Criegee + HONO reaction: the dominant sink of Criegee, and the missing non-photolytic source of OH&bull;

Kumar, Pradeep; Anand, Vishva Jeet; Rai, Philips Kumar

doi:10.5194/egusphere-2025-1364

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https://doi.org/10.5194/egusphere-2025-1364

Preprints

24 Apr 2025

| 24 Apr 2025

Criegee + HONO reaction: the dominant sink of Criegee, and the missing non-photolytic source of OH•

Pradeep Kumar, Vishva Jeet Anand, and Philips Kumar Rai

Abstract. One of the most important puzzles in atmospheric chemistry is a mismatch between observed and modeled concentrations of OH•/HO•₂ in the presence of high concentrations of volatile organic compounds. It is now well established that to fulfill this gap, one needs a reaction that is not only capable of producing OH• but also able to act as a sink of HO•₂. In the present work, we are proposing the Criegee + HONO reaction as a possible solution of this puzzle. Our quantum chemical and kinetic calculations clearly suggest that this reaction can not only be an important source of OH radicals but can also act as a sink of HO₂ radicals. Our study also suggests that HONO has the potential to become the most dominant sink of CI surpasses SO₂ and water dimer, even in highly humid conditions.

Received: 22 Mar 2025 – Discussion started: 24 Apr 2025

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Journal article(s) based on this preprint

25 Nov 2025

Criegee + HONO reaction: a bimolecular sink of Criegee, and the missing non-photolytic source of OH^•

Vishva Jeet Anand, Philips Kumar Rai, and Pradeep Kumar

Atmos. Chem. Phys., 25, 16713–16727, https://doi.org/10.5194/acp-25-16713-2025,https://doi.org/10.5194/acp-25-16713-2025, 2025

Short summary

Pradeep Kumar, Vishva Jeet Anand, and Philips Kumar Rai

Interactive discussion

Status: closed

RC1: 'Comment on egusphere-2025-1364', Anonymous Referee #1, 02 Jun 2025

The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-1364/egusphere-2025-1364-RC1-supplement.pdf

Citation: https://doi.org/10.5194/egusphere-2025-1364-RC1
RC2:
'Comment on egusphere-2025-1364', Anonymous Referee #2, 26 Jun 2025
As suggested by the title, the authors explore the possibility of, and conclude that, the HONO + Criegee reaction acting is the dominant sink for Criegee and is the missing non-photolytic source of OH. Their actual results do not properly justify these strong statements of importance. Their own results show that the first part is greatly overstated, while the second part remains speculative given the fairly limited nature of their simulations. There are also significant shortcomings in the methodologies that could result in significant errors. Improved discussion of many aspects is needed.
The following shortcomings in the rate analysis should be addressed:
The description of their rate calculations seems to imply that they evaluate the overall rate constant as a product of a separately calculated rate for the formation of RC1 and a master equation calculation for the branching in the thermal dissociation of RC1 (between forward reaction, kuni, and back dissociation to reactants). This product of terms would be appropriate if RC1 was being formed in the high pressure limit. But it most certainly is not. In this case, the master equation should instead be used to directly obtain the rate constant for proceeding from the reactants to the bimolecular products.

There is also some possibility that the PCi complexes are collisionally stabilized since they are fairly deep wells on their PESs. Such stabilization would be important as it would reduce the rate of forming OH. Thus, properly formulated master equations should include the PCi complexes and some rate for their decomposition.

The authors refer to a KTOOLS code for estimating the formation rate. The authors should also briefly describe the physical assumption behind the calculation in KTOOLS. This point is significant because their formation rates appear to be about an order of magnitude less than what would be expected.

The wells and TSs (and perhaps the reactants) appear to have hindered rotational modes, some of which might have multiple distinct minima. At the very least, the authors should describe how they treated those torsional motions, and whether or not they searched for multiple torsional minima to ensure they had found the global minimum conformational states.

There authors discussion of their electronic structure calculations needs various clarifications:
It appears that the TS energy reported here for CH₂OO + HONO is about 8 kcal/mol below what was reported in an earlier report from the same group. This is rather odd since the electronic structure methods are very similar. Should I presume that the uncertainty in the energy is truly that large. Some comment on this discrepancy is needed, and ideally the authors would provide some indication of the expected uncertainty in their energies.

