From the journal Environmental Science: Atmospheres Peer review history

14-Oct-2021

Dear Dr Hansel:

Manuscript ID: EA-ART-09-2021-000072
TITLE: The INNpinJeR: A new wall-free reactor for studying gas-phase reactions

Thank you for your submission to Environmental Science: Atmospheres, published by the Royal Society of Chemistry. I sent your manuscript to reviewers and I have now received their reports which are copied below.

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************

Reviewer 1

The manuscript by Scholz et al describes the construction and calibration of a flow reactor designed to study gas-phase reactions such as oxidation of VOCs without the influence of heterogeneous processes on reactor walls. The apparatus is characterized by CFD simulations and proof-of-principle experiments are conducted on tetramethylethylene (TME) ozonolysis to validate the performance. The products of TME ozonolysis are detected by an ammonium cluster chemical ionization mass spectrometer to validate the reactor performance. The reactor is described in detail, and attempts to solve a difficult and important challenge: to enable the study of complex atmospheric reactions, possibly leading to condensation and aerosol formation, without any wall effects. I think it has the potential to be a useful experimental platform. However, I have several concerns and suggestions for the authors before publication:

One significant concern is that it is unclear how the chemistry model interacts with the transport model in the reactor simulations. In a laminar flow reactor one can have radial diffusion of reactive species from slow-moving (“old”) gas in the peripheral regions into the fast-moving (“young”) gas along the centerline, strongly influencing the chemistry there. Section 2.3 and the Results imply that CFD simulations are chemistry simulations are uncoupled. It is not clear how the chemistry model is initialized and propagated. Is it a 0D chemistry simulation (numerically forced to a pre-determined VOC concentration field) or a 2D chemistry model? Why are accretion reactions included, but other secondary reactions (like reactions of OH) excluded? I am afraid that if chemistry/transport interactions are neglected, then the simulations are qualitatively incomplete and quantitatively incorrect.

Secondly, I would like to see a better explanation of the difference between this reactor and the TROPOS reactor, which appears very functionally similar and gives similar results. What is the big innovation here that would elevate this paper from a routine new setup to a significant experimental advance?

In addition, I have some minor questions:
- How and at what locations is the temperature in the reactor measured?
- For what molecules exactly is the correction factor of 1.25 used and why is the same factor used for multiple molecules (line 213)? This is not clear from the description of calibration factors either in the main text or in the SI section S1.4.
- In the intro, comparison with other flow reactors would be more useful if T, P, and residence times were stated. For example, the Caltech flow reactor is intended for residence times of many minutes rather than a few seconds, so a direct comparison is not very informative. They Caltech reactor is not intended to solve the same problem as this reactor.
- In lines 325 – 329, Fig 4b, and numerous other places in the paper it is stated that enhanced reactant concentration at the impingement point leads to enhanced products. Although I can see this as justified for the accretion products from RO2 + RO2, it is not clear why this is the case for the primary products acetone and acetonyl peroxy. If anything, there is less [O3] at impingement point than further downstream; if those small products are created by VOC + O3, their production rate should be decreased in the first second or so.
- Fig 2 – green line in 2b and both lines in 2c need to be labeled.
- The whole idea of an “apparent reaction time” seems questionable since this “apparent time” will be different for every chemical system and even for every starting reaction concentration, whereas the flow field parameters in the reactor will hopefully be constant. It seems more helpful to discuss the observed concentrations in terms of a chemical model (which explicitly accounts for non-linear production rate as a function of axial distance) rather than an apparent reaction time.
- Section 3.3 begins with the assertion that the observed [acetone] and [RO2] are nearly equal, but fig. 4a makes in clear that at low TME reaction rate [RO2] is a factor of 2 lower than [acetone] and the discrepancy looks to be systematically dependent on TME reaction rate. Please explain why this may be.
- Fig. 5b does not have anything plotted in it.
- line 497 – 499 – what’s the reasoning behind thinking that higher T in the present reactor is responsible for higher accretion product formation rate coefficient? The accretion products are formed on complex PESs with barrierless initial association and have been suggested to result from ISC – what’s the evidence that the rate coefficient will have a positive T-dependence?

