the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 22 Nov 2024
Kinetics of the reactions of OH with CO, NO, NO2 and of HO2 with NO2 in air at 1 atm pressure, room temperature and tropospheric water vapour concentrations
Abstract. The termolecular reactions of hydroxyl radicals (OH) with carbon monoxide (CO), nitric oxide (NO) and nitrogen dioxides (NO2) and the termolecular reaction of hydroperoxyl radicals (HO2) with NO2 greatly impact the atmospheric oxidation efficiency. Few studies have directly measured the pressure dependent rate coefficients in air at 1 atm pressure and water vapour as third collision partners. In this work, rate coefficients were measured with a high accuracy (<5 %) at 1 atm pressure, room temperature and in humidified air using laser flash photolysis and detection of the radical decay by laser-induced fluorescence. The rate coefficients derived in dry air are: (2.39±0.11) · 10-13 cm3 s-1 for the OH reaction with CO, (7.3±0.4) · 10-12 cm3 s-1 for the OH reaction with NO, (1.23±0.04) · 10-11 cm3 s-1 for the OH reaction with NO2, and (1.56±0.05) · 10-12 cm3 s-1 for the HO2 reaction with NO2. For the OH reactions with CO and NO, no dependence on water vapour was observed for the range of water partial pressures tested (3 to 22 hPa), and for NO2, only a weak increase of 3 % was measured in agreement with the study by Amedro et al. (2020). For the rate coefficient of HO2 with NO2 an enhancement of up to 25 % was observed. This can be explained by a faster rate coefficient of the reaction of the HO2-water complex with NO2 having a value of (3.4±1.1) · 10-12 cm3 s-1.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Atmospheric Chemistry and Physics. There are no other conflicts to declare.
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.-
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RC1: 'Comment on egusphere-2024-3550', Anonymous Referee #1, 09 Dec 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-3550/egusphere-2024-3550-RC1-supplement.pdf
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AC1: 'Reply on RC1', Hendrik Fuchs, 27 Jan 2025
We thank the reviewer for the positive comments.
Comment: Line 18: remove ) from “in which HO2) “
Response: We changed the text accordingly.
Comment: Line 97: for completeness you might want to cite a very recent paper describing a new FAGE-type instrument for measuring OH and HO2 kinetics at high pressures: “New Instrument for Time-Resolved OH and HO2 Quantification in High-Pressure Laboratory Kinetics Studies, Leonid Sheps and Kendrew Au, JPC A (2024), doi:10.1021/acs.jpca.4c00994” You might also want to cite the following paper which uses a FAGE instrument to study the water dependence of a rate constant under atmospheric conditions, similar to the current work: “Water Vapor does not Catalyze the Reaction between Methanol and OH Radicals, W. Chao et al., Angewandte Chemie International Edition (2019), doi:10.1002/anie.201900711”
Response: We added the suggested references.
Comment: Line 146: when you give the number for the detection limit (1e6 cm-3) in 1 minute, is this the detection limit for the pulsed generation, which you use in this paper, i.e. the detection limit after averaging over 60 laser pulses or is this the detection limit for continuous OH generation?
Response: The mentioned limit of detection in the order of 106 cm−3 in 1 atm of air with a measurement time of 1 min refers to field measurements of ambient OH concentrations (Lou et al., 2010). In the present work, OH decays were averaged over typically 1000 photolysis laser pulses, resulting in an effective integration time of 1s for photon counting in each 1ms time bin. The corresponding limit of detection is in the order of 7-8 x 106 cm−3. To clarify what this means for the OH reactivity measurement, we added: “In the present, study, in which OH reactivity is measured, typically 1000 OH decays were averaged resulting in an integration time for photon counting of about 1s for each 1ms time bin. The resulting limit of detection (< 10^7 cm−3) made it possible to follow the decays over 2 to 3 orders of magnitude, allowing an accurate fit of the decay time."
Comment: Line 210: Concerning the zero-air loss of both radicals, it is surprising that both radicals give the same loss rate. It would be expected that the HO2 loss is slower due to lower reactivity and also lower diffusion compared to OH. Any idea what might be the reason?
