the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 28 Mar 2025
A comprehensive review of tropospheric background ozone: Definitions, estimation methods, and meta-analysis of its spatiotemporal distribution in China
Abstract. Background ozone (O3) refers to O3 concentrations that remain unaffected by direct local anthropogenic emissions, critical for comprehending tropospheric O3 pollution, as it defines the baseline levels without local anthropogenic emissions. Accurately estimating background O3 is essential for determining the maximum achievable reductions in O3 through anthropogenic precursor emissions control and for developing effective air quality management strategies. This review synthesizes the definition and estimation methods for background O3, including in situ measurement, statistical analysis, numerical modeling, and integrated method. A meta-analysis of the spatiotemporal distribution of background O3 across China from 1994 to 2020 reveals substantial spatial variability, with the highest concentrations in the Northwest region (48 ppb) and the lowest in the Northeast and Central regions (~33 ppb). The national average background O3 concentration is approximately 40 ppb, contributing 77 % to the tropospheric maximum daily 8-hour average ozone. Estimation methods show notable discrepancies: in situ measurement and statistical analysis methods yield higher estimates, while integrated method provide lower yet more consistent values. On a global scale, background O3 concentrations in China are ranked medium-to-high and exhibit an increasing trend. This review, from a global perspective, highlights the need for integrated estimation methods to improve accuracy, underscores the importance of international collaboration in addressing long-range pollutant transport, and calls for further research on the interactions between background O3 and climate change. By advancing the understanding of background O3 dynamics, this study provides critical insights for atmospheric chemistry research and air pollution control efforts in China and beyond.
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RC1: 'Comment on egusphere-2025-687', Anonymous Referee #1, 05 Jun 2025
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General comments
Chen et al. review the methods for estimating background ozone concentrations, which determine the maximum achievable reduction in ozone pollution through mitigation of local anthropogenic emissions, with a specific focus on background ozone in China. The paper is informative and organized, but it is overly long in some sections. It is also important for the authors to say why this review is needed when there are so many other similar recent papers reviewing ozone in China (Liu et al., 2020; Lu et al., 2019; Sahu et al., 2021; Wang et al., 2022; Xu, 2021). I think the key is that this paper analyzes background ozone specifically, but that concept is also addressed quite thoroughly in most papers reviewing drivers of ozone concentrations and trends.
I would also suggest that the authors significantly reduce the length of Section 4, which is quite long-winded and repetitive. Section 5.2 and Figure 5 are the most interesting pieces of the review, and the discussion seemed too short. Why is the integrated method a much smaller range? Is there regional variability in the background ozone estimated from the integrated method? Should we “trust” the results of the integrated method above the other methods and, if so, why? I think this information is discussed more in Section 4.4, but I would move this information to the section where Figure 5 is discussed.
In addition, I would expect the paper to have more discussion of the seasonality of background ozone, which is significant in a region like China (Chen et al., 2023). Finally, I suggest adding the following citations from potentially relevant studies. Han et al. (2019) quantify the influence of foreign ozone on East Asia using GEOS-Chem simulations. Colombi et al. (2023) found that subsidence of elevated free troposphere ozone drove surface background ozone in East Asia in May to exceed 50 ppb, which is much higher than the background values in North America and Europe. Wang et al. (2022) showed that the high free tropospheric ozone in East Asia has been increasing in the last two decades, which may lead to increased background ozone. Ma et al. (2025) diagnose causes of high free tropospheric ozone. Luo et al. (2024) also discuss STE as a source of background ozone in the North China Plain. There may be many other papers I am missing, but I leave the decision of whether to add these citations up to the authors’ discretion.
Overall, I think this review is potentially useful, but would benefit from shortening Section 4 and more clearly stating why this review is different and necessary given the many other reviews of ozone in China from the last 5 years.
Specific comments
- I suggest adding a sentence that clearly describes the concept of background ozone to the Introduction of the paper? For example, around line 68 in the introduction, I think a sentence is needed that says something like: “Background surface ozone is defined as the ozone that would be present in a given region in the absence of local anthropogenic emissions, and can be influenced by local natural factors or distant anthropogenic and natural factors.” It is stated in the abstract, but should be defined again in the introduction.
- Figure 1 is a nice visualization. I like how the beaker implies chemical reactions occurring in the atmosphere, and shows how anthropogenic emissions add ozone on top of the background.
