Changes in satellite retrievals of atmospheric composition over eastern China during the 2020 COVID-19 lockdowns

Field, Robert D.; Hickman, Jonathan E.; Geogdzhayev, Igor V.; Tsigaridis, Kostas; Bauer, Susanne E.

doi:https://doi.org/10.5194/acp-21-18333-2021

Articles | Volume 21, issue 24

https://doi.org/10.5194/acp-21-18333-2021

© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/acp-21-18333-2021

© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 21, issue 24

Research article

|

17 Dec 2021

Research article |

| 17 Dec 2021

Changes in satellite retrievals of atmospheric composition over eastern China during the 2020 COVID-19 lockdowns

Robert D. Field, Jonathan E. Hickman, Igor V. Geogdzhayev, Kostas Tsigaridis, and Susanne E. Bauer

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Final revised paper (published on 17 Dec 2021)
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Preprint (discussion started on 16 Jul 2020)
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Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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RC1: 'Report on the manuscript by Field et al.', Anonymous Referee #2, 17 Aug 2020
- RC2: 'Interactive comment on “Changes in satellite retrievals of atmospheric composition over eastern China during the 2020 COVID-19 lockdowns”', Anonymous Referee #1, 20 Aug 2020
AC1: 'Response to reviewers', Robert Field, 05 Oct 2020

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

AR by Robert Field on behalf of the Authors (05 Oct 2020) Author's response Manuscript

ED: Referee Nomination & Report Request started (27 Oct 2020) by Michel Van Roozendael

RR by Anonymous Referee #1 (12 Nov 2020)

RR by Anonymous Referee #2 (27 Nov 2020)

Suggestions for revision or reasons for rejection

Review of the revised manuscript by Field et al. "Changes in satellite retrievals of atmospheric composition over eastern China during the 2020 COVID-19 lockdowns"

I am glad that the authors acknowledge the obvious flattening of the observed decrease of SO2, NO2 and AOD in recent years. Nevertheless, they persist in using their wrong reference period (2011-2019) as basis for their extrapolation to 2020. I agree that they do present an analysis of the impact of different reference periods (Fig.6). But their abstract and conclusions are still centred on results which ignore the flattening of the trend, e.g. "OMI NO2 in 2020 over central east China was (...) only 17% less than what would be expected from trends", which is misleading.

The flattening is not an accident. Among several possible explanations for it, the most straightforward is that the potential for further reduction obviously decreases when the column decreases. This is something that the assumption of a linear decrease simply cannot capture. A better regression model would be an exponential decrease. This is simple to achieve: only perform a linear fit of the logarithm of the column, as for example in the recent study of Diamond and Wood (2020). Note that the linear trend of -0.056 DU yr-1 inferred for SO2 (Figure 5) corresponds to a relative trend of -14% yr-1 in 2011, which increases to -100% yr-1 in 2018 and explodes in 2019. Using such a basis for extrapolation to 2020 is meaningless (I acknowledge that negative columns are common in DOAS retrievals, but if, as the authors suggest, they are likely "below detection limits", then I think they should not be used for any extrapolation). Things are less dramatic for NO2, but qualitatively similar, as the NO2 column decreases by almost a factor of 2 between 2011 and 2019. Interestingly, the NO2 trend calculated from 2016-2019 data is 3 times lower than the trend calculated from 2011-2019. This implies that even the relative trend has diminished in amplitude over the period. The 2016-2019 period is very likely a much more realistic basis for extrapolation to 2020 than 2011-2019.

In conclusion, I do not require further analysis of the data, but I strongly recommend that the authors do better emphasize the existence and importance of the flattening. One important consequence is that the best estimate of the difference attributed to COVID is larger than is currently claimed in this article.

Reference : Diamond, M. S., and Wood, R., Geophys. Res. Lett., 47, e2020GL088913, https://doi.org/10.1029/2020GL088913, 2020.

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ED: Publish subject to minor revisions (review by editor) (25 Jan 2021) by Michel Van Roozendael

AR by Robert Field on behalf of the Authors (19 Jul 2021) Author's response Author's tracked changes Manuscript

ED: Reconsider after major revisions (09 Aug 2021) by Michel Van Roozendael

AR by Robert Field on behalf of the Authors (30 Sep 2021) Author's response Author's tracked changes Manuscript

ED: Publish as is (15 Nov 2021) by Michel Van Roozendael

AR by Robert Field on behalf of the Authors (16 Nov 2021) Author's response Manuscript

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Short summary

In this study, we examined changes in atmospheric composition over China from satellite measurements during the COVID lockdowns of 2020. We found that interpreting changes in 2020 depended strongly on the background period and whether trends were accounted for. In most cases, pollution levels in 2020 were lower than during the previous few years when pollution levels appear to have stabilized.