Modeling urban pollutant transport at multi-resolutions: Impacts of turbulent mixing

Yang, Zining; Du, Qiuyan; Yang, Qike; Zhao, Chun; Li, Gudongze; Xia, Zihan; Xu, Mingyue; Yuan, Renmin; Li, Yubin; Xia, Kaihui; Gu, Jun; Feng, Jiawang

doi:https://doi.org/10.5194/egusphere-2024-3890

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https://doi.org/10.5194/egusphere-2024-3890

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16 Jan 2025

| 16 Jan 2025

Modeling urban pollutant transport at multi-resolutions: Impacts of turbulent mixing

Zining Yang, Qiuyan Du, Qike Yang, Chun Zhao, Gudongze Li, Zihan Xia, Mingyue Xu, Renmin Yuan, Yubin Li, Kaihui Xia, Jun Gu, and Jiawang Feng

Abstract. Air pollution in cities seriously impacts public health and regional climate. Turbulent mixing plays a crucial role in pollutant formation and dissipation, yet current atmospheric models struggle to accurately represent it. The intensity of turbulent mixing varies with model resolution, which has rarely been analyzed. To investigate turbulent mixing variations at multi-resolutions and their implications for urban pollutant transport, we conducted numerical experiments using WRF-Chem at 25 km, 5 km, and 1 km resolutions. The simulated meteorological fields and black carbon (BC) concentrations are compared with observations. Differences in turbulent mixing across multi-resolutions are more pronounced at night, resulting in noticeable variations in BC concentrations. BC surface concentrations decrease as resolution increases from 25 km to 5 km and further to 1 km, but are similar at 5 km and 1 km resolutions. Enhanced planetary boundary layer (PBL) mixing coefficients and vertical wind flux at higher resolutions reduce the overestimation of nighttime BC surface concentrations. The 1 km resolution parameterized lower PBL mixing coefficients than 5 km but resolved more small-scale eddies, leading to similar near-surface turbulent mixing at both resolutions, while the intensity at higher altitudes is greater at 1 km. This caused BC to be transported higher and farther, increasing its atmospheric lifetime and column concentrations. Variations in mixing coefficients are partly attributed to differences in land use and terrain, with higher resolutions providing more detailed data that enhanced PBL mixing coefficients. This study interprets the impacts of turbulent mixing on simulated urban pollutant diffusion at multi-resolutions.

Received: 10 Dec 2024 – Discussion started: 16 Jan 2025

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Journal article(s) based on this preprint

13 Aug 2025

Modeling urban pollutant transport at multiple resolutions: impacts of turbulent mixing

Zining Yang, Qiuyan Du, Qike Yang, Chun Zhao, Gudongze Li, Zihan Xia, Mingyue Xu, Renmin Yuan, Yubin Li, Kaihui Xia, Jun Gu, and Jiawang Feng

Atmos. Chem. Phys., 25, 8831–8857, https://doi.org/10.5194/acp-25-8831-2025,https://doi.org/10.5194/acp-25-8831-2025, 2025

Short summary

Zining Yang, Qiuyan Du, Qike Yang, Chun Zhao, Gudongze Li, Zihan Xia, Mingyue Xu, Renmin Yuan, Yubin Li, Kaihui Xia, Jun Gu, and Jiawang Feng

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2024-3890', Leon Kuhn, 30 Jan 2025

Please refer to the attached PDF document.

Citation: https://doi.org/10.5194/egusphere-2024-3890-RC1
- AC2: 'Reply on RC1', Zining Yang, 18 Apr 2025
  
  We have carefully considered all the comments and have made comprehensive revisions to address each point raised. Our detailed responses to the reviewers' comments are provided in the supplement.
  
  Citation: https://doi.org/10.5194/egusphere-2024-3890-AC2
RC2:
'Comment on egusphere-2024-3890', Anonymous Referee #2, 03 Feb 2025

Please refer to the file attached.

Citation: https://doi.org/10.5194/egusphere-2024-3890-RC2
- AC3: 'Reply on RC2', Zining Yang, 18 Apr 2025
  
  We have carefully considered all the comments and have made comprehensive revisions to address each point raised. Our detailed responses to the reviewers' comments are provided in the supplement.
  
