the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 24 Aug 2022
What caused ozone pollution during the 2022 Shanghai lockdown? Insights from ground and satellite observations
Abstract. Shanghai, one of China’s most important economic centres, imposed a citywide lockdown in April and May 2022 to contain a resurgence in cases of coronavirus disease 2019. Compared with the 2020 lockdown, the 2022 lockdown occurred in a warm season and lasted much longer, thereby serving as a relevant real-world test of the response of ambient ozone (O3) concentrations to emission reductions in a high-O3 season. In this study, we analysed surface observations of O3 and nitrogen dioxide (NO2) concentrations and satellite-retrieved tropospheric NO2 and formaldehyde (HCHO) column concentrations in the first 5 months of 2022 with comparisons to the year 2021. During the 2-month 2022 lockdown, the maximum daily 8-h average (MDA8) O3 concentrations at one or more of the city’s 19 sites exceeded China’s air quality standard of 160 µg/m3 21 times, with the highest value being 200 µg/m3. The city-average MDA8 O3 concentration increased by 28 % in April–May 2022 year-on-year, despite sharp declines in NO2 surface and column concentrations (both by 49 %) and a 19 % decrease in the HCHO column concentration. These results show that the reductions in O3 precursors and other pollutants during the 2022 lockdown did not prevent ground-level O3 pollution. An analysis of meteorological data indicates that there were only small changes in the meteorological conditions and there was little transport of O3 from the high-O3 inland regions during the 2022 lockdown, neither of which can account for the increased and high concentrations of O3 that were observed during this period. The mean HCHO/NO2 ratio in April–May increased from 1.11 in 2021 to 1.68 in 2022, and the correlation between surface O3 and NO2 concentrations changed from negative in 2021 to positive in 2022. These results indicate that the high O3 concentrations in 2022 were mainly due to large reductions in the emissions of NOx and that the decrease in the concentrations of volatile organic compounds (VOCs) could not overcome the NO titration effect. During the 2022 lockdown, Shanghai’s urban centre remained VOC-sensitive, whereas its semi-rural areas transitioned from VOC-limited to VOC–NOx co-limited regimes. Our findings suggest that future emission reductions similar to those that occurred during the lockdown, such as those that will result from electrifying transportation, will not be sufficient to eliminate O3 pollution in urban areas of Shanghai without the imposition of additional VOC controls or substantial decreases in NOx emissions.
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Notice on discussion status
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
- Preprint
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Supplement
(2248 KB) - BibTeX
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- Final revised paper
Journal article(s) based on this preprint
Interactive discussion
Status: closed
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RC1: 'Comment on egusphere-2022-738', Anonymous Referee #1, 24 Aug 2022
This study is well done and the paper clear and concise. However, the focus is rather local.
One recommendation is to put the findings of the paper into better context. For instance, the NO titration effect is well documented in the literature over the last 30+ years in the US and Europe and more recently in China. So, is it a surprise that you see this effect in Shanghai and other large Chinese cities? Not really as you indicate. Other large world cities have struggled with the issue of decreasing emissions enough to begin seeing reductions in ozone. It would be worth discussing the literature on this topic.
It would also be helpful to discuss better (a few paragraphs) on how HCHO/NO2 and ozone trends have evolved over the satellite record. You only looked at TROPOMI, but the OMI data record is much longer (2005-present). How has the HCHO/NO2 evolved over time in Shanghai and other Chinese cities? This analysis would help to put the magnitude of the 2022 LCD into historical context and help policy-makers to know what VOC and NOx emissions reductions will be required to see ozone decrease. There are studies in the literature that have looked at the historical HCHO/NO2 in China, so you may not need to do your own analysis.
These two recommendations will help you to more broadly discuss the implications of your work for other Chinese cities and to better put the importance of your work (e.g., Summary and Implications) into context. I'm recommending this since your local focus on Shanghai may not fit the aims of ACP: "The journal scope is focused on studies with important implications for our understanding of the state and behaviour of the atmosphere. Articles with a local focus must clearly explain how the results extend and compare with current knowledge."