Response: A full quantitative model description of the radical wall loss in our flow tube has not been attempted and is not required to obtain the results presented. However, some plausibility considerations can be made. The volume irradiated by the photolysis laser beam and filled with radicals has a diameter of 3.0 cm. According to Einstein's relation the diffusion length within the radical lifetime (1/k0) is 0.69 cm for OH and 0.56 cm for HO2, which is less than the radius (1.5 cm) of the photolysis volume. It means that not all radicals in the sampled volume are subject to diffusive loss, which reduces the sensitivity of k0 to differences in the diffusion coefficients. In addition, it is possible that small turbulences produced at the gas inlet of the photolysis tube, have not fully decayed in the laminar tube flow and enhance lateral transport. Such eddy diffusion would be the same for OH and HO2. Since the wall loss is mainly limited by transport, differences in wall surface reactivity of OH and HO2 should not play a role.
Comment: Line 296: 2.5 hPa is probably 22.5 hPa?
Response: Thanks for noticing the typo. We changed the text accordingly.
Comment: Line 384: (3.3+/-2.2) %, I imagine?
Response: Thanks for noticing the missing unit. We changed the text accordingly.
Comment: Line 435: Even though the absolute calibration of HO2 is not really important for this work, I wonder how sure you are that the FAGE is equally sensitive to the complexed and the free HO2, because there is no clear information in the cited reference Cho et al 2023
Response: We agree that the work of Cho et al. (2023) does not provide an explicit explanation. We have therefore deleted the reference. The relevant information can be found in Fuchs et al. (2011) which was also referenced in the original manuscript (line 438).
Citation: https://doi.org/10.5194/egusphere-2024-3550-AC1
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AC1: 'Reply on RC1', Hendrik Fuchs, 27 Jan 2025
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RC2: 'Comment on egusphere-2024-3550', Anonymous Referee #2, 18 Dec 2024
The manuscript describes a study of the kinetics of OH radical reactions with CO, NO, and NO2 and of HO2 + NO2 using a modified instrument designed for field measurements of OH reactivity. Experiments were performed at 1 atm and room temperature, and as a function of relative. The experiments are well described and results are presented well. Comparisons are made to previous studies under similar conditions, and the high precision of the experimental approach is noted. In some cases, discrepancies were found between results of this work and some previous studies, and between current IUPAC and JPL-NASA recommendations, but the discussion of such discrepancies is somewhat limited and suggestions are made for further measurements.
The manuscript provides useful data for the atmospheric modelling community. I have only minor comments, listed below, which should be addressed prior to publication.
Line 2: ‘hydroperoxy’ is generally preferred over ‘hydroperoxyl’.
Line 9 (and elsewhere): It would be helpful to give the relative humidity as well as the partial pressure of water.
Line 14: ‘reactions of the OH’ to ‘reactions of the OH radical’ or ‘reactions of OH’.
Line 17: ‘HO2)’.
Line 49: HOx has already been defined.
Line 67: What are the uncertainties in the recommendations? Do the factors of 1.3 and 1.8 fall within the combined uncertainties?
Lines 83-93: Are the details regarding OH reactivity relevant to this work beyond the dual use of the instrument?
Lines 107 & 177: A residence time of 1.8 s and 266 nm pulse repetition frequency of 1 Hz will result in gas mixtures being photolysed more than once, leading to potential for photolysis of reaction products. Please comment on any tests or model simulations that were performed to ensure there was no impact of photolysis of reaction products.
Line 133: The Q1(3) line is less intense than Q1(2), is there any specific reason the Q1(3) transition was selected?
Line 141: Please quantify ‘almost the same detection sensivity’.
Line 147: Is the sensitivity of 1 min measurements relevant here? What is the sensitivity for the time resolution and number of averages typically used in this study?
Line 204: What is the cause of the deviation from single exponential behaviour for t < 10 ms in experiments measuring OH and not HO2? There appears to be some growth in signal in the examples given in Figure 2, although the scale makes this difficult to see, is there any growth evident and, if so, what is the cause?