Reviewer 2

Scholz et al. report the first results from a new reactor designed to study gas-phase chemical reactions with minimal wall interactions. The authors present experimental results from two systems, O3 + TME and O3 + cyclohexene, and compare the latter results with those measured in the TROPOS reactor. The results from the new INNpinJeR reactor are in accord with those obtained in the TROPOS reactor, within experimental uncertainties. The authors perform multiple types of modelling simulations to determine the characteristics of the flow in the reactor, and to simulate the effect of [RO2] and [HO2] on measurement of RO2 accretion rates. The authors present a clear explanation of the apparatus, and the tests they have performed to characterize it. However, there are some details in the manuscript that are lacking, specifically related to experimental uncertainties. Some suggestions follow that would make the paper clearer to the reader.

- In the introduction, you state that RO2 isomerization is fast (presumably, you mean under atmospherically pertinent conditions ?). The rate coefficients in the literature for RO2 isomerization vary by orders of magnitude, and so it would be helpful to specify a range of rates that have been determined thus far, or caveat this statement somehow.

- What uncertainty in the RO2 concentration do you anticipate arises from using the hexanone calibration factor for RO2? You mention in the caption of Table 4 that the uncertainty in the accretion rate coefficients is ~ a factor of 2, largely due to the uncertainty in RO2 quantification, but these uncertainties are not clearly stated (or explained) in the manuscript.

- In Figure 6, the axis markers on the y and x axes are different, making the plot somewhat confusing to read. Error bars should be added to the data points.

- There are no error bars on the data presented in Figure 7, nor an uncertainty in the determined rate coefficient given. Similarly, for Figures S2, S9, S10 and S11. Please address this.

- It is not clear what the results of the simulations (Fig. 8) would be if the authors assumed RO2 isomerization rates that are slower than 1 s-1 (which would be consistent with many measured RO2 isomerization rates in the literature). Please can you elaborate on this?

- You find evidence for an RO2 with the chemical formula C5H9O3, but do not suggest a formation pathway. Do you anticipate that C5H9O3 could result from fragmentation of e.g., the C6H11O3 RO2 ?

First of all, we would like to thank both reviewers for the positive reception of our work and the constructive comments that helped to improve the quality of the manuscript.

REVIEWER REPORT(S):
Referee: 1

One significant concern is that it is unclear how the chemistry model interacts with the transport model in the reactor simulations. In a laminar flow reactor one can have radial diffusion of reactive species from slow-moving (“old”) gas in the peripheral regions into the fast-moving (“young”) gas along the centerline, strongly influencing the chemistry there.
Section 2.3 and the Results imply that CFD simulations and chemistry simulations are uncoupled. It is not clear how the chemistry model is initialized and propagated. Is it a 0D chemistry simulation (numerically forced to a pre-determined VOC concentration field) or a 2D chemistry model?
Why are accretion reactions included, but other secondary reactions (like reactions of OH) excluded? I am afraid that if chemistry/transport interactions are neglected, then the simulations are qualitatively incomplete and quantitatively incorrect.

We added a section about a 2D chemistry simulation on a cross section of the 3D steady state flow field in sec. S1.6 in the SI.
With this simulation we show that diffusion of reactive species from slow-moving gas in the peripheral regions into the fast-moving gas along the centerline occurs, but only in the close vicinity of the centerline (< 1 cm distance) and barely influences the final results.

The paragraph also includes a few words on OH (sec. S1.6, lines 117 ff):
“For peroxy radicals formed via OH+VOC, the OH can be calculated similarly, considering its production and loss terms.
We do not discuss the OH reactions in great detail throughout the text, because basically all OH formed from cyclohexene ozonolysis is reacting with cyclohexene again to form the peroxy radical with sumformula HO-C6H10O2, that then undergoes reactions with RO2 and HO2. We already showed in Hansel et al., 2018 that we detect the peroxy radicals with a concentration close to the simulated total OH produced. For the TME + O3 system, reaction conditions are similar.”