It is well known that CH₂OO has significant multireference character that often disappears in the TSs for its reaction. This commonly results in about a 1 kcal/mol raising of the barrier heights relative to CCSD(T)/CBS estimates. Some comment on this shortcoming in their estimates would be helpful.

The authors claim that 0.04 angstrom geometry errors clearly suggest that M062X geometries are accurate. For this statement to be true, the authors should provide some estimate of how large an error could arise from such bond length errors. In principle, that is straightforward from some consideration of typical force constants. Simply stating that the geometry errors are small is not helpful. Similarly, the authors claim that 250 cm^-1 frequency errors imply that M062X is appropriate for frequency calculations. From my experience, those sorts of frequency errors are extraordinarily large, and would make me wonder if I had done something wrong. My expectation is that they could yield order of magnitude sorts of errors in the predicted rates. Some more appropriate discussion of the meaning of those shortcomings is needed.

The discussion of the meaning of their calculated rates also needs improvement:
The focus on just the bimolecular rate constants in their discussion of the effective rate constants is misleading. For the (CH₃)₂COO case, the unimolecular decomposition rate near room temperature is about 400 s^-1, which swamps their effective bimolecular rates.

With that in mind, their suggestion that the CI + HONO reactions are the major sink for the CI requires some indication as to how rapidly the unimolecular decay rates decrease with temperature.

It would be helpful to have some estimate of the expected uncertainty in their rate predictions.

The actual HONO concentration used in their keff calculations should be explicitly stated.

The model simulations are limited enough in scope that I consider them to be highly speculative at best.
Citation: https://doi.org/10.5194/egusphere-2025-1364-RC2
AC1: 'Comment on egusphere-2025-1364', Pradeep Kumar, 03 Aug 2025

The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-1364/egusphere-2025-1364-AC1-supplement.pdf

Citation: https://doi.org/10.5194/egusphere-2025-1364-AC1
AC2: 'Comment on egusphere-2025-1364', Pradeep Kumar, 03 Aug 2025

The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-1364/egusphere-2025-1364-AC2-supplement.pdf

Citation: https://doi.org/10.5194/egusphere-2025-1364-AC2

Interactive discussion

Status: closed

RC1: 'Comment on egusphere-2025-1364', Anonymous Referee #1, 02 Jun 2025

The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-1364/egusphere-2025-1364-RC1-supplement.pdf

Citation: https://doi.org/10.5194/egusphere-2025-1364-RC1
RC2:
'Comment on egusphere-2025-1364', Anonymous Referee #2, 26 Jun 2025
As suggested by the title, the authors explore the possibility of, and conclude that, the HONO + Criegee reaction acting is the dominant sink for Criegee and is the missing non-photolytic source of OH. Their actual results do not properly justify these strong statements of importance. Their own results show that the first part is greatly overstated, while the second part remains speculative given the fairly limited nature of their simulations. There are also significant shortcomings in the methodologies that could result in significant errors. Improved discussion of many aspects is needed.
The following shortcomings in the rate analysis should be addressed:
The description of their rate calculations seems to imply that they evaluate the overall rate constant as a product of a separately calculated rate for the formation of RC1 and a master equation calculation for the branching in the thermal dissociation of RC1 (between forward reaction, kuni, and back dissociation to reactants). This product of terms would be appropriate if RC1 was being formed in the high pressure limit. But it most certainly is not. In this case, the master equation should instead be used to directly obtain the rate constant for proceeding from the reactants to the bimolecular products.

There is also some possibility that the PCi complexes are collisionally stabilized since they are fairly deep wells on their PESs. Such stabilization would be important as it would reduce the rate of forming OH. Thus, properly formulated master equations should include the PCi complexes and some rate for their decomposition.

The authors refer to a KTOOLS code for estimating the formation rate. The authors should also briefly describe the physical assumption behind the calculation in KTOOLS. This point is significant because their formation rates appear to be about an order of magnitude less than what would be expected.

