Dry Deposition in Urban Green Spaces: Insights from Beijing and Shanghai

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Title: Dry deposition in urban green spaces: Insights from Beijing and Shanghai

1.      Few lines in the “Introduction” need more clarity

2.      How did you validate the dry deposition model in your study?

3.      How did you consider these factors for the seasonal variations in dry deposition velocities for gaseous pollutants and particulate matter?

4.      Why do particulate matter deposition rates remain relatively stable throughout the year?

5.      Point out the key drivers influencing the dry deposition of air pollutants in urban green spaces.

6.      In what all ways do climatic conditions and stomatal conductance of vegetation impact the dry deposition flux of gaseous pollutants?

7.      Which tree species were found to be most effective in dry deposition in Beijing and Shanghai, and why?

8.      What is the significance of species selection and strategic green space planning in urban air quality management?

9.      Do meteorological factors like wind speed, temperature, humidity, and precipitation influence the dry deposition rates, if so, how?

10.   What are the implications of your study for urban green space management and air pollution mitigation strategies in other cities?

11.   How can urban planning be optimized based on your findings to improve air quality and public health?

 

12.   Highlight the applications and utility of the work, what is novel in this study.

Author Response

(1).Few lines in the “Introduction” need more clarity.

Response: Thank you for the reviewer’s feedback. To enhance the clarity of the introduction, we have revised it to explicitly state the background, objectives, and how the study addresses existing gaps. Below is our revised content:

Modified Section: 1.Introduction

Pages: 2-3

Lines: 33-90

1. Introduction

The rapid advancement of urbanization and industrialization has escalated air pollution into a critical global issue. As a primary source of environmental degradation, air pollutants not only degrade the ambient atmosphere but also significantly contribute to the global disease burden. With over half the world's population residing in cities [1], urban centers have become the epicenters for air pollution and associated health risks [2]. Outdoor air pollution annually accounts for millions of premature deaths, particularly in urban settings where over two-thirds of such deaths occur [3]. Addressing air pollution is imperative for the sustainable evolution of urban landscapes and for safeguarding human health. Notoriously, criteria pollutants including PM_2.5, PM₁₀, O₃, SO₂, NO_X, and CO are omnipresent and pose substantial threats to public health and environmental integrity [4].

Mitigating air pollutants effectively can be achieved through integrated pollution removal technologies or, more naturally and sustainably, through vegetation-based solutions [5,6]. Urban green infrastructures (GIs), such as parks and street trees, function as vital barriers against pollution, mitigating the impact of traffic emissions and other sources on adjacent areas [7]. These GIs are ecosystems that interact closely with urban residents, providing a plethora of ecosystem services such as carbon capture, microclimate regulation, and notably, the removal of airborne and waterborne pollutants [8]. Among these services, the dry deposition of air pollutants onto urban green spaces stands out as a critical mechanism for air quality enhancement [9,10].

Existing monitoring and modeling studies often fail to reflect the variations in the dry deposition capacity of green spaces and lack refinement at the tree species level, hindering comprehensive evaluations and the formulation of effective management strategies. Therefore, this study aims to quantitatively assess the dry deposition capacity of six major air pollutants (O₃, CO, NO₂, SO₂, PM_2.5, PM₁₀) in the urban green spaces of Beijing and Shanghai, utilizing an improved dry deposition model and tree species-specific data. Our findings provide valuable insights into the factors influencing deposition rates and the effectiveness of different tree species, offering theoretical support for the fine management of urban green spaces and scientific strategies for air pollution prevention and control. This research is crucial for promoting sustainable urban development.

Dry deposition is the second-most significant natural process for removing pollutants from the troposphere, pivotal for local air quality and health [11,12]. In urban green spaces, dry deposition entails the removal of particulates via surfaces and the absorption of gaseous pollutants through plant stomata, sometimes leading to their incorporation into organic compounds that support plant growth [13,14]. The factors influencing dry deposition are complex, encompassing pollutant concentrations, atmospheric physicochemical conditions, meteorological factors, and traits specific to different plant species, intertwining physical, chemical, and biological processes. Consequently, measuring dry deposition values directly is challenging, often necessitating indirect calculations via model simulations [15,16].

