Approaches to Understand Historical Changes of Mercury in Tree Rings of Japanese Cypress in Industrial Areas

Round 1

Reviewer 1 Report

The authors have revised the manuscript according to my suggestions, leaving only minor comments: Fig. 2C it is necessary to remove decimal places on the ordinate axis; it is desirable to move Fig. 1 and 2 to the "Materials and Methods" section.

Author Response

We appreciate your valuable time and critical comments. We agree with your suggestions and comments. Therefore, we have revised the manuscript according to your suggestions.

The authors have revised the manuscript according to my suggestions, leaving only minor comments:

 

- Fig. 2C it is necessary to remove decimal places on the ordinate axis.

► We have removed decimal places on the ordinate axis in Figure 2C. (Line 116)

 

- it is desirable to move Fig. 1 and 2 to the "Materials and Methods" section.

► We have moved Figures 1 and 2 to the "Materials and Methods" section. (Lines 101-105 and Lines 116-118)

Author Response File: Author Response.docx

Reviewer 2 Report

Dear Authors,

I really enjoyed the new version of this manuscript. I recommend accepting the latest document in its present form.

I congratulate the authors for their excellent work and study.

Best regards

Author Response

We appreciate your valuable time and comments.

Author Response File: Author Response.docx

This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.

Round 1

Reviewer 1 Report

Dear authors!

I’ve read with interest your article, which presents new data about of the Hg concentrations in the tree rings of Japanese cypress (Chamaecyparis obtusa) at sampling sites located at different distances from the steel mill in the Gwangyang industrial areas. But I have some comments and questions.

 

Line 134. At each sampling site 1 or more trees were selected? Have you analyzed the Hg concentrations in the annual rings of one or more trees?

The mercury concentrations in the two sampling sites (Namhae and Suncheon) no significant difference (p > 0.05) between 1968 and 1983 (Figure 2). Why was the Hg concentrations in the tree rings significantly higher in the sampling site in Namhae than one in Suncheon between 1984 and 1992? How can this be explained.

I have a few editorial comments:

There is no need to provide numerical data in the text, which is shown in table 1.

References to figures and table 1 are too often repeated in the text

The same information is repeated many times in the text of the article. The text should be shortened by removing repetitions.

Author Response

Response to Reviewer 1 Comments We appreciate your valuable time and critical comments. We agree with your suggestions and comments. Therefore, we have revised the manuscript according to your suggestions. Revised sentences are highlighted in yellow in the revised manuscript. ********************* Dear authors! I’ve read with interest your article, which presents new data about of the Hg concentrations in the tree rings of Japanese cypress (Chamaecyparis obtusa) at sampling sites located at different distances from the steel mill in the Gwangyang industrial areas. But I have some comments and questions. • Line 134. At each sampling site 1 or more trees were selected? Have you analyzed the Hg concentrations in the annual rings of one or more trees? ► We collected one disc from one Japanese cypress tree (Chamaecyparis obtusa) at each sampling site in this study. (Lines 121-122) • The mercury concentrations in the two sampling sites (Namhae and Suncheon) no significant difference (p > 0.05) between 1968 and 1983 (Figure 2). Why was the Hg concentrations in the tree rings significantly higher in the sampling site in Namhae than one in Suncheon between 1984 and 1992? How can this be explained. ► Hg concentrations at Namhae and Suncheon sites in 1967-1977 showed no significant difference (p > 0.05) (Table 1). However, average Hg concentrations in tree rings during 1978-2014 at Yeosu, Namhae, and Suncheon sites were 11.15 ng/g, 6.54 ng/g, and 4.34 ng/g, respectively. Hg concentrations of tree rings at sampling sites in 19782014 reflected spatial distributions of Hg concentrations associated with wind direction and distance from industrial areas (p < 0.017). The Yeosu site located nearest to Yeosu and Gwangyang industrial areas showed the highest Hg concentrations in tree rings. In contrast, concentrations of Hg in tree rings were the lowest at Suncheon, which was farthest away from Yeosu and Gwangyang industrial areas. In addition, wind in Gwangyang industrial area was predominantly in northwestern, western, southwestern, and northeastern directions in 1978-2014. The site of Suncheon, located to the west of the Gwangyang industrial areas, was relatively less affected by wind transmission of Hg. Although Namhae site is located far in the east of Yeosu and Gwangyang industrial areas, Hg has input from two sources in Yeosu and Gwangyang industrial areas due to wind directions. Especially, abruptly elevated Hg concentrations in tree rings at Namhae site coincided with the operation of steelworks in 1987 at Gwangyang industrial areas. (Lines 248-266) I have a few editorial comments: • There is no need to provide numerical data in the text, which is shown in table 1. ► We thank the reviewer for pointing this out and we agree with the reviewer. Therefore, numerical data in the text have been reduced in Results and Discussion sections. • References to figures and table 1 are too often repeated in the text. ► Repeated references to Figures and Table 1 have been reduced in the text of the revised manuscript. • The same information is repeated many times in the text of the article. The text should be shortened by removing repetitions. ► Repeated same information was deleted from the text of the revised manuscript.