Line 222: "wug" is not a word. "with"?
Citation: https://doi.org/10.5194/egusphere-2022-738-RC1 -
AC1: 'Final author comment (AC) on behalf of all co-authors, responding to RC1 and RC2', Tao Wang, 24 Oct 2022
We would like to thank both reviewers for their helpful, detailed, and constructive comments. We can include all suggestions in the following revision of the manuscript.
Attached is a PDF file with our detailed point-by-point responses to both RC1 and RC2.
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AC1: 'Final author comment (AC) on behalf of all co-authors, responding to RC1 and RC2', Tao Wang, 24 Oct 2022
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RC2: 'Comment on egusphere-2022-738', Hongbo Fu, 06 Sep 2022
This study describes a study in 2022 Shanghai lockdown by investigating the spatiotemporal variations in surface MDA8 O3 and NO2 concentrations, TROPOMI-based HCHO and NO2 concentrations, and the cause of high O3 concentrations during the 2022 LCD from two aspects: meteorological conditions and O3 formation regimes. It have great insight on O3 concentration increased in 2022 LCD, having greatly detailed data and substantial content. I am glad that the author can report the recent ozone pollution situation to us in a short time and make an in-depth analysis. But there are some problems, which must be solved before it is considered for publication. Below I have reported the lines in the text that require further investigation and clarification.
- L25: the abbreviations of nitrogen should be NOxrather than NOx. Elsewhere the abbreviation is correct
- 2.1 section: There is a lack of description of contaminant monitoring instruments and their detection limits.
- L63: 3.3.1 title “Effects of meteorological conditions on O3 concentrations in Shanghai”, I suggest changing the “O3 concentrations” to “O3 pollution”.
- L164: “A large body of literature has indicated that meteorological conditions can significantly affect O3 concentrations …”. there are only two references in the back, which seems a little weak. Suggest adding more references.
- Fig.2: the inset figures show the monthly average concentrations, but the y-axis is missing units.
- L126: based on the coverage in Shanghai that 2020 LCD date from 23 Jan to 23 Mar, but in this paper is 23 Jan to 12 Feb. Hope you can reconfirm it.
- L209 (Fig.9): in this picture, your x-axis is HCHO/NO2 ratio, and y-axis is MDA8 O3 (ug/m3), but OMI HCHO/NO2 and high-O3 Probability (%) in your reference paper, respectively (Wang et al., 2021). Can you explain this difference? Weather the change of axis will affect O3 formation regimes?
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AC1: 'Final author comment (AC) on behalf of all co-authors, responding to RC1 and RC2', Tao Wang, 24 Oct 2022
We would like to thank both reviewers for their helpful, detailed, and constructive comments. We can include all suggestions in the following revision of the manuscript.
Attached is a PDF file with our detailed point-by-point responses to both RC1 and RC2.
-
AC1: 'Final author comment (AC) on behalf of all co-authors, responding to RC1 and RC2', Tao Wang, 24 Oct 2022
We would like to thank both reviewers for their helpful, detailed, and constructive comments. We can include all suggestions in the following revision of the manuscript.
Attached is a PDF file with our detailed point-by-point responses to both RC1 and RC2.
Interactive discussion
Status: closed
-
RC1: 'Comment on egusphere-2022-738', Anonymous Referee #1, 24 Aug 2022
This study is well done and the paper clear and concise. However, the focus is rather local.
One recommendation is to put the findings of the paper into better context. For instance, the NO titration effect is well documented in the literature over the last 30+ years in the US and Europe and more recently in China. So, is it a surprise that you see this effect in Shanghai and other large Chinese cities? Not really as you indicate. Other large world cities have struggled with the issue of decreasing emissions enough to begin seeing reductions in ozone. It would be worth discussing the literature on this topic.
