Photochemical mechanism&ndash;dependent ozone formation and precursor sensitivity under varying NOₓ conditions

Liu, Yuelin; Gong, Yuanjun; Wang, Nan; Shi, Guangming; Yang, Fumo; Zhang, Yuanhang; Lu, Keding

doi:10.5194/egusphere-2026-1786

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https://doi.org/10.5194/egusphere-2026-1786

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30 Apr 2026

| 30 Apr 2026

Photochemical mechanism–dependent ozone formation and precursor sensitivity under varying NOₓ conditions

Yuelin Liu, Yuanjun Gong, Nan Wang, Guangming Shi, Fumo Yang, Yuanhang Zhang, and Keding Lu

Abstract. Photochemical mechanisms are core components of air quality models, yet differences among them introduce substantial uncertainty in ozone (O₃) simulations. Here, we systematically evaluate three widely used mechanisms (CB06, SAPRC07, and RACM2) using the CMAQ model during an O₃ pollution episode in Chengdu, China. To ensure consistency, a customized volatile organic compounds emission inventory was developed for RACM2. The results show pronounced inter-mechanistic differences in simulated O₃, radical chemistry, and precursor sensitivities under distinct NO_x conditions. SAPRC07 and RACM2 produced higher and comparable O₃ levels (~95 ppbv) with better performance in urban areas, whereas CB06 simulated lower O₃ (~80 ppbv) and performed better under low-NOₓ conditions. These differences are linked to variations in radical concentrations and reaction pathways, with RACM2 yielding the highest OH levels, SAPRC07 better capturing HO₂, and CB06 simulating lower RO₂. Despite these discrepancies, all mechanisms consistently reproduced the dominant O₃ formation pathway, with RO₂+NO contributing over 50 % of net production. However, radical sources and termination pathways differed substantially between urban and suburban environments, reflecting regime-dependent chemistry. Sensitivity analysis further shows that anthropogenic aromatics and alkenes dominate urban O₃ formation, whereas biogenic emissions dominate in suburban areas. Notably, O₃ sensitivity to biogenic emissions is higher in urban high-NO_x conditions, highlighting an amplified biogenic contribution. These findings demonstrate that mechanism choice fundamentally affects O₃ formation and precursor sensitivities, emphasizing the need for multi-mechanism approaches to improve model reliability and support effective emission control strategies.

Received: 30 Mar 2026 – Discussion started: 30 Apr 2026

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Yuelin Liu, Yuanjun Gong, Nan Wang, Guangming Shi, Fumo Yang, Yuanhang Zhang, and Keding Lu

Status: final response (author comments only)

RC1:
'Comment on egusphere-2026-1786', Anonymous Referee #1, 20 Jul 2026
General comments
This paper presents the comparison of three different chemical mechanisms (CB06, SAPRC07, RACM2) embedded in the CMAQ model frame focussing on ozone and its precursors. Model results are compared to trace gas and radical measurements conducted at an urban and a suburban site in Chengdu, China. Eventually, a budget analysis of ozone and ROx radicals and a sensitivity analysis of ozone to NOx and VOC emissions is presented.
Trends found in the ozone production and loss pathways are not unexpected. Differences in the chemical mechanisms arise mainly from variations in radical concentrations and mechanism-specific reaction pathways, which however are not discussed in detail.
Overall, this paper showcases the impact of using different chemical mechanisms in modelling air quality in China. Previous studies already investigated the intercomparibility of the different chemical mechanisms in different regions, however, to the reviewer’s knowledge there is no study in the Chengdu region yet. Even though, the presented data set is comparably small, it complements the previous studies and gives insights into the the sensitivity of air quality modelling in China to the chosen chemical mechanism.
Therfore, the reviewer recommends this paper to be considered for publication after major changes have been made.

Major comments
In the introduction, the atmospheric photochemistry is extensively described which rather fits in Section 2.4. A schematic of the NOx and ROx radical cycle may help the reader to follow more easily. In addition, the description can be shortened, as currently it rather reads like a textbook. It is striking that RO2 isomerisation was not mentioned at all. Since the paper is focused on photochemistry it should be stressed that ozone is mainly formed from the oxidation of VOCs by OH. In line 62 for example, it should be made clear that the OH oxidation of VOCs is an important source of primary ROx radicals, with OH being formed through the photolysis of O3 and HONO. Later in Section 3.3.2, the H2O2/HNO3 ratio is mentioned as a measure for the chemical regime which should be introduced in Section 2.4 as well.

