the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 18 Aug 2026
River-induced circulations alter ozone distribution, source contributions, and chemical sensitivity in a river–valley city along the Yangtze River
Abstract. Ground-level ozone (O3) pollution in river–valley cities is influenced by interactions among photochemistry, boundary-layer processes, and terrain-induced circulations, yet their combined effects on O3 distributions remain insufficiently understood. Using WRF–CAMx coupled with source apportionment and process analysis, we investigated the three-dimensional distribution, source contributions, and formation characteristics of O3 over the Nanjing section of the Yangtze River. The altitude of the O3 maximum decreased from ~5.0 km in winter to ~2.0 km in summer, indicating stronger near-surface photochemical production and boundary-layer processes during warm seasons. Distinct spatial heterogeneity was observed between the river corridor and adjacent urban areas. Daytime near-surface O3 development was weaker over the river in summer, whereas the high-O3 layer extended downward to greater depths in spring and autumn. Regional background transport accounted for most total O3, while suburban contributions increased during high-O3 episodes, reaching 45.5 % under O3 concentrations of at least 160 µg/m3 in July. Transport and diffusion associated with river-breeze circulations played important roles in regulating O3 variability within the river corridor. O3 formation remained volatile organic compound (VOC)-limited in spring and autumn, whereas river-breeze-induced nitrogen oxide (NOx) dilution shifted the river corridor to NOx-limited conditions approximately two hours earlier than in the surrounding urban area in summer. These findings demonstrate that river-induced circulations can substantially modify O3 distributions, source contributions, and chemical sensitivity within river–valley cities, and that city-averaged and monthly mean characterizations may not fully capture local O3 variability in complex terrain.
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RC1: 'Comment on egusphere-2026-4158', Anonymous Referee #1, 27 Aug 2026
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AC1: 'Reply on RC1', Min Shao, 04 Sep 2026
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We sincerely thank the reviewer for the careful reading of our manuscript and for the constructive comments. We have carefully considered all the suggestions and revised the relevant text accordingly. Since a revised manuscript cannot be uploaded at the open-discussion stage, the corresponding revisions are provided explicitly below for clarity.
1) The authors refer several times to their previous studies for model configuration, meteorological evaluation, and local circulation characteristics. It would be helpful to clarify more explicitly what analyses and findings are newly presented in the current manuscript compared with Shao et al. (2024a, b), so that the novelty of this study can be more clearly identified.
Response:
Thank you for this helpful comment. Our previous studies mainly focused on the effects of precursor emission reductions on PM2.5 and O3 (Shao et al., 2024a) and the role of local thermal circulations in PM2.5 formation in the lower Yangtze River valley (Shao et al., 2024b). In contrast, the present study focuses specifically on the three-dimensional behavior of O3 by integrating vertical-profile analysis, source apportionment, process analysis, and chemical sensitivity diagnosis.
To clarify this distinction, we have added the following sentence at the end of the penultimate paragraph of the Introduction, immediately before the paragraph beginning with “To better understand…”:
“Building on our previous studies of emission-control effects (Shao et al., 2024a) and local thermal circulations in PM2.5 formation (Shao et al., 2024b), the present study extends the existing modeling framework to investigate the three-dimensional O3 structure, source contributions, process budgets, and chemical sensitivity.”
2) In Section 3.4, the high contribution attributed to Region 8 is interpreted as reflecting favorable ozone retention and enhanced daytime ozone formation. Since source apportionment mainly identifies the contribution associated with tagged source regions, the distinction among source attribution, transport, retention, and local chemical production should be described more carefully. Please clarify the interpretation of the Region 8 contribution and moderate the wording where necessary.Response:
We appreciate this important clarification. We agree that the source-apportionment contribution attributed to Region 8 should not be interpreted directly as local O3 retention or local chemical production. In the CAMx source-apportionment analysis, the Region 8 contribution represents the fraction of O3 attributed to precursor emissions originating from the river-corridor source region. Its magnitude may be affected by chemical production, transport, and meteorological redistribution, and source apportionment alone cannot quantitatively separate these processes.
We have therefore revised the relevant discussion in Section 3.4 to distinguish source attribution from the mechanistic interpretation provided by the subsequent process analysis. The revised text reads:
“Within this local fraction, the highest contribution from Region 8 occurs in April, exceeding 60%. It should be noted that the source apportionment result represents O3 attributed to precursor emissions originating from Region 8 and therefore should not be interpreted solely as local O3 production or retention. Instead, the relatively high contribution reflects the combined effects of precursor emissions, atmospheric transport, chemical transformation, and local meteorological conditions within the river corridor. At 14:00 LST, the contribution attributed to Region 8 increases further, reaching 65.2% in April and 59.7% in July, indicating an enhanced relative importance of precursors originating from the river-corridor region during the daytime photochemically active period.”
