Tracking Particulate Matter Accumulation on Green Roofs: A Study at Warsaw University Library

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This manuscript, titled "Tracking Particulate Matter Accumulation on Green Roofs: A Comparative Study at Warsaw University Library," aims to assess the capacity of green roofs to mitigate air pollution. The authors have designed an effective methodology to assess aerosols deposited onto plant leaves across different seasons. However, a key weakness of this research is its limited response to the research objectives. Specifically, there is a lack of corresponding reports and discussions on air pollution and meteorological factors, which are crucial for establishing the interconnections between plants' particulate matter (PM) capture capacity and air pollution. Additionally, meteorological factors such as rainfall, wind speed, surface temperature, humidity, and air pressure would be helpful in discussing the dispersion, accumulation, and rain-washing of air pollution.

The green roof's performance is also influenced by factors such as fertilization, irrigation, and soil conditioning, all of which are sources of particulate matter and heavy metals. Therefore, it is difficult to assess the extent to which the green roof mitigates air pollution without accounting for the effects of gardening activities and meteorological conditions.

Furthermore, I have several recommendations that I would like the authors to consider:

1. The title should be revised since this is not comparative study.
2. Section 2.1 Study Area and Plant Material: The authors should provide a brief description of the local climate using an international climate classification scheme, such as Köppen. This would make the information more accessible to a broader audience. Additionally, it would be helpful to provide details on the morphology of the plants’ leaves (e.g., surface texture, surface roughness, trichome type and density, foliage density, etc.) used in this study. This information would enhance the discussion section.
3. Section 2.2.1 Filter Preparation Protocol: Please specify the grade number of the filters used.
4. Section 2.2.2 Extraction of Surface Particulate Matter: Please mention if pressurized filtration or syringe filtration was used in the extraction process.
5. Section 2.2.4 Leaf Surface Area Measurement: It is unclear how taking photos of the leaves using SkyLeaf software can be related to measuring epicuticular wax mass. Please briefly explain the principle behind this assessment.
6. Section 2.2.5 Determination of Trace Elements in Leaves: I am concerned that heavy metals in the leaves could largely be taken up from the soil.
7. It would be useful if the authors could provide information about the green roof's gardening activities and schedules. If these activities are consistent across different seasons, we could expect similar influences, whereas variations in air pollution and meteorological conditions could account for the differences.
8. Conclusion and Discussion: The authors conclude that “species such as S. spectabile and S. japonica exhibited a pronounced increase in wax particulate matter (wPM) retention during autumn, suggesting that enhanced wax deposition functions as an adaptive mechanism to stabilize PM capture during periods of reduced growth.” This conclusion may be biased. Although the wax content was higher for S. japonica, it was lower for the other species (Figure 5). Additionally, wPM retention is typically lower than sPM retention in autumn, including for S. japonica (Figure 4).
9. In the conclusion and discussion on seasonal variation in PM retention, the authors focus heavily on the influence of leaf wax. However, I suggest that rain-washing may play a role in lower PM retention in spring, gardening activities in summer may contribute to higher PM retention, and falling leaves may explain the reduced PM retention in autumn. The interesting finding is that Betula pendula, Cornus sericea, Hemerocallis xhybrida, and Sedum spectabile do not follow this hypothesis. It would be beneficial to discuss this finding further.

My suggestion is that the authors should focus more on the key mechanisms of PM capture by plant leaves (including stomatal deposition, surface adhesion and impact, rain-wash, surface texture/structure, and falling leaves) without directly linking them to air pollution mitigation.

Author Response

This manuscript, titled "Tracking Particulate Matter Accumulation on Green Roofs: A Comparative Study at Warsaw University Library," aims to assess the capacity of green roofs to mitigate air pollution. The authors have designed an effective methodology to assess aerosols deposited onto plant leaves across different seasons. However, a key weakness of this research is its limited response to the research objectives. Specifically, there is a lack of corresponding reports and discussions on air pollution and meteorological factors, which are crucial for establishing the interconnections between plants' particulate matter (PM) capture capacity and air pollution. Additionally, meteorological factors such as rainfall, wind speed, surface temperature, humidity, and air pressure would be helpful in discussing the dispersion, accumulation, and rain-washing of air pollution.