In the conclusions, the overall comparison with results from previous studies, mentioned in the introduction, is missing. How does Chengdu compare with, e.g. Japan. There are more studies about the comparison of different chemical mechanisms in CMAQ, e.g.: Shareef et al. (Appl. Sci. 12, 8576 (2022)), Gupta and Mohan (Atmos. Env. 122, 220-229 (2015))

Overall, the impact of the different mapping strategies for the different chemical mechanisms on the results is not clear to the reader. Fig. 9 indirectly shows the different VOC distribution. Would it make sense to show this comparison in the beginning?

For the reader, it is challenging to connect all the results in the different subsections. In Section 3.2 it would help to interpret the results also based on the results presented in Section 3.2. Another example is Section 3.1.2 in which the impact of the strong overestimation of the measured OH radical concentration by the models is unclear. A too high simulated OH radical concentration leads to a faster VOC oxidation rate which may be the reason for or at least partly contributes to the underestimation of measured isoprene by the model.

Even though RO2 radical concentrations were not measured at the suburban site, the diurnal RO2 radical concentrations simulated by the different chemical mechanisms should be shown in Fig. 5 to get a full picture of the radical chemistry. It is unclear to the reader why measured and modelled HO2 radical concentrations agree very well but the OH radical concentration is so different. This hints towards different simulated HO2/RO2 ratios. What is the main difference in the RO2 chemistry of the three chemical mechanisms? Why does the SAPRC07 mechanism simulate a comparably high loss of ROx through HO2+RO2 (Fig. 8), even though RACM2 predicts similar HO2 and RO2 radical concentrations?

Details about meteorological parameters (temperature, relative humidity, jNO2) and instruments measuring the trace gases are missing in the site description (Section 2.1).

Technical corrections
Overall, the language should be improved. Prepositions and articles are often missing.

Line 13: It should be emphasised that surface/tropospheric ozone is simulated.

Line 17: The NOx conditions should be specified.

Lines 34-35: Ozone is typically not the main oxidant regulating atmospheric lifetimes. This of course depends on the boundary conditions. A large part of this paper focuses on the photochemical radical chemistry forming ozone, when the OH radical usually determines the removal of pollutants. The hydroxyl radical should be introduced as the main daytime oxidant in the troposphere driving the ozone formation during the day.

Line 41: NOx was not introduced yet.

Lines 47, 51: A verb is missing here: “In theory, once O3 is formed, …”

Line 48: It is misleading that R1 to R3 are mentioned as an example.

Line 91-92: The reactions are not complete or false.

Line 135: A quantitative comparison would be beneficial here.

Line 142: References are missing.

1a: The position of the XJ site is not visible as it has the same colour as the map. It would be consistent to plot the NOx values in b in the same colours as the locations in plot a.

Line 192: Add Fuchs et al. (Rev. Sci. Instrum. 8, 084104 (2008))

Line 234: References to the reactions help here.

Line 242: The fraction of RO2+NO forming organic nitrate should be excluded here.

Line 243: Only O1D+H2O forming 2OH should be considered in the first term.

Figure 3: The abbreviations should be introduced.

Figure 4: Is there a reason why the model-measurement comparison of NO is missing?

Line 337-338: It sounds like the HCHO photolysis is the main radical source which is not true. Naming the relative contribution to P(ROx) will clarify its role.

