Chemical Characterization of PM_2.5 at Rural and Urban Sites around the Metropolitan Area of Huancayo (Central Andes of Peru)

Round 1

Reviewer 1 Report

Dear Authors,

See my review as an attached pdf file.

Reviewer

Comments for author File: Comments.pdf

Author Response

Point 1: Table 3: correctly: “(ug g-1)”, instead of “(ug. g-1)”, for each case

Response 1:

Thanks for your observation. We have fixed that for each case.

Point 2: Table 3: column “Elements”: As I see, LOD and LOQ for the trace elements listed here are expressed either in % or without %. For example, those for Al is expressed in %, while As without %. It would be better expressing them for all trace elements uniformly either in % or without %.

Response 2:

Thanks for your observation. We have fixed that and also we have standardized the units of all elements in µg g^-1.

 

Point 3: Page 3, row 17: “A” at the end of Table 1 should be an exponent here.

Response 3:

Thanks for your observation. The reviewer has reason. We have fixed that as recommended by the reviewer.

 

Point 4: Table 3: I do not understand the expression below the table, standing alone: “Non-reported”. Please interpret and justify!

Response 4:

Thanks for your observation. Non-reported represents non-certified and/or non-measured elements from SRM 1648 (page 7, line 173).

.

Point 5: Section “2.5 Statistical analyses”: You mention here that after having applied one-way ANOVA and you find significant differences in the means, then you use Tukey test in order to determine that exactly which means differ significantly from each other when pairwise comparisons. However, I do not find the results of ANOVA+Tukey tests anywhere in the text. Regarding Table 4, Figure 2 and Figure 3, I suggest to perform the following ANOVA+Tukey tests:

Response 5:

Thanks for your observation. In Table 4 are presented the results of the ANOVA+ Tukey test of both trace elements and water-soluble ions. Tukey test is represented by letters in the rows and the ANOVA results are shown in the last column. Tukey test represented by letters at each row, where different letters mean significant difference, and equal letter indicate do not exist significant difference In the column of ANOVA is represent by *, **, or *** that indicate the probability level, 0.05, 0.01, and 0.001, respectively.

Results of ANOVA + Tukey test in Figure 2 and 3 are shown below.

 

For Table 4: for the station-related mean mass concentrations of each trace element (15 ANOVA+Tukey tests) and water-soluble ion (7 ANOVA+Tukey tests), respectively (row by row).

 

 

For Figure 2: for the station-related mean mass concentrations of PM2.5 (1 ANOVA+Tukey tests).

Figure. 2. Boxplot of mean mass concentration ± standard deviation (S.D.) of PM_2.5 at each monitoring station. Means with the same letter and color (a and b) code are not significantly different (Tukey multiple comparisons of means, p < 0.05). Red line = 15 µg m^-3 value of Estándares de Calidad Ambiental (ECA) from Peru.

 

For Figure 3: for the station-related mean mass concentrations of PM2.5, for the dry season (May to September), as well as the wet season (March, April, October, and November), separately. This latter means (2 ANOVA+Tukey tests).

Figure 3. Boxplot mean mass concentration ± standard deviation (S.D.) of PM2.5 at each monitoring station for the dry season (May to September) and the wet season (March, April, October, and November). Means with the same letter (a and b) code are not significantly different (Tukey multiple comparisons of means, p < 0.05).

 

Point 6: For Figure 3: for the station-related mean mass concentrations of PM2.5, for the dry season (May to September), as well as the wet season (March, April, October, and November), separately. This latter means (2 ANOVA+Tukey tests). Therefore, altogether 15+7+1+2=25 ANOVA+Tukey tests are to be performed. Even though you were not familiar with the above procedure, you could perform the above ANOVA+Tukey tests analyses online, very easily. To this end, please use the following link: http://astatsa.com/OneWay_Anova_with_TukeyHSD/

When applying ANOVA, and you will find significant differences between the pairwise means of the pollutants concentrations then and only then you should perform the Tukey test. As mentioned above, altogether 15+7+1+2=25 ANOVA are to be performed. If you will find significant differences between the pairwise means by using the Tukey test, you are asked to indicate them by an X in its section (see Table Z, as just an example).

Table Z

Significant differences between the pairwise means of the pollutants concentrations, based on the Tukey test for Metropolitan area of Huancayo city, Junín, Peru (in X: significant at p < 0.05, in X: significant at p < 0.01; in X: significant at p < 0.001).