• Response: Thank you for your thoughtful comment. We acknowledge that incorporating meteorological factors such as rainfall, wind speed, temperature, humidity, and air pressure could provide deeper insights into the processes of PM dispersion, accumulation, and wash-off. Unfortunately, in this study, we did not have the necessary equipment to measure these parameters directly. However, we agree that this is an excellent suggestion for future research to better understand the interplay between environmental factors and PM retention on green roofs.

We also believe that providing general meteorological or air pollution data for the city may not yield meaningful conclusions in this context. PM accumulation and dispersion are highly localized processes, influenced by microclimatic conditions that can vary significantly even between neighboring buildings. Additionally, green roofs are unique in their design, location, and surrounding environment, which means that no two green roofs experience identical conditions. This variability highlights the importance of site-specific studies when evaluating the effectiveness of green roofs for air pollution mitigation. Thank you for raising this important point, which reinforces the complexity and localized nature of green roof systems.

The green roof's performance is also influenced by factors such as fertilization, irrigation, and soil conditioning, all of which are sources of particulate matter and heavy metals. Therefore, it is difficult to assess the extent to which the green roof mitigates air pollution without accounting for the effects of gardening activities and meteorological conditions.

• Response: Thank you for highlighting this important point. We agree that factors such as fertilization, irrigation, and soil conditioning can influence the performance of green roofs by contributing additional sources of particulate matter and heavy metals. Without accounting for the effects of these gardening activities and meteorological conditions, it is indeed challenging to fully assess the extent to which the green roof mitigates air pollution. In response to your suggestion, we have incorporated this aspect into the discussion section of our manuscript. By acknowledging the potential contributions of gardening practices and environmental factors, we aim to provide a more comprehensive evaluation of the green roof's effectiveness in air pollution mitigation. We appreciate your insightful feedback, which has helped us enhance the depth and clarity of our study.

Furthermore, I have several recommendations that I would like the authors to consider:

The title should be revised since this is not comparative study.

• Response: Thank you for your valuable feedback regarding the title. We appreciate your observation that the original title, including the phrase "A Comparative Study," may not accurately reflect the scope of our research. Based on your suggestion, we have revised the title to better align with the content and objectives of the study.

Section 2.1 Study Area and Plant Material: The authors should provide a brief description of the local climate using an international climate classification scheme, such as Köppen. This would make the information more accessible to a broader audience. Additionally, it would be helpful to provide details on the morphology of the plants’ leaves (e.g., surface texture, surface roughness, trichome type and density, foliage density, etc.) used in this study. This information would enhance the discussion section.

• Response: Thank you for your valuable feedback. In response to your suggestions, we have revised the manuscript to include additional details about the local climate and leaf morphology. Specifically, we have added a description of the climate in Warsaw using the Köppen climate classification (Dfb), which provides a globally recognized framework for understanding the city’s humid continental climate. This addition enhances the accessibility of the climate information to a broader audience. Furthermore, we have included a new table summarizing the key morphological traits of the plant species analyzed in this study

Section 2.2.1 Filter Preparation Protocol: Please specify the grade number of the filters used.

• Response: We appreciate the suggestion. We have added the requested information regarding the filter grade number for greater clarity and reproducibility. Specifically, the grade number of the filters used has been explicitly stated in the revised manuscript. Thank you for pointing this out.

Section 2.2.2 Extraction of Surface Particulate Matter: Please mention if pressurized filtration or syringe filtration was used in the extraction process.

• Response: We thank the reviewer for their insightful comment. In our study, pressurized filtration was used during the extraction process. This information has now been included in the manuscript to provide greater clarity and ensure transparency in our methodology. We appreciate the opportunity to improve the manuscript with this addition.

Section 2.2.4 Leaf Surface Area Measurement: It is unclear how taking photos of the leaves using SkyLeaf software can be related to measuring epicuticular wax mass. Please briefly explain the principle behind this assessment.

• Response: We appreciate the question regarding the connection between leaf surface area measurement and epicuticular wax mass. The leaf surface area, measured using the SkyLeaf software, was crucial for normalizing the epicuticular wax mass to a per-unit-area basis (e.g., µg/cm²). This normalization allowed for accurate comparisons between species, as it accounts for differences in leaf size and ensures that wax deposition is expressed relative to the surface area. We have clarified this principle in the revised manuscript for better understanding. Thank you for highlighting this point.

