| 08 Jul 2026
An Underappreciated Aqueous Pathway for Particle Oxidative Potential (OP): Mechanistic Insights into OP-Relevant Products from α-Dicarbonyl and Reduced Nitrogen Reactions
Abstract. Aqueous-phase reactions between α-dicarbonyls and reduced nitrogen species represent an understudied pathway generating secondary organic aerosols that exhibit oxidative potential (OP). In this study, we investigated the OP of products formed from methylglyoxal (MG) or glyoxal (GX) reacting with ammonium sulfate (AS) or glycine (Gly) under varying pH (3–7) and reaction times (4–144 h). OP was quantified using dithiothreitol (DTT) depletion and hydroxyl radical (•OH) production assays, while molecular composition was characterized using high-resolution mass spectrometry and nuclear magnetic resonance spectroscopy. The results demonstrate that OP is strongly dependent on precursor identity, pH, and reaction time. Products from the MG+Gly system consistently exhibit the highest OPDTT and OPOH. Molecular analysis indicates that CHO species, including conjugated carbonyls and quinones, are primarily responsible for DTT activity, whereas N-heterocycles with unprotonated N-bases drive •OH formation. Electron-withdrawing substituents adjacent to the unprotonated N atoms enhance electron-transfer capability, thereby increasing the ability of N-heterocycles to generate •OH. N-heterocycles also act synergistically with quinone-like species to promote DTT oxidation, highlighting their dual role in modulating OP. Collectively, these findings reveal that aqueous reactions between α-dicarbonyls and reduced nitrogen species can produce secondary organic aerosols with substantial oxidative capacity, which may contribute to oxidative stress and cause adverse health effects. This work advances mechanistic understanding of aerosol OP and emphasizes the importance of considering aqueous-phase chemistry when evaluating the health-relevant oxidative properties of ambient particulate matter.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Atmospheric Chemistry and Physics.
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Pang and co-authors present an interesting study on the oxidative potential (OP) generated entirely by organic compounds formed in binary mixtures of glyoxal, methylglyoxal, ammonia, and glycine. The authors show that the composition and oxidative properties of the reaction products vary substantially with pH. They further propose that COOH-substituted N-heterocycles are responsible for ROS formation. While the OP measurements are convincing, the chemical characterisation of the reaction products is incomplete and selectively presented. In my opinion, the manuscript requires major revision before it can be considered for publication.
1) One of my major concerns is that the manuscript does not provide sufficient NMR and LC-MS data to evaluate the reliability of the structural assignments. At minimum, I would expect the authors to provide representative NMR spectra and a summary table of LC-MS results including exact masses, molecular formulae, tentative structures, and Sirius confidence/ranking scores. Without these data, it is difficult to assess what fraction of the detected organic material is represented by the discussed N-containing structures.
In addition, the NMR experiments were performed without removing water and with the addition of a considerable amount of non-deuterated DMSO. Consequently, the HDO and DMSO signals are expected to dominate the spectra and reduce the dynamic range, particularly in neighbouring chemical-shift regions. This limitation should be discussed, as it affects confidence in peak assignment.
2) The limitations of the analytical methodology are not discussed sufficiently. Although the entire reaction mixture was collected, only the ionisable fraction was analysed by LC-MS under the selected ionisation conditions. Consequently, the reported molecular composition represents only a subset of the reaction products. Furthermore, the discussion of individual heterocycle classes is potentially misleading. The reported percentages may be interpreted either as the fraction of identified compounds or as their relative abundance. Since LC-MS peak areas are only semi-quantitative, an increase in peak intensity can reflect an increase in concentration for a given compound, but peak intensities cannot be directly compared between different molecular species because ionisation efficiencies differ substantially. Moreover, the manuscript does not indicate what fraction of assigned molecular formulae could be structurally annotated, making it difficult to evaluate how representative the proposed structures are.
3) I am not fully convinced by the proposed mechanism presented in Figure 6c. First, the authors propose that an N-containing imine intermediate is crucial for the formation of dimethylglyoxal-derived products, yet this intermediate was neither detected nor experimentally supported. Therefore, this part of the mechanism remains speculative. Second, the authors should compare their proposed reaction pathways and identified structures with those reported by Haan et al. (2009).
Finally, I do not believe that the current evidence is sufficient to conclude that COOH-substituted N-heterocycles are directly responsible for OH formation. The presented experiments demonstrate a correlation between the presence of these compounds and increased OP, but they do not exclude alternative explanations. For example, while N-heterocycles may accelerate ascorbate oxidation, the observed OH production could still arise from traces of metals. The observation that several model heterocycles produce OP signals supports their involvement but does not establish a metal-independent mechanism. A control experiment using a strong metal chelator (e.g., EDTA) would substantially strengthen this conclusion. If the reaction is genuinely metal-independent, the addition of such chelators should not significantly affect the measured OP.
Other comments:
1) The authors have a list of tentative structures obtained from the Sirius analysis. I recommend performing a rank-correlation analysis between the peak areas of all assigned structures and both OP measurements. Such an analysis would help identify which molecular species correlate most strongly with oxidative potential. This analysis should include not only N-heterocycles but also oxygenated compounds, as the latter may also contribute to OP. Notably, none of the tested model heterocycles exhibited OP comparable to Cu2+, whereas the synthetic aerosol extracts exceeded the reference.
2) The manuscript lacks appropriate control experiments for both OP and LC-MS analyses. Although the authors state that the parent compounds exhibited negligible OP, it is unclear whether aged glyoxal or methylglyoxal solutions without nitrogen-containing precursors were analysed. Some reported products (e.g., C6H8O3) could potentially form independently of glycine or ammonia.
3) The Experimental section does not specify the aliquot volumes used for NMR, LC-MS, and OP measurements. Were all analyses performed using 200 μL aliquots?
4) The authors used the original AImod expression, although it was later revised by Koch et al. (https://analyticalsciencejournals.onlinelibrary.wiley.com/doi/10.1002/rcm.7433). In the corrected equation, nitrogen contributes to the numerator because it affects the hydrogen deficiency calculation. Alternatively, I recommend avoiding AImod completely in this context, as it assumes that all oxygen atoms are present in carboxyl groups, an approximation that is unlikely to hold for these reaction products.