 

Response 6:

Thanks for your recommendation to use the link http://astatsa.com/OneWay_Anova_with_TukeyHSD/, but we prefer to use R with package FSA to calculate ANOVA test and package multcomp to compute Tukey test because is easier to apply. We did not the Table Z because is hard work (we should build 25 Tables in total, and each table containing the comparison between samples and this total of Tables will be impossible introduces into the manuscript). With R we performed the, for example, the Tukey test for Ca and the results are showed as below.

 
Anova Table (Type II tests) 
Response: Ca          Sum Sq Df F value    Pr(>F)Site      395.83  3  16.281 4.20e-08 *** this sign was putted in Table 4 column ANOVA)Residuals 559.19 69---Signif. codes:  0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1

 

Linear Hypotheses:

                Estimate Std. Error t value Pr(>|t|)   

CHI - UNCP == 0   5.0874     0.8795   5.784  < 0.001 ***

HYO - UNCP == 0   2.8346     0.9548   2.969  0.02057 * 

IGP - UNCP == 0  -0.5186     0.9724  -0.533  0.95052   

HYO - CHI == 0   -2.2528     0.9354  -2.408  0.08449 . 

IGP - CHI == 0   -5.6060     0.9532  -5.881  < 0.001 ***

IGP - HYO == 0   -3.3532     1.0231  -3.277  0.00862 **

---

Signif. codes:  0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1

(Adjusted p values reported -- single-step method)

 

library(multcomp)

Cam = glht(modelCa, mcp(Site="Tukey"))

Cams = summary(Cam, test = adjusted("single-step"))

cld(Cams, level = 0.05, decreasing = TRUE)

UNCP  CHI  HYO  IGP

 "b"  "a"  "a"  "b"

The same letters indicate no significant difference, while different letter indicate significant difference. I think added all this information of each element is a bad idea because is a lot of information. Please If you have a model how design all information in a Table will be excellent.

 

Point 7: As you write in “2.5 Statistical analyses”, you use hierarchical cluster analyses in the paper. What was the reason to choose this method? Has this method any advantage compared to non-hierarchical cluster analysis? However, I think, it would be worth to compare the results having received after the use of these two procedures.

 

Response 7:

Thanks for your observation. The reason to use Hierarchical cluster analysis was that it is a better-known method and easy to apply. I believe that the success or advantage of both clustering methods varies according to the dataset applied.

 

Point 8: All in all, I suggest a substantially extended methodology section and, within this, separated sub-sections, such as:

 

2.5 Methodology

2.5.1 Analysis of variance (ANOVA) and Tukey test

2.5.2 Principal Component Analysis (PCA)

2.5.3 Hierarchical Cluster Analysis (HCA) and Non-Hierarchical Cluster Analysis

(NHCA)

 

Response 8:

Thanks for your suggestion. The methodology was extended and divided as suggested by the reviewer (pages 8-9, lines 181-210).

 

Point 9: This latter sub-section would include among others the theoretical bases of calculating the optimum number of clusters for both procedures and then the comparison of the goodness of the traditionally used Euclidean metric and the Mahalanobis metric. Furthermore, the presentation of the advantages and disadvantages of these clustering technics is expected.

Response 9:

Thanks for your recommendation. As recommended by the reviewer information about calculating and procedures between Euclidean metric and Mahalanobis were added in the section 2.5.3 (page )

 

Point 10: As an example, I suggest you insert the sub-section below, written in bold within quotation marks into the section 2.5 Methodology:

Response 10:

Thanks for your suggestion. As suggested by the reviewer information and details about methodology were inserted in the manuscript (page     ).

 

Point 11: This latter sub-section would include among others the theoretical bases of calculating the optimum number of clusters for both procedures and then the comparison of the goodness of the traditionally used Euclidean metric and the Mahalanobis metric. Furthermore, presentation of the advantages and disadvantages of these clustering technics is expected.

Response 11:

Thanks for your observation. In Table 4 are presented the results of ANOVA of both trace elements and water-soluble ions row by row (Tukey test represented by letters at each row, different letters mean significant difference, and equal letter do not exist significant difference) and ANOVA test in the column ANOVA with their respective probability level. For Figure 2 and 3, the ANOVA + Tukey test was carried out as recommended by the reviewer.

