Black Carbon in a City of the Atacama Desert before and after the Start of the COVID-19 Lockdown: Ground Measurements and MERRA-2 Reanalysis

Round 1

Reviewer 1 Report

This study compared black carbon in Tacna, Peru, by comparing the measured values of PAX BC before and after the start of the Covid-19 lockdown with the data of MERRA-2 BC. As a result, it was confirmed that the actual value came out higher than the value using MERRA-2.

The summary of this thesis is as follows.

In the introduction section, black carbon was explained in detail and the necessity and importance of this paper was clearly expressed.

In materials and methods, the method was explained in detail by expressing the measurement area, measuring instrument, MERRA-2 reanalysis, and weather.

The results are easy to see and compare the PAX BC and MERRA-2 data well, but it is necessary to show two clear differences in one figure.

In the summary and conclusion section, the results are well synthesized, but a more explicit discussion of the limitations of MERRA-2 is needed. It would also be nice to have an explanation of future directions for incorporating the results of PAX BC into MERRA-2.

In conclusion, this paper clearly explains the difference in black carbon values between PAX and MERRA-2, and is worthy of publication in remote sensing as a valuable paper. However, a well-organized conclusion in the conclusion section is required.

Author Response

The authors thank the reviewers for their exhaustive review and their interesting comments, questions, and suggestions. We value your contribution, which has allowed us to improve this work.

Author Response File: Author Response.pdf

Reviewer 2 Report

Black carbon (BC) plays an important role in physicochemical processes in the atmosphere. The manuscript analyzed the temporal variations of BC in Tacna, Peru using ground measurements obtained with a photoacoustic extinctiometer (PAX BC) and NASA’s MERRA-2 reanalysis data (MERRA-2 BC), which can provide useful information for air pollution control. However, the results of the manuscript should be further elaborated and better organized. Some specific comments on the manuscript are as follows.

1. The manuscript reported a large discrepancy between PAX BC and MERRA-2 BC. Could the authors provide some possible solution to reconcile this disparity, or to adjust the MERRA-2 BC?

2. As the manuscript focuses on the periods before and after the start of the COVID-19 lockdown, the seasonal and daily variations of BC should be classified into two categories: during the COVID-19 lockdown and outside the COVID-19 lockdown period. Then the outcome of both categories should be compared to investigate any differences.

3. Conducting statistical tests of BC concentration by considering wind speed and wind direction may provide more reliable and significant information on the influence of wind speed.

4. HYSPLIT backwards trajectories analyses were conducted at 100 m, 1000 m, and 2000 m above ground level. However, the PAX inlet was located at a height of approximately 14 meters above ground level. How can the source identified by HYSPLIT be sufficient to explicate the PAX BC concentration?

5. Apart from two weeks and one week, it is better to test trends over a longer period both before and after the start of the lockdown.

6. The significance level (α) of the Mann-Kendall test is usually set at 0.05.

7. It is better to plot the domain of the grid cell 18.0–18.5°S and 70.000–70.625°W with the Tacna City in the map.

8. It is better to include a scale bar in Figure 1.

There are some minor errors in the manuscript, such as in line 306 where “overestimating o underestimating” should be “overestimating or underestimating”.

Author Response

The authors thank the reviewers for their exhaustive review and their interesting comments, questions, and suggestions. We value your contribution, which has allowed us to improve this work.

Author Response File: Author Response.pdf

Reviewer 3 Report

Ground measurements were obtained with a photoacoustic 23 extinctiometer (PAX BC) and NASA´s MERRA-2 reanalysis data. The data was thoroughly analysed and compared in order to present results on their differences and similarities. Various weather and temporal variables were analysed in order to provide a clear picture of the issue. The effect of the COVID-19 lockdown was analysed in order to show the BC downward trend after lockdown started. This also shows how different policies can have a direct effect on climate issues.

Overall, the results present an interesting solution which can be helpful during further data analysis work and policy development and implementation. The results are in most part clearly presented and thoroughly described. There is some room for improvement concerning some paragraphs and presentation of the results.

Comments:

1. It would be good to further elaborate the references where there are three references in a single sentence, i.e. to reference them separately (e.g. line 42, 43, etc.).

2. Paragraph in lines 54 -63 should be reorganised. I propose that you put the first part of the paragraph at the start, and the rest in the first paragraph of the paper.

3. A table at the end of the methods section with a comparison of the key features of PAX and MERRA-2 would be good.

4. In line 302, it would be good to start the sentence with an author name and not the year. The same goes for line 452.

5. Is it possible to improve readability of Figure 9?

6. Figure 10 is referenced in the text after the Figure. It should be referenced before. I propose to move the paragraph in lines 620 – 635 before Figure 10.