  Citation: https://doi.org/10.5194/egusphere-2024-3890-AC3
AC1: 'Comment on egusphere-2024-3890', Zining Yang, 18 Apr 2025

We have carefully considered all the comments and have made comprehensive revisions to address each point raised. Our detailed responses to the reviewers' comments are provided in the supplement.

Citation: https://doi.org/10.5194/egusphere-2024-3890-AC1
AC4: 'Comment on egusphere-2024-3890', Zining Yang, 20 May 2025

We have carefully considered all the comments and have made comprehensive revisions to address each point raised. Our detailed responses to the reviewers' comments are provided in the supplementary material.

Citation: https://doi.org/10.5194/egusphere-2024-3890-AC4

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2024-3890', Leon Kuhn, 30 Jan 2025

Please refer to the attached PDF document.

Citation: https://doi.org/10.5194/egusphere-2024-3890-RC1
- AC2: 'Reply on RC1', Zining Yang, 18 Apr 2025
  
  We have carefully considered all the comments and have made comprehensive revisions to address each point raised. Our detailed responses to the reviewers' comments are provided in the supplement.
  
  Citation: https://doi.org/10.5194/egusphere-2024-3890-AC2
RC2:
'Comment on egusphere-2024-3890', Anonymous Referee #2, 03 Feb 2025

Please refer to the file attached.

Citation: https://doi.org/10.5194/egusphere-2024-3890-RC2
- AC3: 'Reply on RC2', Zining Yang, 18 Apr 2025
  
  We have carefully considered all the comments and have made comprehensive revisions to address each point raised. Our detailed responses to the reviewers' comments are provided in the supplement.
  
  Citation: https://doi.org/10.5194/egusphere-2024-3890-AC3
AC1: 'Comment on egusphere-2024-3890', Zining Yang, 18 Apr 2025

We have carefully considered all the comments and have made comprehensive revisions to address each point raised. Our detailed responses to the reviewers' comments are provided in the supplement.

Citation: https://doi.org/10.5194/egusphere-2024-3890-AC1
AC4: 'Comment on egusphere-2024-3890', Zining Yang, 20 May 2025

We have carefully considered all the comments and have made comprehensive revisions to address each point raised. Our detailed responses to the reviewers' comments are provided in the supplementary material.

Citation: https://doi.org/10.5194/egusphere-2024-3890-AC4

Peer review completion

AR: Author's response | RR: Referee report | ED: Editor decision | EF: Editorial file upload

AR by Zining Yang on behalf of the Authors (18 Apr 2025) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (29 Apr 2025) by Zhonghua Zheng

RR by Leon Kuhn (04 May 2025)

Suggestions for revision or reasons for rejection

I would like to thank the authors for preparing a compelling response to my initial review, in which they clearly demonstrate that my technical concern regarding the parametrization of vertical mixing strength only applies to gases, but not to aerosols. I believe that the modelling community will benefit from articles that point out these hidden model aspects as clearly as possible, which this article now does.

I have two more minor comments, and a few proposals for language improvements.

Minor comments:

1. I still wonder how the findings of this article should be interpreted in the context of Du et al. (2021). Figure 7 shows a range of nighttime mixing coefficients in the range of 0-3 m²/s. As pointed out by the authors, the clipping of mixing strength does not apply to black carbon (BC), but Du et al. (2021) have nonetheless shown that perhaps it should, because their clipping at 5 m²/s resulted in significantly more realistic surface BC. There is also no reason why trace gases should undergo a different mixing routine than aerosols. Based on these two arguments, it seems plausible that the exclusion of aerosols from the clipping procedure was an oversight. The authors need not concern with this question directly, but they should attempt to answer the following (perhaps in the discussion of Figure 7):

Assuming it were appropriate to apply the clipping also to the mixing of aerosols, would this not potentially change the findings of this article? After all, the entire variance of the nighttime mixing values would vanish if the values were clipped to 5 m²/s as suggested by Du et al. (2021). At daytime, where mixing coefficients are higher anyways, this logic would not apply.

