Long-Term Variation Study of Fine-Mode Particle Size and Regional Characteristics Using AERONET Data

Round 1

Reviewer 1 Report

The manuscript by Shin et al. describes the long-term variation of particle sizes and regional characteristics using AERONET data. They applied a method that separates optical depth as dust and coarse-fine mode pollution particles using size distribution and the linear depolarization ratio. They show the decreasing trends of aerosol particle sizes, globally. The manuscript is well written and logically presented, and I am happy to support the publication. I have only a few suggestions /requests for clarification.

C1: In the abstract, authors need to show the long-term trends including when and how it changes, and to add one short sentence about by what is was caused

C2: It is unclear why there is no data for all American and European sites. And three sites for Europe are not suitable for representing Europe. It would be better to replace another words to denote.

C3: Section 3.2 and Table S1. To investigate the annual trends, Table S1 shows the number of observation days each year for the 17 AEROSNET sites. Here, did you select the same month for each year? The table shows various numbers of days and this means that containing data indicates the different measured months (summer month includes more etc.).  Did you deseasonalize the measurement data?

Technical Corrections/comments:

1.    P4, L135 – 138, Authors need to explain more clearly why you use δ2 as the minimum values.

2.    P7, Fig.2 (d) Capo Verde → Cape Verde

Author Response

We appreciate your advice and have revised the paper based on your advice.

Please see the attachment.

Author Response File: Author Response.docx

Reviewer 2 Report

please see attached file

Comments for author File: Comments.pdf

Author Response

In this paper, trends in regional AOD based on separating aerosols into dust and coarse-and fine-mode pollution particles for 17 AERONET observation sites which representative of regional characteristics are reported. In order to identify the long-term trend of particle size variation, aerosol optical depth (AOD, τ) separated as dust (τD) and coarse-(τPC) and fine-pollution particles (τPF) depending on emission sources and size were analyzed. I recommend publication after the authors address the minor revisions outlined below.

Point 1: Since more than nine years data from 17 AERONET sites were used this study,

validation of data is very important. Please clarify validation of data.

Response 1: We think the data have been sufficiently verified since it is the version3 and level2 data of AERONET.

Point 2: Please describe detailed information of study sites especially which are located in Europe and Southeast Asia.

Response 2: We described the site information in part 2.1. Study sites, but we added the detailed information according to your comments.

(Ln.84) Venice is an urbanized and industrial area, having large harbors and power plants [38-41], and an important city that has the most important wetland sites showing human interferences to the ecosystems [42]. Thessaloniki is the second largest city in Greece and populated [43].

Point 3: Page 3, line 103, 104: citations are needed.

Response 3: We added the references as your comments.

(Ln. 104) Seoul is downwind of westerly winds from Beijing and thus often affected by long-range transport of pollution particles from China [40-42]. Seoul also has a high concentration of locally produced pollution particles. Osaka is a coastal city that produces less anthropo-genic pollution than China and Korea [40,41]. The effect of long-range transport of pollution on Taipei is relatively low [43].

Point 4: Page 7: The ratio of dust and pollution particles was discussed. It is hard to find value of the ratio.

Response 3: We tried to classify AODs as dust and find/coarse pollution particles. Those originated from different sources, so we can find the reasons and solutions for aerosol variation.

Point 5: Page 10, line 327: Please explain author’s exception.

Response 3: We added the explanation.

(Ln 340) The exception is Ilorin, where we find -0.27% (-0.0001 ) because this region is located in the southern part among measurement site in North Africa; thus, the least affected by Saharan dust.

Point 6: Page 10, line 334: The annual average values of τPC slightly increased in Europe is interesting. More explanation is needed, however you mentioned that it is difficult to confirm the evident change of τPC value in Europe.

Response 3: It is difficult to find the change in the value of tau since tau is very low itself. However, we can see the apparent change as percent variations. To avoid confusion, we changed the sentence.

(Ln. 351) Especially, it is difficult to confirm the evident change of  values in Europe and India since  are low values itself, but the percent variation in Thessaloniki and Ballia was high as 11.94% and 10.53%, respectively.

Point 7: Page 15, line 505, 509: Please aware your assumption.

Response 3: We modified the sentence.

(Ln. 537) Particle size and characteristics are essential to understanding air pollution and visibility and have changed over the past decade or more, but few studies about those properties. The information on the characteristics of aerosols helps to find emission sources and how to remove them effectively. Therefore, we need more studies paying attention to changes in the size and quantity of fine-mode pollution particles to reflect air pollution policy.

Reviewer 3 Report

The authors performed trend analyses on total AOD, dust AOD, coarse- and fine-pollution AOD using 17 AERONET sites. But I have a number of concerns on the methodology and analyses as well as the presentation of them in this paper. Since many studies have investigated the temporal variation and the trends of AOD from both ground- and satellite-based observations, it is necessary to clearly outline the objectives of the manuscript and the novelty of the work in the Introduction section. What is the main issue to address? Additionally, I feel that some quantitative information should be synthesized and reported in Introduction, in order to put the results presented in this manuscript into a context. To my evaluation, this paper needs substantial improvements in the analysis of the data and organization of the results from the analysis before it can be accepted for publication in Remote Sensing.

Specific comments

1. How did you get the multi-year mean AOD at the 17 AERONET sites? The values of total AOD at some sites in Table 1 are much higher than previous publications (Yoon et al., 2012; Li et al., 2014; Fan et al., 2018). Please double check the results.

2. The analyses of trends on AOD are one of important results in the manuscript. There are a large number of missing values in the AOD time series that can introduce biases in estimating the appropriate trends, which can lead to large uncertainties in the AOD trend analysis. Additionally, the AOD time series associated the seasonal variability and frequently display statistically significant serial correlation. To eliminate the influence of the serial correlation and preserve the true trend in AOD, it is necessary to prewhitening and de-seasonalizing the time series before estimating the annual trends. However, the authors have not mentioned or presented any pre-processing of the AOD time series in the manuscript, which need to be provided before discussing the trends in AOD.  

3. Line 27-28: “The particle size became smaller, as seen by values of Angstrom exponent decreased by -3.30 to - 30.47% in Europe, North Africa, and the Middle East”. Please explain the result in more detail, since the Angstrom exponent provides information on the particle size (the smaller the particle size, the larger the exponent).

4. Line 62: please describe the goals of your study in more detail.

5. Line 267: why is not Osaka site in the main transport pathway of dust? Please discuss in more detail and add references.

6. Line 315: Pilahome et al. (2020) reported an increase of AOD at 550 nm across Thailand based on analyzing MODIS (Moderate Resolution Imaging Spectroradiometer) data for 316 the years 2006 to 2016. But Table 2 showed total AOD in Chiang Mai, and Bangkok decreased with the values of -0.0039 and -0.0012 per year. Please explain the inconsistency.

7. Line 467-468: The dust aerosols are dominated by coarse-mode particles. Why did the Angstrom exponent show an increase when dust AOD increasing in Beijing?

Minor comments:

1. Line 127: change “the linear particle depolarization ratio” to “the particle linear depolarization ratio”.

2. Line 264: “March, April, and [55]”, The word of May was lost.

3. Line 265: references needed.

Author Response

We appreciate your advice and have revised the paper based on your advice.

Please see the attachment.

Author Response File: Author Response.docx