Lines 215-218: Do the observed k0 values show agreement with values expected from diffusion?
Line 220: The dynamic range is relatively low compared to those typically used in other studies. While the potential advantages of low reactant/reagent concentrations are discussed, are there any potential disadvantages that are introduced by a low dynamic range?
Line 403: While the change in NO2 concentration may be relatively small, is there any potential impact of the NO or O formed by photolysis of NO2?
Figure 11: The caption refers to OH reactivity, should this be HO2 reactivity here?
Citation: https://doi.org/10.5194/egusphere-2024-3550-RC2 -
AC2: 'Reply on RC2', Hendrik Fuchs, 27 Jan 2025
We thank the reviewer for the positive comments.
Comment: Line 2: ‘hydroperoxy’ is generally preferred over ‘hydroperoxyl’.
Response: We changed the text accordingly.
Comment: Line 9 (and elsewhere): It would be helpful to give the relative humidity as well as the partial pressure of water.
Response: We added relative humidity values in the description of the experiments.
Comment: Line 14: ‘reactions of the OH’ to ‘reactions of the OH radical’ or ‘reactions of OH’.
Response: We changed the text accordingly.
Comment: Line 17: ‘HO2)’.
Response: We changed the text accordingly.
Comment: Line 49: HOx has already been defined.
Response: We cancelled the definition in Line 49.
Comment: Line 67: What are the uncertainties in the recommendations? Do the factors of 1.3 and 1.8 fall within the combined uncertainties?
Response: The uncertainties of the values are listed and discussed in the respective sections of the manuscript. In Line 67 we added a short statement.
Comment: Lines 83-93: Are the details regarding OH reactivity relevant to this work beyond the dual use of the instrument?
Response: We have the feeling that a 1-sentence statement about the relevance of OH reactivity measurements in field experiments is appropriate for an introduction although we agree that this is not relevant for the reported work.
Comment: Lines 107 & 177: A residence time of 1.8 s and 266 nm pulse repetition frequency of 1 Hz will result in gas mixtures being photolysed more than once, leading to potential for photolysis of reaction products. Please comment on any tests or model simulations that were performed to ensure there was no impact of photolysis of reaction products.
Response: The formed products (CO2, HONO, HNO3, HO2NO2) are exposed to only one photolysis laser shot. Of the products, only the nitrogen containing compounds absorb at 266 nm. They have absorption cross sections in the order of 10-19 to 10-20 cm2, which is much smaller than that for O3 (~ 10-17 cm2). In the experiments, less than 10-3 product molecules will absorb a photon under the given conditions. Their concentrations are at most as high as the initial OH concentration of (2 - 9) x 109 cm-3. Thus, the photolysis of the OH reaction products results in photolysis product concentrations of less than 107 cm-3, whose OH reactivity is completely negligible even at the highest possible OH reaction rate constant. We added in Line 156: “including the potential photolysis of the OH reaction products by the 266nm radiation”.
Comment: Line 133: The Q1(3) line is less intense than Q1(2), is there any specific reason the Q1(3) transition was selected?
Response: The absorption cross sections are similar. There is no particular reason for the selection.
Comment: Line 141: Please quantify ‘almost the same detection sensitivity’.
Response: We added “within 5%”.
Comment: Line 147: Is the sensitivity of 1 min measurements relevant here? What is the sensitivity for the time resolution and number of averages typically used in this study?
Response: See our response to Referee #1 who asked a related question.
Comment: Line 204: What is the cause of the deviation from single exponential behaviour for t < 10 ms in experiments measuring OH and not HO2? There appears to be some growth in signal in the examples given in Figure 2, although the scale makes this difficult to see, is there any growth evident and, if so, what is the cause?
