the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 12 Sep 2025
Response relationship between atmospheric O3 and its precursors in Beijing based on smog chamber simulation and a revised MCM model
Abstract. Ozone (O3) pollution has been receiving increasing attention, but its simulation performance in models remains unsatisfactory. This study characterized the response relationship between O3 and its precursors in the atmospheric relavant condition through a combination of smog chamber experiments and Master Chemical Mechanism (MCM) box model. By adding chamber wall related reaction mechanisms, the model achieved significant improvement in simulating O3 with an Normalized Mean Bias (NMB) value changing from -76.1 % to -12.7 %. The enhanced model was subsequently extended to the ambient atmospheric conditions in the Daxing District of Beijing, incorporating the parameterization of ground related reactions, heterogeneous reactions of Nitrogen Dioxide (NO2), and unidentified NO2 sinks. Compared to basic model, the resulting revised model demonstrated substantially enhanced accuracy in simulating ambient O3 concentrations with an Normalized Mean Bias (NMB) value changing from 113.8 % to -5.2 % and enhanced O3 formation sensitivity to Volatile Organic Compounds (VOCs) in Daxing District. These findings underscore that incorporating interface mediated chemical processes and accounting for unidentified NO2 sinks into model is critical for determining the sensitivity of O3 formation and optimizing regional O3 pollution control strategies.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2025-3956', Anonymous Referee #1, 02 Oct 2025
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RC2: 'Comment on egusphere-2025-3956', Anonymous Referee #2, 26 Nov 2025
This manuscript outlines attempts to improve the modelling of O3 concentrations, and the production of NOx-VOC isopleths, by box modelling simulations. The paper has two halves. The first half describes a series of chamber experiments in which a chamber-specific mechanism is produced to improve O3 concentration predictions. The second half attempts to use the developed chamber-specific mechanism to account for model biases in a box model of the ambient environment, suggesting that heterogeneous reactions could account for over-predictions in ozone concentrations from the base simulation.
The experimental procedures for the chamber experiments seem sound, and while there is some room for improvement in the explanation of the method by which the chamber-specific mechanism rates are selected (as explained later), the produced mechanism appears to be in line with existing literature and reproduces the observations well.
However, I believe that there are numerous issues with the implementation of the ambient box models, including the author’s focus on NO2 uptake as the sole explanation for model discrepancies. Some of these issues may be clarified with additional context in the explanation, but others may require deeper consideration. I have organised my comments in the attached document into 4 sections: NO2 Sink, General Comments, Specific Comments, and Minor Comments. I do believe there is value in efforts to improve O3 predictions and that the potential role of NO2 uptake is an important consideration, and I hope that addressing these comments can provide the improvements in the manuscript that I believe are necessary for publication.
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RC3: 'Comment on egusphere-2025-3956', Anonymous Referee #3, 09 Dec 2025
This manuscript presents the relationship between O3 and its precursors using smog chamber experiments and a revised MCM box model. The authors improve O3 simulation by accounting for chamber wall effects under experimental conditions and unidentified NO2 sinks under ambient conditions, highlighting the sensitivity of O3 formation to VOCs and the implications for mitigation strategies in Daxing, Beijing.
However, the mechanisms related to chamber wall effects appear to have negligible influence when applied to real atmospheric conditions, whereas the unidentified NO2 sinks required for model-measurement agreement are unrealistically large. As a result, the revisions offer limited insights for ambient applications. Moreover, the simplified mechanisms used in the box model do not adequately represent interactions with meteorology or emissions. Therefore, I do not recommend the publication of this paper in ACP.
Major Comments
- The revised model (O3 SVR) shows better agreement with chamber results than the base model, primarily due to the introduction of OH generation associated with chamber wall effects. While such a mechanism may be justified within a chamber, there is minimal physical basis for applying this wall-induced radical source to ambient atmospheric conditions. Ground-related reactions on atmospheric chemistry do not mimic chamber wall reactions, and extending this mechanism to the atmosphere is inappropriate.
- Reaction rate constants associated with wall effects in Table 2 are key parameters, but their optimization process is insufficiently described (Page 6, Line 159; Page 7, Line 162 and Line 167). For example, how the reported 𝐽NO2 = 0.0015 ppbv s−1 (range 0.00075-0.0030 ppbv s⁻¹) in Angove et al. is converted to 1.2 × 106 molecule cm−3 s−1 in this study (reaction 1: hν + wall → OH).
The optimized rate constants may be chamber-specific. A controlled validation experiment using a small, clean plastic chamber (e.g., 1 m3) under similar conditions is therefore recommended. Is the revised model, with these parameters, able to reproduce results from such test-chamber conditions?
Upper and lower limits for relevant parameters should also be provided, and a sensitivity analysis is recommended to assess how uncertainties in each parameter affect model-measurement discrepancies.
- Other experiments beyond Iso&Tol02 and Iso&Tol04 should be briefly described in the Supplement, including their experimental design and key results beyond simply displaying NMB values (Page 8, Line 185).
- The authors introduced a constant NO2 sink to correct the model-measurement bias (Page 12, Line 270). However, the magnitude of this sink lacks physical justification and may simply force agreement for the wrong reasons. The authors should discuss alternative explanations for the observed bias beyond invoking an NO2 uptake, and evaluate whether other processes could plausibly account for the discrepancy.
- The attribution of O3 overestimation in the SVR NO2-sink model on 16-17 Aug to prevailing winds and associated air mass changes (Page 12, Line 277) requires further validation. The model overestimates O3 continuously from late afternoon on Aug 15 through early morning Aug 16, during which wind speed decreased by more than a factor of four. Statistical analysis is needed to support the proposed explanation.
Minor Comments
- Subtitles for each section should be more concise.
- Table 1. Please explain why NOx,0 is even lower than NO0 under some experimental conditions?
- Please clarify what J1-J56 correspond to in Table S1, and explain why J4 is selected in AtChem2-MCM in Table S2 (Supplement Page 6).
- Units should be provided for all variables in equations (1) and (2).
- Please discuss whether effects other than wall loss may contribute to model-measurement discrepancies (Page 6, Line 144).
- Figure 1. Please explain why, despite the large base-model/measurement differences for both Iso&Tol02 and Iso&Tol04 throughout the process, the base-model/measurement difference at 6 h is much smaller for Iso&Tol04 than for Iso&Tol02?
- Figure 2 and Figure 4. Using a consistent colorbar range across subplots is recommended to improve comparability.
- Table 3. Please clarify whether 0.007 s⁻¹ represents 𝐽NO2 at noon and provide full definitions for kgn, kan, Sa, and other variables.
Technical Corrections
- Grammar issues should be corrected, including “the complex of atmospheric conditions” (Page 2, Line 39), “studying how secondary pollutants like O3 formation” (Page 2, Line 48), and “details information” (Page 5, Line 116).
Citation: https://doi.org/10.5194/egusphere-2025-3956-RC3
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This manuscript presents a well-structured and scientifically rigorous study that combines smog chamber experiments with a revised MCM box model to improve the simulation of O3 formation and its sensitivity to precursors. The work is highly relevant to current air quality challenges, particularly in regions like Beijing suffering from severe O3 pollution. The methodological approach is sound, the results are clearly presented, and the conclusions are well-supported by the data. The inclusion of chamber wall effects and unidentified NO2 sinks represents a valuable contribution to the field. I recommend acceptance after minor revisions.
General comments
Technical comments