To date, the vegetation's capability to scavenge air pollutants has garnered extensive research interest, yet studies have predominantly focused on natural, homogeneous vegetation fields, often overlooking the heterogeneity inherent in urban green spaces [17]. This gap hinders our quantitative understanding of dry deposition in urban ecosystems and the formulation of species-specific management and regulatory practices. Despite a 20.77-fold increase in green spaces within Chinese urban areas from 1981 to 2019, air pollution persists, exacerbated by swift urban and industrial expansion. The role of urban green spaces in dry deposition, a significant pollution abatement pathway, remains underexplored in China, where variations in population density, geography, climate, hydrology, vegetation, industry, and economic development greatly influence deposition characteristics.

Our study bridges this knowledge gap by examining Beijing and Shanghai, two megacities emblematic of the diverse climatic zones and hydrothermal contrasts within China. These cities also reflect differences in urban management and vegetation profiles. Through vegetation surveys and the integration of dry deposition models with sensitivity analysis, we discern the key drivers of urban dry deposition, laying a scientific groundwork for enhanced green space management and air pollution mitigation strategies in these metropolises, with implications extending throughout China's urban landscapes.

 

(2).How did you validate the dry deposition model in your study?

Response: We validated the dry deposition model by comparing the model outputs with empirical measurements obtained through atmospheric gradiometry. This approach involved setting up sampling points at different heights above the ground, measuring the concentration gradients of O₃, and using a diffusion equation to estimate the deposition velocity. The correlation between the model-simulated and empirically measured deposition velocities demonstrated the model’s accuracy.

Modified Section: 2.8 Model validation

Pages: 7

Lines: 229-243

 

(3).How did you consider these factors for the seasonal variations in dry deposition velocities for gaseous pollutants and particulate matter?

Response: Seasonal variations in dry deposition velocities were considered by incorporating meteorological data (temperature, humidity, wind speed) and vegetation characteristics (stomatal conductance, leaf area index) into the model. Higher temperatures and increased solar radiation in summer reduce stomatal resistance, enhancing gaseous pollutant uptake, while particulate matter deposition remains relatively stable due to consistent vegetation interception throughout the year. The model was designed to simulate deposition rates under different seasonal conditions.

Modified Section: 3.2 Seasonal variations of the simulated dry deposition velocities (V_d)

Pages: 8-10

Lines: 261-306

 

(4).Why do particulate matter deposition rates remain relatively stable throughout the year?

Response: Particulate matter deposition rates remain stable because the removal process via vegetation interception occurs year-round, day and night. Unlike gaseous pollutants influenced by seasonal stomatal activities, particulate matter deposition relies on physical interception by plant surfaces, which is constant throughout the year.

Modified Section: 3.2 Seasonal variations of the simulated dry deposition velocities (V_d)

Pages: 8-10

Lines: 261-306

 

(5).Point out the key drivers influencing the dry deposition of air pollutants in urban green spaces.

Response: The key drivers influencing dry deposition in urban green spaces include meteorological conditions (temperature, wind speed, humidity), vegetation characteristics (stomatal conductance, leaf area index), and pollutant concentrations. Seasonal changes, vegetation type, and coverage also play crucial roles in determining the dry deposition rates.

Modified Section: 4.2 The main factors influencing the dry deposition of air pollutants in urban green spaces

Pages: 23-24

Lines: 652-690

 

(6).In what all ways do climatic conditions and stomatal conductance of vegetation impact the dry deposition flux of gaseous pollutants?

Response: Climatic conditions such as temperature and wind speed affect aerodynamic and boundary layer resistances, which facilitate or hinder pollutant transport to vegetation. High temperatures and increased solar radiation reduce stomatal resistance, enhancing pollutant uptake. Conversely, low temperatures and reduced solar radiation increase stomatal resistance, reducing uptake. Stomatal conductance influences the degree of stomatal opening, regulating gaseous pollutant absorption efficiency.