Response: In fact, a rapid increase in the OH signal is observed before the signal decreases slowly exponentially. Several factors play a role in the beginning. Before the photolysis laser pulse, the OH concentration in the detection cell is initially zero. After the photolysis pulse, OH is swept into the detection cell and the OH signal builds up over a period of 1-2 ms. Another effect influencing the initial phase are possible reflections of the photolysis beam at the metallic inlet nozzle. The reflections probably lead to an enhancement of the OH density near the nozzle. Furthermore, inhomogeneities due to the photolysis laser beam profile are conceivable, which are homogenized by subsequent diffusion. However, this does not affect the single exponential behaviour, as this portion of the data is not used for the fitting process and thus does not influence the observed kinetics. We added in Line 204: “due to inhomogeneities in the initial OH concentration”
Comment: Lines 215-218: Do the observed k0 values show agreement with values expected from diffusion?
Response: The displacement of OH radicals is 0.7 cm within the OH lifetime in zero air in the flow tube. This is similar to the mean lateral distance needed to leave the reaction volume. Therefore, there is consistency with the diffusion rate and the observed zero loss rate.
Comment: Line 220: The dynamic range is relatively low compared to those typically used in other studies. While the potential advantages of low reactant/reagent concentrations are discussed, are there any potential disadvantages that are introduced by a low dynamic range?
Response: We agree that the applied reactivities in our work are relatively small compared to values used in other kinetics studies. Our method has one disadvantage that we mentioned in line 167 in the original manuscript. Since the measurements take place on a relatively slow time scale of 1 s, there is a higher possibility of secondary unimolecular reactions becoming more important. This is explained in detail in Section 3.3 with respect to the unimolecular decay of the association reaction product HOONO. We add a corresponding reference in line 167.
Comment: Line 403: While the change in NO2 concentration may be relatively small, is there any potential impact of the NO or O formed by photolysis of NO2?
Response: Any NO formed by phtotolysis would react with HO2 forming OH, but this is not an additional HO2 loss as the OH radical produced reacts back to HO2 in the presence of the excess CO in the flow tube during this experiment. Oxygen atoms rapidly react with oxygen molecules forming ozone in the presence of high oxygen concentrations like in our experiments. As ozone is already present in a much higher concentration, this would not affect our results. We added in Line 403: “Although NO produced in the photolysis of NO2 can react with HO2, this does not affect the observed HO2 decay because the OH produced reacts back to HO2 in the reaction with excess CO.”
Comment: Figure 11: The caption refers to OH reactivity, should this be HO2 reactivity here?
Response: We thank the reviewer for noticing this typo and corrected the text in the caption accordingly.
Citation: https://doi.org/10.5194/egusphere-2024-3550-AC2
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AC2: 'Reply on RC2', Hendrik Fuchs, 27 Jan 2025
Interactive discussion
Status: closed
-
RC1: 'Comment on egusphere-2024-3550', Anonymous Referee #1, 09 Dec 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-3550/egusphere-2024-3550-RC1-supplement.pdf
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AC1: 'Reply on RC1', Hendrik Fuchs, 27 Jan 2025
We thank the reviewer for the positive comments.
Comment: Line 18: remove ) from “in which HO2) “
Response: We changed the text accordingly.
Comment: Line 97: for completeness you might want to cite a very recent paper describing a new FAGE-type instrument for measuring OH and HO2 kinetics at high pressures: “New Instrument for Time-Resolved OH and HO2 Quantification in High-Pressure Laboratory Kinetics Studies, Leonid Sheps and Kendrew Au, JPC A (2024), doi:10.1021/acs.jpca.4c00994” You might also want to cite the following paper which uses a FAGE instrument to study the water dependence of a rate constant under atmospheric conditions, similar to the current work: “Water Vapor does not Catalyze the Reaction between Methanol and OH Radicals, W. Chao et al., Angewandte Chemie International Edition (2019), doi:10.1002/anie.201900711”
Response: We added the suggested references.
Comment: Line 146: when you give the number for the detection limit (1e6 cm-3) in 1 minute, is this the detection limit for the pulsed generation, which you use in this paper, i.e. the detection limit after averaging over 60 laser pulses or is this the detection limit for continuous OH generation?