To maintain consistent terminology later in this section, we have also revised the related interpretation as follows:
“During the daytime, the contribution attributed to Region 8 itself is the largest, reaching 31.5% in January and 60.1% in July, indicating a greater relative contribution from sources within the river-corridor region, with cross-bank contributions from surrounding regions accounting for a correspondingly smaller share”and“These patterns suggest that thermally driven local circulations modulate the diurnal partitioning between contributions from the river-corridor source region and transport from the regions along both banks, thereby shaping the source composition of O3 within the Yangtze River corridor”.
3) The definition of the high-O3 conditions in Figure 6 could be clarified. Section 3.1 indicates that observations from nine monitoring stations were used for model evaluation, while Figure 6 refers to hours when the observed O3 concentration exceeded 120, 160, or 200 μg m-3. Please briefly specify how the observational concentration used for this screening was derived from the nine stations (e.g., citywide mean), and confirm that these thresholds refer to hourly O3 concentrations.Response:
Thank you for pointing out this ambiguity. In this study, hourly O3 concentrations from the nine monitoring stations in Nanjing were first averaged to obtain the citywide mean hourly O₃ concentration. Hours with mean observed O3 concentrations ≥120, ≥160, and ≥200 μg/m3 were then identified, and the corresponding modeled source contributions of the identified hours were averaged for each category.
We have clarified this procedure in Section 3.4 by adding the following description:
“High-O3 periods were identified based on the citywide mean hourly observed O3 concentration averaged across the nine monitoring stations. Hours with mean O3 concentrations ≥120, ≥160, and ≥200 μg/m3 were selected separately, and the corresponding modeled source contributions were averaged for each category.”
The caption of Figure 6 has also been revised as follows:
“Figure 6. Monthly mean proportions (%) of local source contributions to O3 along the Yangtze River (Region 8) under different temporal and pollution conditions. Daytime is defined as 08:00-18:59 LST, while nighttime includes 00:00-07:59 and 19:00-23:59 LST. ‘14:00’ represents the monthly mean source-contribution proportions at 14:00 LST. ≥120, ≥160, and ≥200 represent the mean source-contribution proportions during hours when the citywide mean hourly observed O3 concentration, averaged across the nine monitoring stations, was ≥120, ≥160, and ≥200 μg /m3, respectively.”
4) The earlier transition to NOx-limited conditions in the river corridor is attributed to river-breeze-induced NOx dilution. The temporal pattern is consistent with this interpretation, but the causal wording may be slightly stronger than the evidence directly presented. I suggest briefly clarifying how the onset and weakening of the river breeze were identified, or alternatively moderating the wording to “associated with” or “consistent with” river-breeze-induced NOx dilution.Response:
We agree with the reviewer that the original wording may imply stronger causality than can be established from the present analysis alone. We have therefore adopted the latter suggestion and moderated the causal language. The temporal correspondence between the sensitivity transition and the development of the river-breeze circulation is now described as being consistent with or associated with river-breeze-induced NOx dilution, while acknowledging that other concurrent meteorological and chemical processes may also contribute.
The relevant discussion in Section 3.5 has been revised as follows:
“…approximately two hours later at 11:00 LST. This earlier and stronger sensitivity shift in Region 8 is consistent with the development of the Yangtze River breeze. The river breeze may dilute NOx concentrations along the river corridor, thereby favoring an earlier shift of the VOC/NOx balance toward NOx-limited conditions compared with the broader urban area. The subsequent convergence of the two curves after ~18:00 LST coincides with the weakening of the river breeze, further suggesting an association between local circulation and the sensitivity transition. ”
For consistency, the corresponding statement in the Abstract has been revised to:
“O3 formation remained volatile organic compound (VOC)-limited in spring and autumn, whereas an earlier shift to nitrogen oxide (NOx)-limited conditions in the river corridor, approximately two hours ahead of the surrounding urban area, was consistent with river-breeze-induced NOx dilution in summer.”
5) There is a minor inconsistency in the discussion of ozone sensitivity. January is excluded from the CPA diagnosis because photochemical activity is negligible, but the Conclusions state that ozone production is “predominantly VOC-limited throughout the year.” This statement should be revised to distinguish the diagnosed VOC-limited conditions in spring and autumn from the weak wintertime photochemical activity and the summer transition toward NOx-limited conditions.Response:
Thank you for identifying this inconsistency. We agree that the phrase “throughout the year” is not appropriate because the CPA indicator is not informative in January due to negligible photochemical activity. We have therefore revised the summary of the CPA results and the corresponding statement in the Conclusions to distinguish the VOC-limited conditions diagnosed in April and October, the summer transition toward NOx limitation, and the limited applicability of the CPA indicator in January.