2. The authors discuss the correlation between surface types / land usage and mixing strength. In doing so, they mainly focus on how turbulence is affected by surface roughness. But there is another major mechanism to be discussed: The differences in radiative uptake among different surface types. Some surfaces absorb more of the actinic flux, and subsequently transfer this energy through sensible heat to the air above, which significantly contributes to the day-time convective boundary layer.

Comments on language / writing style:

l. 101: change to "Previous research has indicated (...)". The use of the plural "researches" appears in other parts of the article as well and appears false to me.

l. 101-106: This research summary is only helpful if the readers are made aware of the differences between the model code in the presented article and the referenced literature. For example, the study by Kuhlmann et al. (2003) is over 20 years old, and uses a different model at far lower resolutions. I would argue that such literature has limited relevance for the article's research question.

l. 163: Here and in other places, the term "multi-resolutions" is used, which I would recommend to replace by "multiple resolutions", "different resolutions", or "various resolutions".

l. 177: The sentence with "contains some treatments" is not very informative. The authors could write instead: "WRF-Chem treats photochemistry of trace gases and aerosol-related processes with various different schemes (e.g. the Statewide Air Pollution Research Center (SAPRC99) photochemical mechanism and the Model for Simulating Aerosol Interactions and Chemistry (MOSAIC))."

l. 190: remove "also".

l. 313: Split into two sentences: "This study primarily focuses on BC. The spatial distribution of BC emissions is shown in Figure 22."

l. 322: Is Zhang et al. (2021) a good reference to the MEGAN model? As far as I know, MEGAN is usually cited as:

Guenther, A., Karl, T., Harley, P., Wiedinmyer, C., Palmer, P. I., and Geron, C.: Estimates of global terrestrial isoprene emissions using MEGAN (Model of Emissions of Gases and Aerosols from Nature), Atmos. Chem. Phys., 6, 3181–3210, https://doi.org/10.5194/acp-6-3181-2006, 2006.

This refers to the original version of MEGAN, and newer versions have been published. Regardless, Guenther et al. should be cited in the context of MEGAN.

l. 350: Change "derived" to "used".

l. 391: "We believe this approach (...)" should be rephrased to a more objective statement.

l. 489: Change to "Figure S8b demonstrates (...)".

l. 647: It is unclear to me what "intension" means in this context.

l. 671: Remove "vertically upward".

l. 863: Change to "dynamical framework of the model".

l. 865: Change to "The resolution of dynamic processes reduces (...)".

Hide

RR by Anonymous Referee #2 (07 May 2025)

ED: Publish subject to minor revisions (review by editor) (11 May 2025) by Zhonghua Zheng

Please find the reviewer’s comments below.

I would like to thank the authors for preparing a compelling response to my initial review, in which they clearly demonstrate that my technical concern regarding the parametrization of vertical mixing strength only applies to gases, but not to aerosols. I believe that the modelling community will benefit from articles that point out these hidden model aspects as clearly as possible, which this article now does.

I have two more minor comments, and a few proposals for language improvements.

Minor comments:

1. I still wonder how the findings of this article should be interpreted in the context of Du et al. (2021). Figure 7 shows a range of nighttime mixing coefficients in the range of 0-3 m²/s. As pointed out by the authors, the clipping of mixing strength does not apply to black carbon (BC), but Du et al. (2021) have nonetheless shown that perhaps it should, because their clipping at 5 m²/s resulted in significantly more realistic surface BC. There is also no reason why trace gases should undergo a different mixing routine than aerosols. Based on these two arguments, it seems plausible that the exclusion of aerosols from the clipping procedure was an oversight. The authors need not concern with this question directly, but they should attempt to answer the following (perhaps in the discussion of Figure 7):

Assuming it were appropriate to apply the clipping also to the mixing of aerosols, would this not potentially change the findings of this article? After all, the entire variance of the nighttime mixing values would vanish if the values were clipped to 5 m²/s as suggested by Du et al. (2021). At daytime, where mixing coefficients are higher anyways, this logic would not apply.

2. The authors discuss the correlation between surface types / land usage and mixing strength. In doing so, they mainly focus on how turbulence is affected by surface roughness. But there is another major mechanism to be discussed: The differences in radiative uptake among different surface types. Some surfaces absorb more of the actinic flux, and subsequently transfer this energy through sensible heat to the air above, which significantly contributes to the day-time convective boundary layer.

