Inter-annual variations of wet deposition in Beijing from 2014–2017: implications of below-cloud scavenging of inorganic aerosols

Ge, Baozhu; Xu, Danhui; Wild, Oliver; Yao, Xuefeng; Wang, Junhua; Chen, Xueshun; Tan, Qixin; Pan, Xiaole; Wang, Zifa

doi:https://doi.org/10.5194/acp-21-9441-2021

Articles | Volume 21, issue 12

https://doi.org/10.5194/acp-21-9441-2021

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

https://doi.org/10.5194/acp-21-9441-2021

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

Articles | Volume 21, issue 12

Research article

|

22 Jun 2021

Research article |

| 22 Jun 2021

Inter-annual variations of wet deposition in Beijing from 2014–2017: implications of below-cloud scavenging of inorganic aerosols

Baozhu Ge, Danhui Xu, Oliver Wild, Xuefeng Yao, Junhua Wang, Xueshun Chen, Qixin Tan, Xiaole Pan, and Zifa Wang

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Final revised paper (published on 22 Jun 2021)
Supplement to the final revised paper
Preprint (discussion started on 27 Nov 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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- Supplement

RC1: 'Comment on acp-2020-1146', Anonymous Referee #1, 25 Dec 2020
- AC1: 'Reply on RC1', Baozhu Ge, 06 Mar 2021
RC2: 'Review of acp-2020-1146', Anonymous Referee #2, 29 Dec 2020
- AC2: 'Reply on RC2', Baozhu Ge, 06 Mar 2021

Peer-review completion

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

AR by Baozhu Ge on behalf of the Authors (24 Mar 2021) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (25 Mar 2021) by Leiming Zhang

RR by Anonymous Referee #1 (05 Apr 2021)

RR by Anonymous Referee #2 (06 Apr 2021)

Suggestions for revision or reasons for rejection

The authors provided a thoughtful response to the major comments. However, some of the information was not incorporated into the paper. To achieve a more balanced discussion on wet deposition of S and N, it is important to discuss in the paper whether wet scavenging of coarse particles and gas-phase S and N compounds are important or not and provide justification. Also, the theoretical explanation on below-cloud and in-cloud scavenging should be included in the paper to support the method used to estimate below-cloud and in-cloud scavenging contributions. Below are some of the specific comments that were partly addressed in the responses but not in the revised paper. Providing additional explanations in the paper would help improve its scientific quality.

L22-24: Why would the Action Plan result in declines in Ca2+ given that it is mainly derived from crustal emissions? - This needs to be explained in the paper.

L30: Was there a continuous year-to-year decline in the percentages, i.e. in 2015 and 2016? If so, this should be stated. If not, the next sentence, which suggests the result is due to the Action Plan, should not be included in the abstract because there is no clear decreasing trend. - In the response, the authors stated there was an overall decline but not a year-to-year decline in the percentages. If there is no year-to-year decline, the percentage decreases from 2013 to 2017 are likely not statistically significant.

L78-80: Each precipitation event is unique in terms rainfall intensity, droplet sizes and distribution, precipitation form (rain, snow, or sleet), air concentrations of chemical components, etc. Meteorology varies with location; some locations are frequently affected by thunderstorms or deep convective scavenging. Can you address whether these factors are taken into consideration when using the sequential precipitation sampling method to estimate in-cloud or below-cloud scavenging contributions? - These uncertainties should be discussed in the paper because it is not clear how these factors affect wet scavenging and the proportions from in-cloud and below-cloud processes.

L141-147: One initial criticism I have on this assumption is why below-cloud and in-cloud processes have to occur one after another instead of simultaneously. A full justification is necessary given that the major results of this study depend on this analysis and interpretation. – The theoretical explanation that was provided in the response should be addressed in the paper as well.

Section 3.1: Why are the results of other inorganic ions not discussed? For a better understanding of the impacts of acidification on ecosystems, wet deposition fluxes should be presented as well. While there may be significant decreases in VWM concentrations, interannual variability in precipitation amounts may result in less significant decreases in wet deposition fluxes. - The paper did not discuss the results for Na+, K+, Mg2+, Cl- and F-, which were measured in the study as stated in the methodology. There was also no discussion on the wet deposition fluxes and how changes in the precipitation amounts can affect trends in the wet deposition fluxes.

Section 3.3: An exponential curve was used to fit the data points and then the horizontal asymptote was used to determine the precipitation concentration representative of rainout. One issue that I have with this approach is that there were only 11 data points or precipitation fractions. Previous studies (e.g. Aikawa and Hiraki, 2009) that conducted sequential sampling of precipitation found that the precipitation concentrations could increase in the much latter fractions of precipitation. It seems that more precipitation fractions need to be collected in order to observe what happens to the precipitation concentrations. – Based on the responses, the authors agree that longer fraction measurements and more detailed analysis on the uncertainties are needed in the future. This should be mentioned in the paper.

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ED: Reconsider after major revisions (08 Apr 2021) by Leiming Zhang

AR by Baozhu Ge on behalf of the Authors (18 May 2021) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (20 May 2021) by Leiming Zhang

RR by Anonymous Referee #2 (21 May 2021)

ED: Publish as is (21 May 2021) by Leiming Zhang

AR by Baozhu Ge on behalf of the Authors (24 May 2021)

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Short summary

In this study, an improved sequential sampling method is developed and implemented to estimate the contribution of below-cloud and in-cloud wet deposition over four years of measurements in Beijing. We find that the contribution of below-cloud scavenging for Ca²⁺, SO₄ ^2–, and NH₄⁺ decreases from above 50 % in 2014 to below 40 % in 2017. This suggests that the Action Plan has mitigated particulate matter pollution in the surface layer and hence decreased scavenging due to the washout process.