the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 26 Jan 2026
High Yields of Formic Acid and Acetic Acid during Multi-generational Oxidation of Toluene
Abstract. Formic acid and acetic acid are the most abundant gas-phase organic acids in the atmosphere, yet their concentrations are substantially underestimated by both global and regional atmospheric models across diverse environments. In this study, we report unexpectedly high yields of formic acid and acetic acid formed during the multi-generational photooxidation of toluene, a canonical anthropogenic volatile organic compound. Their yields show a strong dependence on hydroxyl radical (•OH) exposure ([•OH] × residence time), increasing from 25 % and 24 % under low exposure (< 0.2 equivalent days) to 74 % and 40 % under elevated exposure (1–3 equivalent days) for formic and acetic acid, respectively. The formation of these organic acids is not significantly affected by NOx concentrations. A modified box model based on MCM v3.3.1 underestimates the peak concentrations of both acids by approximately a factor of five, indicating substantial gaps in current mechanistic understanding. Although both secondary aerosol formation and organic acid production increase with aging within a certain degree of oxidation, their distinct temporal evolutions indicate that particle photodegradation is not the dominant pathway. The contrasting •OH exposure dependence between organic acids and primary carbonyl compounds further suggests that these acids are predominantly multi-generational oxidation products. These findings demonstrate that multi-generational oxidation of aromatic compounds is an important and previously underappreciated source of atmospheric organic acids. The omission of organic acid formation from aromatic oxidation in current chemical mechanisms likely contributes to their widespread underestimation in models, highlighting the need for detailed laboratory studies and updated chemical mechanisms.
- Preprint
(629 KB) - Metadata XML
-
Supplement
(211 KB) - BibTeX
- EndNote
Status: final response (author comments only)
- RC1: 'Comment on egusphere-2026-234', Anonymous Referee #1, 06 Mar 2026
-
RC2: 'Comment on egusphere-2026-234', Anonymous Referee #2, 08 Mar 2026
This study investigates the formation of formic and acetic acid during the photooxidation of toluene using an oxidation flow reactor (OFR). The authors report unexpectedly high peak molar yields at equivalent atmospheric aging times of 1–3 days. The authors concluded that these findings suggest current chemical mechanisms, such as MCM v3.3.1, which underestimate these yields by a factor of five, are missing significant multi-generational oxidation pathways.
However, the authors did not provide sufficient experimental details for me to examine whether their conclusion is valid. The source of OH radical in the OFR experiments of this study was O3. Its concentration was not reported in the paper.
Usually O3 concentrations in such experiments should be very high (compared to ambient concentrations) to sustain strong OH production. If this is the case in this study, the alkenes formed after the first 1-2 steps of oxidation of toluene are likely also ready to react with O3. Reactivity of the ozonides formed is likely insensitive to NOx. It is possible that ozonides and subsequent products (e.g. Criegee Intermediates) convert into organic acids in the aqueous phase. It is an open question whether the abovementioned processes are relevant to the atmosphere. In any case, the authors should discuss the possibility and quantify their contribution to the observed organic acids.
If O3 concentrations used in the experiments of this study were not very high (as in the OH exposure calibration experiments reported in Figure S3), to achieve high aging, the lamps needed to be very strong, and significantly photolyzed toluene and oxygenated alkenes at 254 nm. These processes have been very poorly constrained. They may or may not lead to organic acid formation, but they are experimental artifacts that should be avoided, or at least deducted.
The authors should report more details of their experiments, carefully examine the possibility of the organic acid formation pathways I discussed above, and update their discussions and conclusion in the paper as needed before this paper can be accepted for publication in ACP.
Specific comments:
Line 111: this OFR is much more tube-like than typical OFRs (e.g. PAM). More details are needed to convince me that its wall losses are negligible.Line 142: I can expect NO and NO2 to play very different roles in toluene oxidation chemistry. The authors should discuss the roles of NO and of NO2 in these experiments separately.
Line 214: the organic acid formation pathways I proposed above both become more important relative to multi-generation OH oxidation at lower RH. None of these reactions directly involve water vapor, but OH is produced from it. More organic acid formed at lower RH is a piece of evidence AGAINST OH oxidation pathways.
Line 225: the authors need to clarify that the decay after 3 days of aging was likely due to the fragmentation of the precursors of formic and acetic acids. The acids react with OH too slowly to be consumed at such a low age.