Figure 5: The columns are not labelled with the corresponding mechanism shown. Furthermore, it should be reminded that XJ is the suburban site. In the colour maps, it would be helpful to add the measurement point, similar to Fig. 2.
Citation: https://doi.org/10.5194/egusphere-2026-1786-RC1
RC2:
'Comment on egusphere-2026-1786', Anonymous Referee #2, 16 Aug 2026
This manuscript evaluates the dependence of simulated ozone formation and precursor sensitivity on three gas-phase chemical mechanisms (CB06, SAPRC07, and RACM2) using CMAQ during an ozone pollution episode in Chengdu. The topic is relevant to atmospheric chemistry and air-quality modeling and falls within the scope of ACP. A systematic comparison of mechanism-dependent ozone formation, radical chemistry, and precursor sensitivity under contrasting urban and suburban NOx conditions could potentially provide useful insight into chemical-mechanism uncertainty in regional air-quality simulations.
However, in its current form, the manuscript does not provide the scientific depth, methodological rigor, or level of mechanistic interpretation needed to realize this potential. Much of the Results and Discussion consists of restating values, rankings, and fractional contributions already shown in the figures, with limited analysis of why CB06, SAPRC07, and RACM2 produce different results. Conversely, several chemical explanations are speculative and are not supported by direct model diagnostics or quantitative evidence. Important methodological issues - including differences in the accompanying model configurations, evaluation of the customized RACM2 emission mapping, limited description of mechanism-specific chemical differences, incomplete measurement documentation, and the lack of meteorological model evaluation - further prevent a robust interpretation of the inter-mechanism comparison.
The manuscript also requires substantial reorganization and comprehensive English-language editing. Frequent grammatical errors, awkward or unclear expressions, repeated textbook-level explanations, and poorly structured paragraphs substantially impede readability and, in several places, obscure the intended scientific meaning. These concerns are not limited to isolated sections that could be addressed through routine revision; rather, they affect the analysis, interpretation, scientific contribution, and presentation of the manuscript as a whole. Although the subject is within the scope of ACP, I do not consider the manuscript suitable for publication in its present form. I therefore recommend rejection, while leaving open the possibility of a substantially redeveloped manuscript being considered as a new submission.
The detailed comments supporting this assessment are provided below.
General comments:
A major limitation of the manuscript is that much of the Results and Discussion remains descriptive rather than mechanistic. Many sections primarily restate values, rankings, or relative contributions shown in the figures, while the chemical reasons for the differences among CB06, SAPRC07, and RACM2 are not sufficiently investigated. Conversely, several mechanistic explanations are proposed speculatively without direct evidence from the simulations. The authors should provide quantitative support for their interpretations and more explicitly connect the observed differences to mechanism-specific features, including VOC lumping/speciation, reaction pathways and rate parameters, radical production and propagation, and RO2 composition and fate. Without such analysis, it is difficult to determine why the mechanisms behave differently or to assess the broader scientific significance of the inter-comparison.

The manuscript does not yet demonstrate that the reported differences arise specifically from the gas-phase mechanisms. The three simulations use different accompanying CMAQ configurations, including different aerosol modules, and RACM2 relies on a customized VOC emission mapping. The authors should quantitatively evaluate conservation of total VOC emissions, carbon mass, and VOC reactivity in the mapping, clarify the influence of the accompanying configurations, and provide a systematic comparison of relevant mechanism-specific chemistry. These details are necessary for interpreting differences in simulated radicals and ozone.

The manuscript requires substantial English-language editing and reorganization. The Introduction contains frequent grammatical errors, awkward sentence structures, incorrect article and verb usage, and expressions that obscure the intended meaning. Similar issues occur elsewhere. Several Results subsections also combine general findings, numerical description, and interpretation without clear paragraph structure. The technical corrections below identify examples but are not exhaustive. All authors should carefully review the entire manuscript, simplify overly long or repetitive passages, and obtain editing by a fluent scientific English speaker.

Specific comments:
Lines 45-93: The description of fundamental ozone and ROx-NOx chemistry is overly detailed for the Introduction, including numerous elementary reactions that constitute textbook knowledge. This section could be substantially condensed, allowing greater emphasis on how differences in VOC lumping, radical propagation, reaction rate coefficients, and termination pathways among CB06, SAPRC07, and RACM2 may influence modeled ozone formation and precursor sensitivity.

Reaction R2: NO is missing from the products of NO2 photolysis.

Reaction R9: The reaction as written represents only H abstraction. OH addition to alkenes or aromatics does not produce H2O. Please revise the generalized VOC + OH reaction accordingly.

Line 66: O3 photolysis does not directly produce OH; it first generates O(1D), which subsequently reacts with water vapor to form OH. Therefore, please change “…by O₃ photolysis at wavelengths below 320 nm (R6–R7) in the troposphere” to “…through O₃ photolysis at wavelengths below 320 nm (R6–R7), followed by the reaction of O(1D) with water vapor in the troposphere.