Section 2.2.5 Determination of Trace Elements in Leaves: I am concerned that heavy metals in the leaves could largely be taken up from the soil.

• Response:

It would be useful if the authors could provide information about the green roof's gardening activities and schedules. If these activities are consistent across different seasons, we could expect similar influences, whereas variations in air pollution and meteorological conditions could account for the differences.

• Response: We appreciate the reviewer’s interest in the gardening activities and schedules of the green roof. Unfortunately, it is not possible to obtain precise and consistent data on these activities, as they are managed by a third party, and different gardening companies have been involved over time. We reached out to the property manager for more information, but no specific or reliable records of gardening schedules or practices were available. The only consistent information we could confirm is that the garden is closed to visitors during the winter months, from November to March, which reduces human activity and maintenance during this time. While we acknowledge the value of this data, no additional details were added to the manuscript, as the lack of reliable records would make any assumptions speculative. We appreciate your understanding of these limitations.

Conclusion and Discussion: The authors conclude that “species such as S. spectabile and S. japonica exhibited a pronounced increase in wax particulate matter (wPM) retention during autumn, suggesting that enhanced wax deposition functions as an adaptive mechanism to stabilize PM capture during periods of reduced growth.” This conclusion may be biased. Although the wax content was higher for S. japonica, it was lower for the other species (Figure 5). Additionally, wPM retention is typically lower than sPM retention in autumn, including for S. japonica (Figure 4).

• Response: Thank you for your valuable feedback. We agree with your comments and have revised the conclusion and discussion to address the concerns raised. Specifically, we clarified the variability in wax content and particulate matter retention across species and acknowledged that surface-bound PM remains the dominant mechanism in autumn. The revised text avoids overgeneralizations and aligns more closely with the data presented. We appreciate your constructive suggestions, which have helped improve the clarity and accuracy of our manuscript.

In the conclusion and discussion on seasonal variation in PM retention, the authors focus heavily on the influence of leaf wax. However, I suggest that rain-washing may play a role in lower PM retention in spring, gardening activities in summer may contribute to higher PM retention, and falling leaves may explain the reduced PM retention in autumn. The interesting finding is that Betula pendula, Cornus sericea, Hemerocallis xhybrida, and Sedum spectabile do not follow this hypothesis. It would be beneficial to discuss this finding further.

• Response: Thank you for your thoughtful comments and suggestions regarding the seasonal variation in PM retention. We have carefully revised the discussion to incorporate your valuable insights. Specifically, we now address the influence of rainfall in spring, gardening activities in summer, and leaf senescence in autumn as additional factors affecting PM retention. Furthermore, we have highlighted the observed deviations in species such as Betula pendula, Cornus sericea, Hemerocallis xhybrida, and Sedum spectabile, suggesting that species-specific traits may play a more significant role in these patterns. We believe these revisions provide a more comprehensive and balanced interpretation of the results. Thank you again for helping us improve the clarity and depth of our manuscript.

My suggestion is that the authors should focus more on the key mechanisms of PM capture by plant leaves (including stomatal deposition, surface adhesion and impact, rain-wash, surface texture/structure, and falling leaves) without directly linking them to air pollution mitigation.

• Response: Thank you for your valuable and insightful suggestions regarding the focus on key mechanisms of PM capture by plant leaves, such as stomatal deposition, surface adhesion, rain-wash, leaf texture, and senescence. We appreciate the importance of these aspects, as they play a fundamental role in understanding plant-particulate matter interactions. We have made an effort to address your suggestion by incorporating a clearer description of these mechanisms in the revised version of the manuscript, particularly in the discussion. However, we believe that a detailed analysis of the specific processes involved in PM capture, such as the role of stomata or surface structures, would require a more focused study that extends beyond the scope of the current work. The primary aim of this manuscript was to emphasize the role of green roofs as effective accumulators of PM in urban environments, showcasing their practical value in retaining particulate matter over different seasons. While many existing studies delve deeply into the mechanisms of PM retention, here we wanted to highlight the overall contribution of green roofs as a system for improving urban environments through PM accumulation. That said, your suggestion provides a compelling direction for future research, and we plan to explore these processes in greater detail in subsequent studies, with a specific focus on plant traits and mechanisms influencing PM capture efficiency. We sincerely thank you for your thoughtful feedback, which has allowed us to clarify the scope of our study while acknowledging important avenues for further investigation.