Additionally, we added one sentence in the main text (lines 504 ff.):
“The reaction conditions are such that the produced OH is mainly reacting with cyclohexene to form the peroxy radical HO-C6H10O3.”

We have a strong focus on the accretion products because determining their rate coefficients is a crucial test comparing the performances of the two reactors in detail. Also, the accretion rate coefficients have not been published previously.


Secondly, I would like to see a better explanation of the difference between this reactor and the TROPOS reactor, which appears very functionally similar and gives similar results. What is the big innovation here that would elevate this paper from a routine new setup to a significant experimental advance?

We added lines 71 ff. in the Introduction, introducing the differences between the reactors:
“The reactors are similar in that both use only a short reaction time in favor of negligible wall contacts. Therefore, particles have no time to form under atmospherically relevant reactant concentrations and the reactors are not suited to study gas-to-particle conversion. However, the details of the two reactors and the internal flow fields are different: The TROPOS flow system uses one movable free jet carrying ozone that quickly mixes into the surrounding flow (air + VOC) due to its fast velocity and a carefully designed nozzle shape. Its inner metal inlet line is therefore rather long. In our reactor, on the other hand, a fast micromixing followed by a slower mesomixing of the reactants in the center flow is achieved by four freely impinging jets of air carrying the organics of interest. The other reactant (here: ozone) enters the reactor at the very beginning, diluted in the main flow. Thereby the inlet for the radicals is substantially shorter and is made out of glass. Therefore, also HO2 can easily enter the INNpinJeR, which is mostly destroyed in the long metal inlet line of the TROPOS reactor. Additionally, by using four freely impinging jets, the shape of the nozzles is not so critical in our case. The TROPOS reactor requires 100 slpm total flow to produce a stable and efficient mixing. The four impinging jets help us to reduce the overall flow down to 33 slpm. The comparison here will be a critical test of the effects that a different flow system has on the product formation and whether using an "effective reaction time", is in general possible for this type of reactors.”

We also added lines 668 ff. in the Conclusion, discussing the innovative potential of studying RO2 + HO2 reactions in the INNpinJeR. By introducing O3 (HO2) in the beginning of the reactor, we can vary the HO2 concentration as discussed in the SI (sec. S2.3.1, lines 279 ff.), while the TROPOS reactor is designed to keep HO2 small utilizing a long metal inlet.


In addition, I have some minor questions:

- How and at what locations is the temperature in the reactor measured?

We added the following lines in the Methods (lines 163 ff. ):
“The temperature of the reaction gas was measured at the outflow of the reactor. The whole setup, the compressed-air tank, the adsorbers and the whole tubing is at laboratory temperature, so that we expect the whole gas-stream to have the same temperature. We estimate a temperature uncertainty of +/- 2K.”


- For what molecules exactly is the correction factor of 1.25 used and why is the same factor used for multiple molecules (line 213)? This is not clear from the description of calibration factors either in the main text or in the SI section S1.4.

We changed the lines 213 (now 241 ff.) to clarify this issue:
“Because Zaytsev et al. 2019 showed that the sensitivity of hexanone is only 80% of the kinetic limit in ammonium mode, we correct the experimentally obtained calibration factor from hexanone by a factor of 1.25 for each experiment.”

and lines 248ff:
“More functional groups typically increase the bonding strength between the ammonium and the analyte, thus we use the measured hexanone-sensitivity multiplied by a factor of 1.25 to account for the maximum ionization efficiency at the kinetic limit.
For all oxidation products of cyclohexene, including the peroxy radicals we used the hexanone calibration factor, which was corrected by a factor of 1.25 to represent the kinetic limit and therefore the maximum sensitivity. “

Please be aware, that by using the kinetic limit for calibration, we give lower limit concentrations of the products, as stated in line 255:
„Concentrations calculated in this way represent lower limit values.“

To be also clear in the SI, we added in the caption of table S1:
„The calibration factor of decanone represents the kinetic limit sensitivity in ammonium mode according to Zaytsev et al. 2019. This sensitivity was used to convert measured ion signals to concentrations for all larger, more oxidized molecules.“

- In the intro, comparison with other flow reactors would be more useful if T, P, and residence times were stated. For example, the Caltech flow reactor is intended for residence times of many minutes rather than a few seconds, so a direct comparison is not very informative. They Caltech reactor is not intended to solve the same problem as this reactor.

