| 04 Dec 2025
Chemical Precursors to Wintertime Carbonyl and Ozone Formation
Abstract. Using the Framework for 0-D Atmospheric Modeling (F0AM), a zero-dimensional box model designed for simulating atmospheric chemistry, we simulated winter O3 formation in the Uinta Basin, Utah, with four chemical mechanisms: the Master Chemical Mechanism (MCMv331), Statewide Air Pollution Research Centre Mechanism (SAPRC07), Regional Atmospheric Chemistry Mechanism (RACM2), and Carbon Bond Mechanism (CB6). Our purpose was to (1) identify key carbonyl precursors that act as important precursors to winter O3 formation and determine how they form, (2) determine the extent to which carbonyl compounds were primarily or secondarily produced, (3) assess O3 production potential, and (4) analyze how different hydrocarbon groups influence both carbonyl and O3 formation. The final emission flux for carbonyls was near zero, indicating that they were mostly secondary photochemical products. MCMv331 identified formaldehyde and acetaldehyde as the dominant O3 precursors, contributing 0.20 and 0.06 ppb/h, respectively, to the O3 production rate. SAPRC07 and RACM2 showed similar trends, while CB6 emphasized the generic group “ketones” as key contributors. Across all mechanisms, alkanes were the most influential precursor group for the formation of carbonyls and O3. Including heterogeneous chemistry in the model resulted in a modest (1 ppb) decrease in O3 levels without altering the relative importance of precursors. This study highlights the importance of primarily emitted organic groups in winter O3 production and provides insights into O3 reduction strategies in the Uinta Basin and similar regions.
This study investigates the potentially important role of carbonyl compounds in wintertime ozone formation using observational data from the Uinta Basin and box model simulations. The comparison among several widely used chemical mechanisms is also a valuable aspect of this work, which could provide useful information for future modelling research. Overall, the topic is scientifically interesting and of significance.
Although chemical budgets have been added to the revised manuscript, the presentation and use of these results could be improved. A large number of tables are provided in the Supplement, listing the top source and sink pathways and their contributions for O3, HOx and carbonyls. However, the key information is difficult to extract, as readers need to repeatedly consult multiple tables to identify the main controlling processes. A more synthesized presentation in the main text, for example using multi-panel figures to highlight the dominant budget terms, together with a focused discussion of how these budget characteristics differ among chemical mechanisms and relate to the sensitivity results, would greatly enhance the clarity and diagnostic value of the budget analysis.
The manuscript is generally clearly written. However, the Methods section and the final paragraph of the Introduction rely heavily on highly repetitive sentence structures, with many consecutive sentences beginning with similar formulations such as “we did…”. This repetitive writing style substantially reduces readability and makes these sections unnecessarily monotonous. A thorough revision to streamline the text, reduce redundancy, and introduce greater syntactic variety is strongly recommended.
Based on these comments, I believe this paper has scientific merit and the potential to make a useful contribution to ACP. However, substantial revisions are required, particularly to deepen the chemical analysis, strengthen the mechanistic interpretation and improving readability, before the manuscript can be considered for publication.
More specific comments are as follows (line numbers are denoted as “Lx”):
1) Title: The title is rather vague and does not clearly convey the specific focus or scope of the study. In particular, it is unclear what is meant by “chemical precursors”. Clarifying the title to better reflect the main objectives and analyses of the manuscript would improve its clarity.
2) Introduction: When introducing chemical reactions related to O3 and carbonyls, please indicate in the text that CH4 and HCHO are presented as example species in Reactions 1-8.
3) L121: “Lyman et al. (Lyman et al., 2020)” should be revised to “Lyman et al. (2020)”. Please ensure the correct format of references.
4) L134: “followed by” should be “following”
5) L137-139: This sentence is vague and should be revised to more precisely describe which aspects of the chemistry are explicitly included.
6) L141: Please provide more details on the uptake coefficients, including their parameterizations and approximate values.
7) L143: Since the comparison among results obtained using different chemical mechanisms is one of the highlights in this paper, it would be helpful to give more details on the main differences among these mechanisms and to explicitly link these differences to the relevant discussions. This would provide clearer context for understanding why the mechanisms lead to different sensitivities and identified contributions.
In addition, a large number of species abbreviations from different chemical mechanisms are used in the main text and the Supplement, which makes cross-mechanism comparison difficult to follow. Providing a summary table in the Supplement that maps analogous species across different mechanisms would greatly improve clarity and facilitate interpretation of the budgets and inter-mechanism comparisons.
8) L155-156: The expression “aggregated their mixing ratios as appropriate” is ambiguous and should be clarified.