The wells and TSs (and perhaps the reactants) appear to have hindered rotational modes, some of which might have multiple distinct minima. At the very least, the authors should describe how they treated those torsional motions, and whether or not they searched for multiple torsional minima to ensure they had found the global minimum conformational states.

There authors discussion of their electronic structure calculations needs various clarifications:
It appears that the TS energy reported here for CH₂OO + HONO is about 8 kcal/mol below what was reported in an earlier report from the same group. This is rather odd since the electronic structure methods are very similar. Should I presume that the uncertainty in the energy is truly that large. Some comment on this discrepancy is needed, and ideally the authors would provide some indication of the expected uncertainty in their energies.

It is well known that CH₂OO has significant multireference character that often disappears in the TSs for its reaction. This commonly results in about a 1 kcal/mol raising of the barrier heights relative to CCSD(T)/CBS estimates. Some comment on this shortcoming in their estimates would be helpful.

The authors claim that 0.04 angstrom geometry errors clearly suggest that M062X geometries are accurate. For this statement to be true, the authors should provide some estimate of how large an error could arise from such bond length errors. In principle, that is straightforward from some consideration of typical force constants. Simply stating that the geometry errors are small is not helpful. Similarly, the authors claim that 250 cm^-1 frequency errors imply that M062X is appropriate for frequency calculations. From my experience, those sorts of frequency errors are extraordinarily large, and would make me wonder if I had done something wrong. My expectation is that they could yield order of magnitude sorts of errors in the predicted rates. Some more appropriate discussion of the meaning of those shortcomings is needed.

The discussion of the meaning of their calculated rates also needs improvement:
The focus on just the bimolecular rate constants in their discussion of the effective rate constants is misleading. For the (CH₃)₂COO case, the unimolecular decomposition rate near room temperature is about 400 s^-1, which swamps their effective bimolecular rates.

With that in mind, their suggestion that the CI + HONO reactions are the major sink for the CI requires some indication as to how rapidly the unimolecular decay rates decrease with temperature.

It would be helpful to have some estimate of the expected uncertainty in their rate predictions.

The actual HONO concentration used in their keff calculations should be explicitly stated.

The model simulations are limited enough in scope that I consider them to be highly speculative at best.
Citation: https://doi.org/10.5194/egusphere-2025-1364-RC2
AC1: 'Comment on egusphere-2025-1364', Pradeep Kumar, 03 Aug 2025

The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-1364/egusphere-2025-1364-AC1-supplement.pdf

Citation: https://doi.org/10.5194/egusphere-2025-1364-AC1
AC2: 'Comment on egusphere-2025-1364', Pradeep Kumar, 03 Aug 2025

The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-1364/egusphere-2025-1364-AC2-supplement.pdf

Citation: https://doi.org/10.5194/egusphere-2025-1364-AC2

Peer review completion

AR – Author's response | RR – Referee report | ED – Editor decision | EF – Editorial file upload

AR by Pradeep Kumar on behalf of the Authors (03 Aug 2025) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (12 Aug 2025) by Kelvin Bates

RR by Stephen Klippenstein (25 Aug 2025)

Suggestions for revision or reasons for rejection

The authors have responded effectively to some of the concerns, but by no means all of the concerns, of the prior referee reports. It requires further major revision before it can be considered appropriate for publication.

As pointed out by the first reviewer the present work only suggests that the Criegee + HONO reaction is the dominant sink of Criegee for (CH3)2COO and not at all for CH2OO. And even for (CH3)2COO it is only dominant at best at temperatures below 225 K. For CH2OO, Fig. 3 shows that the CH2OO + SO2 reaction is always a larger sink than the title reaction. For (CH3)2COO, the unimolecular loss is the dominant sink except perhaps below 225 K. The authors need to make a much more serious effort to remove all of their overstatements. For example, the title needs to be revised, the concluding statements need to be further revised, and the plot in Fig. 4 must include the unimolecular loss rate. The authors must reread the whole manuscript to make sure that any such overstatements have been removed. It is important for a paper to have a correct physical interpretation and tone in addition to correct data.

