the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 22 Aug 2025
Temperature and Stagnation Effects on Ozone Sensitivity to NOx and VOC: An Adjoint Modeling Study in Central California
Abstract. Extreme weather events like heatwaves and stagnation are increasing with climate change. While their effects on ozone levels have been extensively studied, how extreme weather alters O3-NOx-VOC sensitivity and optimal mitigation strategies is less explored. Here, we apply the CMAQ adjoint model over central California to quantify ozone sensitivity to spatiotemporally resolved precursor emissions under three meteorological scenarios (baseline, high-T, and stagnation) and three emission years (2000, 2012, and 2022). Results show that meteorology-induced changes in sensitivity are comparable in magnitude to those from decadal emission reductions. Higher temperature (+5 °C) amplifies ozone sensitivity to both NOx and VOC, with the largest relative increase in biogenic VOC sources. High-T conditions shift ozone chemistry toward NOx limitation under a VOC-limited emission scenario, but increase the relative importance of VOC control for a NOx-limited scenario. Stagnation consistently pushes ozone chemistry toward VOC limitation across emission scenarios, increasing VOC sensitivity by a factor of ~3–4. Stagnation also spatially shifts influential source areas, especially for NOx, and temporally amplifies prior-day emission impacts due to enhanced pollutant carryover. As the study domain transitions to a NOₓ-limited regime over time, we identify a growing subset of "climate-resilient" source targets that remain impactful across meteorological scenarios, along with spatial convergence in optimal locations for NOx and VOC emission control. These findings underscore both the need and feasibility to consider meteorological extremes in the design of ozone mitigation strategies for a warming climate.
Competing interests: Co-author Yuan Wang is a member of the editorial board of Atmospheric Chemistry and Physics.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.- Preprint
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2025-3629', Anonymous Referee #1, 22 Sep 2025
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RC2: 'Comment on egusphere-2025-3629', Anonymous Referee #2, 10 Oct 2025
The manuscript “Temperature and Stagnation Effects on Ozone Sensitivity to NOx and VOC: An Adjoint Modeling Study in Central California” quantifies the sensitivity of ozone exposure to NOx and VOC emissions in the San Juan Valley over different emission scenarios and temperature/meteorological conditions. The authors determine how these sensitivities change with respect to temperature due to a changing climate while also accounting for emission reductions. While the analysis presented provides a strong narrative with important results, the paper would benefit from additional work on comparing the choice of methods and evaluation of the results. After the detailed comments (which constitute minor revisions) below are addressed, I would recommend the article for publication.
Main Comments
Line 84: While not exactly the same as the methods used here, there have been prior studies of the role of meteorological factors on O3 sensitivities using adjoint modeling. The CMAQ adjoint paper (Hakami et al., 2007) included explicit calculation of sensitivities with respect to temperature (via the role of temperature on chemical kinetics); this idea was expanded upon in Zhao et al. (GRL, 2013, https://doi.org/10.1002/2013GL057623) which examined the estimation of ozone climate penalties using adjoint sensitivities. Works such as Park et al. (Atmos. Environ., 2018, https://doi.org/10.1016/j.atmosenv.2018.08.006) investigated both chemical and meteorological influences on ozone using adjoint sensitivities. Lastly, while not a study of meteorological impacts, a paper that does though seem pertinent to the use of adjoint modeling to explore variability in O3 isopleths is that of Ashok and Barrett (Atmos. Environ., 2016, https://doi.org/10.1016/j.atmosenv.2016.03.025). In summary, there may be more prior work on this sort of topic using adjoint sensitivity analysis than the authors have indicated, though I still recognize that their particular approach does seem to be unique.
Section 2.4: Please state the temporal resolution used for the sensitivities and how they relate to emissions – are sensitivities calculated hourly, before being aggregated temporally? Are all emissions also hourly, or are some constant throughout the day (which could pose an issue given the analysis here)?
Section 2.4, Paragraph 3: It’s important to state the limitations of assuming a first-order contribution of emissions to Ox, especially given the nonlinearity in the underlying chemical mechanisms. While less pertinent in the year-to-year comparative analyses, results discussing peak sensitivities and spatial contributions do need this qualification.
Section 2.5, Paragraph 5: Please provide some statistics from the studies you mentioned that analyzed CMAQ performance, like mean bias and standard deviation of ozone concentrations over, if possible, the SJV, just to give context on how well the model calculates ozone exposure.
Section 3.3, paragraph 3: Could this increase in VOC sensitivity be due to less transport of VOCs from outside of SJV in Stagnant conditions that could otherwise lead to ozone formation? Is that something you could quantify?
Sentence spanning lines 370-371: Cite the fact that extreme weather events will increase in frequency with climate change.