Lines 72-73: The statement that each complete ROx cycle converts two NO molecules to NO2 is too general. This stoichiometry applies only to an idealized propagation sequence involving successive RO2 + NO and HO2 + NO reactions, whereas actual VOC oxidation may follow competing pathways and produce different radical and ozone yields.

Lines 80-93 and Reaction R13: The description of high-NOx radical termination appears chemically inaccurate. Organic nitrates are generally formed through a branching channel of RO2 + NO, whereas reactions of RO2 with NO2 more commonly form peroxy nitrates or PAN-type reservoir species, depending on the RO2 structure. These products should not necessarily be treated as permanent radical termination because they can thermally decompose or otherwise recycle NOx and radicals. Please correct Reaction R13 and distinguish irreversible termination from temporary radical and NOx sequestration.

Lines 94-114: The discussion presents air-quality modeling primarily from the perspective of the chemical mechanism, making the model appear conceptually similar to a photochemical box model. Regional ozone simulations also depend strongly on emissions, meteorology, transport, mixing, deposition, boundary conditions, and spatial resolution. Please acknowledge these major sources of uncertainty and clarify that the chemical mechanism is one component of a broader modeling system. This context is important because the study aims to isolate mechanism-dependent differences.

Lines 97-98: The phrase 'O3 regional characteristics' is vague. Please clarify whether this refers to the spatial distribution, temporal variability, or regional formation processes of ozone.

Lines 152: It is unclear what is meant by stating that ozone and its precursors 'accumulate and recycle' in the local atmosphere. While accumulation under stagnant basin conditions is understandable, 'recycle' could refer either to meteorological recirculation or to chemical recycling of reactive species. Please clarify the intended process.

Figure 1b: Showing only mean concentrations is not sufficient to characterize the NOx conditions at the two sites. Please consider adding error bars, such as standard deviation, or replacing the bars with box-and-whisker plots to show measurement variability and distributions. NO2 should also be typeset with a subscript in the panel.

Lines 185-191: The manuscript lists the measured species, but except for OH and HO2, no information is provided on measurement techniques, detection limits, or measurement uncertainties/accuracies. Because these observations are used to evaluate model performance, this information should be provided, at least in the Supplement.

Sections 2.1 and 2.2: Because meteorology strongly affects regional ozone through transport, mixing, temperature, and biogenic emissions, please provide standard meteorological performance metrics, including temperature, wind speed/direction, and humidity, at least in the Supplement.

Lines 196-211: The three simulations use different CMAQ chemical configurations, including different aerosol modules (AE7/AE7i versus AE6), in addition to different gas-phase mechanisms. Please clarify to what extent the reported inter-mechanistic differences can be attributed specifically to gas-phase chemistry rather than to differences in the accompanying model configurations.

Section 2.2: The manuscript provides considerable detail on VOC mapping but very limited information on how the chemical mechanisms themselves differ. Please provide a systematic comparison of the three mechanisms, including differences in reaction-rate coefficients and whether important pathways such as chlorine chemistry, RO2 chemistry, and intramolecular isomerization are represented.

Section 2.3: Please provide a quantitative evaluation of the customized RACM2 emission mapping. For example, how well are total VOC emissions, carbon mass, and VOC reactivity conserved relative to the original inventory? This is important because the customized inventory is central to the inter-mechanism comparison.

Figure 2 and Lines 250-254: Please mark XJ in Figure 2. The simulation period is also inconsistent between the Figure 2 caption (August 9-18) and the text (August 9-19). Please clarify and use a consistent period throughout.

Figure 3: Please clarify which monitoring sites were included in the statistical analysis and whether XJ is included. If the statistics combine urban and suburban sites, the results should be separated by site type because the preceding discussion shows substantially different model performance between these environments. Please also use a consistent evaluation framework for Figures 3 and 4, or explain why Figure 4 separates urban and suburban sites and uses daytime data whereas Figure 3 does not.

Lines 281-283: The attribution of lower ozone in CB06 to insufficient NO2 recycling from nitrates appears too definitive. This explanation is cited from a previous study but is not demonstrated here, and the manuscript later shows that NO2 is generally overestimated by all three mechanisms. Please provide evidence from the present simulations or soften the interpretation.