Comments on language / writing style:

l. 101: change to "Previous research has indicated (...)". The use of the plural "researches" appears in other parts of the article as well and appears false to me.

l. 101-106: This research summary is only helpful if the readers are made aware of the differences between the model code in the presented article and the referenced literature. For example, the study by Kuhlmann et al. (2003) is over 20 years old, and uses a different model at far lower resolutions. I would argue that such literature has limited relevance for the article's research question.

l. 163: Here and in other places, the term "multi-resolutions" is used, which I would recommend to replace by "multiple resolutions", "different resolutions", or "various resolutions".

l. 177: The sentence with "contains some treatments" is not very informative. The authors could write instead: "WRF-Chem treats photochemistry of trace gases and aerosol-related processes with various different schemes (e.g. the Statewide Air Pollution Research Center (SAPRC99) photochemical mechanism and the Model for Simulating Aerosol Interactions and Chemistry (MOSAIC))."

l. 190: remove "also".

l. 313: Split into two sentences: "This study primarily focuses on BC. The spatial distribution of BC emissions is shown in Figure 22."

l. 322: Is Zhang et al. (2021) a good reference to the MEGAN model? As far as I know, MEGAN is usually cited as:

Guenther, A., Karl, T., Harley, P., Wiedinmyer, C., Palmer, P. I., and Geron, C.: Estimates of global terrestrial isoprene emissions using MEGAN (Model of Emissions of Gases and Aerosols from Nature), Atmos. Chem. Phys., 6, 3181–3210, https://doi.org/10.5194/acp-6-3181-2006, 2006.

This refers to the original version of MEGAN, and newer versions have been published. Regardless, Guenther et al. should be cited in the context of MEGAN.

l. 350: Change "derived" to "used".

l. 391: "We believe this approach (...)" should be rephrased to a more objective statement.

l. 489: Change to "Figure S8b demonstrates (...)".

l. 647: It is unclear to me what "intension" means in this context.

l. 671: Remove "vertically upward".

l. 863: Change to "dynamical framework of the model".

l. 865: Change to "The resolution of dynamic processes reduces (...)".

Hide

AR by Zining Yang on behalf of the Authors (20 May 2025) Author's response Author's tracked changes Manuscript

ED: Publish as is (01 Jun 2025) by Zhonghua Zheng

AR by Zining Yang on behalf of the Authors (04 Jun 2025) Manuscript

Journal article(s) based on this preprint

13 Aug 2025

Modeling urban pollutant transport at multiple resolutions: impacts of turbulent mixing

Zining Yang, Qiuyan Du, Qike Yang, Chun Zhao, Gudongze Li, Zihan Xia, Mingyue Xu, Renmin Yuan, Yubin Li, Kaihui Xia, Jun Gu, and Jiawang Feng

Atmos. Chem. Phys., 25, 8831–8857, https://doi.org/10.5194/acp-25-8831-2025,https://doi.org/10.5194/acp-25-8831-2025, 2025

Short summary

Zining Yang, Qiuyan Du, Qike Yang, Chun Zhao, Gudongze Li, Zihan Xia, Mingyue Xu, Renmin Yuan, Yubin Li, Kaihui Xia, Jun Gu, and Jiawang Feng

Supplement

https://doi.org/10.5194/egusphere-2024-3890-supplement

Zining Yang, Qiuyan Du, Qike Yang, Chun Zhao, Gudongze Li, Zihan Xia, Mingyue Xu, Renmin Yuan, Yubin Li, Kaihui Xia, Jun Gu, and Jiawang Feng

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This study investigates the impact of turbulent mixing on black carbon (BC) concentrations in urban areas using WRF-Chem at 25, 5, and 1 km resolutions. Significant variations in BC and turbulent mixing occur mainly at night. Higher resolutions reduce BC overestimation due to enhanced PBL mixing coefficients and vertical wind fluxes. Small-scale eddies at higher resolutions increase BC lifetime and column concentrations. Land use and terrain variations across multi-resolutions affect PBL mixing.


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