What is the cause(s) of ozone trends in three megacity clusters in eastern China during 2015&ndash;2020?

Hu, Tingting; Lin, Yu; Liu, Run; Xu, Yuepeng; Wang, Boguang; Zhang, Yuanhang; Liu, Shaw Chen

doi:https://doi.org/10.5194/acp-2022-781

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https://doi.org/10.5194/acp-2022-781

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03 Jan 2023

| 03 Jan 2023

Status: this preprint was under review for the journal ACP but the revision was not accepted.

What is the cause(s) of ozone trends in three megacity clusters in eastern China during 2015–2020?

Tingting Hu, Yu Lin, Run Liu, Yuepeng Xu, Boguang Wang, Yuanhang Zhang, and Shaw Chen Liu

Abstract. Thanks to a strong emission control policy, major air pollutants in China, including PM_2.5, SO₂, NO₂ and CO had shown remarkable reductions in 2015–2020. However, ozone (O₃) had increased significantly and emerged as a major air pollutant in eastern China at the same time. The annual mean concentration of maximum daily 8-hour average O₃ (MDA8) in three megacity clusters in eastern China, namely Beijing-Tianjin-Hebei (BTH), Yangtze River Delta (YRD) and Pearl River Delta (PRD), showed alarming large upward linear trends of 2.4, 1.1 and 2.0 ppb yr^-1, respectively during the period 2015–2020. Furthermore, drastic trends of approximately three-fold increase in the number of O₃-exceeding days (defined as MDA8 O₃ >75 ppb) were observed during the same period. Our analysis of the upward trends of the annual mean concentration of MDA8 found that the trends were almost entirely attributable to the increase in the number of consecutive O₃-exceeding days. In addition, a widespread expansion of high O₃ from urban centers to surrounding rural regions was found in 2015–2017, which had made the O₃ spatial distribution becoming more uniform after 2017. Finally, we found that the O₃ episodes with four or more consecutive O₃-exceeding days in the three megacity clusters were closely associated with the position and strength of the West Pacific subtropical high (WPSH), which contributed to the meteorological conditions characterized by clear sky, sinking motion and high vertical stability in the lower troposphere, and high solar radiation and positive temperature anomaly at the surface. These meteorological conditions were highly conducive to O₃ formation. Hence we hypothesize that the cause of the worsening O₃ trends in BTH, YRD and PRD from 2015 to 2020 is attributable to the increased occurrence of meteorological conditions of high solar radiation and positive temperature anomaly under the influence of WPSH, tropical cyclones as well as mid-high latitude wave activities.

Received: 15 Nov 2022 – Discussion started: 03 Jan 2023

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Tingting Hu, Yu Lin, Run Liu, Yuepeng Xu, Boguang Wang, Yuanhang Zhang, and Shaw Chen Liu

Status: closed

AC1: 'Erratum', Run Liu, 06 Jan 2023

This note aims to correct an error in Figure 7 of Hu et al. (2023). In the version of this article initially published, Figure 7 was erroneously pasted. The interpretation of the results remains unchanged.

Citation: https://doi.org/10.5194/acp-2022-781-AC1
RC1:
'Comment on acp-2022-781', Anonymous Referee #2, 20 Jan 2023

This paper analyzes the causes of the 2015-2020 surface ozone increases over three megacity clusters in China (BTH, YRD, PRD) and concludes that increasing Western Pacific Subtropical High (WSPH) conditions are responsible for the ozone increase rather than changes in emissions.

I found the paper difficult to read because it is so chatty, hand-waving, and qualitative and weak in its argumentation. Its central thesis that the ozone trend is driven by the WSPH is in my opinion unsupported and flies in the face of ample literature showing that ozone trends in China over the past decade are anthropogenically driven including as evidence (1) removal of meteorological influence using statistical models, (2) broadening of the ozone season, (3) surge of ozone following the Covid shutdown, (4) consistency with model ozone increases when using anthropogenic emission trends as input. The paper largely ignores this literature, but if it is to make the contrary claim that the ozone trend is in fact driven by meteorology rather than emissions it either needs to refute or show consistency with these different strands of evidence. It does not.

The proposed evidence for a WSPH driver of ozone trends is in my opinion very flimsy. The first piece of evidence proposed is that ozone pollution episodes are becoming more regional, but (a) this does not imply a meteorological trend (witness the US in the 1980s when the same phenomenon was observed), (b) it could reflect the well-known broadening of the ozone season in China (a big weakness of this paper is not resolving the seasonal variation of ozone). The second piece of evidence proposed is the correlation of ozone with temperature and SSR, combined with the trends in these meteorological variables over 2015-2020, but (a) this correlation with meteorological variables is well known, (b) past studies have removed statistically the influence of meteorology on the ozone trend, as is very standard practice.

