Round 1

Reviewer 1 Report

Manuscript review: Substantial Decreases in U.S. Cities Ground-Based NO2 Concentrations During COVID-19 from Reduced Transportation (sustainability-1254273).

Recenzowany manuskrypt dotyczy wpływu ograniczonego transportu w trakcie pandemii COVID19 na emisje NO2.

1. The abstract should be redrafted. It is incomprehensible. It will be more understandable if it includes: the sentence of introduction with justification of the presented studies, the purpose of the work, the test methods and the results obtained. The most important results achieved are lacking in the abstract. The abstract lacks specific numerical values of the studies conducted. What type of engine was in the vehicles analyzed? What type of fuels were used in them? What share of the surveyed car population were electric cars? Please add this information in the abstract. This is extremely relevant to the analysis.
2. A review of literature is absolutely not enough. Please highlight the knowledge gap, justify the purpose of your research, and write how it differs from other authors? The literature presented in the manuscript does not include the latest publication achievements. Most of the literature is from ten years ago. I have a feeling that this article was prepared some time ago. The authors do not consider the car fleet at all. Without this it is difficult to interpret the results. The premise in itself (less cars - less emissions) is obvious, what's new about that?
3. Line numbers are missing from the manuscript. It is difficult for me to refer to a specific place in the manuscript. The authors write, "Additionally, NO2 concentrations are assessed and compared to truck traf-fic volumes, a good metric for vehicular emissions in cities, in the Indianapolis study re-gions." Were only trucks considered? Later in the manuscript, the authors write: „we focused on total vehicular traffic, total cars, and classification of motorcycle, car, pickup, and bus as a sub-category (1-4)”
4. The authors write about EPA sensors. After all, on the basis of homologation tests we are able to precisely determine emissions for conventional fuels (does not apply to bio additives in the fuel mixture). Such analyses of estimates of traffic emissions have been made for a great many years. I still don't see the novelty of the manuscript.
5. The authors write, "For the remaining 8 cities we averaged NO2 data over two fixed sensors each for 2020 and 2015-2019. How was this done?
6. Laterinthemanuscript, theauthorswrite: „we focused on total vehicular traffic, total cars, and classification of motorcycle, car, pickup, and bus as a sub-category (1-4)” How many individual vehicles were there?
7. How many people were in the study location at the time of the measurements? By how much did the population decrease (people went out of town for the pandemic)? Contrast this with the results in Tables 1 and 2 and Figure 1.
8. The manuscript lacks basic answers to the "why" question. Ok, concentrations of emissions are shown, but the authors have not asked themselves why these emissions are the way they are. Certainly the number of cars on the road had an impact, but surely that's all it was? Absolutely not. This manuscript is not scientific in nature. It lacks elements of novelty.

Author Response

Please see attached file.

Author Response File: Author Response.pdf

Reviewer 2 Report

1.  Introduction

The health crisis derived from COVID-19 has forced the establishment of restrictions on mobility in most countries, as well as a limitation on certain productive activities. These restrictions have resulted in lower levels of traffic and, consequently, a decrease in the levels of pollution recorded at the air quality measurement stations.

The comparison of the air quality levels obtained in the periods in which traffic restrictions have been present with other periods allows knowing the impact that the measures derived from the air quality improvement plans could have on a certain locality.

The analysis of these decreases in pollution in different locations can allow us to better understand the impact that these measures could have, based on the relationships with the characteristics of the car fleet, the orographic conditions of the area and its meteorology.

In this article, not only the concentration levels of pollutants observed in different locations are used, but also other values ​​related to the volume of traffic are used to obtain the relationship between this and the levels of contamination.

The ratios found can be used by other researchers at the beginning of their studies. Questions not resolved in the study are justified in the Discussion section.

2.  Major observations

None.

3.  Minor observations

• The next paragraph expresses the idea of uncertainty in an incomplete way.

Materials and Methods

NO2 and Vehicle Miles Travelled (VMT) data

Based on Federal Audits required by the Environmental Protection Agency (EPA), generally a difference of < 15 percent difference is an acceptable federal standard that the states have to adhere to.

The following wording is suggested:

Based on Federal Audits required by the Environmental Protection Agency (EPA), the uncertainty associated with the measurement (sum of possible deviations due to the different sources of error that may appear) must remain below 15%.

• The tables use a different number of decimal places. It is advisable to unify it.
• Table 3 header can be misleading due to the text alignment format.
• Include at least one graph of the individual correlation between NO2 and VMT in March and April 2020, to better understand the behaviour between these variables and the linearity found between both.
• The next paragraphs, with which the Discussion section begins, are more typical of the Introduction section. It is suggested that it be displaced.

“High vehicular emissions can result in corridors of heavy air pollution [26] in rural and urban regions. NO2 pollution, a tracer for vehicular emissions, has been linked to ad-verse health effects for increased asthma events in predominantly urban areas [27]. A 20 ppb increase in NO2 has been found to increase chronic obstructive pulmonary disease (COPD) hospital visits, cardiovascular disease, lung cancer in adults, and respiratory mor-tality [9,10]. The onset of COVID-19, and the stay at home orders in March and April, presented an opportunity to examine the changes in NO2 concentrations and their rela-tionship to VMT in 11 cities in the U.S. with implications for local health outcomes.