The authors need to either improve their discussion of why they believe the M06-2X/aug-cc-pVTZ is an effective approach for determining the stationary point geometries and vibrational frequencies, or simply remove all of that discussion. An error in the geometry of 0.04 A converts to a 2 kcal/mol error for a typical force constant of 2000 N/m. Is that what the authors deem an appropriate error for an accurate geometry? Similarly, an error of 250 cm-1 in the frequency correlates with anywhere from a few % error to a factor of two or more error in the partition function (depending on which frequency is in error by that amount). Is a factor of two error in the rate prediction an indication that the method is adequate for kinetics calculations? And that is for just one vibrational mode. What is the cumulative effect of such errors for many modes? Oddly, I actually believe that the M06-2X/aug-cc-pVTZ is a suitable enough method for predicting the kinetics for this reaction, but the authors arguments are so poorly presented that one can readily come to the opposite conclusion. A correct statement would be that, prior literature studies suggest 2sigma uncertainties of ~2 kcal/mol in CCSD(T)/CBS//M06-2X barrier heights and factor of two sorts of uncertainties in partition function ratios. It would be much better to simply say something like that.

The description of the VTST calculations with KTOOLS is missing key details. Ordinarily a VTST calculation requires some description of both the potential energies along some form of a minimum energy path and the vibrational frequencies for the orthogonal motions along the reaction path. The latter are not described at all. Furthermore, the authors provide no indication of the electronic structure method that was used to map the PES. Nor do they describe the method used to scan the PES – is it some sort of distinguished reaction coordinate analysis?

The ILT for the reaction from the PC to the products also needs some description of the presumed temperature dependence.

It’s nice to know that the torsional scans did not find lower global minimal conformers. Nevertheless, some description should still be provided regarding how extensive your search for conformers was initially. Was some manual search of the conformational space performed? Did you just accept whatever conformer was first found? …

Why would the L-J parameters for a CH2OO…HONO complex correspond to the average of those for CH2OO and HONO. Shouldn’t they correspond more closely to some sort of sum of the parameters. Certainly the size of the complex is not the average of the individual sizes of the components. This treatment should be revised and the master equation calculations repeated.

On. p. 5 – why do you describe the reaction as occurring in two steps? Isn’t it three steps – formation of RC1, passage through TS1 to form PC1, then decomposition of PC1 to products. This proper physical description as three steps is important because there may be some stabilization of PC1.
On a related note, the authors should mention whether or not there is there any stabilization of either reactant or product complexes? With 45 kcal/mol exothermicity and 27 kcal/mol bond dissociation energies there is particularly some possibility of the product complexes being stabilized.

On line 146 RC should be RC1. On Line 158 RC should be RC2.

The capture rates reported in Table S3 are still smaller than I would have naively guessed by about a factor of 4. This is likely an artifact of some limitation in the VTST calculation, but since those are not described in appropriate detail it is hard to judge what the issue is.

A proper citation (i.e., likely to one of his websites) should be provided to the actual source of the Ruscic et al. thermochemical values for the reaction energy.

It appears that the rate calculations were performed without the CCSDT(Q) correction. Why? Do the authors somehow believe that including that correction would not improve the accuracy of the predictions. If so, they should state that.

The discussion of the uncertainties on p. 6 lacks any discussion of the uncertainties arising from inadequate partition function evaluations. I would estimate that such uncertainties are at least a factor of two and probably more like a factor of 4. Indeed, as I mention above, I expect that the true capture rate would probably be a factor of four larger, and almost certainly a factor of two larger. It appears that their uncertainty analysis does not consider the uncertainties in this capture rate. Furthermore, I strongly suspect that the barrier height 2 sigma uncertainties are more like 2 kcal/mol. Also, since the more relevant part of the prediction is for low temperature, they should indicate how the uncertainties change with decreasing temperature.

The paragraph on p. 7 starting on line 219 first misleads the reader to think that the CH2OO + HONO is dominant at combined low temperature and low RH and then corrects that to indicate that CH2OO + SO2 is always dominant over CH2OO + HONO. Just start off with the proper statement that CH2OO + HONO is never the dominant sink, or even major sink (it looks like the maximum is about 20% at about 240 K and low RH). Then the details can be discussed. That is the proper primary finding of your calculations and the more clearly it is stated the better.

