Articles | Volume 25, issue 18
https://doi.org/10.5194/acp-25-10731-2025
© Author(s) 2025. This work is distributed under
the Creative Commons Attribution 4.0 License.Roles of pH, ionic strength, and sulfate in the aqueous nitrate-mediated photooxidation of green leaf volatiles
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- Final revised paper (published on 18 Sep 2025)
- Supplement to the final revised paper
- Preprint (discussion started on 04 Mar 2025)
- Supplement to the preprint
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Status: closed
Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor
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- RC1: 'Comment on egusphere-2025-570', Anonymous Referee #1, 18 Mar 2025
- RC2: 'Comment on egusphere-2025-570', Anonymous Referee #2, 27 Apr 2025
- AC1: 'Comment on egusphere-2025-570', Theodora Nah, 04 Jun 2025
- AC2: 'Comment on egusphere-2025-570', Theodora Nah, 07 Jun 2025
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AR: Author's response | RR: Referee report | ED: Editor decision | EF: Editorial file upload
AR by Theodora Nah on behalf of the Authors (04 Jun 2025)
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ED: Referee Nomination & Report Request started (18 Jun 2025) by Ryan Sullivan
RR by Anonymous Referee #1 (19 Jun 2025)
RR by Anonymous Referee #2 (25 Jun 2025)
ED: Publish subject to minor revisions (review by editor) (25 Jun 2025) by Ryan Sullivan

AR by Theodora Nah on behalf of the Authors (26 Jun 2025)
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ED: Publish as is (01 Jul 2025) by Ryan Sullivan
AR by Theodora Nah on behalf of the Authors (03 Jul 2025)
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General comments
This manuscript describes measurements that aim to understand the SOA-forming capacity of green leaf volatiles (GLV) that react with the products of the aqueous phase photolysis of nitrate. Specifically, the work describes measurements of the overall reaction rate constants (kobs) for 4 specific GLVs via high-resolution time-of-flight electrospray ionization mass spectrometer (HR-ToF-ESI) and separate experiments to determine the SOA yields. In order to investigate both cloud/fog and aerosol-like conditions, both the ionic strength and pH of the solutions were varied. Importantly, ammonium sulfate was used to control the ionic strength of the solutions, which led to complications in the interpretation of the results. Under dilute cloud/fog-like conditions, the four GLVs had higher kobs at lower pH, which could be attributed to the pH-dependent formation of OH and other reactive species from nitrate photolysis. Ionic strength and sulfate had insignificant effects on kobs. In contrast, under concentrated aqueous aerosol-like conditions, the four GLVs had higher kobs at higher pH, as well as higher kobs values at higher ionic strength and sulfate concentration. These effects are explained by the expected nitrate photolysis-initiated processes as well as the unexpected role of sulfate-related oxidation processes. Higher SOA yields under both cloud/fog and aerosol-like conditions were observed at lower pH, which was attributed to acid-catalyzed accretion reactions.
Because of the importance of the study in helping to refine the formation mechanisms of SOA from such precursors as GLVs, this work will be of interest to general readers of EGUsphere. The experiments are rationally designed, thoroughly analyzed, and the manuscript is generally well written. However, the work is difficult to assess as it seems that it was designed as a careful study of the nitrate photolysis-initiated processes, but that design was compromised by the presence of unanticipated sulfate photolysis mechanisms. The authors admit that the ionic strength dependence of the nitrate photolysis-initiated processes needs to be reinvestigated with a non-sulfate species. The finding of sulfate photolysis-related processes is very important and worth reporting but is likewise complicated by the concurrent nitrate photolysis mechanism. Therefore, it is quite obvious to the reader that new experiments should be designed that isolate the nitrate and sulfate photolysis processes. Nonetheless, even though the work was not able to achieve its original goals of determining a rigorous quantitative understanding of the nitrate photolysis-related processes, it is still valuable as a qualitative outline of the combined importance of the nitrate and sulfate photolysis pathways.
There are several items that should be addressed in a revised version of the manuscript:
Line 235: It would have been relatively to test this hypothesis with a separate experiment that generated OH exclusively. Is there a reason this was not done?
Line 246: Why couldn’t the inorganic salts be separated before analysis?
Line 377: This is a very out of date set of references for acid catalyzed SOA processes. I suggest adding:
Epoxides: Cooke et al. ES&T, 58, 10675-10684, 2024
Acetals: Presberg et al. ACS Earth and Space Chem., 8, 1634-1645, 2024
Oligomers: Maben et al., Environ Sci Process Impacts, 25, 214-228, 2023
Line 379: Why would there be enhanced formation of organonitrates from RO2 + NO at high ionic strength?
Technical comments
Line 64: typo “ideal”
Line 154: extraneous “the” between “from” and “before”
Line 313: typo in subscript for ionic strength “total”