Response: The mentioned limit of detection in the order of 106 cm−3 in 1 atm of air with a measurement time of 1 min refers to field measurements of ambient OH concentrations (Lou et al., 2010). In the present work, OH decays were averaged over typically 1000 photolysis laser pulses, resulting in an effective integration time of 1s for photon counting in each 1ms time bin. The corresponding limit of detection is in the order of 7-8 x 106 cm−3. To clarify what this means for the OH reactivity measurement, we added: “In the present, study, in which OH reactivity is measured, typically 1000 OH decays were averaged resulting in an integration time for photon counting of about 1s for each 1ms time bin. The resulting limit of detection (< 10^7 cm−3) made it possible to follow the decays over 2 to 3 orders of magnitude, allowing an accurate fit of the decay time."
Comment: Line 210: Concerning the zero-air loss of both radicals, it is surprising that both radicals give the same loss rate. It would be expected that the HO2 loss is slower due to lower reactivity and also lower diffusion compared to OH. Any idea what might be the reason?
Response: A full quantitative model description of the radical wall loss in our flow tube has not been attempted and is not required to obtain the results presented. However, some plausibility considerations can be made. The volume irradiated by the photolysis laser beam and filled with radicals has a diameter of 3.0 cm. According to Einstein's relation the diffusion length within the radical lifetime (1/k0) is 0.69 cm for OH and 0.56 cm for HO2, which is less than the radius (1.5 cm) of the photolysis volume. It means that not all radicals in the sampled volume are subject to diffusive loss, which reduces the sensitivity of k0 to differences in the diffusion coefficients. In addition, it is possible that small turbulences produced at the gas inlet of the photolysis tube, have not fully decayed in the laminar tube flow and enhance lateral transport. Such eddy diffusion would be the same for OH and HO2. Since the wall loss is mainly limited by transport, differences in wall surface reactivity of OH and HO2 should not play a role.
Comment: Line 296: 2.5 hPa is probably 22.5 hPa?
Response: Thanks for noticing the typo. We changed the text accordingly.
Comment: Line 384: (3.3+/-2.2) %, I imagine?
Response: Thanks for noticing the missing unit. We changed the text accordingly.
Comment: Line 435: Even though the absolute calibration of HO2 is not really important for this work, I wonder how sure you are that the FAGE is equally sensitive to the complexed and the free HO2, because there is no clear information in the cited reference Cho et al 2023
Response: We agree that the work of Cho et al. (2023) does not provide an explicit explanation. We have therefore deleted the reference. The relevant information can be found in Fuchs et al. (2011) which was also referenced in the original manuscript (line 438).
Citation: https://doi.org/10.5194/egusphere-2024-3550-AC1
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AC1: 'Reply on RC1', Hendrik Fuchs, 27 Jan 2025
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RC2: 'Comment on egusphere-2024-3550', Anonymous Referee #2, 18 Dec 2024
The manuscript describes a study of the kinetics of OH radical reactions with CO, NO, and NO2 and of HO2 + NO2 using a modified instrument designed for field measurements of OH reactivity. Experiments were performed at 1 atm and room temperature, and as a function of relative. The experiments are well described and results are presented well. Comparisons are made to previous studies under similar conditions, and the high precision of the experimental approach is noted. In some cases, discrepancies were found between results of this work and some previous studies, and between current IUPAC and JPL-NASA recommendations, but the discussion of such discrepancies is somewhat limited and suggestions are made for further measurements.
The manuscript provides useful data for the atmospheric modelling community. I have only minor comments, listed below, which should be addressed prior to publication.
Line 2: ‘hydroperoxy’ is generally preferred over ‘hydroperoxyl’.
Line 9 (and elsewhere): It would be helpful to give the relative humidity as well as the partial pressure of water.
Line 14: ‘reactions of the OH’ to ‘reactions of the OH radical’ or ‘reactions of OH’.
Line 17: ‘HO2)’.
Line 49: HOx has already been defined.
Line 67: What are the uncertainties in the recommendations? Do the factors of 1.3 and 1.8 fall within the combined uncertainties?
Lines 83-93: Are the details regarding OH reactivity relevant to this work beyond the dual use of the instrument?