Section 3.4: The spatial results could be supported with details on if NOx and AVOC sectors align for most optimal controls, like if road transport reductions dominate in rural regions versus industrial emissions in the cities, or it is just one sector that dominates. If CARB’s inventory provides sectoral emission distributions I would consider adding this analysis.
Section 3: Overall, it is unclear what the range in result values pertains to. Are these confidence intervals? Or the range in values in the SJV?
It would be beneficial to quantify the advantage of using the adjoint model as opposed to a “brute force” method either in the discussion or where the adjoint method is introduced, something like “where the brute force method would require X simulation runs, our adjoint model only requires 9 to provide the same results”
Minor Edits
Figure 3: missing percentages on 2000 NOx, is this on purpose?
Figure 5: Consider including geographic regions in “non-local” category, could be interesting to see how, e.g., SF Bay’s contribution changes in a graphical presentation with respect to other areas, or to see if there is an outside region that contributes most in Baseline but becomes negligible in Stagnation.
Line 29: Use of Oxford comma here, but omitted on Line 24, please keep consistent. Can also omit “by” at the beginning of each dependent phrase.
Line 35: Repetitive “while” use, I’d try to use more unique wording, or omit line all together since it is not necessary.
Introduction, Paragraph 2: Seems out of place. I would move to after Paragraph 3 to introduce ozone formation mechanisms and terminology before its sensitivities to model parameters.
Line 40: HOx is never defined, either define it or simply say “hydrogen oxide radicals” since HOx is not used that often in the manuscript.
Section 2.3/2.4: The choice to use pop-weighted 8-hr average Ox as opposed to just O3 should be stated here.
Line 190: Forgot to make subscript x on Ox.
Citation: https://doi.org/10.5194/egusphere-2025-3629-RC2
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General comments
This is a very nice paper telling us the impacts of heatwaves and stagnation on O3-NOx-VOC relationships. It presents some new findings, fits the scope of ACP, and has strong policy implications. I would recommend it to be published after some revisions.
Major comments:
I think a very important point that could be improved is the separate analysis of heatwave and stagnation. Now we are more and more concerned on “compound events” (e.g., heatwave and stagnation happen simultaneously) and it is believed that such events are likely to increase in a climate-change world. I understand that it may be time-consuming to have new modeling for “compound events”, but I recommend that authors to have some analysis and discussions in some ways they prefer.
In the introduction part (around line 45-line 70), the authors have a nice literature review on previous studies using different methods with different findings (even opposite). I think the authors should add some discussions to clarify if the findings in this study are different or not and what the key influencing factors are (e.g., research area, models, metrics…). I think a comprehensive comparison would improve the insights of the current study on how to do this kind of research on O3-NOx-VOC relationships in the future.
The modeling finds that stagnation reduces NOx sensitivity and amplifies AOC sensitivity (e.g., Figure 2). Why are they opposite? Some chemical or meteorological explanations (or both them) are needed.
I also recommend the authors to add some analysis on PAN since it is an important intermediate for O3 formation, especially by transporting to downwind areas.
I am a little bit confused about the opposite sign of NOx and AVOC sensitivity changes due to temperature increase at grid scale (Fig. 4). Because in Fig.2, it seems that the changes are in the same direction, but in Fig.4, it is completely different. I am not an expert on adjoint approach, but please explain it and have more detailed explanations in the context.
I recommend the authors to have some discussions on how this study would provide new insights for studies in other regions and other scales (since this study focuses on a very specific region), such as East Asia and Europe. Discussing limitations and uncertainties would be appreciated.
Minor comments:
Line 94: Please add references for “The year 2000 represents a more VOC-limited environment, whereas year 2022 reflects cleaner, NOx-limited conditions following major NOx emission reductions.”
Line 108: A altitude map of the area would be nice (maybe in SI). It helps readers to understand the wind flows and accumulation of air pollutants.
Line 190: The authors write “We denote a chemical regime as “NOx-limited” when the Ox sensitivity to NOx exceeds that to anthropogenic VOC, and as “VOC-limited” otherwise.” So, there is no “transitional regime” defined here? Also, please add a map to tell the readers the spatial distributions of chemical regimes at grid level.
Figure 3: In 2022, stagnation increases NOx sensitivity compared to baseline. It is opposite to Figure 2. Why?
Line 286: “Under high-T conditions, sensitivities to these three groups increase by similar percentages (+22-40%), with no single group showing disproportionately larger temperature impacts.” I think the small differences are because the anthropogenic emissions are unchanged with meteorological conditions, as compared with biogenic emissions. However, there are many studies showing increased anthropogenic emissions as temperatures rise (e.g., Wu et al., 2024). This should be discussed.
Wu, W., Fu, T. M., Arnold, S. R., Spracklen, D. V., Zhang, A., Tao, W., ... & Yang, X. (2024). Temperature-dependent evaporative anthropogenic VOC emissions significantly exacerbate regional ozone pollution. Environmental Science & Technology, 58(12), 5430-5441.