the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 20 Nov 2023
Quantifying SO2 oxidation pathways to atmospheric sulfate by using stable sulfur and oxygen isotopes: laboratory simulation and field observation
Abstract. The formation of secondary sulfate in the atmosphere remains controversial, and it is urgent to seek for a new method to quantify different sulfate formation pathways. Thus, SO2 and PM2.5 samples were collected from 4 to 22 Dec. 2019 in Nanjing. Sulfur and oxygen isotope compositions were synchronously measured to study the contribution of SO2 homogeneous and heterogeneous oxidation to sulfate. Meanwhile, the correlation of δ18O values between H2O and sulfate from SO2 oxidation by H2O2 and Fe3+/O2 were investigated in the lab. Based on isotope mass equilibrium equations, the ratios of different SO2 oxidation pathways were calculated. The results showed that secondary sulfate constituted higher than 80 % of total sulfate in PM2.5 during the sampling period. Laboratory simulation experiments indicated that δ18O of sulfate was linearly dependent on δ18O of water, and the slopes of linear curves for SO2 oxidation by H2O2 and Fe3+/O2 were 0.43 and 0.65, respectively. The secondary sulfate in PM2.5 was mainly ascribed to SO2 homogeneous oxidation by OH radicals and heterogeneous oxidation by H2O2 and Fe3+/O2. SO2 heterogeneous oxidation was generally dominant during sulfate formation, and the contribution of SO2 heterogeneous oxidation was about 52 %. Especially, SO2 oxidation by H2O2 predominated in SO2 heterogeneous oxidation reactions with an average ratio around 55 %. This study provided an insight into precisely evaluating sulfate formation pathways by combining stable sulfur and oxygen isotopes.
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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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RC1: 'Comment on egusphere-2023-2554', Anonymous Referee #1, 22 Nov 2023
In recent years, air pollution is seriously threatening the health of millions of people in China. Sulfate is one of the major chemical species in PM2.5, and play a critical role in human health, and environmental chemistry. However, its formation in the atmosphere remains controversial. In this study, both observational data (δ34S and δ18O values) and laboratory simulation are used to constrain SO2 oxidation pathways. The authors found that the sulfate in PM2.5 was mainly formed from the oxidation of SO2 by OH, H2O2 and TMI. This work provides a valuable dataset of δ34S and δ18O that add critical constraints for sulfate formation pathways. The specific modification suggestions can be seen in the attachment.
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AC1: 'Reply on RC1', Zhaobing Guo, 16 Dec 2023
General Comments:
In recent years, air pollution is seriously threatening the health of millions of people in China. Sulfate is one of the major chemical species in PM2.5, and play a critical role in human health, and environmental chemistry. However, its formation in the atmosphere remains controversial. In this study, both observational data (δ34S and δ18O values) and laboratory simulation are used to constrain SO2 oxidation pathways. The authors found that the sulfate in PM2.5 was mainly formed from the oxidation of SO2 by OH, H2O2 and TMI. This work provides a valuable dataset of δ34S and δ18O that add critical constraints for sulfate formation pathways.
Specific Comments:
1. Page 4, Lines 83-84 The authors mention that the sulfur isotopic fractionation factor of SO2 oxidation by OH determined with laboratory experiments by Harris et al. (2012) was 1.0087. However, they discussed that “It is reported that sulfur isotope fractionation about SO2 was -9‰ for homogeneous oxidation process (Tanaka et al., 1994)”. The sulfur isotopic fractionation factor for homogeneous pathway (SO2+OH) obtained by Tanaka et al. (1994) is different from the laboratory results by Harris et al. (2012). The authors need to compare these two values and explain which to be used for their discussion.
Response: Thanks for Reviewer’s rigorous work. Harris et al. (2012) measured sulfur isotopic fractionation factor (αOH) of SO2 oxidation by OH radicals, which was from the photolysis of water vapor at 30% relative humidity and 184.9 nm. They found that αOH was negatively correlated to the temperature and described as αOH=(1.0089±0.0007)−((4±5)×10−5)T (◦C). In the revised manuscript, we cancelled αOH values of SO2 oxidation by OH, O3/H2O2 and iron catalysis and emphasized their differences of αOH values.