This is entirely correct. Therefore, we deleted that sentence.

Also lines 48 ff now read:
“Lambe et al. found for two oxidation flow reactors to study secondary organic aerosol formation (with reaction times in the order of minutes) that the SOA yield depends on the reactor design.”


- In lines 325 – 329, Fig 4b, and numerous other places in the paper it is stated that enhanced reactant concentration at the impingement point leads to enhanced products. Although I can see this as justified for the accretion products from RO2 + RO2, it is not clear why this is the case for the primary products acetone and acetonyl peroxy. If anything, there is less [O3] at impingement point than further downstream; if those small products are created by VOC + O3, their production rate should be decreased in the first second or so.

In order to clarify this, we added a plot in the SI (fig. S2 a) and adjusted the lines 360 ff in the main text to:
„Ozone is uniformly distributed over the entire volume of the flow tube and its deviation from the final steady-state concentration is therefore smaller than that of the VOC. This imbalance close to the impingement point leads to enhanced product formation rates (see fig. S2 a) in the first centimeters. “

- Fig 2 – green line in 2b and both lines in 2c need to be labeled.
We added colored labels for better readability.


- The whole idea of an “apparent reaction time” seems questionable since this “apparent time” will be different for every chemical system and even for every starting reaction concentration, whereas the flow field parameters in the reactor will hopefully be constant. It seems more helpful to discuss the observed concentrations in terms of a chemical model (which explicitly accounts for non-linear production rate as a function of axial distance) rather than an apparent reaction time.

As the name „apparent“ reaction time might imply that it is somehow variable or uncertain, which is not the case, we changed the term to „effective“ reaction time.
We explain the concept of the effective reaction time now in a longer paragraph in lines 405 ff. in the main text.

Furthermore, we used the results from the 2D chemistry simulations for two very different products (Acetone or RO2 without wall losses and ROOR) to determine the effective reaction times (sec. S1.6, lines 102 ff.) and show the results in the updated fig. 4B in the main text. and also changed lines 441 ff. to:
“In fig. 4.b) the effective reaction time, determined from simulated product concentrations (see sec. S1.6, is plotted against the residence time in the reactor. The determined effective reaction times are very similar for both products (see table 2). The ROOR from the RO2 + RO2 accretion reaction gives a slightly smaller effective reaction time. We chose these very different products as extreme examples because they are formed from precursors with different concentration distributions over the reactor. The effective reaction times for both compounds are nonetheless very similar (and within the experimental uncertainties), which demonstrates that the effective reaction time is a useful parameter to summarize the flow and mixing effects in this type of reactors. The nonlinearity of the effective reaction time can be attributed to the enhanced reactant concentrations close to the impingement point and varying diffusional losses along the flow direction coordinate z. Please find more details in sec. S1.6.”
and lines 466 ff.:
“… we get a theoretical effective reaction time of 9.0-9.3 seconds in very good agreement with the measured value. This comparison shows, that using the effective reaction time as a parameter for the 0D box model calculations for the reactor, will not infer a large error. “

Summarizing, the effective reaction time parametrizes the transport conditions in the reactor quite well. We showed, that even for two molecules that have differently distributed direct precursors, the effective reaction time is only slightly different in our reactor.
It is therefore valid for any chemical system and different reactant concentrations, as long as the flow conditions remain and reactants are used highly diluted so that they do not affect the flow field and the diffusivity.


- Section 3.3 begins with the assertion that the observed [acetone] and [RO2] are nearly equal, but fig. 4a makes it clear that at low TME reaction rate [RO2] is a factor of 2 lower than [acetone] and the discrepancy looks to be systematically dependent on TME reaction rate. Please explain why this may be.

