the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 06 Dec 2024
What makes the less urbanized city a deeper ozone trap: implications from a case study in the Sichuan Basin, southwest China
Abstract. The urban-rural gradient of surface ozone concentration is widely reported in global megacities. As yet, quantitative analyses for this gradient pattern have been lacking. Using near surface atmospheric pollutant reanalysis and remote sensing measurements, we demonstrate a dipole-like urban surface ozone trap pattern in two megacities (Chengdu and Chongqing) in the Sichuan Basin. During the study period of 2013–2019, the urban-rural gradients of surface ozone level in Chongqing were higher than in Chengdu despite Chongqing’s lower urbanization level. In winter, the ozone level in the core area of Chongqing/Chengdu is 16.4/22.1 μg m-3, with an increasing rate of 8.98 %/5.19 % per 10 km towards the surrounding suburban area. However, the nitrogen dioxide level in Chengdu is higher than in Chongqing. Besides, the concentration levels of formaldehyde and ultraviolet-absorbing aerosol did not show comparable differences between these two cities. Regarding the meteorological conditions, atmospheric visibility, sunshine duration, and nighttime wind speed in Chongqing were all lower compared to Chengdu, the ozone trap pattern aligns more with meteorological condition rather than chemical condition. Our study characterized the ozone trap pattern for two megacities with different urbanization levels, providing a novel perspective on urban atmospheric environment assessment.
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RC1: 'Comment on egusphere-2024-3180', Anonymous Referee #1, 11 Mar 2025
General Comments
Wang and coauthors present on urban-rural gradients of O3 and NOx extending from two cities within the Sichuan Basin. Although the authors do not introduce entirely novel concepts, the clarity of the analysis and the unique geographical context of the study make the manuscript a useful contribution to the field. While the potential causes for the observed gradient are clearly outlined, a more thorough discussion on NOx-limited vs. VOC-limited chemistry would greatly enhance the depth of the analysis and could help explain seasonal differences in the observed trapping patterns. Specifically, expanding the focus beyond O3 titration and loss in the city center to explore how the O3 production sensitivity regime varies across the urban-rural gradient would add significant value. A simple and brief way to approach this could be by examining how the formaldehyde to NO2 ratio changes from VOC-limited to NOx-limited conditions across the gradient for the two cities seasonally. With this addition, along with addressing the specific comments below, I believe the manuscript would be suitable for publication.
Specific Comments
Line 45: This could be more quantitative. What is the projected increase in ozone and what are the climate and emission changes?
Lines 50 – 55: This is also too qualitative. There should be numbers for discrepancies, increases, etc.
Line 67: There should be a new paragraph that sets up the manuscript. The transition to introducing the study is not clear. In particular, the statement that starts with “However, by presenting…” should be rewritten to introduce your study.
Figure 4/Table 1: How many CAQRA sites are within your study region? The plots in figure 4 seem like smooth bilinear interpolations which could be biasing your slopes. Can you either report some statistics of distance between sites (e.g., median, mean, max) or a validation of measurements from your sites within your domain rather than the whole network?
Line 183: While you do present figures and some ranges, there should be numbers backing your statement that the concentration range became wider in higher ISA fraction bins either by a normalized IQR or select ranges for ISA bins.
Line 222-223: How far away from the city core area are the slopes calculated for? In Figure 8 there are two distinct slopes for relative concentration and distance from city center.
Line 229-231: The fact that Chongqing has a deeper O3 trap at lower NO2 levels compared to Chengdu makes me want to see the differences in production sensitivity regimes as per my general comment.
Line 245, Figure A5: What about formaldehyde columns in the summertime? Do they increase and would that help explain O3 production and a decrease in the urban-rural slope in the summertime?
Line 270-271: It is unclear how you got to this conclusion that the meteorological conditions align more closely than chemical conditions based on the preceding paragraph. Was this determined from shorter daylight hours and nighttime wind speeds? While there may not be a difference across the gradient and between cities in the same season, shouldn’t seasonal surface/air temperature also be a cause for higher O3 since it increases emissions of both anthropogenic and biogenic VOCs in a VOC-limited region?
I am also unclear on the windspeed relationship since only nighttime windspeeds are presented. From what I am reading, Chengdu has a higher nighttime windspeed and thus more dispersed and lower NO2 and higher O3. Wouldn't dilution also affect O3 to the same extent? Why is only nighttime presented and what about daytime dilution?
Line 278-280: It is not clear why PM would be a cause for O3 changes rather than just correlated. When the boundary layer is low in the winter PM and NOx concentrations may be higher due to minimized vertical dilution, with NOx titration removing the O3, not PM. Can you explain briefly why PM could “modulate the balance of NO titration and O3 production”? Is it through uptake of HNO3 and H2O2?
Lines 325-326: You state here that winds are lower during the cold season, but only nighttime data is presented. A more thorough and quantitative discussion of meteorological conditions in the paragraph of lines 270-271 would be helpful for supporting the claim that meteorology rather than chemistry drives this seasonal pattern.
Technical Corrections
Line 34: change “condition” to “conditions”
Line 38: In general, when referring to compounds broadly, an article is not needed. For example “Conversely, the NO…” should change to “Conversely, NO…”. This should be applied to the rest of the manuscript.
Line 41: Citation error for Anenberg et al. Remove “Susan”.
Line 185: There is a stray T at the start of a sentence.
Line 196: Figures should be presented in order. So, if presenting the contents of Figure 9 at this point you should briefly describe it here and label it as Figure 7.
Line 290: Change to “VOC-limited”.