In contrast, Tanaka et al. (1994) estimated αhom to be 0.991 during homogeneous oxidation of SO2 by OH radicals by Ab initio calculations using transition state theory. The discrepancy between these two values may be explained by different research methods and/or temperature-dependence of fractionation factor. Generally, sulfate enriched light sulfur isotope (αhom<1) during SO2 homogeneous oxidation for this process was only related to kinetic fractionation. Therefore, we used αhom=0.991 and αhet=1.0165 to study the contribution of SO2 heterogeneous and homogeneous oxidation to sulfate in our study.
2. Page 17, lines 345-348 Their calculations displayed that the H2O2 pathway is predominated during heterogeneous oxidation of SO2. Could the authors discuss the sources of H2O2 in atmosphere if it plays an important role in heterogeneous oxidation of SO2?
Response: H2O2 production in the relatively clean atmosphere is ascribed to self-reaction of HO2 radicals that mainly come from the reactions of OH with CO and volatile organic compounds. It is favorable for H2O2 formation under the conditions of high O3 concentration, strong solar irradiation, and high temperature. We have added the sources of H2O2 in the revised manuscript.
3. The conclusion seems to be a bit dry. I suggest that the important implications for this work should be added, besides summarize the main points.
Response: This is a constructive suggestion, and we have added the following descriptions in Conclusions in the revised manuscript. Sulfur and oxygen isotopes could be used to gain an insight into sulfate formation. Sulfur isotope compositions in SO2 and sulfate were simultaneously measured to quantify the contributions of SO2 homogeneous and heterogeneous oxidation. Combining field observations of oxygen isotope in the atmosphere with the linear relationships of δ18O values between H2O and sulfate from different SO2 oxidation processes can obtain an increased understanding of specific sulfate formation pathways. This study is favorable for deeply investigating sulfur cycle in the atmosphere.
Technical corrections:
4. Page 8, Line 163 Please change “The concentrations of PM2.5, SO42- and SO2” to “Variations in concentrations of PM2.5, SO42- and SO2”.
Response: Thanks for Reviewer’s suggestion. The sentence has been revised in the manuscript.
5. Page 9, Line 197 Please change “Sulfur isotope compositions in sulfate and SO2” to “Variations in sulfur isotope compositions in sulfate and SO2”. In addition, the black solid circles represent the δ34S values of sulfate instead of PM2.5.
Response: Thanks for Reviewer’s suggestion. The sentence has been revised in the manuscript. The black solid circles represent the δ34S values of sulfate in PM2.5, we have revised it in Fig.3.
Citation: https://doi.org/10.5194/egusphere-2023-2554-AC1
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AC1: 'Reply on RC1', Zhaobing Guo, 16 Dec 2023
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RC2: 'Comment on egusphere-2023-2554', Anonymous Referee #2, 05 Dec 2023
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-2554/egusphere-2023-2554-RC2-supplement.pdf
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AC2: 'Reply on RC2', Zhaobing Guo, 16 Dec 2023
This manuscript quantifies the sulfate formation pathways from 4 to 22 December 2019 in Nanjing by proposing a new method of simultaneously measuring sulfur and oxygen isotope compositions. The authors conclude that sulfate in PM2.5 is mainly from a secondary source with SO2 homogeneously oxidized by OH and heterogeneously oxidized by H2O2. Overall, the manuscript is well-written, and the method is reasonable. I have a few points that could be addressed to strengthen the manuscript and some minor comments.
General Comments:
1. The method seems applicable, but the authors need to explain the calculations better. I find it hard sometimes to understand how the result is derived. For example, the authors mentioned that the δ18O value of primary sulfate is about 38 ‰ in Line 296 before they pointed out it was based on Formula (5). That is confusing. Why there are contribution ranges on each day in Table 1, instead of a single number like in Table 2?
