From the journal Environmental Science: Atmospheres Peer review history

18-Aug-2022

Dear Dr Sorooshian:

Manuscript ID: EA-ART-06-2022-000073
TITLE: Stubborn aerosol: Why particulate mass concentrations do not drop during the wet season in Metro Manila, Philippines

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Associate Editor, Environmental Sciences: Atmospheres

************

Reviewer 1

Comments and recommendations are in the attached file. Thank you.

Reviewer 2

I believe the authors have presented a well-written and well-discussed study on supermicrometer particulates in Metro Manila. I think the conclusions made are logical and therefore recommend acceptance.

Manuscript ID: EA-ART-06-2022-000073
TITLE: Stubborn aerosol: Why particulate mass concentrations do not drop during the wet season in Metro Manila, Philippines

We thank the handling editor and the two referees for their helpful comments. We have provided our responses to comments from Referee 1 below in blue.

Referee 1:
This manuscript investigated mass concentration of particulate matters in Metro Manila and tried to explain reasons why particles cannot be efficiently removed by rain. Authors identified factors that contributed to the high mass during rainy season, however I found that the content is not significant. My comments/questions are mainly supported by the three references below:

Cruz, Melliza Templonuevo, et al. "Size-resolved composition and morphology of particulate matter during the southwest monsoon in Metro Manila, Philippines." Atmospheric Chemistry and Physics 19.16 (2019): 10675-10696.

Hilario, Miguel Ricardo A., et al. "Diurnal characteristics of summer precipitation over Luzon Island, Philippines." Asia-Pacific Journal of Atmospheric Sciences 57.3 (2021): 573-585.

AzadiAghdam, Mojtaba, et al. "On the nature of sea salt aerosol at a coastal megacity: Insights from Manila, Philippines in Southeast Asia." Atmospheric Environment 216 (2019): 116922.

Response: The first paper by Cruz et al. (2019)1 only used data spanning July – October 2018 and focused on general characterization and source apportionment. The second paper by Hilario et al. (2020) does not use the MOUDI dataset and analyzed diurnal precipitation patterns across the entire Philippines rather than just Metro Manila. Hilario et al. (2020)2 focused on topographical and seasonal effects on the diurnal cycle of precipitation and does not discuss aerosol chemistry. The third paper by AzadiAghdam et al. (2019)3 does use the MOUDI dataset, but only for July – December 2018, and analyzes sea salt relationships with aerosol chemistry and precipitation.

Compared to the three studies mentioned by the reviewer, our current study uses the full 16-month span of the MOUDI dataset, which was completed on October 20194. We also focus on total-water soluble species, black carbon, and total gravimetric mass, rather than a specific aerosol type like in AzadiAghdam et al. (2019)3. Our study investigates the seasonal consistency in particulate matter (PM) over Metro Manila, a feature first pointed out by Kim Oanh et al.5 that has not been studied to this level of detail before. The MOUDI dataset, though also used in previous studies, has not been used to study this seasonal feature and provides a rich multi-seasonal dataset of size-resolved aerosol composition.

1. The MOUDI data sets used in this manuscript has been published several times before, can you reuse them?

Response: The dataset has been used for several studies, but the fact that multiple studies can use the same dataset to answer different science questions is a testament to the wealth of information contained within the dataset. The repeated use of this MOUDI dataset does not detract from the novelty and importance of this studies’ findings as it answers a science question that is distinct from the previous studies using the same dataset. In this study, we explain the seasonal consistency in particulate matter (PM) over Metro Manila, a feature first pointed out by Kim Oanh et al.5. The MOUDI dataset, though also used in previous studies, has not been used in any previous work to study this seasonal feature and provides a rich multi-seasonal dataset of size-resolved aerosol composition with which to address the science questions listed in the Introduction.

2. The relationship between size-resolved particulate matter and rainfall in Metro Manila has been extensively studied and already in press, anything significantly new here? Can you include hypotheses in the introduction?

Response: Please see our response to comment #1. We have specified hypotheses at the end of the Introduction, as follows:

“To answer these science questions, we propose the following hypotheses: (1) submicrometer particles will be inefficiently scavenged by rain, which is largely light in intensity over Metro Manila; and (2) more conducive conditions for secondary formation will contribute to sustained PM during the wet season.”

3. How did you determine the length of dry/wet season? Does the decision on the cutoff month affect your results? Can you use data to explain?

Response: The monsoon seasons over Southeast Asia are well-established in literature2,6–11. Generally, the seasons compose of two phases: the wet season (i.e., the southwest monsoon) between May and October, and the dry season (i.e., the northeast monsoon) between November and April. We have added citations to the studies in the Introduction as follows:

“In this study, we define the wet (dry) season as May – October (November – April), corresponding roughly to the two phases of the Asian monsoon as established in previous studies 2,6–8,10,11.”

Our decision to use a two season division was also to be consistent and ensure comparability with Kim Oanh et al.5, who first pointed out the seasonal consistency of PM in Metro Manila. The division of the year into two seasons helps to isolate seasonal factors by using the long averaging window, which masks out other factors such as transient aerosol events (discussed in Section 3.1). Furthermore, the longer averaging window allows for more robust statistics by increasing the number of MOUDI sets used in each average. This is important due to the labor-intensive nature of the MOUDI data collection.