Author Response File: Author Response.docx

Reviewer 2 Report

Dear Authors,

This is an interesting study and a well-written manuscript. However, the analysis must be improved to accomplish the authors’ objectives. My recommendation is that this manuscript will be suitable for this journal after a major revision. In my opinion, this study is not accurate on the assessment of the sources. For a proper source analysis (or least as the authors want), it is required more data (more chemical elements), a prolonged studied period, a better meteorological analysis, and/or to prove that other possible sources of Hg are not relevant in the studied area.

I would like to encourage the authors to improve this study and re-send the manuscript. The information contained in this work has high relevance for further environmental policies.

My major doubts about this study are the following:

1. Statistics analysis is not adequate for the studied periods (1967-1972, 1973-1976, 1977-1986, and 1987-2010), as Kruskal-Wallis (or Mann-Whitney) need at least ten (10) values to provide reliable Statistics analysis. All studied periods are shorter than ten years or provide less than 10 values, so they are not statistically comparable.
2. A possible increase in the population could also be a relevant source of mercury. An increase in the population can bring more emission from residential heating, bigger landfill, and other industrial-economical activities not mentioned here. Long-distance sources could also contribute to mercury levels.
3. Authors mentioned that the Hg-intake is through the roots of the tree. A possible change of precipitation could change the concentration of Hg in the tree as a consequence of the difference in the tree intake.
4. A better analysis of the meteorology could help authors to discard natural influences on Hg concentration and to validate that the industrial activity is the primary source of this pollutant.
5. The methodology used to collect the samples can contaminate them with metals and maybe mercury. The best proof for showing adequate sampling is with the blank information. Did the authors collect field or lab blanks?

Minor revision or suggestions

• The reference numbers didn’t follow a continuous pattern. It looks as if they were added randomly.
• Authors have written the same word with and without a hyphen in this document (coalfired and coal-fired). Both ways are acceptable, but it’s best to be consistent.
• Line 31. The word “chemistry” doesn’t seem to fit this context. Consider replacing it to “Chemical”.
• Line 35. The word “during” doesn’t seem to fit this context.
• Line 109, 234, and 236. The word “steel works” seems to be miswritten. Consider replacing by “steelworks”.
• Line 110-113. The next sentence needs a reference “The mill represents the largest steel plant in the world. It manufactures coils used to make bridges, steel structures, cars, refrigerators and other products. Its production capacity averages approximately 18 million tons per year (Figure 2A).”
• It appears that “gas burning” is missing a hyphen. Consider adding the hyphen (gas-burning)
• Figure 3. is not statistically accurate. I recommend using a boxplot chart.
• Table 1. I suggest also including the median. This parameter is a better central tendency than the average for this type of data.

