply
- Definition of “TC Day/Non-TC Day” and “TC influence” is unclear and not reproducible.
The manuscript compares ozone levels on “TC Days” versus “Non-TC Days” (e.g., Fig. 3a; around L154) and uses “periods of TC influence” to derive the 37% reduction applied to solar radiation and photolysis rates in NO_TC (Section 2.3). However, the criteria for classifying a day/time as “TC influence” are not explicitly stated. Please provide a clear, reproducible definition, including: (i) the spatial domain used to detect TC presence (e.g., west of 140°E only？); (ii) the temporal resolution (6-hourly samples vs daily aggregation) and how multiple TCs in one day are handled; (iii) whether post-landfall residual circulation days are counted as TC days; and (iv) objective criteria for the three track types (westward/landfalling/northward). In addition, please explicitly define the spatial boundary of SEC (lat/lon box or a land mask) in the Methods and show it in the figures (at least Fig. 3).
- The linkage between the multi-year statistical hypothesis and the two-case “validation” is incomplete due to missing mechanistic diagnostics in the case simulations.
The study first uses long-term statistics (Figs. 4–5; box/violin plots) to propose a mechanistic framework (radiation/photochemistry, boundary-layer mixing, transport, and wet processes). However, the subsequent case-study simulations (Figs. 10–12 and related discussion) mainly show ozone differences (and limited wind/cross-section information), while key variables invoked by the mechanistic explanations are not shown, such as SSRD/photolysis rates, cloud/precipitation, wet/dry deposition, precursor/NOy transport, and/or process terms. Consequently, the case simulations cannot robustly support the mechanism inferred from the long-term statistical signals (Figs. 4–5), leaving the key mechanistic claims insufficiently justified.
For example, L234–236 states that under STY–SSTY “enhanced wet scavenging begins to outweigh” despite generally favorable photochemical conditions. Yet Fig. 6(d–f) shows predominantly negative RH anomalies over southern China/SEC; RH anomalies alone cannot demonstrate enhanced wet scavenging, and the phrase “wet scavenging of ozone” itself is contentious without quantitative evidence. Similar issues apply to the explains of Figs. 10–12 and Table 2, where only ozone changes are shown but the interpretation relies on unshown changes in radiation, precipitation/deposition, and precursor transport.
- The STY–SSTY “wet scavenging” explanation needs stronger evidence and more precise wording.
The manuscript attributes the limited ozone increase beyond TY primarily to “enhanced wet scavenging/wet deposition” (e.g., around L215 and later). This statement will likely be questioned, because ozone is not typically removed efficiently by precipitation compared with soluble reservoir species, and the current evidence is indirect. Please clarify whether the inhibition under STY–SSTY is due to (a) reduced photolysis under enhanced cloudiness/precipitation, (b) enhanced ventilation/dilution and boundary-layer structural changes, and/or (c) wet deposition of precursors/reservoir species (NOy, peroxides, etc.) that reduces ozone production potential. Quantitative support (precipitation/cloud/photolysis and deposition diagnostics) is needed.
- The 37% reduction of radiation/photolysis in NO_TC is conceptually problematic for attributing “TC impacts” and needs major clarification and/or additional controls.
If the goal is to quantify the impact of the TC vortex itself, then in a vortex-removal experiment the radiation and photolysis fields should be allowed to evolve self-consistently after spin-up, reflecting “no TC vortex under the same large-scale background.” Instead, the authors impose a uniform 37% reduction in solar radiation and photolysis rates in NO_TC based on a climatological comparison between “TC-influence periods” and “periods without northward TCs.” This procedure potentially mixes the TC-vortex effect with differences in background synoptic regimes (e.g., WPSH configuration, cloudiness/moisture background) between TC and non-TC composites, thereby confounding attribution. In other words, NO_TC becomes a hybrid experiment (“vortex removal” + “forcing a non-TC radiative regime”), rather than a clean removal of TC effects under the same environment.
At minimum, the authors should (i) explicitly document how the 37% scaling is implemented (which variable, which hours, and which spatial mask) and whether it is consistent with the region/time window used to derive 37%; (ii) provide spatial maps of SSRD/photolysis differences before/after scaling; and (iii) add at least one additional control to separate dynamical vs radiative effects (e.g., a “vortex-only removal” without radiation adjustment. Without this, the reported ozone reduction (e.g., >10 ppb) cannot be interpreted as a pure “TC impact.”
- Inconsistent TC modification between IT_TC and NO_TC requires justification and validation.
Table 1 indicates that IT_TC strengthens only the TC-vortex horizontal winds (U, V, U10, V10), whereas NO_TC removes the TC vortex from a broader set of variables (including T, H, RH, ps, etc.). The authors should explicitly explain why the “intensification” and “removal” experiments are configured differently. Does this asymmetric design affect the comparability of the experiments and the interpretation of the results? Please provide reasons so that the subsequent mechanistic attribution is supported.
Figures comments:
1, Define the region of SEC in method part and show in Figure 3.
2, Give more details in Figure 5’s captions or make the labels larger in each subplot in Figure 5.
3, Subplots in Figure 12 are too small for comfortably reading.
Citation: https://doi.org/10.5194/egusphere-2025-5765-RC2
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- 1
This manuscript quantitatively evaluates the influence of the intensity and location of northward tropical cyclones (TCs) on ozone in Southeastern China, by using a vortex filtering method in combination with WRF-CMAQ simulations. The work falls well within the scope of ACP. The findings are novel and potentially impactful, providing important insights into the studies on TC-induced ozone pollution. In my view, there are some minor modifications needed to be committed before the publication.
Line 40: Please define the geographical range of the SEC.
Figure 2: Please add the latitude and longitude information.
Section 3.1.1: Ozone concentration during westward TCs also frequently exceeds 50 ppb. However, the analysis mainly focuses on northward TCs, with little discussion of westward TCs. Could you provide further explanation?
Figure 3: It presents the time series of MDA8 ozone concentration and classifies TC tracks into three different types. However, the interannual variability in ozone for each TC type is not clearly shown or discussed. Additional analysis would be helpful.
Lines 164 - 166: The description of “When these TCs make landfall between 18-18.8°N subsequently, the ozone concentration in SEC remains at a relatively high level.” appears inconsistent with Figure 3b, which shows relatively high ozone during TC landfall over a broader latitude range (12~18°N). Please revise accordingly. In addition, the explanation “due to strong winds and precipitation” is not directly supported by the figure and should be either justified or removed.
Figure 4: Landfalling TCs are generally expected to reduce ozone through strong winds and enhanced precipitation. Why does the mean ozone concentration during landfalling TC periods remain higher than on days without TCs (Figure 4b)?
Line 304: The caption for Figure 10 is unclear and potentially misleading. Please revise it for clarity.