Response: We are grateful for Reviewer’s suggestions. We mentioned that δ18O value of primary sulfate was about 38‰, which aimed to calculate the contribution of primary and secondary sulfate in the atmosphere. The δ18O value of 38‰ was cited from the study of Holt and Kumar (1984), and it was not directly from Formula (5). We have added this reference in the revised manuscript.
In addition, we have further explained the calculation method about the contribution of primary and secondary sulfate in PM2.5 and the ratios of different SO2 oxidation pathways in the revised manuscript. We calculated the contribution of primary and secondary sulfate according to the equation: δ18OPM2.5=δ18OPS×(1-fSS)+δ18OSS×fSS. It is known that secondary sulfate was mainly ascribed to SO2 homogeneous oxidation by OH radicals and heterogeneous oxidation by H2O2 and Fe3+/O2 in this study. Therefore, δ18OSS can be obtained based on the following three equations, respectively.
δ18OSS=0.69×δ18Owater+9.5 ‰ (OH)
δ18OSS=0.65×δ18Owater+10.6 ‰ (Fe3+/O2)
δ18OSS=0.43×δ18Owater+12.5 ‰ (H2O2)
As a result, data ranges about the contribution of primary and secondary sulfate in PM2.5 are presented in the original manuscript. To keep consistent with the single ratios of SO2 different oxidation pathways to sulfate in Table 2, we have calculated the average contribution of primary and secondary sulfate on each dayin Table 1 in the revised manuscript.
2. Is it possible to add more data points in Figure 7? It seems three are not robust enough to rerive the linear relationships.
Response: When simulatively studying the linear relationship of δ18O values between H2O and sulfate from SO2 oxidation by H2O2 and Fe3+/O2 in the lab, we selected three kinds of representative water including tap-water, lake water and rainwater. The results showed that the slopes of these two linear curves were 0.43 and 0.65, respectively, which can basically reflect the characteristics of SO2 heterogeneous oxidation mechanisms by H2O2 and Fe3+/O2.
We fully agree with the reviewer, and will provide more data points to precisely study the correlation in the following experimental design.
Minor Comments:
3. Line 69: Define RH here instead of in Line 186.
Response: According to Reviewer’s suggestions, we have defined RH as “relative humidity” in Line 68 and deleted “relative humidity” in Line 187 in the revised manuscript.
4. Line 98/320: I did not find references related to Holt et al.
Response: We are very sorry for our negligence. The reference has been added in the revised manuscript.
Holt, B.D. and Kumar R.: Oxygen-18 study of high-temperature air oxidation of SO2, Atmos. Environ., 18, 2089-2094, https://doi.org/10.1016/0004-6981(84)90194-X, 1984.
5. Line 168-170: I am not sure why high CO is indicative of local emissions. It can be transported by a long range.
Response: As Reviewer said, CO can be transported by a long range due to its stability, and CO is not indicative of local emissions. In the manuscript, the conclusion “High CO concentration indicates that the pollution was mainly from local emissions” was mainly ascribed to the analysis of meteorological conditions. During the sampling period, the wind speed is lower than 3m/s and there is presence of static weather. Therefore, it is hard for CO to transport a long range. We did not explain clearly in the original manuscript, and we have added the analysis of meteorological conditions in the revised one. The detailed description was as “Based on the wind speed is lower than 3m/s and there is presence of static weather during the sampling period, we believed that high CO concentration was mainly from local emissions.”.
6. Line 192: What does the negative -2.9 mean here? Is it possible to have negative values?
Response: Thanks for Reviewer’s suggestion. The negative -2.9 means that the lighter sulfur isotopes were enriched in SO2. It is common to have negative δ34S values in the samples.
7. Figure 3: Legend of PM2.5 is wrong. Should be sulfate.
Response: We are very sorry for our cursoriness. The legend in Figure has been revised.
8. Line 249: The average of 51.6% seems just a little higher than 50%. I suggest to say that most of the days (seems 7 out of 11) have more than 50% contributions from heterogeneous oxidation.
Response: Thanks for Reviewer’s suggestion. The sentence in the manuscript has been revised.
Page 11, Line 249-251: It is observed from Fig. 6 that most of the days (7 out of 11) have more than 50% contributions from SO2 heterogeneous oxidation, which indicated that SO2 heterogeneous oxidation was generally dominant during sulfate formation.