Satellite data, due to its analysis being based on the entire tropospheric column and its spatial and temporal coverage limitations, can mis-report on the ground pollutant measurements. Additionally, urban regions versus remote regions can have its daily NO2 retrievals varying up to 40% [28].”

Author Response

Please see attached.

Author Response File: Author Response.docx

Reviewer 3 Report

The authors have analyzed the relation between NO2 concentrations with Transportation based on data of normal five years and a COVID-19 year. The result found that the NO2 is positively related to Vehicle Miles Travelled (VMT). Actually, many studies have demonstrated their relation; however this manuscript use the data occurred in COVID-19 period, which exclude better the effect of human activities due to stay-at-home policy occurance. Therefore, the data and results have stronger persuasiveness. However, there are still details required to address for a more complete story.

1. P3, L9-13: “A reduction between …..Table A1)”, there are some data results such as 12%-41%, 11%-56% et al. It is difficult to be found. Because Table 1 and Table A1 show the variation between jan to Mar/apr in 2020 and 2015-2019 respectively. I don’t found there are some comparison between 2020 and 2015-2019. Please clarify them
2. P7, table3. The title of table is incompact. Like “NO2 Jan to Mar”, ‘VMT Jan to Mar”, “NO2 Jan to Apr”, they seems to integrated and thus no clear.
3. The explanation is less although I found some explanation to the Houston that its industries affect the NO2 concentration affecting the correlation between NO2 and VMT. However, in P7L1-5: “The percentage change of NO2 of indianaplis… are higher than the VMT, …. For LA, VMT are lower than NO2 …” why?   P8, L1-3:”San Diego have higher significant and LA have lower correlations” why?
4. In P9-10, there are two explanations which confused me, please describe it more clearly. For example, “VMT is tow times NO2 in Indianaplis”. Another is to related to Extrapolating conclusion, how to understand it.

Author Response

Please see attached.

Author Response File: Author Response.pdf

Reviewer 4 Report

This research deals with the experimented changes in NO2 in 11 U.S. cities during the COVID-19 lock-down. NO2 concentrations were related with the vehicular traffic and their reductions were estimated. Results are interesting and deserve to be published. Nevertheless, before publications some comments should be attended.

 

Introduction

If the NOx reduction is related with traffic reduction, the reader expects to know which the contribution of traffic in the ambient air of the eleven cities of the U.S. is, and which is the situation and standard of this pollutant in this country.

 

Methods

Please include the data source addresses. It could be done in supplemental material or in the references. It is important also to include the locations of the sensors. For instance:

authors mentioned that due to excessive number of missing days of data for San Antonio and Austin we utilized data from one sensor each in those locations. It is important to know which was the used sensor?

I do not know if it is a mistake but the reference 15 do not contain the information of the sensor locations in American cities, since that study was performed in the border of U.S.-Canada, then it is important to know which the data sources are.

 

Results

Please include in Table 1 and Table A1, the standard deviations, medians or ranges of NO2 data.

It is not clear how were estimated the reductions in Figure 1. Which were the reference values to estimate the reductions? Please explain this with a better detail.

Which was the significance level used for Spearman correlation? Please include the value in the text and in the Tables.

It is not clear if the evaluated cities do not have a monitoring network for NO2. If those data are available, why they were not used instead data from sensors?.

Author Response

Please see attached.

Author Response File: Author Response.docx

Reviewer 5 Report

Substantial Decreases in U.S. Cities Ground-Based NO2 Concentrations During COVID-19 from Reduced Transportation - Asrah Heintzelman et al., 2021 

The authors investigated the change of atmospheric NO2 concentrations in several cities in the U.S. during the COVID-19 and explore the links between the decrease in NO2 and the drop in transportation. Although many similar studies already show the decrease in NO2 concentration during the COVID-19 lockdown period, this study tried to link with the drop in traffic counts/classification and vehicle miles travelled. I do believe these statistics can help policymakers to better migrate the air pollution.

A major comment:

The emission of the car should not only relate to the VMT (or the car number) but also to the traffic condition and the gas/engine quality. Do you have considered this?

Some specific minor comments:

Introduction: please add the introduction about NO2 emission sections in these cities from the bottom-up inventories., e.g. EDGAR

Section 3.1: In addition to comparing with the 5-years average, I suggest looking at the range (all these years) to address the year-to-year variability.

Figure A1: add the cities in the map

Table 3: -7898?

 

Author Response

Please see attached

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

The manuscript continues to fall short of my expectations.
Cosmetic changes have been made to the literature review as well as the rest of the manuscript.
The scientific value of the manuscript was not added (or highlighted).
The fact that there is reading data in the manuscript does not establish its scientific validity.
The authors do not scientifically analyze why the results are the way they are. If a model (e. g. a mathematical one describing the phenomenon) had been built on the basis of the research results, we could speak of the scientific value of the manuscript.

In summary, the cosmetic changes made to the manuscript by the authors do not convince me to give a positive review.

Reviewer 3 Report

I think that the authors have reviewed what i concern, and  i suggest the manuscript be accepted in current form