Lines 317-320: RACM2 shows the closest agreement with observed HONO, but the origin of this inter-mechanistic difference is not discussed. Because HONO photolysis is subsequently identified as an important ROx source, please explain which HONO-related reactions, sources, or precursor treatments differ among the mechanisms and lead to the higher HONO simulated by RACM2.

Lines 322-329: The phrase 'uncertainties in ISOP-related oxidation chemistry' is too vague. Clarify whether this refers to differences in direct chemical loss of isoprene or to downstream isoprene oxidation chemistry that alters radical recycling and OH. This distinction is important because Figure 5 shows substantial inter-mechanistic differences in OH, which could directly affect simulated ISOP lifetime and concentration.

Lines 330-335: The HNO3 overestimation is attributed mainly to overestimated NO2. However, HNO3 production through OH + NO2 depends on both reactants, and Figure 5 shows that OH is also substantially overestimated, particularly by RACM2, which also shows the largest HNO3 bias. Please discuss the contribution of OH overestimation to excessive HNO3 production. Furthermore, HNO3 concentrations are strongly affected by partitioning to particulate inorganic nitrate and other loss processes. Given that different aerosol modules are used for the three mechanisms, please explicitly examine and discuss HNO3-inorganic nitrate partitioning before attributing the differences primarily to gas-phase chemistry.

Lines 343-345: This statement is unclear and appears inconsistent with the following results. Figure 4 does not show temporal HCHO peaks, so the meaning of the 'superior performance of SAPRC07 in simulating HCHO peaks' is unclear. In addition, Figure 5 shows that RACM2 produces the highest OH while SAPRC07 produces the highest HO2, so the general statement that SAPRC07 has higher radical concentrations is inaccurate. Please clarify the radical species, provide the appropriate figure or section reference, and explain the mechanistic basis for linking the HCHO bias to radical concentrations.

Figure 5: Please clearly indicate which panels correspond to CB06, SAPRC07, and RACM2, respectively, and mark XJ on the maps.

Line 358: The statement that the spatial distributions are 'largely consistent' is too qualitative. Please quantify the degree of agreement or specify the range of inter-mechanistic differences.

Lines 363-365: The explanation of low urban OH focuses only on OH + NO2. Reactions of OH with VOCs, particularly anthropogenic VOCs in the urban core, may also represent an important OH sink and should be mentioned.

Lines 383-387: The conclusion that lower RO2 in CB06 explains its lower ozone is incomplete without discussing the origin of the RO2 difference. Please relate this to mechanism-specific VOC lumping and speciation (Table S3), which can alter OH reaction rates, RO2 yields and identities, and subsequent propagation chemistry. A bulk RO2 comparison alone is insufficient to explain the inter-mechanistic difference in ozone.

Lines 393-410: The discussion is largely descriptive and does not explain the origin of the different P(O3) among the three mechanisms. Please relate these differences to mechanism-specific VOC lumping/speciation (Table S3), VOC reactivity, and subsequent RO2 chemistry. For example, the apparently larger role of alkene-related chemistry in RACM2, particularly at the urban site, warrants further discussion.

Line 417: The description of NOx-ROx interactions as continuously 'generating' HO2 and RO2 is potentially misleading. Please distinguish radical propagation/recycling from the primary ROx production pathways.

Lines 422-425: RACM2 is reported to produce the highest P(ROx), particularly under urban high-NOx conditions, but the chemical origin of this difference is not discussed. Please identify which source pathways or mechanism-specific treatments are primarily responsible. The statements of 'considerable divergence' under urban conditions and 'more consistent results' under suburban conditions should also be supported with corresponding P(ROx) values or inter-mechanistic differences.

Lines 427-440: This paragraph primarily lists the contributions shown in Figure 7 without providing further interpretation. Please focus on mechanistically meaningful differences among the three mechanisms and explain why the ROx source distributions become more similar under suburban low-NOx conditions.

Lines 434-436: The explanation that the higher other-carbonyl photolysis contribution in RACM2 may result from differences in photolysis parameters is not supportive. The manuscript does not describe how the relevant photolysis parameters differ among CB06, SAPRC07, and RACM2. Please identify the specific parameterizations, such as absorption cross-sections, quantum yields, or photolysis frequencies, and provide evidence that they explain the higher contribution in RACM2. Differences in carbonyl concentrations and speciation due to different VOC lumping methods should also be considered.

