the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 12 Feb 2026
Resolving Systematic Errors in Sulfate Source Apportionment: A Field-Validated Kinetic Isotope Fractionation Framework
Abstract. Sulfates represent a critical constituent of atmospheric fine particulate matter (PM2.5), significantly influencing air quality and climate dynamics. Precise quantification of atmospheric sulfate formation mechanisms and emission sources through stable isotope fractionation analysis represents a critical advancement in particulate matter pollution control. Conventional isotopic models relying on idealized complete SO2 oxidation scenarios, while providing preliminary source apportionment estimates, exhibit systematic errors in reaction pathway quantification. Our field-validated approach incorporating actual atmospheric oxidation processes demonstrates that transition-metal ions (TMI)-catalyzed and NO2-mediated pathways dominate sulfate production, with coal combustion (overestimate by 10.8 % in summer) and traffic emissions (underestimated by 8.2 % in summer) constituting primary sources. Comparative analysis reveals that traditional complete-oxidation models disproportionately diminish TMI pathway contributions, highlighting the necessity of kinetic fractionation corrections. These findings establish an improved isotopic tracing framework that resolves longstanding calculation discrepancies, delivering essential constraints for atmospheric sulfur cycle modeling and emission regulation strategies.
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RC1: 'Comment on egusphere-2025-6533', Anonymous Referee #1, 26 Feb 2026
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AC1: 'Reply on RC1', Pengxiang Qiu, 18 Jun 2026
We sincerely appreciate the opportunity to revise our manuscript titled “Resolving Systematic Errors in Sulfate Source Apportionment: A Field-Validated Kinetic Isotope Fractionation Framework”. The editorial team's consideration and the reviewers' insightful comments have been invaluable in improving the quality and clarity of our work. The constructive feedback has significantly strengthened both the scientific rigor and presentation of our findings.
The manuscript has undergone rigorous revision in response to the insightful comments from both editors and reviewers, with each point carefully considered and addressed to strengthen the study's validity and clarity. The comments are in BLACK and our responses are in BLUE.
As our responses include equations, figures, and other detailed explanations, the complete response document is provided in the "Supplement".
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AC1: 'Reply on RC1', Pengxiang Qiu, 18 Jun 2026
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RC2: 'Comment on egusphere-2025-6533', Sanjeev Dasari, 04 May 2026
Review for Guo et al.,
This study is interesting and novel. It does provide a new perspective towards sulfate source apportionment, accounting for newer pathways for sulfate formation.
I suggest the authors to comment on the following and make necessary changes in the manuscript:
1. Role of primary sulfates: I think this aspect has been overlooked. It needs to be quantified as the works of Daie et al. (2019) and Song et al., (2024) reveal nearly half of atmospheric sulfate could be attributed to primary sulfate. Once the authors attribute this fraction, the remaining can be attributed to secondary and as such Fig. 6 needs to show this division clearly with a separate sub-figure.
2. Terrigenous sulfate/mineral dust: Please refer to Dasari et al., 2022 ES&T L and 2024 ES&T Air ( which should be cited as these are also relevant works to this study) wherein the role of long-range transported mineral dust/terr-sulfate has been shown as an important factor. The authros need to apportion this source too.
3. Cluster analysis vs. footprint analysis: Please note the AMBTs can show the air masses arriving from a certain region, the footprint of the contributing regions can be very different. Please refer to Dasari et al., 2020 ES&T for this distinction. As such, solely banking on the AMBTs isnt proof enough of the regional source contributions. The authors should convincingly show that the footprint analysis matches the AMBTs .
4. No2 mediated pathway: Growing evidence suggest this pathway is more active in winter foggy/hazy conditions Wang et al., 2020 Nat. Comm. However, here the authors suggest this pathway is key with contribution in winter and summer nearly the same. This is contradicory to growing body of research (both lab-based and field-based). I suggest the authors reconsider the literature findings and convincingly show this and rethink Fig. 4.
5. Referencing is poor and needs to be updated to correct format and form. Please also add relevant regional works from other parts of the world to address the issue with d34S-based source apportionment of sulfate.
While the study is interesting, there are many caveats (e.g, with pathway attribution ) as such some wording like 'paradigm shift' seem unncessary. I suggest the authors to reconsider such wordings.