Author Response

Response to Reviewer 2 Comments We appreciate your valuable time and critical comments. We agree with your suggestions and comments. Therefore, we have revised the manuscript according to your suggestions. Revised sentences are highlighted in yellow in the revised manuscript. ********************* Dear Authors, This is an interesting study and a well-written manuscript. However, the analysis must be improved to accomplish the authors’ objectives. My recommendation is that this manuscript will be suitable for this journal after a major revision. In my opinion, this study is not accurate on the assessment of the sources. For a proper source analysis (or least as the authors want), it is required more data (more chemical elements), a prolonged studied period, a better meteorological analysis, and/or to prove that other possible sources of Hg are not relevant in the studied area. I would like to encourage the authors to improve this study and re-send the manuscript. The information contained in this work has high relevance for further environmental policies. My major doubts about this study are the following: 1. Statistics analysis is not adequate for the studied periods (1967-1972, 1973-1976, 1977-1986, and 1987-2010), as Kruskal-Wallis (or Mann-Whitney) need at least ten (10) values to provide reliable Statistics analysis. All studied periods are shorter than ten years or provide less than 10 values, so they are not statistically comparable. ► Mercury concentrations in tree rings were compared over the two periods (1967-1977 and 1978-2014) of the last 50 years and between sampling sites located away from Yeosu and Gwangyang industrial areas (Table 1). Between 1978 and 2014, this period had 37 values. They were statistically comparable by Kruskal-Wallis (or Mann-Whitney) test. (Lines 144-151) 2. A possible increase in the population could also be a relevant source of mercury. An increase in the population can bring more emission from residential heating, bigger landfill, and other industrial-economical activities not mentioned here. Long-distance sources could also contribute to mercury levels. ► We have added the distance from Hg sources and population information in study areas (Figure 2C). These effects were explained in the Discussion section as follows (Lines 247-286): Industrial activities in Yeosu and Gwangyang industrial areas were intensified and continued since late 1970s (Figure 3A). In general, atmospheric Hg levels are influenced by wind flow [37]. Mercury is highly volatile. It might be transported in long distances [10,12]. Hg concentrations in tree rings during 19782014 at study sites were elevated with constant concentrations. Spatial distributions of Hg concentrations were associated with decreased distance from industrial areas (p < 0.017) (Table 1). In the period of 19782014, the highest concentration of Hg in the tree ring was found at the Yeosu site near Yeosu and Gwangyang industrial areas. However, the lowest concentration of Hg in the tree ring during 19782014 was found at the Suncheon site, which was far away from those industrial areas. Especially, Hg concentrations in tree rings at Suncheon site after 1987 did not increase. They showed a constant concentration distribution despite operation of the largest steelworks in Gwangyang industrial area in 1987. Wind in Gwangyang industrial area was predominantly in northwestern, western, and northeastern directions in 19872014 (Figure 1). The site of Suncheon located to the west of Gwangyang industrial area was relatively less affected by wind transmission of Hg from the steel mill in Gwangyang industrial area. Hg concentrations in tree rings at Suncheon site were mainly influenced by Yeosu industrial area. However, abruptly elevated Hg concentrations in tree rings at Namhae site coincided with the operation of steelworks in 1987. Although Namhae sites are located far in the east of the Yeosu and Gwangyang industrial areas, Hg has input from two sources of Yeosu and Gwangyang industrial areas due to wind directions. Residential heating seems to play a role in Hg concentration of tree ring due to Hg emission into the atmosphere [13,14]. The population has the largest number in Yeosu and Gwangyang industrial areas. Thus, Hg concentrations in tree rings at Yeosu site were higher in 19782014 compared to those at Namhae and Suncheon sampling sites (Figures 2C and 3B). The Suncheon site, the farthest from industrial areas with limited influence of wind direction, had a 6.3-fold population than the Namhae area. However, its Hg concentration in tree rings during 19782014 was lower than that at the Nimhae site. In 1992 at Suncheon site, Hg concentration in tree rings was abruptly increased, reaching the maximum value. This was thought to be due to Hg emission from residential heating associated with the largest population in the Suncheon area in the early 1990s (Figure 2C). The distribution of Hg concentrations in tree rings during 19782014 was influenced by the population. However, spatial distribution of Hg concentrations was found to be more affected by wind direction and distance from Yeosu and Gwangyang industrial areas. 