From the journal Environmental Science: Atmospheres Peer review history

11-May-2021

Dear Dr Bianchi:

Manuscript ID: EA-ART-03-2021-000016
TITLE: COVID-19 lockdown effects on aerosol particle size distributions in northern Italy

Thank you for your submission to Environmental Science: Atmospheres, published by the Royal Society of Chemistry. I sent your manuscript to reviewers and I have now received their reports which are copied below.

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Dr Nønne Prisle
Associate Editor, Environmental Sciences: Atmospheres

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Reviewer 1

The manuscript employed the in situ observations of aerosols and trace gases to assess the COVID-19 impacts from the traffic sector on the air quality, in particular the aerosol size distribution. I find the results of suppressed primary aerosols but enhanced NPF by the traffic reduction are quite intriguing, shedding light on the physical and chemical mechanisms of NPF. I will recommend the acceptance of the manuscript by Environmental Science: Atmospheres, after my comments below are addressed.

1) For the definitions of NPF, how to quantify the “a sudden burst of particles below 25 nm”?

2) The definition of “primary” aerosols seems not rigorous. Are they just small particles (less than 50 nm) under the non-NPF condition?

3) For Fig. 1 and related results, can the authors elaborate more on the potential impacts by the variations of meteorological conditions? A related question is whether and how meteorology affects the aerosol particle size distribution?

4) The paper has lots of formatting and/or layout issues, e.g. L167, L211-226. Please fix them.

Reviewer 2

COVID-19 lockdown effects on aerosol particle size distributions in northern Italy
Shen Jiali, Bigi Alessandro, Marinoni Angela, Lampilahti Janne, Kontkanen Jenni, Ciarelli Giancarlo, Putaud Jean, Nieminen Tuomo, Kulmala Markku, Lehtipalo Katrianne, Bianchi Federico,
EA-ART-03-2021-000016
The paper entitled “COVID-19 lockdown effects on aerosol particle size distributions in northern Italy” was improving the knowledge of importance of emission sources other than
traffic and of other processes for controlling particulate pollution.
This way the topic is suitable for Atmospheres and the manuscript could be high interest for the scientific community. However, two minor additional points need to be addressed before final publications.

Measurement data
Page 7 line 132 Please correct “distributiondata” to distribution data
Page 8 line 167 in the sentence “As shown in….” is missing reference, please correct this

The response to the comments from reviewers for the Manuscript ID: EA-ART-03-2021-000016
TITLE: COVID-19 lockdown effects on aerosol particle size distributions in northern Italy
Please find below our replies the reviewers comments (in green) and the changes that have been made in the manuscript (in red).


First of all we would like to thank the reviewers and the editor to find the time to evaluate our manuscript.


Referee: 1
Comments to the Author:
The manuscript employed the in situ observations of aerosols and trace gases to assess the COVID-19 impacts from the traffic sector on the air quality, in particular the aerosol size distribution. I find the results of suppressed primary aerosols but enhanced NPF by the traffic reduction are quite intriguing, shedding light on the physical and chemical mechanisms of NPF. I will recommend the acceptance of the manuscript by Environmental Science: Atmospheres, after my comments below are addressed.

1) For the definitions of NPF, how to quantify the “a sudden burst of particles below 25 nm”?

NPF events are characterized by a sudden appearance of nucleation mode particles in the size distribution. There is no exact limit how much the concentration needs to increase (the events can be “weak” or “strong”). However, typically, the concentration of particles below 25 nm increases by several factors or even orders of magnitude in less than 1h (see examples in Fig S6). The NPF “intensity” (how many particles are produced) is quantified by formation rate, J (not included in this study). Additionally, to qualify as regional NPF event day, there needs to be continuous growth of the new mode visible in the size distribution plot for several hours, indicating that the NPF event takes place over a larger geographical region. This growth pattern is not visible e.g. during the morning rush hours, which also leads to increase in the concentration of sub-25nm particles.

To clarify the definition of NPF events days, we changed the sentence: “The days when a sudden burst of particles below 25 nm occurred, followed by their growth, were assigned as NPF event days.”

to

“The regional NPF event days are characterized by a sudden increase in the concentration of particles smaller than 25 nm, followed by a continuous growth of the new particle mode lasting for several hours.”

Additionally, we added the word “regional” in front of the word NPF to several places (marked in red) in the manuscript to clarify that only these “regional type NPF events” are included in the NPF event class and not any local particle bursts that could be related e.g. to traffic or other local sources.