In fig. 4a we show the experimental data from two experiments (TME (a) and TME (b)). While the second one gave very good results, the first experiment suffered from a higher acetone background. While we corrected the data for that background, at low TME reaction rates they are still more uncertain in the first experiment. This we acknowledge by larger error bars for those data points. The discrepancy becomes smaller, because the uncertainty of the acetone background is less relevant at higher reaction rates (and it is a log-log plot).

We changed lines 433 ff to:
“In fig. 4 a) we plot measured concentrations of the two ozonolysis products (determined using the acetone calibration factor for both compounds and correcting for background signals and radical wall losses in the short inlet line) against the TME reaction rate. “


- Fig. 5b does not have anything plotted in it.
That must have been a compatibility issue. We avoid it now by including that image as a png file.


- line 497 – 499 – what’s the reasoning behind thinking that higher T in the present reactor is responsible for higher accretion product formation rate coefficient? The accretion products are formed on complex PESs with barrierless initial association and have been suggested to result from ISC – what’s the evidence that the rate coefficient will have a positive T-dependence?
Thank you for noticing. We agree and deleted that sentence.



Referee: 2

Comments to the Author
Scholz et al. report the first results from a new reactor designed to study gas-phase chemical reactions with minimal wall interactions. The authors present experimental results from two systems, O3 + TME and O3 + cyclohexene, and compare the latter results with those measured in the TROPOS reactor. The results from the new INNpinJeR reactor are in accord with those obtained in the TROPOS reactor, within experimental uncertainties. The authors perform multiple types of modelling simulations to determine the characteristics of the flow in the reactor, and to simulate the effect of [RO2] and [HO2] on measurement of RO2 accretion rates. The authors present a clear explanation of the apparatus, and the tests they have performed to characterize it. However, there are some details in the manuscript that are lacking, specifically related to experimental uncertainties. Some suggestions follow that would make the paper clearer to the reader.

- In the introduction, you state that RO2 isomerization is fast (presumably, you mean under atmospherically pertinent conditions ?). The rate coefficients in the literature for RO2 isomerization vary by orders of magnitude, and so it would be helpful to specify a range of rates that have been determined thus far, or caveat this statement somehow.

Yes, we wanted to keep it short, but it‘s important, especially in connection with your 2nd-last comment. We therefore added the following lines as a paragraph in the introduction (lines 6 ff.):

“Recently, RO2 radical isomerization, followed by the incorporation of O2, referred as autoxidation, has been discovered to play an important role in the atmosphere (Crounse et al. 2013 and Ehn et al. 2014), leading to higher-functionalized RO2 radicals.
Theoretically calculated rates of intramolecular hydrogen-shifts are structure dependent with rate coefficients covering orders of magnitude (10⁻⁶ – 10² s⁻¹) (Møller et al., 2020, Møller et al., 2019). Under atmospheric conditions these rates start to compete against bimolecular reactions with NO, HO2 or other peroxy radicals (R'O2) when the effective rate of isomerization is larger than 0.01 s-1. Fast isomerization rates in the order of ~1 s⁻¹ or higher can lead to the formation of highly oxidized organic molecules (HOMs) even in polluted environments (Bianchi et al., 2019 and Wennberg, et al., 2018) as is the case for some of the peroxy radicals from different alkenes like isoprene (Wennberg et al, 2018), monoterpenes or also cyclohexene (Berndt et al., 2015), as well as for some bicyclic peroxy radicals (Wang et al., 2017) and certainly many other compounds. “


- What uncertainty in the RO2 concentration do you anticipate arises from using the hexanone calibration factor for RO2? You mention in the caption of Table 4 that the uncertainty in the accretion rate coefficients is ~ a factor of 2, largely due to the uncertainty in RO2 quantification, but these uncertainties are not clearly stated (or explained) in the manuscript.