Reviewer 2 Report

Comments and Suggestions for Authors

This is an excellent manuscript describing a study to measure and compare the ability of green roof plant species to capture PM and trace elements in different size fractions across seasons.  Strengths include robust study design, an efficient and comprehensive methodology for collecting and measuring PM and metals, and evaluation of diverse plant species with different characteristics.  The authors also do a good job with data interpretation, providing insightful analysis of growing season dynamics and PM sources.  Most usefully, they provide recommendations for green roof design to enhance PM mitigation efforts.  I have a few comments for the authors to consider:

1. L217: Please clarify – is total PM = sPM + wPM?
2. L263-264: This is an incomplete sentence – what is meant here?

Author Response

This is an excellent manuscript describing a study to measure and compare the ability of green roof plant species to capture PM and trace elements in different size fractions across seasons.  Strengths include robust study design, an efficient and comprehensive methodology for collecting and measuring PM and metals, and evaluation of diverse plant species with different characteristics.  The authors also do a good job with data interpretation, providing insightful analysis of growing season dynamics and PM sources.  Most usefully, they provide recommendations for green roof design to enhance PM mitigation efforts.  I have a few comments for the authors to consider:

Response: Thank you for your kind and thoughtful feedback on the manuscript. We're delighted to hear that you found the study design robust, the methodology comprehensive, and the evaluation of diverse plant species insightful. It is especially gratifying to know that our interpretation of the data and the practical recommendations for green roof design were useful and resonated with you. We greatly appreciate your time and effort in reviewing our work and welcome your comments for further improvement.

1. L217: Please clarify – is total PM = sPM + wPM?

• Response: Thank you for your question. Yes, to clarify, total PM is the sum of sPM (solid particulate matter) and wPM (water-soluble particulate matter). We have added this clarification to the manuscript to ensure it is clear for all readers. We appreciate you bringing this to our attention.

1. L263-264: This is an incomplete sentence – what is meant here?

• Response: Thank you for pointing this out. That sentence was intended to be removed in an earlier revision, and we have now ensured it has been deleted in the current version of the manuscript. We appreciate your careful review and attention to detail.

Reviewer 3 Report

Comments and Suggestions for Authors

The background information is insufficient, including how plants behave seasonally, environmental PM and conditions reporting, and a control. The conclusion is that certain species are better for green roofs and collection of PM, but there is no control and no baseline. These scientific deficiencies must be addressed prior to publication.

Comments for author File: Comments.pdf

Author Response

The background information is insufficient, including how plants behave seasonally, environmental PM and conditions reporting, and a control. The conclusion is that certain species are better for green roofs and collection of PM, but there is no control and no baseline. These scientific deficiencies must be addressed prior to publication.

• Response: Thank you for your observations. We appreciate your concern about the background information, control, and baseline. Background Information: We acknowledge the importance of providing comprehensive background details. In the manuscript, we have included a detailed explanation of how plants behave seasonally, their physiological adaptations, and their influence on PM retention (Sections 1 and 4). Seasonal variation in particulate matter retention is discussed extensively, supported by quantified data, which highlights the interaction of plant traits and environmental conditions (e.g., Fig. 3). Environmental PM and Conditions Reporting: The study carefully reports PM characteristics, including size fractions and trace elements, across seasons. The methodology (Section 2) details the sampling conditions and local climatic influences, such as wind and precipitation, which were accounted for during the study. Additionally, the location's environmental context is outlined in the introduction, highlighting typical urban pollution dynamics in Warsaw. Control and Baseline: While it is challenging to implement a direct "control" in green roof studies due to the inherent nature of outdoor experiments, our methodology ensures a systematic comparison of plant species under identical environmental conditions on the same green roof. This uniformity provides a baseline for evaluating interspecies and seasonal variability. The baseline pollution levels in the study area are implicitly provided through local environmental data, further contextualizing our findings.

We have revised the manuscript to clarify these points and ensure the conclusions are firmly rooted in the data. If further enhancements are necessary, we would be glad to incorporate additional suggestions. Thank you for your valuable feedback.