We have performed sensitivity tests by changing the bounds of the dry & wet seasons below. We find no significant differences that would affect the conclusions of the study.

Original (Fig. 1). Wet season: May to October (N: 32 for WS, 8 for Mass, BC); dry season: November to April (N: 22 for WS, 3 for Mass, BC).

Sensitivity test #1: Wet season: May to September (N: 29 for WS, 7 for Mass, BC); dry season: November to April (N: 22 for WS, 3 for Mass, BC).

Sensitivity test #2: Wet season: June to September (N: 25 for WS, 6 for Mass, BC); dry season: November to April (N: 22 for WS, 3 for Mass, BC).


4. I think the question why wet scavenging is inefficient in Manila cannot be answered by data analyses. I suggest improving the manuscript by exploring concepts such as (1) interaction between droplet and hydrophobic/hydrophilic particles; (2) chemical diffusion upon droplet evaporation; and (3) interfacial instabilities

Response: Although causation is a natural limitation of any observation-based study without modeling, our study still offers valuable conclusions that address our science questions and point to future topics. We show that seasonal cycles of major PM components (BC and water-soluble species) are opposing and their combined contributions can explain the seasonal consistency in PM observed by Kim Oanh et al.5. Our study further demonstrates using multiple datasets (i.e., MOUDI, AERONET, weather stations) that the presence of rain does not necessitate efficient wet scavenging and we show that rain duration has the clearest relationship with AOD. We also utilize the MOUDI’s size-resolved aerosol composition to identify size ranges most affected by higher rain durations. The concepts recommended by the reviewer are indeed important topics to address in future studies using a more appropriate dataset. 

References
1 M. T. Cruz, P. A. Bañaga, G. Betito, R. A. Braun, C. Stahl, M. A. Aghdam, M. O. Cambaliza, H. Dadashazar, M. R. Hilario, G. R. Lorenzo, L. Ma, A. B. MacDonald, P. C. Pabroa, J. R. Yee, J. B. Simpas and A. Sorooshian, Atmos. Chem. Phys., 2019, 19, 10675–10696.
2 M. R. A. Hilario, L. M. Olaguera, G. T. Narisma and J. Matsumoto, Asia-Pacific J Atmos Sci, , DOI:10.1007/s13143-020-00214-1.
3 M. AzadiAghdam, R. A. Braun, E.-L. Edwards, P. A. Bañaga, M. T. Cruz, G. Betito, M. O. Cambaliza, H. Dadashazar, G. R. Lorenzo, L. Ma, A. B. MacDonald, P. Nguyen, J. B. Simpas, C. Stahl and A. Sorooshian, Atmospheric Environment, 2019, 216, 116922.
4 C. Stahl, M. T. Cruz, P. A. Bañaga, G. Betito, R. A. Braun, M. A. Aghdam, M. O. Cambaliza, G. R. Lorenzo, A. B. MacDonald, P. C. Pabroa, J. R. Yee, J. B. Simpas and A. Sorooshian, Scientific Data, 2020, 7, 128.
5 N. T. Kim Oanh, N. Upadhyay, Y.-H. Zhuang, Z.-P. Hao, D. V. S. Murthy, P. Lestari, J. T. Villarin, K. Chengchua, H. X. Co and N. T. Dung, Atmospheric Environment, 2006, 40, 3367–3380.
6 J. Matsumoto, L. M. P. Olaguera, D. Nguyen‐Le, H. Kubota and M. Q. Villafuerte, Int J Climatol, 2020, joc.6492.
7 F. T. Cruz, G. T. Narisma, M. Q. Villafuerte, K. U. Cheng Chua and L. M. Olaguera, Atmospheric Research, 2013, 122, 609–616.
8 L. M. Olaguera, J. Matsumoto, H. Kubota, T. Inoue, E. O. Cayanan and F. D. Hilario, Atmosphere, 2018, 9, 464.
9 L. M. P. Olaguera and J. Matsumoto, Int J Climatol, 2019, 40, 4203–4217.
10 I. Akasaka, W. Morishima and T. Mikami, Int. J. Climatol., 2007, 27, 715–725.
11 I. Akasaka, Int. J. Climatol., , DOI:10.1002/joc.1975.

30-Aug-2022

Dear Dr Sorooshian:

Manuscript ID: EA-ART-06-2022-000073.R1
TITLE: Stubborn aerosol: Why particulate mass concentrations do not drop during the wet season in Metro Manila, Philippines

Thank you for submitting your revised manuscript to Environmental Science: Atmospheres. I am pleased to accept your manuscript for publication in its current form. I have copied any final comments from the reviewer(s) below.

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Dr Nønne Prisle
Associate Editor, Environmental Sciences: Atmospheres

Reviewer 1

Thank you for your responses. I still think it would be a tremendous help for the journal and the field if you can draw more important and surprising conclusions, but the manuscript is at a publishable level. I recommend accepting for publication.