9. Figure 7: Are the three dots corresponding to three kinds of water? Better to describe it in the texts or figure title.
Response: Thanks for Reviewer’s suggestion. Three dots are corresponding to three kinds of water in Fig.7, and we have made a detailed explanation as “which aims to make clear the relationship of δ18O values between product sulfate and three kinds of water at 10 ℃” in Line 278 in the text.
10. Line 324: fSS-OH+ should be fSS-OH+
Response: Thanks for Reviewer’s suggestion. The formula has been revised in the manuscript.
Citation: https://doi.org/10.5194/egusphere-2023-2554-AC2
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AC2: 'Reply on RC2', Zhaobing Guo, 16 Dec 2023
Interactive discussion
Status: closed
-
RC1: 'Comment on egusphere-2023-2554', Anonymous Referee #1, 22 Nov 2023
In recent years, air pollution is seriously threatening the health of millions of people in China. Sulfate is one of the major chemical species in PM2.5, and play a critical role in human health, and environmental chemistry. However, its formation in the atmosphere remains controversial. In this study, both observational data (δ34S and δ18O values) and laboratory simulation are used to constrain SO2 oxidation pathways. The authors found that the sulfate in PM2.5 was mainly formed from the oxidation of SO2 by OH, H2O2 and TMI. This work provides a valuable dataset of δ34S and δ18O that add critical constraints for sulfate formation pathways. The specific modification suggestions can be seen in the attachment.
-
AC1: 'Reply on RC1', Zhaobing Guo, 16 Dec 2023
General Comments:
In recent years, air pollution is seriously threatening the health of millions of people in China. Sulfate is one of the major chemical species in PM2.5, and play a critical role in human health, and environmental chemistry. However, its formation in the atmosphere remains controversial. In this study, both observational data (δ34S and δ18O values) and laboratory simulation are used to constrain SO2 oxidation pathways. The authors found that the sulfate in PM2.5 was mainly formed from the oxidation of SO2 by OH, H2O2 and TMI. This work provides a valuable dataset of δ34S and δ18O that add critical constraints for sulfate formation pathways.
Specific Comments:
1. Page 4, Lines 83-84 The authors mention that the sulfur isotopic fractionation factor of SO2 oxidation by OH determined with laboratory experiments by Harris et al. (2012) was 1.0087. However, they discussed that “It is reported that sulfur isotope fractionation about SO2 was -9‰ for homogeneous oxidation process (Tanaka et al., 1994)”. The sulfur isotopic fractionation factor for homogeneous pathway (SO2+OH) obtained by Tanaka et al. (1994) is different from the laboratory results by Harris et al. (2012). The authors need to compare these two values and explain which to be used for their discussion.
Response: Thanks for Reviewer’s rigorous work. Harris et al. (2012) measured sulfur isotopic fractionation factor (αOH) of SO2 oxidation by OH radicals, which was from the photolysis of water vapor at 30% relative humidity and 184.9 nm. They found that αOH was negatively correlated to the temperature and described as αOH=(1.0089±0.0007)−((4±5)×10−5)T (◦C). In the revised manuscript, we cancelled αOH values of SO2 oxidation by OH, O3/H2O2 and iron catalysis and emphasized their differences of αOH values.
In contrast, Tanaka et al. (1994) estimated αhom to be 0.991 during homogeneous oxidation of SO2 by OH radicals by Ab initio calculations using transition state theory. The discrepancy between these two values may be explained by different research methods and/or temperature-dependence of fractionation factor. Generally, sulfate enriched light sulfur isotope (αhom<1) during SO2 homogeneous oxidation for this process was only related to kinetic fractionation. Therefore, we used αhom=0.991 and αhet=1.0165 to study the contribution of SO2 heterogeneous and homogeneous oxidation to sulfate in our study.
2. Page 17, lines 345-348 Their calculations displayed that the H2O2 pathway is predominated during heterogeneous oxidation of SO2. Could the authors discuss the sources of H2O2 in atmosphere if it plays an important role in heterogeneous oxidation of SO2?