 

Point 12: As an example, I suggest you insert the sub-section below, written in bold within quotation marks into the section 2.5 Methodology:

Response 12:

Thanks for your suggestion. The information provided was added in the manuscript section 2.5 Methodology. Besides, we added information required by the reviewer.

 

Point 13: Section “3.1. Trace elements contents and water-soluble ion”, paragraph 1: Why three from the seven water-soluble ions are written in bold, I do not find any reference on it in the text.

Response 13:

Thanks for your observation. The words written in bold is does not indicate anything, it was simply an error, but it was already fixed in the manuscript.

 

Point 14: In Table 4, I suggest a modification, so that let it be: Comparison of the mean concentration values…..”;

Response 14:

Thanks for your suggestion. We have modified the sentence as indicated by the reviewer.

 

Point 15: Figure 2: correctly: “Mean mass concentration….” Instead of Mean concentration…..”

Response 15:

Thanks for your observation. We have fixed that.

 

Point 16: Figure 3: correctly: “Mean mass concentration ± standard deviation (S.D.) of PM2.5 at each monitoring station for the dry season (May to September) and the wet season (March, April, October, and November)”, instead of “Mean concentration ± standard deviation (S.D.) of PM2.5 at each monitoring station by season dry (May to September) and wet (March, April, October, and November).”

Response 16:

Thanks for your observation. We have fixed that.

 

Point 17: Figure 3: correctly: “Mean mass concentrations…..” instead of Mean concentration…”

Response 17:

Thanks for your observation. We have fixed that.

 

Point 18: Section 3.2.1 Trace element”, this should be in plural, so correctly: “3.2.1 Trace elements”

Response 18:

Thanks for your observation. We have fixed that.

 

Point 19: Section 3.2.1 Trace elements”. Rows 9-10: Is there any measure of similarity you mention here?

Response 19:

Thanks for your observation. We have fixed that.

 

The resulting dendrogram (Figure 2) suggest that group I may have a mixture of both natural and anthropogenic sources, while group II suggests elements released basically from anthropogenic activities.

 

Point 20: Section 3.3.1 Trace elements”. Rows 15 and 16: Correctly: “Communalities, instead of “commonalities”;

Response 20:

Thanks for your observation. We have fixed that.

 

Point 21: Table 5: Only two extracted factors are shown in this table, not three.

Response 21:

Thanks for your observation. The reviewer has reason and the sentence was fixed.

 

Point 22: Table 5: You write here Loadings superior to 0.60 (in bold) are considered as significant. Comments: (1) If we chose 0.60 as a threshold, then you are asked to indicate, e.g. in bold, those factor loadings that exceed this threshold. (2) However, this threshold may be subjective. Instead, I suggest thresholds at the 5% (indicated by normal letters, e.g. X), 1% (bold, e.g. X) and 0.1% (bold and underlined, e.g. X) probability levels, depending on the number of the element pairs.

Response 22:

Thanks for your observation. Those factor loading higher than 0.60 was put in bold.

 

Point 23: Table 6: See the same comments, taken concerning Table 6 to 5

Response 23:

Thanks for your observation. We have fixed that

 

Point 24: Page 7, rows 51-52: In one sentence there are two “on the other hand”. Please refine the sentence.

Response 24:

Thanks for your observation. We have fixed that (page 14, lines 356-357).

 

Elements, such as Al, K, Mn, and Rb are related to natural origin (geogenic sources), and As, Ca, Cd, Cr, and Fe are related to anthropogenic activities

 

Point 25: Page 8, paragraph 2, pages 11-14: “Five ions (acetate, formate, oxalate, chlorine, and nitrate) were positively correlated with factor loading higher than 63 formed the factor 1. Factor 2 was represented by sulfate and ammonium positively correlated with factor loading higher than 0.85.” In this sentence, the numbers 63 vs 0.85 seem contradictory. Please check them!

Response 25:

Thanks for your observation. We revise the manuscript and found an error in 63 (was changed by 0.60). Likewise to avoid contradiction in both factors only was considered factor loading higher than 0.60 (page 14, lines 356-357).

 

PCA analysis of water-soluble ions segregated two groups. Five ions (acetate, formate, oxalate, chlorine, and nitrate) positively correlated with factor loading higher than 0.60 formed the factor 1. Factor 2 was represented by sulfate and ammonium positively correlated with factor loading higher than 0.60.