Reviewer 4 Report

Comments and Suggestions for Authors

Dear authors,

In the paragraph 2.2 between lines 113 and 118, you make the general presentation of the methodology for measuring PM, but you do not mention anything about the metals that you will also be measuring.

In figures 2, 3, 4 and 5 you talk about the trees. I would say that not all the plants you analysed are trees.

In lines 359-360 you say 'the results demonstrate.....' I couldn't find where or how you measured surface roughness or plant hairs. How do you come to this conclusion?

Author Response

Thank you very much for your detailed and thoughtful review. We greatly appreciate the time and effort you have taken to provide this feedback.

In the paragraph 2.2 between lines 113 and 118, you make the general presentation of the methodology for measuring PM, but you do not mention anything about the metals that you will also be measuring.

• Response: Thank you for your valuable comment. We appreciate your attention to detail. In response, we have clarified the methodology section to explicitly include the procedures for measuring TEs using X-ray fluorescence (XRF) spectrometry. This addition ensures that the description of the methodology comprehensively reflects both PM and TEs analysis. The revised text has been incorporated into the manuscript for clarity and completeness.

In figures 2, 3, 4 and 5 you talk about the trees. I would say that not all the plants you analysed are trees.

• Response: Thank you for bringing this to our attention. You are absolutely correct that not all the plants we analyzed are trees. We have revised the manuscript to ensure consistent and accurate terminology, replacing references to "trees" with "plants" or "species" where appropriate, including in the captions of Figures 2, 3, 4, and 5. This adjustment ensures clarity and better reflects the diversity of the analyzed vegetation. We appreciate your careful review and helpful suggestion.

In lines 359-360 you say 'the results demonstrate.....' I couldn't find where or how you measured surface roughness or plant hairs. How do you come to this conclusion?

• Response: Thank you for your question. You are correct that we did not directly measure surface roughness or plant hairs. Instead, our conclusions in lines 359–360 are based on visual observations and supported by existing literature that links these morphological traits to PM retention. To avoid any misunderstanding, we have revised the text to clarify this point, explicitly stating that our findings regarding surface roughness and plant hairs are inferred from observational data and literature rather than direct measurements. This adjustment ensures the conclusions are accurately represented. We appreciate your attention to this detail.

Round 2

Reviewer 3 Report

Comments and Suggestions for Authors

Thank you for the revision. However, I do not see a detailed response to each of my comments left in the attachment attached again. 

Comments for author File: Comments.pdf

Author Response

We sincerely apologize for the oversight in not noticing the file with your detailed comments earlier. We greatly appreciate the time and effort you put into providing these valuable suggestions. Below, we have addressed each of your comments in detail, and where necessary, incorporated the suggested changes into the manuscript.

Thank you once again for your patience and for contributing to the improvement of our work. Please find our responses and revisions below. 

In the paragraph about line 107: "20-30 leaves were collected ... " How long was each leaf exposed to the environment? Are they mature or new leaves? What does "carefully" mean? How do you know that PM wasn't shaken off when handling the leaves? Especially for the smaller PM sizes, aren't they easily disturbed and airborne?

• The collected leaves were mature, as their development under Polish climatic conditions is synchronized, with most leaves emerging in spring and reaching maturity around the same time. This ensures that leaves sampled during the study were of comparable age. We have omitted the word "carefully" in the revised manuscript, as we agree it is self-evident in this context.

We acknowledge the possibility that a small fraction of particulate matter (PM), particularly smaller sizes, could become airborne during leaf handling. However, the leaves were collected with great attention to minimize such disturbances. Despite these precautions, current methodologies do not entirely eliminate this limitation. The collection process followed established practices to ensure consistency and reduce variability, as this approach is widely accepted in similar studies. We have added this clarification to the manuscript to better address potential concerns about PM handling and loss during sampling.

2.2. Methodology, about line 117: "Each 116 fraction was meticulously isolated and quantified ..." There are qualitative words used in this paper that are unnecessary and not quantified. (To name one more in Line 157: carefully.) Suggest removing these phrases and sticking to the work performed. How were the particles isolated?

• Thank you for your valuable suggestion. We have removed unnecessary qualitative words, such as "meticulously" and "carefully," as well as the sentence in question. These terms were redundant and did not provide measurable or specific information about the methodology. Instead, we have revised the text to focus on the precise steps and techniques employed, ensuring clarity and adherence to scientific writing standards.