- Line 163: The conversion from µg L-3 to ppb is strange to me. Why would you assume a temperature of 0˚C? How big of a difference does it make if you assume a more reasonable temperature and pressure rather than STP? Could you use meteorological data to assume a more accurate and precise temperature and pressure?
- Figure 3 is pretty wordy and not easy to absorb the information presented. I suggest significantly revising this figure to make it easier to digest and more visually appealing. At the very least, the authors should remove unnecessary words (e.g., "suitable" is over-used).
- Figure 4: Could you please make the shading of each region be a color that represents the magnitude of the background ozone concentration? For example, a color bar that ranges from 25 to 50 ppb where lower background ozone is a lighter color and higher background is a darker color. It is hard to view the spatial variability of background ozone from the bar charts.
- Figure 5: where are the estimates of background ozone from numerical modeling, statistical methods, and integrated methods coming from? I assume it is from Table S1, but there is no citation or reference to table S1 in figure caption. Is n= at the bottom of each box referring to the number of studies compiled or the number of measurements? Overall, this caption should be more descriptive.
- Section 5.3 and Figure 6: Why is NC the only region that has two separate lines showing the trends in background ozone?
- Same comment for Figure 7 as made for Figure 4. Please consider shading the regions according to the background ozone concentration
- The conclusions section is informative and written well
Technical corrections
- Lines 8-10: The first sentence of the abstract is worded in a confusing way. I suggest: Background ozone (O3) refers to baseline O3 concentrations in the absence of local anthropogenic emissions, critical for understanding and mitigating tropospheric O3 pollution.
Citations:
Chen, Z., Xie, Y., Liu, J., Shen, L., Cheng, X., Han, H., et al. (2023). Distinct seasonality in vertical variations of tropospheric ozone over coastal regions of southern China. Science of The Total Environment, 874, 162423. https://doi.org/10.1016/j.scitotenv.2023.162423
Colombi, N. K., Jacob, D. J., Yang, L. H., Zhai, S., Shah, V., Grange, S. K., et al. (2023). Why is ozone in South Korea and the Seoul metropolitan area so high and increasing? Atmospheric Chemistry and Physics, 23(7), 4031–4044. https://doi.org/10.5194/acp-23-4031-2023
Han, H., Liu, J., Yuan, H., Wang, T., Zhuang, B., & Zhang, X. (2019). Foreign influences on tropospheric ozone over East Asia through global atmospheric transport. Atmospheric Chemistry and Physics, 19(19), 12495–12514. https://doi.org/10.5194/acp-19-12495-2019
Liu, H., Zhang, M., & Han, X. (2020). A review of surface ozone source apportionment in China. Atmospheric and Oceanic Science Letters, 13(5), 470–484. https://doi.org/10.1080/16742834.2020.1768025
Lu, X., Zhang, L., & Shen, L. (2019). Meteorology and Climate Influences on Tropospheric Ozone: a Review of Natural Sources, Chemistry, and Transport Patterns. Current Pollution Reports, 5(4), 238–260. https://doi.org/10.1007/s40726-019-00118-3
Luo, Y., Zhao, T., Meng, K., Hu, J., Yang, Q., Bai, Y., et al. (2024). A mechanism of stratospheric O 3 intrusion into the atmospheric environment: a case study of the North China Plain. Atmospheric Chemistry and Physics, 24(12), 7013–7026. https://doi.org/10.5194/acp-24-7013-2024
Ma, X., Huang, J., Hegglin, M. I., Jöckel, P., & Zhao, T. (2025). Causes of growing middle-to-upper tropospheric ozone over the northwest Pacific region. Atmospheric Chemistry and Physics, 25(2), 943–958. https://doi.org/10.5194/acp-25-943-2025
Sahu, S. K., Liu, S., Liu, S., Ding, D., & Xing, J. (2021). Ozone pollution in China: Background and transboundary contributions to ozone concentration and related health effects across the country. Science of The Total Environment, 761, 144131. https://doi.org/10.1016/j.scitotenv.2020.144131
Wang, T., Xue, L., Feng, Z., Dai, J., Zhang, Y., & Tan, Y. (2022). Ground-level ozone pollution in China: a synthesis of recent findings on influencing factors and impacts. Environmental Research Letters, 17(6), 063003. https://doi.org/10.1088/1748-9326/ac69fe
Xu, X. (2021). Recent advances in studies of ozone pollution and impacts in China: A short review. Current Opinion in Environmental Science & Health, 19, 100225. https://doi.org/10.1016/j.coesh.2020.100225
Citation: https://doi.org/10.5194/egusphere-2025-687-RC1
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