Response: H2O2 production in the relatively clean atmosphere is ascribed to self-reaction of HO2 radicals that mainly come from the reactions of OH with CO and volatile organic compounds. It is favorable for H2O2 formation under the conditions of high O3 concentration, strong solar irradiation, and high temperature. We have added the sources of H2O2 in the revised manuscript.
3. The conclusion seems to be a bit dry. I suggest that the important implications for this work should be added, besides summarize the main points.
Response: This is a constructive suggestion, and we have added the following descriptions in Conclusions in the revised manuscript. Sulfur and oxygen isotopes could be used to gain an insight into sulfate formation. Sulfur isotope compositions in SO2 and sulfate were simultaneously measured to quantify the contributions of SO2 homogeneous and heterogeneous oxidation. Combining field observations of oxygen isotope in the atmosphere with the linear relationships of δ18O values between H2O and sulfate from different SO2 oxidation processes can obtain an increased understanding of specific sulfate formation pathways. This study is favorable for deeply investigating sulfur cycle in the atmosphere.
Technical corrections:
4. Page 8, Line 163 Please change “The concentrations of PM2.5, SO42- and SO2” to “Variations in concentrations of PM2.5, SO42- and SO2”.
Response: Thanks for Reviewer’s suggestion. The sentence has been revised in the manuscript.
5. Page 9, Line 197 Please change “Sulfur isotope compositions in sulfate and SO2” to “Variations in sulfur isotope compositions in sulfate and SO2”. In addition, the black solid circles represent the δ34S values of sulfate instead of PM2.5.
Response: Thanks for Reviewer’s suggestion. The sentence has been revised in the manuscript. The black solid circles represent the δ34S values of sulfate in PM2.5, we have revised it in Fig.3.
Citation: https://doi.org/10.5194/egusphere-2023-2554-AC1
-
AC1: 'Reply on RC1', Zhaobing Guo, 16 Dec 2023
-
RC2: 'Comment on egusphere-2023-2554', Anonymous Referee #2, 05 Dec 2023
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-2554/egusphere-2023-2554-RC2-supplement.pdf
-
AC2: 'Reply on RC2', Zhaobing Guo, 16 Dec 2023
This manuscript quantifies the sulfate formation pathways from 4 to 22 December 2019 in Nanjing by proposing a new method of simultaneously measuring sulfur and oxygen isotope compositions. The authors conclude that sulfate in PM2.5 is mainly from a secondary source with SO2 homogeneously oxidized by OH and heterogeneously oxidized by H2O2. Overall, the manuscript is well-written, and the method is reasonable. I have a few points that could be addressed to strengthen the manuscript and some minor comments.
General Comments:
1. The method seems applicable, but the authors need to explain the calculations better. I find it hard sometimes to understand how the result is derived. For example, the authors mentioned that the δ18O value of primary sulfate is about 38 ‰ in Line 296 before they pointed out it was based on Formula (5). That is confusing. Why there are contribution ranges on each day in Table 1, instead of a single number like in Table 2?
Response: We are grateful for Reviewer’s suggestions. We mentioned that δ18O value of primary sulfate was about 38‰, which aimed to calculate the contribution of primary and secondary sulfate in the atmosphere. The δ18O value of 38‰ was cited from the study of Holt and Kumar (1984), and it was not directly from Formula (5). We have added this reference in the revised manuscript.
In addition, we have further explained the calculation method about the contribution of primary and secondary sulfate in PM2.5 and the ratios of different SO2 oxidation pathways in the revised manuscript. We calculated the contribution of primary and secondary sulfate according to the equation: δ18OPM2.5=δ18OPS×(1-fSS)+δ18OSS×fSS. It is known that secondary sulfate was mainly ascribed to SO2 homogeneous oxidation by OH radicals and heterogeneous oxidation by H2O2 and Fe3+/O2 in this study. Therefore, δ18OSS can be obtained based on the following three equations, respectively.