 

Point 26: Page 8, row 15: correctly: “we may suppose…..”, instead of “we may be supposed….”;

Response 26:

Thanks for your observation. We have fixed that (page 14, line 370)

 

Point 27: English in some context should be improved, furthermore several small corrections are necessary. So, you are asked to have it checked by a native speaker.

Response 27:

Thanks for your recommendation. A native speaker checked our manuscript

 

Author Response File: Author Response.docx

Reviewer 2 Report

The authors measured 24 h  PM_2.5 mass concentrations at three urban sites in Huancayo (Central Andes of Peru) and at one rural site, 13.5 km from  the city,  between March  and November 2017. Fifteen trace elements and seven water-soluble ions were quantified in PM_2.5  samples. The descriptive statistics has been  conducted to show PM_2.5   pollution levels in the city.  Further, the authors  analysed the sources of PM_2.5  using  the Principal Component Analysis and Crustal Analysis.  They use common  methodologies in the source apportionment assessment. From the formal point of view, the manuscript provides new data  on a specific case study.  To the authors’ knowledge, is missing data on the chemical composition of PM_2.5  in Peru and in this sense, the paper could be valuable. Overall, I have a few issues that I hope the authors can address before further considerations.

1)      The paper needs revision to clarify its message and main results. There are objectives (p. 2, l. 28-32) but there is no the aim of the work, i.e. the overall purpose of the study. Please define it clearly and concisely. Aims emphasize what is to be accomplished (not how it is to be accomplished  what is  the task of objectives).

2)      The paper gives overall description of the sampling side but misses some detailed information. The authors collected 151 samples (from Table 4 – 148 samples) during 8 months, thus they should provide information about the sampling periods at every monitoring locations. For clarity of further arguments they  should specify meteorological conditions during measurements. In my opinion,  instead of operation conditions for ICP-MS measurements (Table 2) should appear the detection limits, calculated from the calibration curves. I think that some elements were detected on or below the instrument detection limit. This is important in the subsequent source apportionment assessment by PCA. Some, not key elements for source identification, could be excluded from the analysis and the results would be more clear.

3)      It is difficult to assess the validity of the approach to the  PCA.  It was shown that the trace elements concentrations differed significantly between the sampling points which suggests diversity in the contribution of  different trace elements sources to PM_2.5. The correlations between trace elements concentrations  collected in different sites were not considered. The meteorological conditions that could affect good mixing in the lower troposphere  were also not analysed. In such a situation to take into account all of the results (from urban and rural sites)  does not make sense.

4)      My impression is that the authors did not carry out part of the research, there are missing results from post hoc tests.

5)      The stated Crustal Analysis goal on p. 6, l. 9-10 (“  to segregate the monitoring sites”) differed from the presented results in Figs. 3 and 4 and their discussion (identification of trace elements and ions sources).

Some specific comments:        

p. 1, l. 16 – The purpose ....was to determine PM_2.5 mass concentrations and the contents....in collected samples....

p. 1 l.38 – It is not true that PM₁₀ –aerodynamic diameter between 2.5 and 10 mm.

p. 2 l. 12-15 – this sentence is not clear

p. 2 -  the filter weighing procedure is missing

Table 3 - ambiguous description of units, unclear information – below the Table – non reported.

p. 6, l. 7-8 – The number of components was determined by Kaiser’s criterion – an eigenvalue > 1[24]

p. 7, l. 6-7 – the statement is not supported by the results in Table 4

Figs. 2 and 3 - no x axis designation. It would be more legible to present the results in the form of box-and-whisker plots.

Table 5 and 6  - two not three components were extracted.

                     

Author Response

Point 1: The paper needs revision to clarify its message and main results. There are objectives (p. 2, l. 28-32) but there is no the aim of the work, i.e. the overall purpose of the study. Please define it clearly and concisely. Aims emphasize what is to be accomplished (not how it is to be accomplished what is the task of objectives).

Response 1:

Thanks for your recommendation. We fixed this sentence as showed below (page 2, lines 74-79)

Due to these gaps in knowledge, the overall purpose of the study was to characterize trace elements and water-soluble ions of PM_2.5 samples collected of 4 monitoring stations of three urban areas and one rural area. In addition, the possible sources of these trace elements and water-soluble ions will be identified using hierarchical cluster analysis (HCA) and principal component analysis (PCA).