Line 275: I do not believe that NO:HO2 can reach 1000 with much O3 added, which reacts with NO very rapidly and sustains HO2 concentration through HOx radical recycling. The authors should also report the details of these "high-NOx" experiments.
Line 314: Fittschen et al. (2014) reported a very high overall rate constant but did not conclude that HCOOH is the only C-containing product. Other pathways were also discussed that paper and the branching ratios of different channels were not specified.
Line 364: NO can facilitate glyoxal and methylglyoxal formation through ring-opening. Could their low yields at high exposures simply because almost all NO was oxidized in these experiments?
Technical correction:
Table S1: "Rquivalent" -> "Equivalent" in the first rowCitation: https://doi.org/10.5194/egusphere-2026-234-RC2 -
RC3: 'Comment on egusphere-2026-234', Anonymous Referee #3, 11 Mar 2026
This study investigated the formation of formic acid and acetic acid during multi-generational photooxidation of toluene using an oxidation flow reactor (OFR). The authors found that the yields of these organic acids increased significantly with OH exposure, reaching 74% for formic acid and 40% for acetic acid at 1-3 equivalent atmospheric aging days. This finding challenges the traditional view of relatively low organic acid yields and provides a new explanation for the widespread underestimation of organic acids in atmospheric models. The study design is rigorous, with a wide range of experimental conditions (different RH, OH exposure levels, NOx concentrations), and the data quality is high. However, several issues regarding the generalizability of the conclusions, depth of mechanistic interpretation, and appropriateness of model comparisons need to be clarified or addressed. Until these concerns are adequately resolved, publication in ACP is not recommended.
Major comments
(1) Discussion of "multi-generational oxidation mechanisms" is overly speculative. The title and core conclusion emphasize that "multi-generational oxidation" is key to high organic acid yields, but the paper's discussion of specific chemical mechanisms is relatively weak:
Section 3.3 rules out the dominant roles of SOA photodegradation and known gas-phase pathways (e.g., glyoxal/methylglyoxal oxidation) but does not propose clear multi-generational oxidation pathways. The authors mention that "fragmentation of five-membered oxygen-containing heterocyclic compounds" may yield acetic acid, but note that "this pathway remains controversial." This makes "multi-generational oxidation" sound more like a descriptive label than a mechanistic explanation.
Section 3.2 tests four formic acid formation pathways, with only the CH3O2 + OH pathway contributing significantly, yet the model still underestimates observations by ~5-fold. This suggests the existence of unidentified pathways, but the authors do not further explore potential candidate pathways (e.g., Criegee intermediate chemistry, peroxy radical isomerization). It is recommended to propose more specific mechanistic hypotheses in the discussion based on recent literature, to guide future research.
(2) The observed NOx independence presents a profound conflict with existing mechanistic understanding and is insufficiently explained. The authors report that increasing NO concentrations up to 29 ppbv (corresponding to a NO/HO2 ratio >1000) did not suppress the formation of formic and acetic acid (Section 3.1.2). This finding, while intriguing, presents a fundamental paradox with established atmospheric chemical mechanisms: Traditional atmospheric chemistry holds that organic acids are primarily formed through reactions of RO2 with HO2. Under high-NO conditions, RO2 should preferentially react with NO to form nitrates or carbonyl compounds, which would strongly suppress organic acid formation.
However, authors provide no concrete chemical mechanism to explain how such high yields (up to 74% for formic acid) can persist when traditional RO₂+HO₂ pathways are effectively shut down. The current MCM model underestimates observations by approximately five-fold, confirming that the model captures neither the magnitude nor the NOx-independent nature of the observed production. This represents a critical gap: if the mechanism is truly insensitive to NOx at levels exceeding 1000:1 NO:HO2 ratios, it would require a fundamental rethinking of aromatic oxidation chemistry. Yet the authors offer little speculation on what such a pathway might entail.
Other comments:
(1) In lines 109 - 126, the 2 L small-scale reactor has an extremely high surface-to-volume ratio. Formic acid and acetic acid, as highly polar molecules, may undergo adsorption and reversible release on surfaces, which could significantly affect concentration measurements at the outlet. The authors need to provide recovery rate calibration data (measured or simulated) for these two acids under different humidity conditions.