3. Authors mentioned that the Hg-intake is through the roots of the tree. A possible change of precipitation could change the concentration of Hg in the tree as a consequence of the difference in the tree intake. ► We thank the reviewer for pointing this out and we agree with the reviewer. Therefore, we have added precipitation information for study areas (Figure 2B). The effect of Hg deposition in soils from precipitation was also explained in the Discussion section as follows (Lines 225-235): Although the average precipitation in the Namhae area was approximately 350 mm higher than that in the Suncheon area (Figure 2B), tree ring Hg concentrations were similar between Namhae and Suncheon sites (Figure 3B). There was no relation between precipitation and tree rings Hg concentration in this study. Previous studies have found significant Hg contributions to forest soils via throughfall and litterfall, with the contribution of litterfall being the greatest [3,20]. A soil source would be expected to result in increased Hg concentration over time due to continued deposition of Hg to soil via litterfall [3234]. Godbold and Hutterman (1988) [36] have found that regardless of external Hg concentration, Hg concentration in roots exceeded that in foliage. It remains unclear whether tree rings can serve as reliable proxies for atmosphere Hg concentrations due to Hg uptakes from soils and limits from atmosphere under low atmospheric Hg conditions. 4. A better analysis of the meteorology could help authors to discard natural influences on Hg concentration and to validate that the industrial activity is the primary source of this pollutant. ► We thank the reviewer for pointing this out and we agree with the reviewer. Therefore, we have added wind and temperature information (Figures 1 and 2A) as follows (Lines 261-269 and 270-287): - Wind in Gwangyang industrial area was predominantly in northwestern, western, and northeastern directions in 19872014 (Figure 1). The site of Suncheon, which is located to the west of Gwangyang industrial area, was relatively less affected by wind transmission of Hg from the steel mill in Gwangyang industrial area. Hg concentrations in tree rings at Suncheon site after 1987 did not show increases. Instead, they showed a constant concentration distribution despite the operation of the largest steelworks in Gwangyang industrial area in 1987. Hg concentration in tree rings at Suncheon site is mainly influenced by Yeosu industrial area. However, the abruptly elevated Hg concentration in tree rings at Namhae site coincided with the operation of steelworks in 1987. Although Namhae sites are located far in the east of Yeosu and Gwangyang industrial areas, Hg has input from two sources in Yeosu and Gwangyang industrial areas due to wind directions. (Lines 259-266) - Temperature seems to play a role in Hg concentration of tree rings. Higher temperature seems to stimulate Hg uptake from the atmosphere [14]. Temperatures in Yeosu and Namhae were higher than those in Suncheon. We observed that Hg concentrations in tree rings at Yeosu and Namhae sites were higher than those at Suncheon sites in 19782014 (Figures 2A and 3B). Yeosu site, which was located near those industrial areas, had the highest Hg concentration in tree rings. The distribution of Hg concentrations in tree rings during 1978-2014 was influenced by temperature. However, the spatial distribution of Hg concentrations was found to be more affected by wind direction and distance from Yeosu and Gwangyang industrial areas. (Lines 267-286) 5. The methodology used to collect the samples can contaminate them with metals and maybe mercury. The best proof for showing adequate sampling is with the blank information. Did the authors collect field or lab blanks? ► These disks were collected using a stainless steel saw. To reduce chemical contamination of samples, latex gloves were worn at all time. The stainless steel saw was wiped down with a new Kimwipe wetted with methanol before cutting the tree as well as after every cutting. There was no mercury contamination during the sample collection. (Lines 122-125) ► In addition, we have analyzed blanks. Reference samples were included every 520 samples as follows (Lines 136-140): A calibration curve was generated using a reference material obtained from the National Research Council of Canada Institute for National Measurement Standards MESS-3 (marine sediment, certified value = 90 ± 9 ng/g HgT [dry weight]) with R2 > 0.999. Measurement standard MESS-3 was used to calculate the accuracy and precision. Recovery ranged from 97% to 102%. Machine blanks and reference samples were included every 520 samples. Minor revision or suggestions • The reference numbers didn’t follow a continuous pattern. It looks as if they were added randomly. ► Reference numbers were revised. • Authors have written the same word with and without a hyphen in this document (coalfired and coal-fired). Both ways are acceptable, but it’s best to be consistent. ► We have revised it to be consistent in this document. • Line 31. The word “chemistry” doesn’t seem to fit this context. Consider replacing it to “Chemical”. ► We have revised it accordingly. (Line 27) • Line 35. The word “during” doesn’t seem to fit this context. ► We have changed it to “from” as follows (Lines 46-47); Mercury is also emitted into the atmosphere from coal combustion by thermoelectric power plants. • Line 109, 234, and 236. The word “steel works” seems to be miswritten. Consider replacing by “steelworks”. ► We have revised it to “steelworks”. (Lines 215, 259, and 264) • Line 110-113. The next sentence needs a reference “The mill represents the largest steel plant in the world. It manufactures coils used to make bridges, steel structures, cars, refrigerators and other products. Its production capacity averages approximately 18 million tons per year (Figure 2A).” ► We have added a reference ([25]). (Lines 73-75) • It appears that “gas burning” is missing a hyphen. Consider adding the hyphen (gas-burning) ► We have revised it to “gas-burning”. (Line 46) • Figure 3. is not statistically accurate. I recommend using a boxplot chart. ► Figure 3 was deleted because another reviewer commented that Hg data of tree rings were not suitable for comparison unless they were collected at the same place and the same time. • Table 1. I suggest also including the median. This parameter is a better central tendency than the average for this type of data. ► We have added median values into Table 1. (Lines 149-151)