2) The definition of “primary” aerosols seems not rigorous. Are they just small particles (less than 50 nm) under the non-NPF condition?

Primary particles refer to the particles that are not formed from other gaseous pollutants. They are emitted into the atmosphere directly e.g. from human or natural activities, and combustion process. Traffic can produce particles also via new particle formation process, when the hot exhaust cools down as it is diluted in the atmosphere (these are sometimes called “delayed primary particles” as they are directly connected to traffic emissions, see e.g. Rönkkö and Timonen, 2019). In the section, ‘The impact of the mobility restrictions on particle number size distributions’ we discuss the particles size distributions from ‘primary particle emission from traffic’ and ‘new particle formation and growth’ based on the primary and secondary. In some place, we have used the term “primary” to refer to small particles although some of them could obviously be secondary. Therefore, we have removed the word ‘primary’ in L97, L135, and L343, where we actually mean small particles.
Besides, we try to demonstrate the definition of ‘Primary’ aerosol clearly in the paragraph by add one small sentence (marked in red):

Now the main text reads:

“Traffic is a major source of atmospheric nanocluster aerosols (particles smaller than 3 nm), and it directly emits aerosol particles including the core mode (particles smaller than 10 nm) and soot mode (particles between 30 and 100 nm in diameter) 18-20, what we call primary aerosol particles. In addition, traffic emissions have been shown to significantly produce sub-3 nm particles, and also to contribute to secondary aerosol particle formation by emitting various aerosol precursors21,22, including SO2, VOCs (volatile organic compounds) and NOx.”


3) For Fig. 1 and related results, can the authors elaborate more on the potential impacts by the variations of meteorological conditions? A related question is whether and how meteorology affects the aerosol particle size distribution?

The meteorological conditions have several important effects on the aerosol particle size distribution. For example, precipitation will decrease the number concentration of primary and secondary particles. In addition, solar radiation affects the formation of condensable vapors, resulting in the formation and growth of secondary particles. Besides, convection, wind direction and speed also affect the particle size distribution by transporting and diluting atmospheric emissions.
The effect from meteorological conditions on particle size distribution is minor. Because, the small particles < 100 nm are easy to loss due to the dilution, collision, decomposition, and coagulation.
For the other related results in Fig.1, we compare 2020 levels of pollutants with the mean level of the corresponding period in previous years (2016-2019). In this case, the impacts caused by the variations of meteorological conditions are minor compared to the reduction of traffic. However, we cannot exclude that the meteorological conditions in 2020 compared to the reference period could not have had a minor contribution to the results.

We added a mentioning in the text: “Here we have assumed that any anomaly in the meteorological conditions in year 2020 compared to the reference period had a minor impact compared to the reduction of traffic.”

4) The paper has lots of formatting and/or layout issues, e.g. L167, L211-226. Please fix them.
The layout issues have been fixed in the revised manuscript.

Reference:
T. Ronkko and H. Timonen, Overview of Sources and Characteristics of Nanoparticles in Urban Traffic-Influenced Areas, Journal of Alzheimer's Disease, 2019, 72, 15-28.

Referee: 2
Comments to the Author
The paper entitled “COVID-19 lockdown effects on aerosol particle size distributions in northern Italy” was improving the knowledge of importance of emission sources other than traffic and of other processes for controlling particulate pollution. This way the topic is suitable for Atmospheres and the manuscript could be high interest for the scientific community. However, two minor additional points need to be addressed before final publications.

Measurement data
Page 7 line 132 Please correct “distribution data” to distribution data
Page 8 line 167 in the sentence “As shown in….” is missing reference, please correct this
These minor comments are addressed in the revised manuscript.

07-Jun-2021

Dear Dr Bianchi:

Manuscript ID: EA-ART-03-2021-000016.R1
TITLE: COVID-19 lockdown effects on aerosol particle size distributions in northern Italy

Thank you for submitting your revised manuscript to Environmental Science: Atmospheres. After considering the changes you have made, 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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With best wishes,

Dr Nønne Prisle
Associate Editor, Environmental Sciences: Atmospheres

Reviewer 2

I think that author do a nice job while answering the referee’s comments. In my opinion with this shape this manuscript is adequate for the publication in the Environmental Science: Atmospheres.

Reviewer 1

The authors have adequately addressed my comments.