(2) In lines 386 - 419, the authors do not discuss the potential influence of photolysis. A critical flaw is that important intermediate products of toluene oxidation (such as glyoxal, methylglyoxal, pyruvic acid, etc.) have large absorption cross-sections at 254 nm. In the real atmosphere, these species are primarily degraded via long-wavelength UV photolysis or reaction with OH radicals; however, inside the OFR, the intense 254 nm radiation may induce rapid photolysis of these intermediates and directly release formic acid, leading to artificially enhanced yields. The authors should discuss this potential artifact and, if possible, quantify the contribution of direct photolysis to the observed formic acid production under their experimental conditions. Without such analysis, the reported "multi-generational oxidation" yields may significantly overestimate the true atmospheric formation potential.
(3) The temperature effect should be addressed. What was the internal temperature of the OFR? Did the temperature increase as a function of OH exposure, even with the use of a water jacket?"
(4) Since N2O absorbs a significant amount of photons, have the authors calibrated the OH exposure under high-NOx conditions during N2O injection? The manuscript lacks a description of this. It is important to note that OH exposure at a constant lamp voltage certainly differs significantly between high-NOx and low-NOx conditions. The results shown in Fig. 3 are inconsistent with expectations, suggesting that the same calibration factors may have been applied regardless of whether N2O was injected. If so, this approach would be technically inaccurate, as N2O significantly alters the photolysis environment and OH exposure within the reactor.
(5) Line 167: Please also provide the volume concentrations of the produced SOA.
(6) The wall loss of formic acid, acetic acid, and LVOCs (Low Volatility Organic Compounds) must be accounted for in the yield calculations."
(7) Line 208-209: As shown in Fig. 2, while the decay of toluene increases with OH exposure, it is not completely consumed. A more plausible explanation for the observed organic acid yield curves would be the kinetic competition between acid formation and their subsequent degradation (or other non-acid producing pathways) as OH exposure increases.
Citation: https://doi.org/10.5194/egusphere-2026-234-RC3
Viewed
| HTML | XML | Total | Supplement | BibTeX | EndNote | |
|---|---|---|---|---|---|---|
| 246 | 122 | 24 | 392 | 36 | 15 | 38 |
- HTML: 246
- PDF: 122
- XML: 24
- Total: 392
- Supplement: 36
- BibTeX: 15
- EndNote: 38
Viewed (geographical distribution)
| Country | # | Views | % |
|---|
| Total: | 0 |
| HTML: | 0 |
| PDF: | 0 |
| XML: | 0 |
- 1
The article High Yields of Formic Acid and Acetic Acid during Multi-generational Oxidation of Toluene provides laboratory based evidence for substantial formation of formic acid (FA) and acetic acid (AA) under high OH exposure, equivalent to multi-day atmospheric oxidation. The article is well written, and the main conclusions are generally well supported with figures and discussion. I recommend this article for publication pending a few minor corrections as noted below.
Major Comments:
Although the experimental evidence for FA and AA formation from toluene oxidation is strong, the claim that this pathway could close the budget of missing FA and AA in urban and petroleum emissions needs further discussion or softened. This is particularly true considering that much of the recent literature points to aerosol sources being the main missing FA/AA formation pathway. Two key places to clarify.
What are the limitations imposed by only using one VOC in the flow reactor? Would competitive OH reactions change the yield and/or main conclusions?
Minor Comments:
General: Please provide a brief description of how you calculated % yield from the experiments.
Figure 2 a and d: Why does the % yield start at 20 % and 40 % before any production has occurred? See previous comment about explaining the % yield calculation more clearly.
84: Consider adding Permar et al., 2023 for wildfire emissions (https://doi.org/10.1039/D3EA00098B). This article also supports that FA is rapidly formed in environments with high OH exposure, although the authors see no AA production on the same time scale. The MCM is similarly evaluated and shows the model is missing most FA and AA production.
245: What was the OH exposure in these other studies? How different is it relative to this work?
258-260: Would the presence of other VOCs be expected to similarly decrease yields compared to the toluene only experiments in this work (see major comment)?
261-264: Is there a figure for this?
Figure 5: Can the authors comment on why there is 2x more aerosol mass formed during the 70 % RH experiments relative the 20 % ones? Could this account for some of the lower FA and AA yields at higher RH (Fig 1)?
336-339: Consider moving the clarification at line 352 further up in this discussion as this statement is specific to toluene oxidation and doesn't rule out a broader aerosol pathway under real-world conditions.