Lines 150-152: "The difference in filter mass before and after filtration provided a direct measurement of SPM collected from each sample, ensuring high accuracy in PM quantification." may read "The difference in filter mass before and after filtration provided a direct measurement of SPM collected from each sample."

• Thank you for your valuable feedback. Following your suggestion, we have revised the sentence to remove the phrase "ensuring high accuracy in PM quantification." This change ensures the sentence is concise and focused solely on the methodological aspect, in alignment with scientific writing standards. We appreciate your input in improving the clarity of the manuscript.

Results, the first paragraph: Report ambient PM concentrations before the leaf collection. Are the leaves sprayed with chemicals as part of regular care of the green roof? Will other activities that could impact PM settlement on the leaves? Were all the leaves collected exposed directly to the environment (or were they under other leaves?)?

• Thank you for your question. The plants used in this study were not sprayed with any chemical treatments, such as pesticides or fertilizers, during the sampling period. This ensured that the PM collected on the leaf surfaces was not influenced or altered by any external chemical applications. Additionally, only leaves that were naturally exposed to the environment were selected for sampling. These leaves were chosen from the outer layers of the plant canopy, ensuring they were in direct contact with ambient air and subjected to typical environmental conditions, such as urban air pollution, particulate matter deposition, and weather variability. This approach allowed us to accurately assess the natural PM accumulation on the leaf surfaces without interference from protective or artificial layers.

Section 3.1, the first paragraph, the first statement, "The results of the study reveal substantial variability in total PM. accumulation 217 among species and across seasons, with the highest PM retention generally observed 218 during the summer months.": It is difficult to assess the accuracy of this statement because you didn't state how long the leaves were exposed and if they all had the same exposure period.

• Thank you for your thoughtful comment. As mentioned in an earlier response, the leaves sampled in this study were annual and of the same age. This information has now been explicitly included in the Materials and Methods section of the manuscript for clarity. The study, like many others published in similar contexts, was conducted on living plants growing naturally on the green roof. This approach reflects the real-world conditions of the study area and aligns with the objectives of the research. A specific feature of the temperate climatic zone where this study was conducted is that plants begin their growth and leaf development within a relatively synchronized timeframe in spring. This ensures that the leaves collected were naturally exposed to the environment for comparable periods. While an alternative method, such as growing plants in pots and exposing them simultaneously, could ensure stricter control over exposure periods, it would not align with the aim of this research, which focuses on the natural processes and dynamics of a functioning green roof ecosystem.

Figure 2 caption: y label missing. What do letters a through f mean? It is not clearly explained in this caption. What do the error bars mean?

Thank you for your valuable feedback. We have revised the caption for Figure 2 to address the concerns you raised. The y-axis label has been added to clearly indicate the variable being measured, along with its units. Additionally, the meaning of the letters a through f has been explicitly explained in the caption to ensure clarity. Finally, we have clarified the representation of the error bars.

Figure 3: See comment on Fig 2. Keep the scale on the y-axis the same for comparison. C is the lowest because the smallest particles are more likely to remain airborne longer than bigger particles.

• Thank you for your comments. We have standardized the y-axis scale for easier comparison and revised the caption for Figure 3 for clarity, similar to Figure 2. We also agree with your observation about smaller particles (C) remaining airborne longer and have ensured this is reflected in the discussion. We appreciate your helpful feedback!

Section 3.1, the last paragraph: The example result is not supported since you didn't report the PM the leaves were exposed to. There may have been less PM in the air. How is the increased leaf area in summer supported? It is not explained clearly that there is increased physiological activity and growth in the leaf area during summer. You collected in summer; how does that relate to growth DURING summer?

• Thank you for your feedback. In response to your comment, we have added data regarding general PM concentrations in the air for the city, which may partially explain the observed results. While our study focused on comparing PM accumulation across species under natural conditions, these ambient PM data provide additional context for interpreting seasonal differences. Regarding the size of the leaves, the sampled leaves were mature and had reached their full size in spring. However, during the summer, other factors such as the development of wax layers, trichomes, and other surface structures can significantly influence PM accumulation. Environmental conditions, including temperature, light intensity, and seasonal physiological changes, also play a role by modifying surface properties like wax composition and deposition dynamics. These updates have been included in the manuscript to clarify these points. Thank you for your thoughtful suggestions, which have contributed to improving the overall clarity and depth of the discussion.