7. The results are clearly presented, Figures and Tables are thoroughly explained, and many variables are taken into account and their contribution individually discussed. However, in order to better present the significance of the results, there should be a discussion section (inside the Results section or by itself) where a wider scheme or interaction between various influences is discussed. It is currently more or less listing the results, and there should be an additional interpretation of the results from the authors.

Author Response

The authors thank the reviewers for their exhaustive review and their interesting comments, questions, and suggestions. We value your contribution, which has allowed us to improve this work.

Author Response File: Author Response.pdf

Reviewer 4 Report

This paper compared black carbon concentrations in Tacna, Peru, measured by a photoacoustic extinctiometer and simulated by MERRA-2, and analyzed the diurnal and seasonal variations of BC. The authors also analyzed the impacts from meteorological factors (mixed boundary layer height and wind speed and direction) as well as human activities (such as COVID-19 lockdown). I have two major comments.

(1) What is the purpose of comparing PAX BC and MERRA-2 BC? The two BC datasets are basically not comparable, including their magnitudes, diurnal variations and seasonal patterns. If you assume the PAX measurements are correct, does it mean that MERRA-2 BC simulations are not? The explanations of the huge discrepancy are not convincible. Moreover, what is the exact definition of BC concentration in MERRA-2? Is it similar to the PAX-BC that is derived from light absorption measurements? And whether the MAC parameter used in the paper is selected correctly?

(2) What is (are) the main reason(s) of BC variation according to the analysis? To me, BC emission sources, such as biomass burning and power plant, should be investigated in-depth because we can see the variations of PAX-BC as shown by Figure 4a, and meteorological factors can hardly cause such dramatic changes. The authors did not put emphasize on those. I also don’t understand some of the analysis. For instance, they showed the diurnal variations of mid-boundary layer height of four individual days (Figure 7), but did not show the relevant diurnal variations of BC concentrations of these days. In some sections, the authors did not attempt to give a real explanation, instead, they just described what the figures or tables already told the audience.

 

Some specific comments are listed below:

1) Line 25, “070”à“0.70”

2) Line 85, what does “MERRA-2 underestimates ground measurements” mean? MERRA-2 underestimates surface AAOD or columnar AAOD? Note that AERONET only provides AAOD of the entire atmosphere.

3) Figure 1, the basic mapping elements are missing, e.g., geographic coordinates, scale.

4) Line 206, “at single-level”, which level?

5) Line 233-234, which season does “September” belongs to? Spring or winter?

6) Table 1, what does “2020-2021” mean? If it is the average of two years, why the values of T, RH, WS of summer in 2020-2021 are lower than both years?

7) Line 364-365, if MERRA-2 BC represents the average concentration of the column, then the comparison between MERRA-2 and PAX should take vertical distribution of BC into account because the assumption of the profile influences the result.

8) Figure 5. The extremely high values shown in Figure 4a may be exceptional, and these values are easy to increase the average when calculating the seasonal average, resulting in potential seasonal differences being drowned out, noticing the average of spring, summer, and autumn are quite similar as shown in Figure 5. In contrast, the seasonal differences of MERRA-2 are more obvious. In my opinion, seasonal patterns reflect the combination of climate, emission sources, and other factors, thus PAX-BC and MERRA-2 BC should have general similarity.

9) Line 370-388, what does this paragraph try to convey? It is difficult to understand the purpose of this analysis since it cannot explain the discrepancy between MERRA-2 and PAX..

10) Line 418. “due to the start of anthropogenic activities” at around 3-5 o’clock (local time) in the morning? This is kind of unreal.

11) Line 497-498. It is recommended to indicate whether the main sources of BC emissions from the above directions in these seasons are consistent with the results in relevant studies.

12) Figure 9. The results of the HYSPLIT analysis still do not clearly indicate what have caused the particular high BC values in these days, e,g,, some strong emission sources transmitted to the site. The relevant paragraphs of Figure 9 are mainly devoted to the detailed introduction of each sub-figure, without further in-depth analysis. Moreover, Figure 9 is too vague.

Minor editing of English language required.

Author Response

The authors thank the reviewers for their exhaustive review and their interesting comments, questions, and suggestions. We value your contribution, which has allowed us to improve this work.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Most of the comments in the first round have been addressed by the authors.

There is no major problem with the English language.

Reviewer 4 Report

The authors did make substantial corrections so the paper has clearly improved in the revision. They have also answered to all the comments or questions in the response. This paper can be accepted for publication.