Point 2: The paper gives overall description of the sampling side but misses some detailed information. The authors collected 151 samples (from Table 4 – 148 samples) during 8 months, thus they should provide information about the sampling periods at every monitoring locations. For clarity of further arguments, they should specify meteorological conditions during measurements.

Response 2:

Thanks for your recommendation. Table 4 was clarified and added the number of samples used to analyze water-soluble ions, which totalize 151 samples as described in the manuscript. Information about sampling periods was added in the manuscript (Item 2.2 Sampling method, page 3, lines 104-106).

Aerosol sampling was performed, simultaneously, at four monitoring stations, collecting one sample by week and about five per month with 24 h sampling time.

Point 3: In my opinion, instead of operation conditions for ICP-MS measurements (Table 2) should appear the detection limits, calculated from the calibration curves. I think that some elements were detected on or below the instrument detection limit. This is important in the subsequent source apportionment assessment by PCA. Some, not key elements for source identification, could be excluded from the analysis and the results would be more clear.

Response 3:

Thanks for your recommendation. The detection limits were shown in Table 3 and inside the manuscript (page 7, lines 172-174) is explained as LOD and LOQ were computed. Elements with below LOD were not considered in this work.

The limits of detection (LOD) and quantification (LOQ) were computed as three and ten times the standard deviation of ten blank measurements divided by the slope of the analytical curve.

Point 3: It is difficult to assess the validity of the approach to the PCA. It was shown that the trace elements concentrations differed significantly between the sampling points which suggests diversity in the contribution of different trace elements sources to PM2.5. The correlations between trace elements concentrations collected in different sites were not considered. The meteorological conditions that could affect good mixing in the lower troposphere were also not analyzed. In such a situation to take into account, all of the results (from urban and rural sites) does not make sense.

Response 3:

Thanks for your recommendation. The results obtained by PCA can be explained because the environment from Huancayo city is a mixture from urban and rural areas, where agricultural areas and urban growing coexist. Therefore, the mixture from elements released from urban areas and rural areas (soil-resuspension) are found.

Point 4: My impression is that the authors did not carry out part of the research, there are missing results from post hoc tests.

Response 4:

Thanks for your observation. Postdoc tests were carried out and were presented in Table 4. Post hoc tests were carried out for Figure 2 and Figure 3 also.

Point 5: The stated Crustal Analysis goal on p. 6, l. 9-10 (“to segregate the monitoring sites”) differed from the presented results in Figs. 3 and 4 and their discussion (identification of trace elements and ions sources).

Response 5:

Thanks for your observation. The reviewer has reason; therefore the paragraph was rewritten (page 7, lines186-187).

Furthermore, a Hierarchical Cluster Analysis (HCA) using Ward’s method of linkage and the squared Euclidean distance for similarities was used to segregate associations within the group of trace elements and water-soluble ions to identify possible pollutant sources.

Some specific comments:

Point 6: p. 1, l. 16 – The purpose ....was to determine PM2.5 mass concentrations and the contents....in collected samples....

Response 6:

Thanks for your observation. We have fixed that (page 14, line 370)

 

Point 7: p. 1 l.38 – It is not true that PM10–aerodynamic diameter between 2.5 and 10 mm.

Response 7:

Thanks for your feedback. The reviewer has reason, therefore the paragraph was fixed (page 1, line 38)

“PM₁₀ – particles that have aerodynamic diameters less than or equal to 10 µm”

Point 8: p. 2 1. 12-15 – this sentence is not clear

Response 8:

Thanks for your observation. The sentence was rewritten for a better understanding (page 2, lines 42-47).

“Airborne particulate matter represents a complex mixture of organic and inorganic substances such as chemical elements (i.e., Si, Al, Fe, Ca, Ti, V, Cr, Cu, Zn), volatile organic compounds (VOCs), water-soluble ions (i.e.,                                               , Na⁺, Ca²⁺, Mg²⁺, and K⁺) among others that can threaten living organisms and human health. These pollutants could be released from either natural and/or anthropogenic sources”

Point 9: p. 2 – the filter weighing procedure is missing.

Response 9:

Thanks for your observation. The weighing procedure was added in the manuscript (page 3, lines 112-116).