The summary paragraph at the end of Section 3.5 now reads:
“Overall, O3 formation remained predominantly VOC-limited in April and October, whereas a transition toward NOx-limited conditions occurred in July, particularly over the Yangtze River corridor. In January, weak photochemical activity limited the applicability of the CPA indicator for a meaningful sensitivity diagnosis. These findings highlight the importance of VOC control during photochemically active periods and coordinated VOC–NOx reductions in summer, while accounting for the spatiotemporal effects of local meteorological processes.”
The corresponding paragraph in the Conclusions has been revised to:
“CPA-based sensitivity analysis indicates that O3 formation is predominantly VOC-limited in spring and autumn….”
6) A careful language and terminology check is recommended. For example, the first sentence of the Introduction contains a grammatical error, and some expressions in the discussion of Figure 5 are repetitive. The section titles “O3 transport revealed through source apportionment technology” and “O3 formation revealed through process analysis technology” could also be simplified to more conventional wording.Response:
We have performed an additional language and terminology check throughout the manuscript and corrected the grammatical and repetitive expressions noted by the reviewer. We have also simplified the titles of Sections 3.4 and 3.5.
Specifically, the first sentence of the Introduction has been revised to:
“Air pollution episodes arise from complex interactions between intensive anthropogenic emissions and meteorological conditions and can adversely affect ecosystems and human health.”
The repetitive expression in the discussion of Figure 5 has been revised to:
“Comparing Figures 5(c) and 5(d) highlights the contrasting roles of different local forcing mechanisms at the same local noon: the longitudinal transect (5c) reveals pronounced spatial heterogeneity associated with industrial emissions, localized convective uplift over Baguazhou, and terrain-induced upslope transport near Mt. Zijin, whereas the latitudinal transect (5d) features a more horizontally uniform elevated O3 layer associated with strong convective mixing over the urban core.”
The section titles have been revised to:
“3.4 Source apportionment of O3” and “3.5 Process analysis of O3 formation”
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AC1: 'Reply on RC1', Min Shao, 04 Sep 2026
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Summary
The manuscript investigates the three-dimensional distribution, source contributions, and chemical sensitivity of ozone over the Nanjing section of the Yangtze River using the WRF–CAMx modeling system combined with source apportionment and process analysis. The topic is relevant to understanding ozone pollution in complex river–valley environments, and the manuscript provides useful insights into the roles of boundary-layer dynamics, local circulations, regional transport, and photochemistry. Overall, the manuscript is well organized, and the analyses support the main conclusions. The following comments should be addressed to improve the manuscript's clarity and interpretation.
Specific comments:
1) The authors refer several times to their previous studies for model configuration, meteorological evaluation, and local circulation characteristics. It would be helpful to clarify more explicitly what analyses and findings are newly presented in the current manuscript compared with Shao et al. (2024a, b), so that the novelty of this study can be more clearly identified.
2) In Section 3.4, the high contribution attributed to Region 8 is interpreted as reflecting favorable ozone retention and enhanced daytime ozone formation. Since source apportionment mainly identifies the contribution associated with tagged source regions, the distinction among source attribution, transport, retention, and local chemical production should be described more carefully. Please clarify the interpretation of the Region 8 contribution and moderate the wording where necessary.
3) The definition of the high-O3 conditions in Figure 6 could be clarified. Section 3.1 indicates that observations from nine monitoring stations were used for model evaluation, while Figure 6 refers to hours when the observed O3 concentration exceeded 120, 160, or 200 μg m-3. Please briefly specify how the observational concentration used for this screening was derived from the nine stations (e.g., citywide mean), and confirm that these thresholds refer to hourly O3 concentrations.
4) The earlier transition to NOx-limited conditions in the river corridor is attributed to river-breeze-induced NOx dilution. The temporal pattern is consistent with this interpretation, but the causal wording may be slightly stronger than the evidence directly presented. I suggest briefly clarifying how the onset and weakening of the river breeze were identified, or alternatively moderating the wording to “associated with” or “consistent with” river-breeze-induced NOx dilution.
5) There is a minor inconsistency in the discussion of ozone sensitivity. January is excluded from the CPA diagnosis because photochemical activity is negligible, but the Conclusions state that ozone production is “predominantly VOC-limited throughout the year.” This statement should be revised to distinguish the diagnosed VOC-limited conditions in spring and autumn from the weak wintertime photochemical activity and the summer transition toward NOx-limited conditions.
6) A careful language and terminology check is recommended. For example, the first sentence of the Introduction contains a grammatical error, and some expressions in the discussion of Figure 5 are repetitive. The section titles “O3 transport revealed through source apportionment technology” and “O3 formation revealed through process analysis technology” could also be simplified to more conventional wording.