Author Response File: Author Response.docx

Reviewer 3 Report

This paper is of interest to the broader dendrochemistry community and the study design permits critical evaluation of the data. However, there are several critical factors to be addressed:

1. The introduction lacks a review of Hg behavior in the environment and particularly in tree rings. This reviewer, without a background in Hg, has concerns about the translocation of Hg across growth rings which is wholly ignored throughout the paper and must be addressed.
2. Ring age: Dendroanalysis requires rigorous correlation to determine the age of a ring with confidence. This is not addressed in the paper. If the rings were simply counted, that is ok, but needs to be explicitly stated, 
3. Analysis: The authors omit information regarding limits of detection and external reproducibility of standard data. No errors are presented with the results which is important to interpret the temporal changes presented. 
4. Data interpretation: The interpretation of the temporal data is lacking and makes conclusions that are not fully supported by the data set itself. Please see comments on the pdf. 

Other, minor comments are in the attached pdf.  

Comments for author File: Comments.pdf

Author Response

Response to Reviewer 3 Comments This paper is of interest to the broader dendrochemistry community and the study design permits critical evaluation of the data. However, there are several critical factors to be addressed: • The introduction lacks a review of Hg behavior in the environment and particularly in tree rings. This reviewer, without a background in Hg, has concerns about the translocation of Hg across growth rings which is wholly ignored throughout the paper and must be addressed. ► Previous manuscripts explained that Hg concentration in tree rings was due to Hg uptake from the atmosphere via foliage. However, this revised version was re-written based on the fact that Hg concentration of tree rings was due to uptakes from the atmosphere via foliage and soils via roots. • Ring age: Dendroanalysis requires rigorous correlation to determine the age of a ring with confidence. This is not addressed in the paper. If the rings were simply counted, that is ok, but needs to be explicitly stated. ► We have added information about age determination of tree rings with confidence as follows (Lines 126-128): Each sample disk was scanned. Growth rings were marked with a CooRecorder program (version 7.8; Cybis Elektronik & Data AB, Saltsjobaden, SE). • Analysis: The authors omit information regarding limits of detection and external reproducibility of standard data. No errors are presented with the results which is important to interpret the temporal changes presented. ► We have added the information regarding limits of detection and standard measurement information as follows (Lines 133-140): The analyzer uses thermal decomposition, gold amalgamation, and atomic absorption spectrometry. The Hg detection limit was 0.005 ng. Operation conditions for the DMA-80 were based on Environmental Protection Agency Method 7473 protocol [30]. A calibration curve was generated using a reference material obtained from the National Research Council of Canada Institute for National Measurement Standards MESS-3 (marine sediment, certified value = 90 ± 9 ng/g HgT [dry weight]) with R2 > 0.999. Measurement standard MESS-3 was used to calculate the accuracy and precision. Recovery ranged from 97% to 102%. Machine blanks and reference samples were included every 520 samples. • Data interpretation: The interpretation of the temporal data is lacking and makes conclusions that are not fully supported by the data set itself. Please see comments on the pdf. ► We have revised the interpretation of data according to comments on the pdf. < Other, minor comments are in the attached pdf. > • Line 48. This is misleading - translocation is highly element-dependent and needs to be stated as such. ►We have revised as follows (Lines 51-59): Mercury in tree rings is closely associated with atmospheric Hg since atmospheric Hg is the predominant form of Hg absorbed by foliage [10,12,16]. However, limited work on reliable proxies for atmospheric Hg concentrations has been published regarding records of Hg in tree rings [17]. Plants can uptake Hg from soils via roots and from the atmosphere via leaves [4,1821]. With low atmospheric Hg concentrations, Hg concentrations in tree rings are significantly influenced