Nitrate-dominated PM<sub>2.5</sub> and elevation of particle pH observed in urban Beijing during the winter of 2017

Xie, Yuning; Wang, Gehui; Wang, Xinpei; Chen, Jianmin; Chen, Yubao; Tang, Guiqian; Wang, Lili; Ge, Shuangshuang; Xue, Guoyan; Wang, Yuesi; Gao, Jian

doi:https://doi.org/10.5194/acp-20-5019-2020

Articles | Volume 20, issue 8

https://doi.org/10.5194/acp-20-5019-2020

© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Special issue:

Multiphase chemistry of secondary aerosol formation under...

https://doi.org/10.5194/acp-20-5019-2020

© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 20, issue 8

Research article

|

28 Apr 2020

Research article |

| 28 Apr 2020

Nitrate-dominated PM_2.5 and elevation of particle pH observed in urban Beijing during the winter of 2017

Yuning Xie, Gehui Wang, Xinpei Wang, Jianmin Chen, Yubao Chen, Guiqian Tang, Lili Wang, Shuangshuang Ge, Guoyan Xue, Yuesi Wang, and Jian Gao

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

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RC1: 'Review of Xie et al.', Anonymous Referee #1, 09 Sep 2019
- AC2: 'Reply to comments by referee#1', Y.N. Xie, 25 Dec 2019
RC2: 'Review of ACP 541', Anonymous Referee #2, 16 Oct 2019
- AC1: 'Reply to comments by referee #2', Y.N. Xie, 25 Dec 2019

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

AR by Anna Wenzel on behalf of the Authors (14 Jan 2020) Author's response

ED: Referee Nomination & Report Request started (18 Jan 2020) by Jingkun Jiang

RR by Anonymous Referee #3 (30 Jan 2020)

Suggestions for revision or reasons for rejection

As mentioned in its title and abstract, the key argument made in this paper is that “As nitrate’s fraction significantly elevated, particle pH was also found to increase in winter Beijing given sufficient ammonia (average concentration 7.1μg/m3, 12.9μg/m3 during pollution). During PM2.5 pollution episodes, the particle pH predicted increased from 4.4 (moderate acidic) to 5.4 (near neutral) as nitrate to sulfate molar ratio increased from 1 to 5.”

Currently, this argument is mainly supported by Figure 6 “Scatter plot of simulated pH vs. the molar ratio of nitrate to sulfate”, when only the data during pollution classification and with sufficient aerosol liquid water (above 5μg/m3) was chosen. The authors also use sensitivity studies to exclude the potential effect of crustal elements on the results.

As we all know, correlation does not necessarily imply causation. I suggest the authors make additional sensitivity tests to rule out other possible reasons for the observed correlation between pH and Ratio(N-to-S). The possibilities include: (1) ammonia. Will the authors find a relationship between gaseous NH3 and pH? Previous literature, some of which are citied here, show the positive relationship between pH and log10(NH3); (2) ambient relative humidity RH. Will the authors find a relationship between pH and RH? Higher RH may lead to higher aerosol water amount and then higher pH. (3) Temperature. Higher temperature may lead to lower pH. These sensitivity tests should be conducted before drawing the conclusion that elevated nitrate fraction leads to elevated pH.

In fact, the best way to demonstrating the relationship between nitrate fraction and pH, I suggest, is to replace nitrate with the same moles of sulfate in the input of the thermodynamic equilibrium model, or to replace sulfate with the same moles of nitrate in the input. In this way, all the other model inputs, including RH, temperature, and other cations/anions, are kept the same and assumed as control variables. I highly recommend the authors conduct such sensitivity tests.

These suggested sensitivity analyses imply a major revision to the current paper.

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RR by Anonymous Referee #2 (13 Feb 2020)

ED: Reconsider after major revisions (15 Feb 2020) by Jingkun Jiang

AR by Anna Wenzel on behalf of the Authors (26 Mar 2020) Author's response

ED: Referee Nomination & Report Request started (27 Mar 2020) by Jingkun Jiang

RR by Anonymous Referee #3 (27 Mar 2020)

ED: Publish as is (28 Mar 2020) by Jingkun Jiang

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Short summary

As a result of strict emission control, nitrate-dominated PM_2.5 in pollution episodes was observed in urban Beijing during the winter of 2017–2018. With the help of sufficient ammonia, particle pH could increase to near neutral (5.4) as particulate nitrate fraction increases. Further tests imply that airborne particle hygroscopicity would be enhanced at moderate RH in nitrate-dominated particles, and pH elevation will be accelerated when ammonia and particulate nitrate both increase.

Nitrate-dominated PM2.5 and elevation of particle pH observed in urban Beijing during the winter of 2017

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Nitrate-dominated PM_2.5 and elevation of particle pH observed in urban Beijing during the winter of 2017