Modified Section: 3.3 The annual variation of the related impedance to the dry deposition of air pollutants

Pages: 10-14

Lines: 307-375

 

(7).Which tree species were found to be most effective in dry deposition in Beijing and Shanghai, and why?

Response: In Beijing, Zelkova serrata was found to be the most effective tree species for dry deposition, while in Shanghai, Photinia serratifolia was the most effective. These species have high stomatal conductance and large leaf areas, enhancing their capacity to absorb gaseous pollutants. Additionally, their canopy structure and leaf surface characteristics contribute to their higher deposition efficiency.

Modified Section: 3.11 The impact of urban greening tree species on the dry deposition rate of gaseous pollutants

Pages: 21

Lines: 549-579

 

(8).What is the significance of species selection and strategic green space planning in urban air quality management?

Response: Species selection is crucial as different tree species have varying capacities for pollutant removal. Strategic green space planning can optimize the configuration of urban green spaces to maximize pollutant removal, improve air quality, and provide greater ecological benefits. Proper selection and configuration of tree species with high pollutant absorption capacity can significantly enhance urban air quality management.

Modified Section: 3.11 The impact of urban greening tree species on the dry deposition rate of gaseous pollutants

Pages: 21

Lines: 549-579

 

(9).Do meteorological factors like wind speed, temperature, humidity, and precipitation influence the dry deposition rates, if so, how?

Response: Yes, meteorological factors significantly influence dry deposition rates. Wind speed affects pollutant diffusion rates; higher wind speeds can promote particulate and gaseous pollutant deposition. Temperature and humidity affect stomatal conductance and plant physiological activities; higher temperatures and humidity favor stomatal opening, increasing gaseous pollutant absorption. Precipitation removes atmospheric particulates through wet deposition and can influence the resuspension of particulates on vegetation surfaces.

Modified Section: 3.3 The annual variation of the related impedance to the dry deposition of air pollutants

Pages: 10-14

Lines: 307-375

 

(10).What are the implications of your study for urban green space management and air pollution mitigation strategies in other cities?

Response: Our study provides insights into the effectiveness of different tree species in pollutant removal, emphasizing the importance of strategic species selection and green space planning to maximize air quality improvements. These findings can inform urban greening strategies in other cities to enhance air pollution mitigation efforts. For example, selecting tree species with high stomatal conductance and large leaf areas can improve pollutant removal efficiency in high pollution areas.

Modified Section: 4.3 The improvement of air quality by dry deposition in urban green spaces

Pages: 24

Lines: 691-721

 

(11).How can urban planning be optimized based on your findings to improve air quality and public health?

Response: Urban planning can be optimized by increasing the density of broad-leaved tree species with high stomatal conductance, enhancing vegetation coverage, and strategically placing green spaces in high pollution areas. This approach can enhance pollutant removal and improve public health outcomes. Additionally, considering urban heat island effects and wind corridors can further optimize green space layout, improve air circulation, and reduce pollutant accumulation.

Modified Section: 4.3 The improvement of air quality by dry deposition in urban green spaces

Pages: 24

Lines: 691-721

 

(12).Highlight the applications and utility of the work, what is novel in this study.

Response: This study provides practical recommendations for species selection and green space planning by thoroughly analyzing the dry deposition capacities of different tree species in urban green spaces in Beijing and Shanghai. By integrating field plant data with an improved dry deposition model, we achieved a more accurate assessment of pollutant removal, offering a scientific basis for effective air quality management strategies. The novelty of this study lies in the combination of an improved model and species-specific data, making the assessment more precise and targeted, thereby better guiding urban air quality improvement and enhancing ecological benefits.

