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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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2025-6533', Anonymous Referee #1, 26 Feb 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
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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.