Section 3.2, the first paragraph: Data must be provided to support the change in seasonal conditions. Due to larger sizes of depositing, there are no necessarily other conditions since no evidence has been shown here that the species of leaves, or the leaves themselves, attract PM. Are these the species (S. spectabile?) with the largest surface area? There is no data in this paper showing variations in sizes.

• Thank you for your feedback. We have added data on ambient PM concentrations across seasons to provide context for the observed seasonal differences, noting that conditions vary significantly year to year, particularly in recent times. Our study focuses on evaluating the intrinsic properties of plants in capturing PM under natural conditions. Additionally, a table detailing leaf characteristics has been included in the Methods section to clarify their role in PM retention. It is also important to note that this section is primarily a presentation of the results obtained from the study. Broader interpretations or contextual explanations, such as the factors influencing PM retention or seasonal variability, are addressed according to scientific standards in the Discussion section.

Section 3.3, the first paragraph: "when 270 leaf surface area and metabolic activity are highest." Demonstrated where? Is this common knowledge to readers in the field of air quality? This article doesn't support PM capture on plants that are any different than PM capture on other types of surfaces. What is the control?

• Thank you for your feedback. We agree that the statement "when leaf surface area and metabolic activity are highest" is misplaced in the results section and have removed it. While this is common knowledge in plant physiology, we did not directly study or demonstrate it in this work. Similarly, it is well-established that leaves, due to their surface properties, accumulate more PM than other surfaces, but our study focused on comparing PM retention across plant species, not on non-plant surfaces. Broader discussions on this topic are addressed in the Discussion section. As the study was conducted on a functional green roof, it was not feasible to include a control roof without vegetation under identical conditions. Our objective was to assess interspecies differences in PM accumulation rather than compare plants to non-vegetative surfaces.

Section 4, the first paragraph: "including surface roughness, the presence of wax content, and plant hairs," "plant hairs," "Environmental factors". Not mentioned above.

• We have added a table in the Methods section detailing leaf characteristics, such as surface roughness, waxes, and plant hairs, to highlight their role in PM retention. Baseline PM data were also included earlier for context. Quantitative data on waxes were presented in the Results, while this section references other studies to emphasize their broader significance in PM capture. Thank you for your feedback, which has improved the manuscript.

Section 4, the second paragraph: "others, reinforcing their suitability for green roof systems designed 366 to mitigate urban air pollution" Not supported.

• Thank you for your feedback. We have revised the fragment to clarify and better support the statement. The updated version highlights specific results from our study and explicitly connects the observed traits, such as the dense foliage of S. spectabile, to their potential suitability for green roof systems.

Section 4, the second paragraph: "who highlighted the influence of dense canopies on PM accumulation." How is this better or different than another large flat surface for PM collection?

• Thank you for your question. Dense canopies are more effective than flat surfaces for PM capture due to their larger cumulative surface area, ability to disrupt airflow and enhance deposition, and unique retention properties like waxes and trichomes. This has been extensively discussed in the literature, where numerous studies highlight the superior PM retention capabilities of dense plant canopies compared to flat, non-vegetative surfaces. Additionally, canopies provide shading, cooling, and other multifunctional benefits, enhancing their role in urban air quality improvement.

Section 4. "including increased foliage density, elevated am-378 bient PM concentrations, and peak industrial and vehicular emissions characteristic of this 379 season [42]." This supporting data should be introduced earlier.

We acknowledge that specific data on industrial emissions, and vehicular activity during the study period are not available in our research. Instead, our interpretation is based on findings from other studies, which suggest that these factors typically peak during summer and contribute to higher PM retention levels observed in this season. To address your concern, we have revised the relevant fragment to reflect this reliance on established literature and to avoid potential controversy. The updated text now emphasizes general trends reported in similar studies rather than presenting them as direct findings of our work. Thank you!