by soil Hg concentrations via roots. With high atmospheric Hg concentrations, Hg concentrations in tree rings are dominated by atmospheric Hg uptake via foliage [4,18,19]. Many studies have found that significant Hg contributions are from forest soils via throughfall and litterfall [4,20] and plant uptakes of Hg via roots [19,21]. • Line 83. how are these studies different? ► We have revised as follows (Lines 83-93): A previous study has focused on the record of historical Hg emissions in Yeosu industrial areas of Korea, a region known for phosphate fertilizer production since the late 1970s [12]. However, in that study by Jung and Ahn (2017) [12], the distinction between phosphate fertilizer production and the impact of coal-fired power plants and steel mills on Hg emissions was limited. In particular, Hg concentrations in tree rings shown a gradually increasing patten of low Hg levels during 1960s1970s when there was no industrial activity. Jung and Ahn (2017) [12] did not address whether these low concentrations of Hg in tree rings were uptakes from the soil in the 1960s1970s. In the present study, we measured Hg concentrations in tree rings of Japanese cypress (Chamaecyparis obtusa) and examined historical changes of Hg concentrations in Yeosu and Gwangyang industrial areas of Korea. In addition, this study examined changes from soil versus atmospheric uptake as the primary source of Hg in tree rings of Japanese cypress. • Line 100-102. MUST specify that this is 2017 study. ► Tree ring Hg data of Japanese cypress (Chamaecyparis obtusa) are not results of the previous study by Jung and Ahn (2017) who have studies tree rings of Japanese cedar (Crytomeria japonica). These results are newly analyzed data in this paper. (Lines 152-155) • Line 104-124. Move to introduction ► We have moved this to the Introduction section about industrial development except for topographical and meteorological information of study areas. (Lines 65-82) • Line 138. Add to References. ► We have added a reference ([12]). (Line 130) • Line 140. What is the error on repeat analyses of the standard? Analytical errors are not reported elsewhere and the change in concentrations reported is minimal for some of these time periods. this must be addressed. ► We have added the information regarding limits of detection and standard data as follows (Lines 133-140): The analyzer uses thermal decomposition, gold amalgamation, and atomic absorption spectrometry. The Hg detection limit was 0.005 ng. Operation conditions for the DMA-80 were based on Environmental Protection Agency Method 7473 protocol [30]. A calibration curve was generated using a reference material obtained from the National Research Council of Canada Institute for National Measurement Standards MESS-3 (marine sediment, certified value = 90 ± 9 ng/g HgT [dry weight]) with R2 > 0.999. Measurement standard MESS-3 was used to calculate the accuracy and precision. Recovery ranged from 97% to 102%. Machine blanks and reference samples were included every 520 samples. • Line 159. What do these correlate to? ► Mann-Whitney U-test for pairwise comparison with p adjusted < 0.017 after Bonferroni correction was used to investigate spatial distributions of Hg concentrations associated with decreased distance from industrial areas. (Lines 144-147) • Line 177. What is the error on these measurements? Are these even different numbers? ► This is the range of Hg concentration values in tree rings during 1967-1977 in Figure 3B. It was revised to average value according to another reviewer’s comments as follows (Lines 165-167): Hg concentrations in tree rings between 1967 and 1977 were relatively low. They were 2.51 ng/g and 2.15 ng/g at Namhae and Suncheon sites, respectively, showing no significant difference (p > 0.05). • Line 211. Again, this should be moved to the moved/combined with the introduction. ► These sentences were deleted because they were repeated in the Introduction section. (Lines 65-82) • Line 232. This is not supported by the data. There is a linear increase in concentrations until 1982 - this needs more explanation and is a major hole in the paper. There does appear to be an increase in Hg in the Namhae core coincident with the steel mill starting, but then all tree cores have essentially unchanging concentrations to the end of the core. There is a spike in concentration in the Suncheon core in 1992. What happened then? The discussion of average concentrations for each time range