So I don’t think that this paper should be published in anything close to current form. It could be used to argue wrongly against the urgency for China to decrease VOC emissions, and it will waste the research community’s time in having to debunk it. One interesting result in this paper is the apparent regionalization of ozone pollution in China, which I don’t think has been discussed before. That could provide the basis for a paper but it would need to be better demonstrated.

Specific comments:

Line 65: are only sites with complete records for 2015-2020 used? Otherwise the analysis would be biased by expansion of the network.

Line 83: I didn’t see the criteria for separating clean and polluted sites in Table 1.

Line 125: text and captions don’t match what is actually shown in Figures 5-7. Comparing just two years of data (2017 vs. 2015)as a trend indicator is obviously bad – of course the difference between two individual years could be meteorologically driven.

Line 136, elsewhere: a big weakness of this paper is not resolving the ozone data by season as is standard practice. In particular, it is not clear to me that this regionalization of ozone could not simply reflect the broadening of the ozone season that has been reported in previous papers.

Line 141: why jump to an attribution to weather? This is characteristic of the weak argumentation throughout this paper. Same thing in line 151 – why would the regionalization of BTH and YRD be very unlikely to be driven by emissions?

Line 184: no one is arguing that the increase in ozone is driven by decreasing NO titration (OK, maybe in winter, but ozone is low then anyhow). The argument is that it is driven by NOx emission decreases under VOC-limited conditions.

Line 187: past studies have attributed the ozone increase to PM2.5 decrease only for summer. Again, the paper would need to resolve its analysis by season.

Line 241, elsewhere: there is nothing new in the attribution of ozone pollution episodes to the WSPH. The lengthy discussion of this attribution is just routine.

Citation: https://doi.org/10.5194/acp-2022-781-RC1
- AC3: 'Reply on RC1', Run Liu, 13 Feb 2023
  
  The comment was uploaded in the form of a supplement: https://acp.copernicus.org/preprints/acp-2022-781/acp-2022-781-AC3-supplement.pdf
  
  Citation: https://doi.org/10.5194/acp-2022-781-AC3
- AC4: 'Reply on RC1', Run Liu, 13 Feb 2023
  
  Publisher’s note: this comment is a copy of AC3 and its content was therefore removed.
  
  Citation: https://doi.org/10.5194/acp-2022-781-AC4
RC2:
'Comment on acp-2022-781', Anonymous Referee #1, 21 Jan 2023
The annual mean concentration of MDA8 increased at a high rate in BTH, YRD and PRD during the period 2015-2020. The spatial expansion of high O3 from urban centers to surrounding regions was found during 2015-2017, accompanied by a saturation effect. The authors suggest that the occurrence of meteorological conditions of solar radiation and positive temperature under WPSH and mid-high latitude wave activities are the main reason for the increased O3 episodes with four or more consecutive O3-exceeding days in the three megacity clusters. The paper is logical and informative. It is a novel and interesting topic. I suggest it to be accepted after addressing the following comments.

L91-93, The increase in the contribution of O3-exceeding days is the primary contributor to the large increase in the annual mean O3. The contribution of O3-exceeding days is affected by exceeding days and concentrations. Therefore, I think the mean concentration of O3-exceeding days should be shown.

L162-163, The contribution of the increased O3 concentration and the number of O3-exceeding days is +20% and +34%, respectively. Please elaborate more about this method, how is the contribution calculated? L214-219, and the difference in O3 between 2017 and 2015 can be attributed to the large number of days and higher average concentration with four or more consecutive O3-exceeding days and those with less than four consecutive O3-exceeding days. âHow are the respective contributions distinguished?

L181-184, The emission of air pollutants and O3 precursors isn’t a main cause of the expansion and saturation in BTH. Is the result the same in PRD and YRD?

The values of SSR and T2m of the O3 episodes with four or more consecutive O3-exceeding days is lower than those of O3 episodes with less than four consecutive O3-exceeding days in PRD during 2017-2020, which are different with in BTH and YRD. In addition, O3-exceeding days in PRD are mostly decoupled from those in BTH and YRD. Does it imply that the cause of worsening O3 trend in PRD is different with BTH and YRD?

L280-284, The annual weighted SSR/T2m can be performed by multiplying the difference between the four or more consecutive O3-exceeding days and clean days of SSR/T2m with the frequency of occurrence of O3 episodes with four or more consecutive O3-exceeding days each year. Is that right? Has this approach been used in previous studies? Do the months inside the parentheses in Figure 12 represent the months when O3-exceeding days happen in this region?