Citation: https://doi.org/10.5194/egusphere-2025-6533-RC2 -
AC2: 'Reply on RC2', Pengxiang Qiu, 18 Jun 2026
We sincerely appreciate the opportunity to revise our manuscript titled “Resolving Systematic Errors in Sulfate Source Apportionment: A Field-Validated Kinetic Isotope Fractionation Framework”. The editorial team's consideration and the reviewers' insightful comments have been invaluable in improving the quality and clarity of our work. The constructive feedback has significantly strengthened both the scientific rigor and presentation of our findings.
The manuscript has undergone rigorous revision in response to the insightful comments from both editors and reviewers, with each point carefully considered and addressed to strengthen the study's validity and clarity. The comments are in BLACK and our responses are in BLUE.
As our responses include equations, figures, and other detailed explanations, the complete response document is provided in the "Supplement".
-
AC2: 'Reply on RC2', Pengxiang Qiu, 18 Jun 2026
Status: closed
-
RC1: 'Comment on egusphere-2025-6533', Anonymous Referee #1, 26 Feb 2026
General Comments:
Sulfate is an important part of atmospheric aerosol and has a significant influence on air quality. Investigation of sulfate sources and formation using stable isotopes is important for air pollution control. In this manuscript, it makes an important contribution by correcting a long-standing oversimplification in isotopic source apportionment. The comparison between complete-oxidation and incomplete-oxidation scenarios clearly demonstrates the biases introduced by traditional assumptions. However, several major concerns require clarification to improve its clarity and impact.
Specific Comments:
- Lines 31-32 What do the letters “a, b, c, d” in the text “δ34S= a, δ34S= b, δ34S= c, δ34S=d ” represent?
- Lines 35-38 Please clarify the distinction between complete and incomplete SO2 oxidation frameworks.
- Lines 65-69 The uncertainties for water-soluble ions, δ34S and δ18O are necessary to show in this part. In addition, please add the standard materials for measurements.
- Lines 66-67 The sentence “The δ34S value of sulfate was determined by precipitating BaSO4 via BaCl2 addition, followed by selective dissolution of residual BaSO3 with 1 MHCl” is vague. Please rewrite it.
- Lines 194-201 Wang et al. (2016) suggest that the aqueous oxidation of SO2 by NO2 is key to efficient sulfate formation, but is only feasible under two atmospheric conditions: on fine aerosols with high relative humidity and NH3neutralization or under cloud conditions. How to prove that the NO2-mediated oxidation pathway is ubiquitous during the sampling period?
Reference：Wang, et al. 2016. Persistent sulfate formation from London Fog to Chinese haze, PNAS, 113, 13630-13635.
6. Lines 215-225: The concluding paragraph is effective but could be strengthened by mentioning policy implications for sulfate control.
Technical corrections:
- Line 28 Remove the number “2”
- Line 39 The sentence “Drawing conceptual inspiration from these insights” is vague.
- Line 43 Please change “sulfur-oxygen isotope analysis” to “sulfur and oxygen isotope analyses ”.
- Line 58 Please remove “(08:00-08:00 local time)”.
- Line 60 and line 68 Please keep consistent with temperature units (450℃ V.S. 1073K).
- Line 191 Nanjing is in eastern China, not central. Please change “central China” to “eastern China”.
- References
Several references have incomplete DOIs (e.g., Han et al., 2022 has a PNAS DOI that doesn't match the journal)
Mang et al., 2018- check author names
Sinha, 2013 – incomplete reference (missing journal, volume, pages)
Ensure all in-text citations match reference list
Please correct language and technical errors throughout.
Citation: https://doi.org/10.5194/egusphere-2025-6533-RC1 -
AC1: 'Reply on RC1', Pengxiang Qiu, 18 Jun 2026
We sincerely appreciate the opportunity to revise our manuscript titled “Resolving Systematic Errors in Sulfate Source Apportionment: A Field-Validated Kinetic Isotope Fractionation Framework”. The editorial team's consideration and the reviewers' insightful comments have been invaluable in improving the quality and clarity of our work. The constructive feedback has significantly strengthened both the scientific rigor and presentation of our findings.