Citation: https://doi.org/10.5194/egusphere-2024-3180-RC1 -
RC2: 'Comment on egusphere-2024-3180', Anonymous Referee #2, 21 Apr 2025
Review for Wang et al.
In this study, the authors analyzed the spatial pattern of ozone levels across the Sichuan Basin of China and further investigated the urban-rural intra-pattern of ozone in several cities using Chinese air quality reanalysis (CAQRA) and satellite data products. However, I do not believe this paper offers a substantial contribution to the existing body of knowledge, and its overall structure lacks clarity and organization. Although the manuscript is titled "What makes the less urbanized city a deeper ozone trap: implications of a case study in the Sichuan Basin, southwest China", the authors fail to convincingly address this central question with robust evidence or clear analysis. The spatial distribution of ozone and its precursors in the Sichuan Basin has already been well-documented in prior studies. What novel insights or significant differences does this work present in comparison? Moreover, the analysis relies solely on basic statistical methods to assess ozone and meteorological factors, which limits the scientific rigor of the study. As such, I don't think that this paper merits publication in a high-standard journal as ACP.
Major comments:1) The most critical flaw of this work is its neglect of the impact of complex urban anthropogenic emission changes on the unique deep-basin topography of the Sichuan Basin. Although the authors aim to address the question "What makes the less urbanized city a deeper ozone trap: implications from a case study in the Sichuan Basin, southwest China", they overlook the fact that ozone concentrations in highly urbanized areas are primarily influenced by precursor emissions. This is especially important considering the significant reduction in NOₓ emissions in the Sichuan Basin since the implementation of China’s Air Pollution Prevention and Control Action Plan (APPCAP) in 2013. It is widely acknowledged that meteorological conditions typically affect ozone concentrations only under extreme weather or climate events (such as the ozone surge in the Sichuan Basin during the summer of 2022), while long-term ozone levels in urban areas tend to be more sensitive to changes in precursor emissions. It is undoubtedly insufficient to explain the phenomenon of a deeper ozone trap solely based on a few simple meteorological variables. However, the authors simply skip over the quantitative assessment of the impact of anthropogenic emission processes.
2) The data processing approach is fundamentally flawed. In Line 120, the authors state that they “resampled the CAQRA data to a horizontal resolution of 1 km × 1 km via the LAF algorithm under CDO.” However, such interpolation using CDO undoubtedly neglects the spatial heterogeneity of ozone as an air pollutant. This method lacks scientific rigor, especially considering that ozone distribution in urban centers typically exhibits strong spatial variability.
3) Meteorological data: The authors used a limited number of ground-based meteorological stations for visibility and sunshine duration. However, for two key factors influencing ozone concentration—surface wind speed and surface temperature—they selected ERA5 reanalysis data and MODIS satellite data, respectively. This inconsistency is very confusing to the readers. The Sichuan Basin already has an extensive network of ground-based observation stations that provide comprehensive and direct measurements of 10-meter wind speed and 2-meter air temperature, which are significantly more accurate than satellite or reanalysis data. Without thorough validation of the reliability of surface wind and temperature data, especially the MODIS-derived temperature, I doubt the results derived from such a mixture of data sources.
4) Another major concern is the lack of innovation. The mechanism proposed by the authors to explain the urban-rural ozone gradient has already been well-documented in previous studies. On the one hand, differences in anthropogenic activities determine the emission levels of ozone precursors—vehicular and industrial emissions in urban areas are generally much higher than in rural areas—resulting in distinct ozone formation regimes and consequently affecting ozone concentrations. On the other hand, urbanization-induced heat island effects may alter local atmospheric circulation, thereby influencing the transport of air pollutants. However, most of the conclusions in this study are drawn from basic statistical analysis and lack robust evidence to support the underlying mechanisms discussed.
5) The study also fails to fundamentally address the core question posed in the title: "What makes the less urbanized city a deeper ozone trap: implications from a case study in the Sichuan Basin, southwest China." The mechanism proposed in Figure 9 is merely a simplistic textbook-level extension. The authors neither tested the feasibility of this hypothesis using box modeling approaches such as PBM-MCM nor validated the proposed mechanism through three-dimensional chemical transport models (WRF-Chem or WRF-CMAQ). Such an unvalidated and systematically unassessed hypothesis falls well below the standards expected by Atmospheric Chemistry and Physics (ACP).
6) Line 245: The authors mention TROPOMI NO₂ and HCHO column data and claim that the HCHO data do not exhibit featured spatial patterns. This statement is completely incorrect. Formaldehyde (HCHO), due to its short atmospheric lifetime, high yield from the oxidation of non-methane volatile organic compounds (NMVOCs), and detectability by satellite instruments, has been widely used to characterize the intensity of anthropogenic activities. Moreover, meteorology-corrected TROPOMI HCHO column data can effectively reflect long-term trends in anthropogenic VOC emissions, which is particularly important for explaining ozone variations in urban areas such as Chengdu and Chongqing.
7) The authors utilized the Chinese Air Quality Reanalysis (CAQRA) dataset for their analysis, yet they did not validate the accuracy or reliability of this dataset using surface ozone concentration observations from the China National Environmental Monitoring Center (CNEMC). How do the authors ensure the quality of the CAQRA data and its capability to accurately capture ozone concentrations in the Sichuan Basin?
8) The TROPOMI NO2 and HCHO products and quality control process are not mentioned in Methods section.
9) The manuscript contains numerous grammatical and phrasing errors, making it difficult to follow.
Citation: https://doi.org/10.5194/egusphere-2024-3180-RC2
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