Why is the Indo-Gangetic Plain the region with the largest NH<sub>3</sub> column in the globe during pre-monsoon and monsoon seasons?

Wang, Tiantian; Song, Yu; Xu, Zhenying; Liu, Mingxu; Xu, Tingting; Liao, Wenling; Yin, Lifei; Cai, Xuhui; Kang, Ling; Zhang, Hongsheng; Zhu, Tong

doi:https://doi.org/10.5194/acp-20-8727-2020

Articles | Volume 20, issue 14

https://doi.org/10.5194/acp-20-8727-2020

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

https://doi.org/10.5194/acp-20-8727-2020

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

Articles | Volume 20, issue 14

Research article

|

23 Jul 2020

Research article |

| 23 Jul 2020

Why is the Indo-Gangetic Plain the region with the largest NH₃ column in the globe during pre-monsoon and monsoon seasons?

Tiantian Wang, Yu Song, Zhenying Xu, Mingxu Liu, Tingting Xu, Wenling Liao, Lifei Yin, Xuhui Cai, Ling Kang, Hongsheng Zhang, and Tong Zhu

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

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RC1: 'Review of Wang et al. 2019', Anonymous Referee #1, 18 Jan 2020
RC2: 'Review of “Why the Indo-Gangetic Plain is the region with the largest NH3 column in the globe during summertime?”', Anonymous Referee #2, 21 Jan 2020
AC1: 'Authors' responses to reviewers 1 and 2', SONG YU, 15 Apr 2020

Peer-review completion

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

AR by SONG YU on behalf of the Authors (16 Apr 2020) Author's response Manuscript

ED: Referee Nomination & Report Request started (21 May 2020) by Barbara Ervens

RR by Anonymous Referee #1 (08 Jun 2020)

Suggestions for revision or reasons for rejection

This study aims to explore the reasons behind the elevated levels of ammonia observed over the Indo-Gangetic Plain (IGP). This is an important and scientifically relevant question, particularly since the ammonia burden has significant implications on inorganic aerosol concentrations over the region. The authors use the WRF-Chem model to investigate the physics and thermodynamics underlying the atmospheric fate of NH3 and the resulting analysis provides useful insights into some of the factors driving the high concentrations over the region. Specifically, the authors suggest that despite other regions (such as the North China Plain) having higher ammonia emissions, the IGP has a higher burden of ammonia primarily due to:
1) The lower amounts of NOx and SO2 emissions over the region compared to the NCP (limiting aerosol formation)
2) The terrain and meteorology of the region that result in weak horizontal advection and the accumulation of NH3

While the physical transport and meteorology simulated by the model is validated and constrained, the ammonia simulation and the underlying processes are not explicitly validated. The authors contend that this is due to the lack of available data in the region. However, as a result, while the study provides useful sensitivity analyses, the conclusions are largely model-driven estimations. With this in mind, the study could be strengthened by a more detailed discussion of the underlying uncertainties associated with the different model processes that drive the pathways that the authors identify as being most important to the elevated ammonia burden. The authors could also add a more specific discussion about future work that might observationally validate these conclusions and improve model NH3 representation over this region.

I also suggest the removal of the statement on line 44 - “This is the first study to analyze the causes of high NH3 loading over the IGP considering all possible factors.” – since the study does not exhaustively evaluate various important processes (such as deposition, regional atmospheric chemistry, etc.) that could play an important role.

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ED: Publish subject to minor revisions (review by editor) (09 Jun 2020) by Barbara Ervens

AR by SONG YU on behalf of the Authors (12 Jun 2020) Author's response Manuscript

ED: Publish as is (18 Jun 2020) by Barbara Ervens

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Short summary

Satellite measurements have revealed that the Indo-Gangetic Plain (IGP) has the global maximum ammonia concentrations, with a peak from June to August. Here, we studied the reasons for this phenomenon through computer simulations. Low sulfur dioxide and nitrogen oxides emissions and high air temperature over the IGP weaken the swallowing of gaseous ammonia by acidic gases. Additionally, the barrier effects of the Himalayas, like a windshield, are also conducive to the accumulation of ammonia.

Why is the Indo-Gangetic Plain the region with the largest NH3 column in the globe during pre-monsoon and monsoon seasons?

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Why is the Indo-Gangetic Plain the region with the largest NH₃ column in the globe during pre-monsoon and monsoon seasons?