Section 4. "The role of environmental factors such as wind and precipitation also cannot be overlooked." And yet, it was not reported in the article. "species like S. spectabile demonstrated rela-386 tively consistent PM retention into autumn" Not supported. "These findings have important implications for urban phytoremediation strategies," No control present. Why not measure PM on the roof and on the roof next door without a canopy? Elsewhere in city away from the green room? "Reduced leaf area, decreased metabolic activity, and senescence likely contribute" First mention.  "for green roofs, ensuring optimized year-round 431 air purification in urban environments." This study hasn't shown that the green roof actually "purifies" the urban environment because no environmental PM data was collected or reported. Collection on 30 leaves doesn't then extrapolate to a hectare green roof. "The accumulation of TEs, including Mn, Fe, Cu, and Zn, demonstrated significant" Does capture of these from the air have health benefits?

Thank you for your thoughtful feedback. Below, we address your concerns and provide clarifications:

Meteorological Data: We acknowledge the absence of meteorological data, such as wind and precipitation, in this study. While we recognize their importance in influencing PM retention, these data were not collected during the study period and cannot be retrospectively reconstructed. We agree that including such data would enhance future research, and we plan to integrate these factors in subsequent studies. Findings on S. spectabile: To avoid controversy, we have revised the discussion regarding the performance of S. spectabile to ensure it aligns more closely with the available data and reflects its specific characteristics without overgeneralization. Control and Study Context: As noted previously, our study focuses on a living green roof system in a specific urban context—namely, a rooftop in the heart of a major European capital. This is emphasized in the study's title. Given the unique nature of this environment, finding a suitable control, such as a comparable roof without vegetation, is not feasible. Our primary goal is to compare the PM retention capacity across plant species rather than evaluate the overall impact of the green roof on air quality. Air Purification and PM Capture: It is important to emphasize that when PM particles are retained on leaf surfaces, they are effectively removed from the air, contributing to improved air quality. While it is not feasible to collect and analyze all leaves from the roof to quantify total PM captured, our results from selected samples provide a reliable basis for understanding species-specific retention capacities. This approach aligns with common practices in ecological and environmental research. Health Impacts of PM and TE Removal: Removing particulate matter, including trace elements (TE), from the air has well-documented health benefits. Reduced PM levels have been linked to lower rates of respiratory and cardiovascular diseases and increased life expectancy, as supported by numerous studies. While our study does not directly quantify these health impacts, it contributes to the broader understanding of how urban vegetation can mitigate air pollution and enhance public health.

Conclusions. "Evidence presented in this study underscores the value of green roofs as a sustainable solution for improving urban air quality." Not a claim that is supported given the gaps in data not reported. The dirt that plants are grown in are sources of PM and other substances.

• Thank you for your thoughtful feedback. We agree that the original conclusion may have overstated the findings, as our study does not provide direct evidence to fully support the impact of green roofs on urban air quality. To address this, we have revised the relevant statement in the manuscript to reflect a more measured interpretation of our results. Regarding the concern about soil being a potential source of PM on green roofs, we respectfully disagree with the implication that this is a major contributor in the context of our study. While it is not entirely impossible for PM to originate from the substrate, this largely depends on the type and composition of the soil used. For the green roof in this study, the substrate was specifically selected and designed to minimize such risks, making significant contributions from the soil unlikely. Following this reasoning, it is essential to recognize that questioning PM retention due to potential substrate contributions would also imply challenging the broader notion of PM pollution being primarily attributed to human activities, such as transportation and industrial emissions. While substrate management is indeed critical, we believe that the role of plants in capturing airborne PM remains an important aspect of urban air quality strategies. To further address this concern, it is worth noting that best practices for green roof design often include multi-layered vegetation systems. These can help ensure that any PM washed off by rainfall is captured and prevented from becoming airborne again. Such practices are already employed in many European cities to enhance the effectiveness of green roofs in mitigating urban air pollution. However, evaluating substrate contributions or optimizing multi-layered systems was beyond the scope of this study.

Round 3

Reviewer 3 Report

Comments and Suggestions for Authors

Thank you for addressing my detailed comments thoroughly and thoughtfully. Especially making clear that this paper is focused on PM accumulation and doesn't try to make claims that green roofs can impact surrounding air quality. More research would be needed to support the larger claim.

Author Response

Thank you for your kind words and for taking the time to provide such detailed feedback. We appreciate your recognition of the paper’s focus on PM accumulation and agree that it is important to clarify that our study does not aim to make broader claims about the impact of green roofs on surrounding air quality. We acknowledge that further research would be necessary to support such conclusions. Your thoughtful comments have greatly contributed to strengthening the clarity and precision of our work.