It would also be helpful to discuss better (a few paragraphs) on how HCHO/NO2 and ozone trends have evolved over the satellite record. You only looked at TROPOMI, but the OMI data record is much longer (2005-present). How has the HCHO/NO2 evolved over time in Shanghai and other Chinese cities? This analysis would help to put the magnitude of the 2022 LCD into historical context and help policy-makers to know what VOC and NOx emissions reductions will be required to see ozone decrease. There are studies in the literature that have looked at the historical HCHO/NO2 in China, so you may not need to do your own analysis.
These two recommendations will help you to more broadly discuss the implications of your work for other Chinese cities and to better put the importance of your work (e.g., Summary and Implications) into context. I'm recommending this since your local focus on Shanghai may not fit the aims of ACP: "The journal scope is focused on studies with important implications for our understanding of the state and behaviour of the atmosphere. Articles with a local focus must clearly explain how the results extend and compare with current knowledge."
Line 222: "wug" is not a word. "with"?
Citation: https://doi.org/10.5194/egusphere-2022-738-RC1 -
AC1: 'Final author comment (AC) on behalf of all co-authors, responding to RC1 and RC2', Tao Wang, 24 Oct 2022
We would like to thank both reviewers for their helpful, detailed, and constructive comments. We can include all suggestions in the following revision of the manuscript.
Attached is a PDF file with our detailed point-by-point responses to both RC1 and RC2.
-
AC1: 'Final author comment (AC) on behalf of all co-authors, responding to RC1 and RC2', Tao Wang, 24 Oct 2022
-
RC2: 'Comment on egusphere-2022-738', Hongbo Fu, 06 Sep 2022
This study describes a study in 2022 Shanghai lockdown by investigating the spatiotemporal variations in surface MDA8 O3 and NO2 concentrations, TROPOMI-based HCHO and NO2 concentrations, and the cause of high O3 concentrations during the 2022 LCD from two aspects: meteorological conditions and O3 formation regimes. It have great insight on O3 concentration increased in 2022 LCD, having greatly detailed data and substantial content. I am glad that the author can report the recent ozone pollution situation to us in a short time and make an in-depth analysis. But there are some problems, which must be solved before it is considered for publication. Below I have reported the lines in the text that require further investigation and clarification.
- L25: the abbreviations of nitrogen should be NOxrather than NOx. Elsewhere the abbreviation is correct
- 2.1 section: There is a lack of description of contaminant monitoring instruments and their detection limits.
- L63: 3.3.1 title “Effects of meteorological conditions on O3 concentrations in Shanghai”, I suggest changing the “O3 concentrations” to “O3 pollution”.
- L164: “A large body of literature has indicated that meteorological conditions can significantly affect O3 concentrations …”. there are only two references in the back, which seems a little weak. Suggest adding more references.
- Fig.2: the inset figures show the monthly average concentrations, but the y-axis is missing units.
- L126: based on the coverage in Shanghai that 2020 LCD date from 23 Jan to 23 Mar, but in this paper is 23 Jan to 12 Feb. Hope you can reconfirm it.
- L209 (Fig.9): in this picture, your x-axis is HCHO/NO2 ratio, and y-axis is MDA8 O3 (ug/m3), but OMI HCHO/NO2 and high-O3 Probability (%) in your reference paper, respectively (Wang et al., 2021). Can you explain this difference? Weather the change of axis will affect O3 formation regimes?
-
AC1: 'Final author comment (AC) on behalf of all co-authors, responding to RC1 and RC2', Tao Wang, 24 Oct 2022
We would like to thank both reviewers for their helpful, detailed, and constructive comments. We can include all suggestions in the following revision of the manuscript.
Attached is a PDF file with our detailed point-by-point responses to both RC1 and RC2.
-
AC1: 'Final author comment (AC) on behalf of all co-authors, responding to RC1 and RC2', Tao Wang, 24 Oct 2022
We would like to thank both reviewers for their helpful, detailed, and constructive comments. We can include all suggestions in the following revision of the manuscript.
Attached is a PDF file with our detailed point-by-point responses to both RC1 and RC2.
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Yue Tan
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
- Preprint
(4050 KB) - Metadata XML
-
Supplement
(2248 KB) - BibTeX
- EndNote
- Final revised paper