We use the sensitivity of hexanone multiplied by 1.25 (correction factor, from Zaytsev et al.2019) to determine the kinetic limit sensitivity (see SI, sec. S1.4, lines 27 ff.).
We added a whole paragraph to discuss all error margins in section S1.4 (lines 38 ff.) that lead to the uncertainty discussion of the accretion reaction coefficients in the new section S1.4.1.

from lines 38ff. in S1.4:
“Because the products of cyclohexene have a similar carbon backbone as hexanone, but have more functional groups, we expect that the uncertainty of the detection efficiency does not exceed +/-20 %.”


- In Figure 6, the axis markers on the y and x axes are different, making the plot somewhat confusing to read. Error bars should be added to the data points.

We have updated the figure with error bars and a grid for better readability.


- There are no error bars on the data presented in Figure 7, nor an uncertainty in the determined rate coefficient given. Similarly, for Figures S2, S9, S10 and S11. Please address this.

We have updated the figures with error bars based upon the calculations in sec. S1.4.1
(The numbering changed in the SI because we included new figures. They are now fig. S4, S11-S13)

- It is not clear what the results of the simulations (Fig. 8) would be if the authors assumed RO2 isomerization rates that are slower than 1 s-1 (which would be consistent with many measured RO2 isomerization rates in the literature). Please can you elaborate on this?

We added a short section in the SI (sec. S2.5) and added the lines 596 ff. in the main text:
“For our simulations we assume fast RO2 isomerization rates (r > 1s⁻¹, tau << t_eff = 9.4 s) in accordance with results from Berndt et al., 2015 for peroxy radicals from cyclohexene ozonolysis, so that the peroxy radicals increase nearly parallel within the reactor as their ratio is determined within the first second. Therefore, the effect of isomerization on accretion product formation rates is small for the cyclohexene system. In sec. S2.5 we shortly discuss uncertainties in case of unknown isomerization rates.”


- You find evidence for an RO2 with the chemical formula C5H9O3, but do not suggest a formation pathway. Do you anticipate that C5H9O3 could result from fragmentation of e.g., the C6H11O3 RO2 ?

Yes, we do not want to speculate how it is formed here.
But most parts of the signal are unlikely due to a fragment of C6H11O3, because C5H9O3 is in large amounts (80%) formed from ozonolysis (while C6H11O3 is produced only by OH) as tested using a propane scavenger. In addition, C5H9O3 forms in reaction with NO the corresponding organonitrate, and dimerization products of the peroxy radicals with other known peroxy radicals are clearly measured. Therefore, we expect the compound to be at least partly a peroxy radical. This is described in sec. S2.1.

Furthermore, we performed a collision induced dissociation voltage ramp (CID ramp) to see, if we are able to fragment the peroxy radical ammonium clusters. We observed that their fragmentation takes place at significantly higher voltage settings than what is needed to fragment the NH4+-water cluster, demonstrating that the binding energy between the peroxy-radicals and ammonium is higher than that of the NH4+-water cluster. These experiments show that under normal (soft) voltage settings peroxy-radicals ammonium clusters do not break up in our instrument.

We added the data of the CID ramp in fig. S6 as well as the lines 149 ff in the SI:
“By performing ramps of the collision-induced dissociation voltages, we did not observe a decay of any of the larger peroxy radicals, that might have fragmented on the exact mass of C5H9O3 before the ammonium-water clusters decay. This suggests, that the peroxy radicals themselves as well as their ammonium clusters are stable enough to survive the ionization and transfer into the TOF.”

13-Dec-2021

Dear Dr Hansel:

Manuscript ID: EA-ART-09-2021-000072.R1
TITLE: The INNpinJeR: A new wall-free reactor for studying gas-phase reactions

Thank you for submitting your revised manuscript to Environmental Science: Atmospheres. After considering the changes you have made, I am pleased to accept your manuscript for publication in its current form. I have copied any final comments from the reviewer(s) below.

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Reviewer 2

The authors have satisfactorily addressed the points raised in the original reviews, and this manuscript should be accepted for publication.

Reviewer 1

The authors have addressed my initial comments fully and convincingly, so I recommend publishing the paper in its current form. Thank you.