the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 04 Mar 2025
Roles of pH, ionic strength, and sulfate in the aqueous nitrate-mediated photooxidation of green leaf volatiles
Abstract. Biotic and abiotic stresses can lead to terrestrial green plants releasing green leaf volatiles (GLVs), which can partition into atmospheric aqueous phases where they can undergo oxidation to form aqueous secondary organic aerosols (aqSOA). Anthropogenic emission changes have resulted in nitrate becoming an increasingly important component of atmospheric aqueous phases, which has significant implications for aqSOA formation since nitrate photolysis produces oxidants. Nevertheless, sulfate remains the main inorganic aqueous component in most regions, and thus controls the pH and ionic strength of atmospheric aqueous phases. We report results from laboratory investigations of the effects of pH, ionic strength, and sulfate on the reaction kinetics and aqSOA formation of the aqueous nitrate-mediated photooxidation of four GLVs, cis-3-hexen-1-ol, trans-2-hexen-1-ol, trans-2-penten-1-ol, and 2-methyl-3-buten-2-ol. Our results showed that the aqueous reaction medium conditions, i.e., dilute cloud/fog vs. concentrated aqueous aerosol conditions, governed the effects that pH, ionic strength, and sulfate have on the GLV degradation rates and aqSOA mass yields. Most notably, reactions initiated by sulfate photolysis will have significant effects on the GLV degradation rates and aqSOA mass yields in aqueous aerosols, but not in cloud/fog droplets. In addition to providing new insights into aqSOA formation from the aqueous reactions of GLVs in regions with substantial concentrations of nitrate in cloud, fog, and aqueous aerosols, this study highlights how nitrate and sulfate photochemistries can couple together to influence the reactions of water-soluble organic compounds and their aqSOA formation in aqueous aerosols, which have implications for our evaluations of aqueous organic aerosol lifetimes and composition.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Atmospheric Chemistry and Physics.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this preprint. The responsibility to include appropriate place names lies with the authors.- Preprint
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RC1: 'Comment on egusphere-2025-570', Anonymous Referee #1, 18 Mar 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”
Citation: https://doi.org/10.5194/egusphere-2025-570-RC1
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