The manuscript has undergone rigorous revision in response to the insightful comments from both editors and reviewers, with each point carefully considered and addressed to strengthen the study's validity and clarity. The comments are in BLACK and our responses are in BLUE.
As our responses include equations, figures, and other detailed explanations, the complete response document is provided in the "Supplement".
-
RC2: 'Comment on egusphere-2025-6533', Sanjeev Dasari, 04 May 2026
Review for Guo et al.,
This study is interesting and novel. It does provide a new perspective towards sulfate source apportionment, accounting for newer pathways for sulfate formation.
I suggest the authors to comment on the following and make necessary changes in the manuscript:
1. Role of primary sulfates: I think this aspect has been overlooked. It needs to be quantified as the works of Daie et al. (2019) and Song et al., (2024) reveal nearly half of atmospheric sulfate could be attributed to primary sulfate. Once the authors attribute this fraction, the remaining can be attributed to secondary and as such Fig. 6 needs to show this division clearly with a separate sub-figure.
2. Terrigenous sulfate/mineral dust: Please refer to Dasari et al., 2022 ES&T L and 2024 ES&T Air ( which should be cited as these are also relevant works to this study) wherein the role of long-range transported mineral dust/terr-sulfate has been shown as an important factor. The authros need to apportion this source too.
3. Cluster analysis vs. footprint analysis: Please note the AMBTs can show the air masses arriving from a certain region, the footprint of the contributing regions can be very different. Please refer to Dasari et al., 2020 ES&T for this distinction. As such, solely banking on the AMBTs isnt proof enough of the regional source contributions. The authors should convincingly show that the footprint analysis matches the AMBTs .
4. No2 mediated pathway: Growing evidence suggest this pathway is more active in winter foggy/hazy conditions Wang et al., 2020 Nat. Comm. However, here the authors suggest this pathway is key with contribution in winter and summer nearly the same. This is contradicory to growing body of research (both lab-based and field-based). I suggest the authors reconsider the literature findings and convincingly show this and rethink Fig. 4.
5. Referencing is poor and needs to be updated to correct format and form. Please also add relevant regional works from other parts of the world to address the issue with d34S-based source apportionment of sulfate.
While the study is interesting, there are many caveats (e.g, with pathway attribution ) as such some wording like 'paradigm shift' seem unncessary. I suggest the authors to reconsider such wordings.
Citation: https://doi.org/10.5194/egusphere-2025-6533-RC2 -
AC2: 'Reply on RC2', Pengxiang Qiu, 18 Jun 2026
We sincerely appreciate the opportunity to revise our manuscript titled “Resolving Systematic Errors in Sulfate Source Apportionment: A Field-Validated Kinetic Isotope Fractionation Framework”. The editorial team's consideration and the reviewers' insightful comments have been invaluable in improving the quality and clarity of our work. The constructive feedback has significantly strengthened both the scientific rigor and presentation of our findings.
The manuscript has undergone rigorous revision in response to the insightful comments from both editors and reviewers, with each point carefully considered and addressed to strengthen the study's validity and clarity. The comments are in BLACK and our responses are in BLUE.
As our responses include equations, figures, and other detailed explanations, the complete response document is provided in the "Supplement".
-
AC2: 'Reply on RC2', Pengxiang Qiu, 18 Jun 2026
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General Comments:
Sulfate is an important part of atmospheric aerosol and has a significant influence on air quality. Investigation of sulfate sources and formation using stable isotopes is important for air pollution control. In this manuscript, it makes an important contribution by correcting a long-standing oversimplification in isotopic source apportionment. The comparison between complete-oxidation and incomplete-oxidation scenarios clearly demonstrates the biases introduced by traditional assumptions. However, several major concerns require clarification to improve its clarity and impact.
Specific Comments:
Reference：Wang, et al. 2016. Persistent sulfate formation from London Fog to Chinese haze, PNAS, 113, 13630-13635.
6. Lines 215-225: The concluding paragraph is effective but could be strengthened by mentioning policy implications for sulfate control.
Technical corrections:
Several references have incomplete DOIs (e.g., Han et al., 2022 has a PNAS DOI that doesn't match the journal)
Mang et al., 2018- check author names
Sinha, 2013 – incomplete reference (missing journal, volume, pages)
Ensure all in-text citations match reference list
Please correct language and technical errors throughout.