Direct thermal enhancement dominates over emission-mediated pathways in heatwave-induced O<sub>3</sub> and SOA increases across China

Wang, Peng; Yu, Wenxuan; Zhang, Zhaolei; Lu, Jianyan; Li, Shenxin; Zhang, Hongliang

doi:10.5194/egusphere-2026-1101

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https://doi.org/10.5194/egusphere-2026-1101

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11 May 2026

| 11 May 2026

Status: this preprint is open for discussion and under review for Atmospheric Chemistry and Physics (ACP).

Direct thermal enhancement dominates over emission-mediated pathways in heatwave-induced O₃ and SOA increases across China

Peng Wang, Wenxuan Yu, Zhaolei Zhang, Jianyan Lu, Shenxin Li, and Hongliang Zhang

Abstract. Heatwaves are major drivers of ozone (O₃) and secondary organic aerosol (SOA) pollution. High temperatures directly accelerate photochemical reaction rates and concurrently enhance emissions of biogenic volatile organic compounds (BVOCs) and soil nitric oxide (SNO). However, the individual contributions of these direct and emission-mediated pathways to pollution formation remain poorly constrained. This study explicitly quantifies the distinct roles of these two pathways during heatwave events in China. Results show that high temperatures drive over 80 % of the O₃ and SOA increases nationally, primarily through favorable weather conditions and enhanced atmospheric oxidation capacity. The O₃-temperature dependence is strongest in the Yangtze River Delta (0.66 ppb °C^-1) and Pearl River Delta (0.95 ppb °C^-1). Furthermore, high-temperature-induced BVOC emissions significantly exacerbate O₃ in VOC-limited regions like the North China Plain. These findings underscore the importance of climate mitigation by illustrating its critical role in alleviating temperature-driven secondary pollution.

Received: 26 Feb 2026 – Discussion started: 11 May 2026

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Peng Wang, Wenxuan Yu, Zhaolei Zhang, Jianyan Lu, Shenxin Li, and Hongliang Zhang

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RC1:
'Comment on egusphere-2026-1101', Anonymous Referee #1, 12 May 2026 reply
The manuscript addresses an important and timely question: how heatwaves enhance O3 and SOA through direct meteorological effects and emission-mediated pathways. The topic fits well within the journal scope. The WRF-CMAQ sensitivity design is generally appropriate, and the main conclusion that direct high-temperature meteorological effects dominate the national O3/SOA increases is interesting and policy relevant. The regional finding that BVOC-related effects are particularly important for O3 in the VOC-limited North China Plain is also valuable. I recommend publication after minor revisions.
The attribution method should be explained more clearly. The authors state that nonlinear interactions lead to a residual term and that percentage contributions are normalized against the sum of absolute changes from separated scenarios. This treatment may influence the reported “over 80%” meteorological contribution, so a clearer explanation or sensitivity discussion is needed. See p. 4, lines 101–105, and p. 1, lines 18–21.

The use of summer 2021 as the baseline for the 2022 heatwave should be discussed as a limitation. Interannual differences may reflect not only heatwave effects, but also circulation variability, emissions, and boundary conditions. A short discussion, or comparison with a multi-year baseline if available, would strengthen the attribution. See p. 4, lines 90–99.

The uncertainty in MEGAN-based BVOC and SNO emissions should be discussed more explicitly, since these emissions are central to the indirect pathway analysis. This is especially important for regional conclusions regarding NCP BVOC sensitivity and SNO effects. See p. 3, lines 75–82, and p. 5, lines 106–120.

The model evaluation is acceptable, but the SOA-related conclusions would benefit from clearer validation or caveats. The current evaluation focuses mainly on meteorology, O3, PM2.5, and NO2, while direct observational constraints on SOA are not clearly shown. Please clarify whether OA/SOA observations are available, or explicitly state this limitation. See p. 5–6, lines 121–135.

The nighttime O3 mechanism is interesting but could be clarified. The explanation involving NO3 chemistry, reduced NO titration, and boundary-layer stability should better distinguish chemical effects from vertical mixing or transport effects. Additional diagnostics such as nighttime PBL height, NOx, NO3, or vertical diffusion changes would be helpful if available. See p. 10–11, lines 220–225.

Some figures contain dense information and relatively small labels. Improving figure readability and keeping scenario names consistent would make the manuscript easier to follow, especially for Figs. 3–6.

Overall, this is a useful and timely study. After addressing the above minor issues, the manuscript should be suitable for publication.

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Citation: https://doi.org/10.5194/egusphere-2026-1101-RC1
RC2:
'Comment on egusphere-2026-1101', Anonymous Referee #2, 23 May 2026 reply
This manuscript presents a well-designed modeling study on the drivers of O₃ and SOA increases during heatwaves in China. The experimental design is appropriate, and the findings regarding the dominant role of direct temperature effects and the heightened VOC sensitivity in the North China Plain are valuable for understanding climate air quality interactions. The paper is generally well written and organized. However, several minor issues remain that should be addressed before publication. Overall, the manuscript is suitable for acceptance by ACP after minor revision.
In the BVOC22 sensitivity simulation, the authors state that isoprene, terpenes, and sesquiterpenes are selected as the dominant BVOC species. Please provide the relative emission proportions of these three groups under the baseline (2021) and heatwave (2022) conditions. This information would help readers assess which biogenic precursors are most responsible for the observed O₃ and SOA changes.

Section 3.6 reports that the PRD region has the highest O₃ temperature sensitivity, yet Figure 1 shows that summer temperatures in the PRD slightly decreased from 2021 to 2022. Please clarify how a sensitivity slope can be derived under a net cooling condition. Is the sensitivity estimate driven by spatial variability rather than interannual temperature change? A brief explanation would help readers interpret this result correctly.

Section 3.4 reports that the direct meteorological contribution (MET22) to O₃ in the North China Plain is more pronounced at night than during the day, attributed to temperature inversion trapping surface O₃. However, nocturnal O₃ is typically depleted by NO titration. Please explain why the trapped O₃ is not rapidly titrated by NO under the stagnant, high-pressure conditions described.

In Figure 2 caption, the difference term is written as “BASE22-BASE2”. It should be “BASE22-BASE21”. Please add the missing digit “1”.

Section 3.4 defines daytime as 8:00 to 20:00 and nighttime as 20:00 to 8:00. Please clarify whether these times refer to local standard time or Beijing Time (UTC+8). Additionally, please confirm whether the model simulations accounted for time zone differences across the study domain (e.g., western China versus eastern China) when aggregating diurnal patterns.

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Citation: https://doi.org/10.5194/egusphere-2026-1101-RC2

Peng Wang, Wenxuan Yu, Zhaolei Zhang, Jianyan Lu, Shenxin Li, and Hongliang Zhang

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Short summary

Heatwaves worsen ozone and secondary organic aerosol pollution. We quantified how much comes from heat speeding up chemical reactions versus increasing natural emissions in China. High temperatures drive over 80 % of the pollution increase nationwide by making the atmosphere more chemically reactive. The effect was strongest in the Yangtze River Delta and Pearl River Delta. Limiting climate change is essential for controlling future pollution and protecting health.


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Direct thermal enhancement dominates over emission-mediated pathways in heatwave-induced O3 and SOA increases across China

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