Why did the expansion and saturation occur mostly during 2015-2017? O3 concentrations have increased in 2015-2019. The annual mean concentration of MAD8 decreased significantly in 2020. In BTH, YRD and PRD, the number of days of all O3-exceeding days has increased from 2015 to 2019. However, the number of O3-exceeding days decreases in 2020. Is this change also due to the influence of WPSH and mid-high latitude wave activities?

Figure 7 is the same as Figure 6. Please check it.
Citation: https://doi.org/10.5194/acp-2022-781-RC2
- AC2: 'Reply on RC2', Run Liu, 11 Feb 2023
  
  The comment was uploaded in the form of a supplement: https://acp.copernicus.org/preprints/acp-2022-781/acp-2022-781-AC2-supplement.pdf
  
  Citation: https://doi.org/10.5194/acp-2022-781-AC2

Status: closed

AC1: 'Erratum', Run Liu, 06 Jan 2023

This note aims to correct an error in Figure 7 of Hu et al. (2023). In the version of this article initially published, Figure 7 was erroneously pasted. The interpretation of the results remains unchanged.

Citation: https://doi.org/10.5194/acp-2022-781-AC1
RC1:
'Comment on acp-2022-781', Anonymous Referee #2, 20 Jan 2023

This paper analyzes the causes of the 2015-2020 surface ozone increases over three megacity clusters in China (BTH, YRD, PRD) and concludes that increasing Western Pacific Subtropical High (WSPH) conditions are responsible for the ozone increase rather than changes in emissions.

I found the paper difficult to read because it is so chatty, hand-waving, and qualitative and weak in its argumentation. Its central thesis that the ozone trend is driven by the WSPH is in my opinion unsupported and flies in the face of ample literature showing that ozone trends in China over the past decade are anthropogenically driven including as evidence (1) removal of meteorological influence using statistical models, (2) broadening of the ozone season, (3) surge of ozone following the Covid shutdown, (4) consistency with model ozone increases when using anthropogenic emission trends as input. The paper largely ignores this literature, but if it is to make the contrary claim that the ozone trend is in fact driven by meteorology rather than emissions it either needs to refute or show consistency with these different strands of evidence. It does not.

The proposed evidence for a WSPH driver of ozone trends is in my opinion very flimsy. The first piece of evidence proposed is that ozone pollution episodes are becoming more regional, but (a) this does not imply a meteorological trend (witness the US in the 1980s when the same phenomenon was observed), (b) it could reflect the well-known broadening of the ozone season in China (a big weakness of this paper is not resolving the seasonal variation of ozone). The second piece of evidence proposed is the correlation of ozone with temperature and SSR, combined with the trends in these meteorological variables over 2015-2020, but (a) this correlation with meteorological variables is well known, (b) past studies have removed statistically the influence of meteorology on the ozone trend, as is very standard practice.

So I don’t think that this paper should be published in anything close to current form. It could be used to argue wrongly against the urgency for China to decrease VOC emissions, and it will waste the research community’s time in having to debunk it. One interesting result in this paper is the apparent regionalization of ozone pollution in China, which I don’t think has been discussed before. That could provide the basis for a paper but it would need to be better demonstrated.

Specific comments:

Line 65: are only sites with complete records for 2015-2020 used? Otherwise the analysis would be biased by expansion of the network.

Line 83: I didn’t see the criteria for separating clean and polluted sites in Table 1.

Line 125: text and captions don’t match what is actually shown in Figures 5-7. Comparing just two years of data (2017 vs. 2015)as a trend indicator is obviously bad – of course the difference between two individual years could be meteorologically driven.

Line 136, elsewhere: a big weakness of this paper is not resolving the ozone data by season as is standard practice. In particular, it is not clear to me that this regionalization of ozone could not simply reflect the broadening of the ozone season that has been reported in previous papers.

Line 141: why jump to an attribution to weather? This is characteristic of the weak argumentation throughout this paper. Same thing in line 151 – why would the regionalization of BTH and YRD be very unlikely to be driven by emissions?

Line 184: no one is arguing that the increase in ozone is driven by decreasing NO titration (OK, maybe in winter, but ozone is low then anyhow). The argument is that it is driven by NOx emission decreases under VOC-limited conditions.

Line 187: past studies have attributed the ozone increase to PM2.5 decrease only for summer. Again, the paper would need to resolve its analysis by season.

