| 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.
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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.