ignores some of this detail and is a detriment to the work. The only real conclusion of the paper that is supported is the spatial relationship and levels of contamination at each site. The temporal record needs a re-evaluation. ► Patterns of a linear increase in Hg concentration during 1967-1977 and elevated and constant Hg concentrations in 1978-2014 were explained in the Discussion section of the revised manuscript as follows (Lines 182-246): Mercury uptakes in trees are bidirectional and variable depending on atmospheric Hg conditions. With low atmospheric Hg concentrations, Hg concentrations in tree rings are significantly influenced by soil Hg concentrations via roots, whereas with high atmospheric Hg concentrations, Hg concentrations in tree rings are dominated by atmospheric Hg uptake via foliage [4,18,19]. The pathway of Hg uptake at sampling sites could be divided into: (1) a linear increase in Hg concentration due to low Hg levels originated from soils via roots in 19671977, and (2) an elevated and constant concentration with spatial variation due to foliar uptake from atmosphere in 19782014 (Figure 3B). Between 1967 and 1977, when shrubs and vegetation senesced each year, there was an annual source of Hg in soils due to continuous deposition of Hg to soil via litterfall and debris. Thus, Hg concentration was increasing over time. Previous studies have found significant Hg contributions from forest soils via throughfall and litterfall, with contribution of litterfall being the greatest [3,20]. The residence time of Hg in soil is quite long. Thus, we anticipate limited loss of Hg from soil pools [35]. During these periods, Hg concentrations in tree rings reflect Hg uptakes from roots under young forest and low atmosphere Hg conditions. Intensified chemical plants and steel mills have continued throughout Yeosu and Gwangyang industrial areas since late 1970s, resulting in high Hg emissions. Hg concentrations in tree rings in 19782014 showed elevated and constant levels. In addition, we observed elevated and consistent concentrations of Hg in tree rings since late 1970s (Figure 3B). The radial pattern of elevated Hg concentrations in tree rings since late 1970s supports the hypothesis of an atmospheric source. ► The reason for the abruptly increase in Hg concentration at the Suncheon site in 1992 was explained as follows (Lines 278-281): The Suncheon site is the farthest from industrial areas with limited influence from wind direction (Figure 1). In 1992 at Suncheon site, Hg concentrations in tree rings showed abrupt increases, reaching the maximum value. This was thought to be due to Hg emission from residential heating associated with the population in Suncheon area in the early 1990s which had the largest number in that area (Figure 2C). • Line 245. This is not a convincing argument in this paper. ► Our dendrochronological record shows that a linear increase in low Hg concentrations in tree rings before 1977 cannot serve the proxy for atmospheric Hg concentrations from nonindustrial and initial industrial activities. These Hg trends reflected the pathway of tree rings before 1977 caused by Hg uptakes from soils via roots under low atmospheric Hg conditions. A soil source would be expected to result in increased Hg concentrations over time due to continued deposition of Hg to soil via litterfall. However, tree ring Hg concentrations in 19782014 are indicative of regional trends associated with industrial activities. These elevated and constant Hg levels were consistent with anthropogenic emissions following intensified industrial activities known to continue to release high Hg levels. ► We explained as follows: - Between 1967 and 1977, tree rings could not serve as reliable proxies for atmosphere Hg concentrations due to Hg uptakes from soils and limits from atmosphere under low atmospheric Hg conditions. (Lines 220-235) - Between 1978 and 2014, tree rings might provide appropriate information for Hg deposition affected by intensified industrial activities in Yeosu and Gwangyang areas. (Lines 236-246) • Line 252. This does not add anything to the paper unless these data were collected in the same place/time. ► We have deleted it.

Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

Dear Authors,

I really enjoyed the new version of this manuscript. I recommend accepting the latest document in its present form.

I congratulate the authors for their excellent work and study.

Best regards
Francisco