Modified Section: 5. Conclusions

Pages: 24-25

Lines: 722-765

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The work is interesting since it allows us to learn more about the role of dry deposition in urban green areas and even at the species level. Although the introduction provides information about the main issues, I suggest modifying a paragraph with general information and leaving the objective details for the last paragraph. The methodology well describes the calculations and model for dry deposition; however, it does not indicate how the species analysis was carried out. The results section seems extensive to me, perhaps you could focus on the most important results and evaluate whether all the figures are necessary. I reiterate, in section 3.10 you talk about the species role but there is no clear background in the methodology of how these results were reached. Finally, section 5 says “results”, when I suppose it should say conclusions. If so, I suggest that a conclusion be indicated for each section of the results since some points are not mentioned; you can even put them all together in a large paragraph. In general, the manuscripts seem to me a good work that requires adding small formatting details (review the attached PDF).

Comments for author File: Comments.pdf

Comments on the Quality of English Language

the quality of English seems good to me

Author Response

Dear Reviewer,

Thank you for your insightful comments and suggestions. We appreciate your positive feedback on our work and your constructive critiques which have greatly helped us improve the manuscript. Below are our responses to your comments:

(1).The work is interesting since it allows us to learn more about the role of dry deposition in urban green areas and even at the species level. Although the introduction provides information about the main issues, I suggest modifying a paragraph with general information and leaving the objective details for the last paragraph.

Response: We have revised the Introduction section to improve clarity and flow. The general information about the study's background has been elaborated in the initial paragraphs, while the specific objectives and the research gap addressed by this study have been detailed in the last paragraph. This restructuring aims to provide a clearer context and highlight the significance of our research.

Modified Section: 1. Introduction

Pages: 1-2

Lines: 33-90

 

(2).The methodology well describes the calculations and model for dry deposition; however, it does not indicate how the species analysis was carried out.

Response: We have added a detailed description of the species analysis methodology in Section 2.4. This includes the criteria for selecting tree species, the measurement of their physical and physiological traits, and the calculation of their dry deposition rates for different pollutants. This addition ensures that the readers can understand how the species-specific results were derived.

Added Section: 2.4 Species Analysis Method

Pages: 4

Lines: 149-157

 

(3).The results section seems extensive to me, perhaps you could focus on the most important results and evaluate whether all the figures are necessary.

Response: We have reviewed and condensed the Results section, focusing on the most significant findings. This revision helps to emphasize the key outcomes of our study without overwhelming the reader with excessive detail.

Modified Section: 3. Results and Discussion

Pages: 7-21

 

(4).I reiterate, in section 3.10 you talk about the species role but there is no clear background in the methodology of how these results were reached.

Response: As noted in our response to Comment 2, we have included a detailed methodology for the species analysis in Section 2.4. This provides a clear background on how the species-specific dry deposition rates were calculated and ensures the results in Section 3.11 (formerly 3.10) are well-supported by the methodology.

Added Section: 2.4 Species Analysis Method

Pages: 4

Lines: 149-157

Modified Section: 3.11 The Impact of Urban Greening Tree Species on the Dry Deposition Rate of Gaseous Pollutants

Pages: 21

Lines: 549-579

 

(5).Finally, section 5 says “results”, when I suppose it should say conclusions. If so, I suggest that a conclusion be indicated for each section of the results since some points are not mentioned; you can even put them all together in a large paragraph.

Response: We have changed the title of Section 5 from "Results" to "Conclusions". We have also revised this section to clearly indicate the conclusions drawn from each major part of the results. This ensures that all critical points are addressed and summarized effectively.

Modified Section: 5. Conclusions

Pages: 24-25

 

By implementing these changes, we aim to enhance the clarity and comprehensiveness of our manuscript, ensuring that each critical point is adequately addressed and all significant findings are summarized effectively.

We hope these revisions meet your expectations and we are grateful for your valuable feedback. Thank you once again for your thorough review and constructive suggestions.

Kind regards,

Siqi Shao

State Key Lab of Subtropical Silviculture, Zhejiang A&F University, Hangzhou 311300, PR China

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

Agree