9) Fig 2: Three blue colors are used in this figure, which are difficult to distinguish, particularly when the figure is viewed at reduced size or under low-quality printing conditions. Using more contrasting colors would improve clarity.
10) L231-251: This paragraph analyzes the contributions of individual carbonyl species, with most of the discussion focusing on benzaldehyde. While the additional discussion on why benzaldehyde shows positive or negative contributions under different mechanisms is interesting, the current emphasis appears disproportionate. Formaldehyde and acetaldehyde are identified as the dominant contributors to ozone production, yet their roles and the reasons for their varying contributions across different chemical mechanisms are only briefly discussed. It would be useful to discuss them in a dedicated paragraph, with a clearer explanation of why their contributions differ substantially among chemical mechanisms. The discussion of benzaldehyde, which plays a more minor role but shows interesting sign changes across mechanisms, could then be presented separately. Such a reorganization would help clarify the relative importance of different species and improve the overall focus of the discussion.
11) L258: Is the limited contribution of heterogeneous uptake related to low uptake coefficients?
12) Sect 3.3: This section requires further improvement in both analysis and organization. Two key questions need to be clearly addressed: (1) how hydrocarbons influence the formation of different carbonyl species (based on MCM), and (2) why the simulated responses differ among the four chemical mechanisms. While four figures are used to illustrate the sensitivities of carbonyls to different hydrocarbon classes, the discussion mainly describes the general features of the sensitivity tests rather than providing mechanistic explanations linked to specific reaction pathways. Even a brief indication of the key reactions involved would greatly help readers understand why certain hydrocarbon classes dominate the responses. Without such explanation, several results remain difficult to interpret, particularly for readers who are not specialists in detailed carbonyl chemistry.
In the manuscript, the responses of formaldehyde to hydrocarbons are summarized and explained, but similar explanations are largely missing for other carbonyl species. Specifically, it remains unclear why alkanes dominate the responses of acetaldehyde, methacrolein, benzaldehyde and MEK. An additional paragraph is devoted to explaining the response of benzaldehyde to alkanes. Finally, the differences among the four mechanisms are only briefly mentioned, but the manuscript does not clearly explain why benzaldehyde is more strongly influenced by aromatics in RACM2, or why the results from CB6 are generally distinct from the other mechanisms.
Overall, this section relies heavily on descriptive interpretation of sensitivity results, without sufficiently linking the responses to specific chemical reactions or mechanism structures. As a result, several key questions remain unresolved after reading this section. A clearer summary of the main findings, together with a more balanced and reaction-based discussion across species and mechanisms, would substantially strengthen both the clarity and the scientific depth of this section.
13) Sect 3.4: The authors conclude that “Alkane emissions from oil and gas extraction play a critical role in wintertime O3 formation” based on the results of O3 sensitivity to hydrocarbon emissions. However, the logic between sensitivity analyses and the final source attribution could be further strengthened. While the sensitivity results indicate a strong role of alkanes in modulating O3 production, this alone does not fully support the conclusion. Firstly, other emission sources in the Uinta Basin may still play a role and are not fully constrained. Secondly, the assumption of zero primary carbonyl emissions in the simulations may not fully reflect real-world conditions, particularly given that oil and gas activities can also emit carbonyls or their immediate precursors.
In addition, the magnitude of the simulated O3 production response to a 50% increase in alkane emissions (on the order of ~0.2–0.4 ppb h⁻¹) appears small compared to the observed ozone levels (over 100 ppb), suggesting that sensitivity alone may not explain the full amplitude of the pollution events. Clarifying the relative roles of emission strength, chemical amplification, and meteorological accumulation would help close the logical gap between sensitivity results and the final attribution.
14) Sect 3.5: While the addition of chemical budget analyses is appreciated, the way these results are incorporated into the manuscript weakens the overall structure. The main body of the study focuses on the sensitivities of O3 to carbonyls, carbonyls to hydrocarbons, and O3 to hydrocarbons. However, the budget analyses are introduced in a separate section at the end, rather than being integrated into the corresponding sensitivity discussions. As a result, the budget results appear disconnected from the main analyses and do not effectively explain the observed sensitivities. Integrating the key budget diagnostics into the relevant sections, and using them directly to interpret the sensitivity results, would lead to a more coherent structure and a stronger, process-based narrative.
15) Conclusion: The Conclusion section is very brief and largely reiterates the content of the manuscript, without sufficiently distilling the key mechanistic insights. Given the complexity of the sensitivity analyses and chemical mechanism comparisons presented in this study, a more substantive conclusion that clearly summarizes the main findings, highlights the dominant controlling processes, and acknowledges remaining uncertainties would strengthen the overall impact of the paper.