“The mass of the particles was calculated by the difference between the filter weight before and after sampling, and the concentration in the designated size range was computed by dividing the weight gain of the filter by the volume of the air sampled. The filter weighing procedure was carried out in a temperature and humidity controlled clean room on a microbalance capable of making measurements of weight of objects of relatively small mass”

 

Point 10: Table 3 – ambiguous description of units, unclear information – below the Table – non reported.

Response 10:

Thanks for your observation. The units and information in Table 3 were clarified. The non-report sentence was described for better understanding (page 7, line 174).

“Non-reported represents non-certified and non-measured elements from SRM 1648”.

 

Point 11: p. 6, 1. 7-8 – The number of components was determined by Kaiser’s criterion – an eigenvalue > 1 [24].

Response 11:

Thanks for your correction. We have added the phrase fixed as suggested by the reviewer.

The number of components was determined by Kaiser’s criterion – an eigenvalue > 1.

Point 12: p. 7, l. 6-7 – the statement is not supported by the results in Table 4.

Response 12:

Thanks for your observation the statement was rewritten in the manuscript (page 10, lines246-248).

Hierarchical Cluster Analysis (HCA) using Ward’s method of linkage and the squared Euclidean distance for similarities was used to segregate associations within the group of trace elements and water-soluble ions to identify possible pollutant sources.

Point 13: Figs. 2 and 3 - no x axis designation. It would be more legible to present the results in the form of box-and-whisker plots.

Response 13:

Thanks for your observation. The Figure 2 and 3 was plotted again in form of boxplot as recommended by the reviewer and presented below.

 

Figure 2.

Figure 3.

Point 14: Table 5 and 6 - two, not three components were extracted.

Response 14:

Thanks for your observation. It was an error of digitation but already was corrected.

Author Response File: Author Response.docx

Reviewer 3 Report

We read the article Chemical characterization of PM2.5 at rural and urban sites around the Metropolitan area of Huancayo (Central Andes of Peru). 

We have a number of questions for the article 

Page 5 line 15-16

Maybe instead of the word "commonalities" use the word "correlation coefficient"?

Page 6 line 19 

How justified is the use of words "macronutrients" and "micronutrients" for aerosol components?

Page 7 line 14 

There is a repeat of "Cu" in the same sequence.

Page 7 line 43

What does the phrase " Particulate in form of ammonia" mean? Ammonia is gas.

Author Response

Point 1: Page 5 line 15-16. Maybe instead of the word "commonalities" use the word "correlation coefficient"?

Response 1:

Thanks for your observation. We considered using the word communalities because communalities are the proportion of each variable’s variance that can be explained by the factors (e.g., the underlying latent continua). It is also noted as h2 and can be defined as the sum of squared factor loadings for the variables and not correlation coefficient, while correlation coefficient is a statistical measure that calculates the strength of the relationship between the relative movements of the two variables.

Point 2: Page 6 line 19 How justified is the use of words "macronutrients" and "micronutrients" for aerosol components?

Response 2:

Thanks for your observation. We have to write macronutrients and micronutrients because are elements only for the state the order according to concentration levels. Then to avoid bad understandings the word macronutrients and micronutrients were deleted from the manuscript.

Point 3: Page 7 line 14 There is a repeat of "Cu" in the same sequence.

Response 3:

Thanks for your observation. We have fixed that.

 

Point 4: Page 7 line 43. What does the phrase "Particulate in form of ammonia" mean? Ammonia is gas.

Response 4:

Thanks for your observation. In the phrase was lacking one sentence (page 16, lines 414-415).

 

Particulate ammonium mainly originates from ammonia vapor and ammonium salt particles in form of ammonia are found in urban areas where gaseous ammonia reacts chemically with other compounds, leading to the formation of ‘‘smog”

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

Dear Authors,

Your asnwer to my comments can be acceptable: 

BUT: in spite of several completions in the original manuscript, no new references were inserted that could have been necessary.

So, I strongly suggest you inserting references regarding the completed/modified parts of the manuscript. 

Reviewer

Author Response

Point 1: in spite of several completions in the original manuscript, no new references were inserted that could have been necessary.

So, I strongly suggest you inserting references regarding the completed/modified parts of the manuscript. 

Response 1:

Thanks for your suggestion. We inserted the references as suggested by the reviewer.