Lines 450-451: This statement is misleading because OH + NO2 remains the dominant ROx termination pathway at the suburban site, contributing nearly 50% of total loss. Please revise the text to state that HO2 + RO2, HO2 + HO2, and RO2 + RO2 become relatively more important under suburban low-NOx conditions. Please also provide their quantitative contributions, as done for HNO3 and RONO2.

Lines 459-467: The manuscript uses the H2O2/HNO3 ratio to infer VOC-limited and NOx-limited regimes, but no diagnostic threshold or criterion is provided. Please specify the threshold values used to distinguish the regimes and provide an appropriate reference or justification for their application here.

Lines 469–472: The basic description of RO₂/HO₂ + NO chemistry and its role in O₃ formation is repeated multiple times throughout the manuscript. Please substantially reduce this repetitive textbook-level explanation.

Lines 484–486: The statement should be quantitatively supported. Please provide the relative contributions of each VOC class to the total OH + VOC reaction rate.

Figure 10 and Lines 517–524: Please clarify what species are included in the “BVOC” category. If BVOC represents only isoprene, it would be clearer to label it as ISOP. If it includes other biogenic VOCs (e.g., terpenes), the discussion should not focus exclusively on isoprene and should address the contributions of the other BVOC species as well.

Figure 10: It is unclear why the term “sensitivity coefficient” is used in the caption. Please define exactly what this coefficient represents and how it is calculated.

Line 507: “O₃ production” appears to be incorrect here. The DDM analysis in Figure 10 represents the sensitivity of O₃ concentration to precursor emissions, not the sensitivity of the O₃ production rate.

Lines 510–512: This statement appears abrupt and overinterprets the DDM results. Figure 10 shows emission sensitivities, not direct source contributions to O₃ formation. Please revise the wording and clarify how this conclusion follows from the sensitivity analysis.

Lines 512–516 and Figure 10: The interpretation of the reported first-order sensitivities is unclear. Although the sensitivities are given in ppbv, the manuscript does not define the emission perturbation or scaling parameter with respect to which they are calculated. Please provide the mathematical definition and explain what magnitude of emission change the reported sensitivity values correspond to.

Line 517: “Higher OH reactivity of ISOP” is not appropriate here. Figure 9 shows the OH + ISOP reaction rate, not OH reactivity. Please revise the terminology accordingly.

Lines 517–524: The explanation for the greater BVOC sensitivity under urban high-NOₓ conditions is plausible but remains qualitative. Please provide quantitative evidence linking BVOC-derived radical fate or RO₂ + NO chemistry to the enhanced O₃ sensitivity rather than inferring this solely from the general radical termination patterns.

Technical corrections:
Throughout: Use consistent chemical formatting, including appropriate subscripts and superscripts.

Line 38: Please change 'via radiative effect' to 'because of its radiative effects.'

Lines 39-40: Please remove the marked unnecessary article, change 'scientific communities' to 'scientific community,' change 'has remained a focus of sustained attention' to 'has received sustained attention,' and replace 'governments' with 'policymakers.'

Lines 41-43: Please replace 'As' with 'Because of' or 'Due to' and revise the verb agreement in the sentence, including 'ozone concentrations have remained.'

Line 47: Please change 'once O3 formed' to 'once O3 is formed.'

Line 48: Please remove the extra parenthesis and 'e.g.' before R1-R3.

Lines 49-50: Please remove the redundant use of 'radical(s)' in the definitions of HO2 and RO2.

Lines 52-53: Please replace 'Under the action of' with 'Through' and revise 'formed' to 'represented,' as marked in the annotated manuscript.

Line 59: Please revise 'the ROx radicals cycle' to 'the ROx cycle.'

Line 63: Please replace the second 'and' with 'as well as' in the list of primary ROx sources.

Line 65: Please remove the redundant word 'radicals' after OH.

Line 66: O3 photolysis does not directly produce OH; it first generates O(¹D), which subsequently reacts with water vapor to form OH. Therefore, please change 'by O3 photolysis at wavelengths below 320 nm (R6-R7) in the troposphere' to 'through O3 photolysis at wavelengths below 320 nm (R6-R7), followed by the reaction of O(¹D) with water vapor in the troposphere.'

Line 69: Please change 'VOC' to 'VOCs.'

Line 70: The reference to Reaction R9 should be changed to R10.