Table S6 is central to the authors argument that the HONO + CH32COO is the dominant sink at low temperature. Their data for the unimolecular dissociation of (CH3)2COO in that Table needs some citation as to the source of the data. That data also needs to be directly plotted in Fig. 4. Then the discussion of Fig. 4 on p. 8 should again start with the primary conclusion that only below 225 K is the CH32COO + HONO reaction predicted to be the dominant sink of CH32COO. Then more details can be provided.

Again, the statement in the conclusion “By comparing it with other known sinks of CI, we have shown that this reaction can serve as a major sink for Criegee intermediates in most of the atmospheric conditions” is not in fact true.

Hide

RR by Anonymous Referee #1 (25 Aug 2025)

ED: Reconsider after major revisions (29 Aug 2025) by Kelvin Bates

AR by Pradeep Kumar on behalf of the Authors (08 Oct 2025) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (08 Oct 2025) by Kelvin Bates

RR by Stephen Klippenstein (13 Oct 2025)

Suggestions for revision or reasons for rejection

I have only minor further suggestions for improvement. These suggestions are of a pedagogical nature and are not expected to affect their results.

Although it is not clearly stated in the manuscript, it seems that the authors don’t seem to see any stabilization of the RCs or the PCs. In that case, the values used for the LJ parameters are not particularly relevant to the conclusions of this paper. Nevertheless, to avoid future confusion for other authors, I note that the LJ parameters reported here are strange, and the description of how they are calculated seems incomplete. The words suggest that they directly calculate the LJ parameters for the RC…N2 interaction. But then, how can the size, sigma (which presumably represents the distance between the center-of-mass of the RC and N2 at the minimum of the complex) be only 2.6 Angstroms, when the separation between the CI and the HONO in the RC itself is on the order of 3 Angstroms? This makes no sense. Also, if they are directly calculating the LJ parameters for the RC…N2 interaction then why do they report values for the bath gas alone. Those values shouldn’t even enter into the kinetic analysis.

The authors should report what value they presumed or calculated (I still can’t tell if it was calculated or presumed - it seems like it must be the latter since no data is provided for its MEP) for the capture rate for the process from products to the PC complexes. Or do they not include this process in their master equation and simply presume the PC complexes dissociate?

At line 269 the authors state that “keff increases only slightly”. I think this should say “keff decreases only slightly” since the context of the sentence is for increasing temperature.

The left-hand y-axes in Figure 5 need some improvement.

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ED: Publish subject to minor revisions (review by editor) (13 Oct 2025) by Kelvin Bates

AR by Pradeep Kumar on behalf of the Authors (15 Oct 2025) Author's response Author's tracked changes Manuscript

ED: Publish as is (15 Oct 2025) by Kelvin Bates

AR by Pradeep Kumar on behalf of the Authors (17 Oct 2025) Author's response Manuscript

Journal article(s) based on this preprint

25 Nov 2025

Criegee + HONO reaction: a bimolecular sink of Criegee, and the missing non-photolytic source of OH^•

Vishva Jeet Anand, Philips Kumar Rai, and Pradeep Kumar

Atmos. Chem. Phys., 25, 16713–16727, https://doi.org/10.5194/acp-25-16713-2025,https://doi.org/10.5194/acp-25-16713-2025, 2025

Short summary

Pradeep Kumar, Vishva Jeet Anand, and Philips Kumar Rai

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https://doi.org/10.5194/egusphere-2025-1364-supplement

Pradeep Kumar, Vishva Jeet Anand, and Philips Kumar Rai

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In this work, we studied the reaction of simple and dimethyl-substituted Criegee with HONO. By comparing it with other known sinks of CI, we have shown that this reaction can serve as a major sink for Criegee intermediates, even in the presence of high humidity and SO₂. The reaction can be a key nocturnal source of OH•, capable of HO•₂ ↔ OH• recycling. Consequently, this reaction can be key in fulfilling the gap between the observed and modeled concentration of OH radicals.


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