δ18OSS=0.69×δ18Owater+9.5 ‰ (OH)
δ18OSS=0.65×δ18Owater+10.6 ‰ (Fe3+/O2)
δ18OSS=0.43×δ18Owater+12.5 ‰ (H2O2)
As a result, data ranges about the contribution of primary and secondary sulfate in PM2.5 are presented in the original manuscript. To keep consistent with the single ratios of SO2 different oxidation pathways to sulfate in Table 2, we have calculated the average contribution of primary and secondary sulfate on each dayin Table 1 in the revised manuscript.
2. Is it possible to add more data points in Figure 7? It seems three are not robust enough to rerive the linear relationships.
Response: When simulatively studying the linear relationship of δ18O values between H2O and sulfate from SO2 oxidation by H2O2 and Fe3+/O2 in the lab, we selected three kinds of representative water including tap-water, lake water and rainwater. The results showed that the slopes of these two linear curves were 0.43 and 0.65, respectively, which can basically reflect the characteristics of SO2 heterogeneous oxidation mechanisms by H2O2 and Fe3+/O2.
We fully agree with the reviewer, and will provide more data points to precisely study the correlation in the following experimental design.
Minor Comments:
3. Line 69: Define RH here instead of in Line 186.
Response: According to Reviewer’s suggestions, we have defined RH as “relative humidity” in Line 68 and deleted “relative humidity” in Line 187 in the revised manuscript.
4. Line 98/320: I did not find references related to Holt et al.
Response: We are very sorry for our negligence. The reference has been added in the revised manuscript.
Holt, B.D. and Kumar R.: Oxygen-18 study of high-temperature air oxidation of SO2, Atmos. Environ., 18, 2089-2094, https://doi.org/10.1016/0004-6981(84)90194-X, 1984.
5. Line 168-170: I am not sure why high CO is indicative of local emissions. It can be transported by a long range.
Response: As Reviewer said, CO can be transported by a long range due to its stability, and CO is not indicative of local emissions. In the manuscript, the conclusion “High CO concentration indicates that the pollution was mainly from local emissions” was mainly ascribed to the analysis of meteorological conditions. During the sampling period, the wind speed is lower than 3m/s and there is presence of static weather. Therefore, it is hard for CO to transport a long range. We did not explain clearly in the original manuscript, and we have added the analysis of meteorological conditions in the revised one. The detailed description was as “Based on the wind speed is lower than 3m/s and there is presence of static weather during the sampling period, we believed that high CO concentration was mainly from local emissions.”.
6. Line 192: What does the negative -2.9 mean here? Is it possible to have negative values?
Response: Thanks for Reviewer’s suggestion. The negative -2.9 means that the lighter sulfur isotopes were enriched in SO2. It is common to have negative δ34S values in the samples.
7. Figure 3: Legend of PM2.5 is wrong. Should be sulfate.
Response: We are very sorry for our cursoriness. The legend in Figure has been revised.
8. Line 249: The average of 51.6% seems just a little higher than 50%. I suggest to say that most of the days (seems 7 out of 11) have more than 50% contributions from heterogeneous oxidation.
Response: Thanks for Reviewer’s suggestion. The sentence in the manuscript has been revised.
Page 11, Line 249-251: It is observed from Fig. 6 that most of the days (7 out of 11) have more than 50% contributions from SO2 heterogeneous oxidation, which indicated that SO2 heterogeneous oxidation was generally dominant during sulfate formation.
9. Figure 7: Are the three dots corresponding to three kinds of water? Better to describe it in the texts or figure title.
Response: Thanks for Reviewer’s suggestion. Three dots are corresponding to three kinds of water in Fig.7, and we have made a detailed explanation as “which aims to make clear the relationship of δ18O values between product sulfate and three kinds of water at 10 ℃” in Line 278 in the text.
10. Line 324: fSS-OH+ should be fSS-OH+
Response: Thanks for Reviewer’s suggestion. The formula has been revised in the manuscript.
Citation: https://doi.org/10.5194/egusphere-2023-2554-AC2
-
AC2: 'Reply on RC2', Zhaobing Guo, 16 Dec 2023
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Ziyan Guo
Pengxiang Qiu
Mingyi Xu
Zhaobing Guo
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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