Line 80: Please remove the redundant phrase 'mutual self-reaction.'

Line 81: Please replace 'explosive' with an appropriate noun phrase, such as 'explosive growth.'

Line 86: Please change 'ratio' to 'ratios' and revise the corresponding pronoun to 'their.'

Reaction R13: Please change NO2 to NO, consistent with the chemical correction described in Specific comment 5.

Line 97: Please change 'laboratory simulations' to 'laboratory experiments.'

Lines 99-100: Please change 'the air quality model' to 'air quality models' and 'largely' to 'strongly.'

Line 104: Please revise 'As far' to 'So far' or remove the expression.

Line 107: Please remove 'and so on.'

Lines 108-110: Please revise the classification sentence to state that photochemical mechanisms are 'classified as explicit or lumped.' Change 'Explicit reaction mechanism' to 'An explicit reaction mechanism.'

Line 112: Please remove the extraneous words in 'the mainstream choices for in the air quality model.'

Lines 115-116: Please change 'research' to 'studies.'

Lines 119-120: Please remove the redundant word 'requirement,' change 'limit' to 'limits,' and consider citing the recent mechanism-evaluation study https://doi.org/10.5194/gmd-18-8461-2025.

Line 123: Please change 'which often referred' to 'which are often referred.'

Line 126: Please remove the sentence fragment beginning 'And RACM...' or integrate it grammatically into the preceding sentence.

Lines 131-132: Please change 'Previous study has evaluated' to 'Previous studies have evaluated.'

Lines 137-139: Please change 'other study' to 'other studies,' replace 'for more detailed VOC treatment' with 'due to/owing to the more detailed VOC treatment,' and change 'a recent comparative study using box models have demonstrated' to 'a recent comparative study using box models has demonstrated.'

Lines 141-143: Please revise to: 'However, few studies have investigated uncertainties in simulations of O3 pollution over China using three-dimensional air-quality models that arise from different photochemical mechanisms.'

Line 144: Please write the area as '14,335 km².'

Lines 145-146: Please correct 'GPD' to 'GDP' and identify the currency used for '2.35 trillion dollars' (e.g., CNY).

Lines 147-149: Please revise the description to 'as a major regional center in western China,' change 'expected' to 'is expected to become,' and change 'Its features' to 'It features.'

Figure 1b: Please use a subscript in NO2.

Line 216: Please check whether 'RADM2' should be 'RACM2.'

Figure 2: Please mark XJ and use a consistent simulation period (August 9-18 versus August 9-19).

Line 276: Please change 'In specific' to 'Specifically.'

Line 281: Please change 'Previous study' to 'A previous study.'

Line 377: Please change 'literature' to 'a literature value' or another grammatically appropriate expression.

Line 429: Please change 'In specific' to 'Specifically' and replace 'in Zhou's research (Zhou et al., 2024)' with 'consistent with Zhou et al. (2024).'

Lines 430-431: Please revise the sentence connection by replacing the period before 'And' with a comma and continuing with 'and.'

Line 433: Please check the marked article and capitalization in 'O3 and Alkenes' and remove the extra 'and' after the HCHO-photolysis contribution.

Lines 434-436: Please provide numerical support for 'significantly higher,' replace 'hydroxyl compounds' with 'carbonyl compounds' if that is the intended meaning, and remove the unnecessary phrase 'rate parameters of.'

Line 451: Please replace 'OH-HO2-RO2 radical system' with 'ROx radical system.

Section 3.3.1: Please reorganize the text into logically distinct paragraphs separating general findings, quantitative comparisons, and mechanistic interpretation
Citation: https://doi.org/10.5194/egusphere-2026-1786-RC2

Yuelin Liu, Yuanjun Gong, Nan Wang, Guangming Shi, Fumo Yang, Yuanhang Zhang, and Keding Lu

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Yuelin Liu, Yuanjun Gong, Nan Wang, Guangming Shi, Fumo Yang, Yuanhang Zhang, and Keding Lu

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Short summary

Ozone pollution is difficult to predict because its formation depends on complex chemical reactions that vary under different pollution conditions. By comparing multiple modeling approaches during a pollution event in Chengdu, China, we examine how ozone formation and the underlying chemical processes, particularly radical-driven reactions, differ between low- and high-pollution environments.


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