Line 241, elsewhere: there is nothing new in the attribution of ozone pollution episodes to the WSPH. The lengthy discussion of this attribution is just routine.

Citation: https://doi.org/10.5194/acp-2022-781-RC1
- AC3: 'Reply on RC1', Run Liu, 13 Feb 2023
  
  The comment was uploaded in the form of a supplement: https://acp.copernicus.org/preprints/acp-2022-781/acp-2022-781-AC3-supplement.pdf
  
  Citation: https://doi.org/10.5194/acp-2022-781-AC3
- AC4: 'Reply on RC1', Run Liu, 13 Feb 2023
  
  Publisher’s note: this comment is a copy of AC3 and its content was therefore removed.
  
  Citation: https://doi.org/10.5194/acp-2022-781-AC4
RC2:
'Comment on acp-2022-781', Anonymous Referee #1, 21 Jan 2023
The annual mean concentration of MDA8 increased at a high rate in BTH, YRD and PRD during the period 2015-2020. The spatial expansion of high O3 from urban centers to surrounding regions was found during 2015-2017, accompanied by a saturation effect. The authors suggest that the occurrence of meteorological conditions of solar radiation and positive temperature under WPSH and mid-high latitude wave activities are the main reason for the increased O3 episodes with four or more consecutive O3-exceeding days in the three megacity clusters. The paper is logical and informative. It is a novel and interesting topic. I suggest it to be accepted after addressing the following comments.

L91-93, The increase in the contribution of O3-exceeding days is the primary contributor to the large increase in the annual mean O3. The contribution of O3-exceeding days is affected by exceeding days and concentrations. Therefore, I think the mean concentration of O3-exceeding days should be shown.

L162-163, The contribution of the increased O3 concentration and the number of O3-exceeding days is +20% and +34%, respectively. Please elaborate more about this method, how is the contribution calculated? L214-219, and the difference in O3 between 2017 and 2015 can be attributed to the large number of days and higher average concentration with four or more consecutive O3-exceeding days and those with less than four consecutive O3-exceeding days. âHow are the respective contributions distinguished?

L181-184, The emission of air pollutants and O3 precursors isn’t a main cause of the expansion and saturation in BTH. Is the result the same in PRD and YRD?

The values of SSR and T2m of the O3 episodes with four or more consecutive O3-exceeding days is lower than those of O3 episodes with less than four consecutive O3-exceeding days in PRD during 2017-2020, which are different with in BTH and YRD. In addition, O3-exceeding days in PRD are mostly decoupled from those in BTH and YRD. Does it imply that the cause of worsening O3 trend in PRD is different with BTH and YRD?

L280-284, The annual weighted SSR/T2m can be performed by multiplying the difference between the four or more consecutive O3-exceeding days and clean days of SSR/T2m with the frequency of occurrence of O3 episodes with four or more consecutive O3-exceeding days each year. Is that right? Has this approach been used in previous studies? Do the months inside the parentheses in Figure 12 represent the months when O3-exceeding days happen in this region?

Why did the expansion and saturation occur mostly during 2015-2017? O3 concentrations have increased in 2015-2019. The annual mean concentration of MAD8 decreased significantly in 2020. In BTH, YRD and PRD, the number of days of all O3-exceeding days has increased from 2015 to 2019. However, the number of O3-exceeding days decreases in 2020. Is this change also due to the influence of WPSH and mid-high latitude wave activities?

Figure 7 is the same as Figure 6. Please check it.
Citation: https://doi.org/10.5194/acp-2022-781-RC2
- AC2: 'Reply on RC2', Run Liu, 11 Feb 2023
  
  The comment was uploaded in the form of a supplement: https://acp.copernicus.org/preprints/acp-2022-781/acp-2022-781-AC2-supplement.pdf
  
  Citation: https://doi.org/10.5194/acp-2022-781-AC2

Tingting Hu, Yu Lin, Run Liu, Yuepeng Xu, Boguang Wang, Yuanhang Zhang, and Shaw Chen Liu

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https://doi.org/10.5194/acp-2022-781-supplement

Tingting Hu, Yu Lin, Run Liu, Yuepeng Xu, Boguang Wang, Yuanhang Zhang, and Shaw Chen Liu

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We hypothesize that the cause of the worsening O₃ trends in the Beijing-Tianjin-Hebei region, the Yangtze River Delta, and the Pearl River Delta from 2015 to 2020 is attributable to the increased occurrence of meteorological conditions of high solar radiation and positive temperature anomaly under the influence of West Pacific Subtropical High, tropical cyclones as well as mid-high latitude wave activities.


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