Lines 107 & 177: A residence time of 1.8 s and 266 nm pulse repetition frequency of 1 Hz will result in gas mixtures being photolysed more than once, leading to potential for photolysis of reaction products. Please comment on any tests or model simulations that were performed to ensure there was no impact of photolysis of reaction products.
Line 133: The Q1(3) line is less intense than Q1(2), is there any specific reason the Q1(3) transition was selected?
Line 141: Please quantify ‘almost the same detection sensivity’.
Line 147: Is the sensitivity of 1 min measurements relevant here? What is the sensitivity for the time resolution and number of averages typically used in this study?
Line 204: What is the cause of the deviation from single exponential behaviour for t < 10 ms in experiments measuring OH and not HO2? There appears to be some growth in signal in the examples given in Figure 2, although the scale makes this difficult to see, is there any growth evident and, if so, what is the cause?
Lines 215-218: Do the observed k0 values show agreement with values expected from diffusion?
Line 220: The dynamic range is relatively low compared to those typically used in other studies. While the potential advantages of low reactant/reagent concentrations are discussed, are there any potential disadvantages that are introduced by a low dynamic range?
Line 403: While the change in NO2 concentration may be relatively small, is there any potential impact of the NO or O formed by photolysis of NO2?
Figure 11: The caption refers to OH reactivity, should this be HO2 reactivity here?
Citation: https://doi.org/10.5194/egusphere-2024-3550-RC2 -
AC2: 'Reply on RC2', Hendrik Fuchs, 27 Jan 2025
We thank the reviewer for the positive comments.
Comment: Line 2: ‘hydroperoxy’ is generally preferred over ‘hydroperoxyl’.
Response: We changed the text accordingly.
Comment: Line 9 (and elsewhere): It would be helpful to give the relative humidity as well as the partial pressure of water.
Response: We added relative humidity values in the description of the experiments.
Comment: Line 14: ‘reactions of the OH’ to ‘reactions of the OH radical’ or ‘reactions of OH’.
Response: We changed the text accordingly.
Comment: Line 17: ‘HO2)’.
Response: We changed the text accordingly.
Comment: Line 49: HOx has already been defined.
Response: We cancelled the definition in Line 49.
Comment: Line 67: What are the uncertainties in the recommendations? Do the factors of 1.3 and 1.8 fall within the combined uncertainties?
Response: The uncertainties of the values are listed and discussed in the respective sections of the manuscript. In Line 67 we added a short statement.
Comment: Lines 83-93: Are the details regarding OH reactivity relevant to this work beyond the dual use of the instrument?
Response: We have the feeling that a 1-sentence statement about the relevance of OH reactivity measurements in field experiments is appropriate for an introduction although we agree that this is not relevant for the reported work.
Comment: Lines 107 & 177: A residence time of 1.8 s and 266 nm pulse repetition frequency of 1 Hz will result in gas mixtures being photolysed more than once, leading to potential for photolysis of reaction products. Please comment on any tests or model simulations that were performed to ensure there was no impact of photolysis of reaction products.
Response: The formed products (CO2, HONO, HNO3, HO2NO2) are exposed to only one photolysis laser shot. Of the products, only the nitrogen containing compounds absorb at 266 nm. They have absorption cross sections in the order of 10-19 to 10-20 cm2, which is much smaller than that for O3 (~ 10-17 cm2). In the experiments, less than 10-3 product molecules will absorb a photon under the given conditions. Their concentrations are at most as high as the initial OH concentration of (2 - 9) x 109 cm-3. Thus, the photolysis of the OH reaction products results in photolysis product concentrations of less than 107 cm-3, whose OH reactivity is completely negligible even at the highest possible OH reaction rate constant. We added in Line 156: “including the potential photolysis of the OH reaction products by the 266nm radiation”.
Comment: Line 133: The Q1(3) line is less intense than Q1(2), is there any specific reason the Q1(3) transition was selected?
Response: The absorption cross sections are similar. There is no particular reason for the selection.
Comment: Line 141: Please quantify ‘almost the same detection sensitivity’.
Response: We added “within 5%”.
Comment: Line 147: Is the sensitivity of 1 min measurements relevant here? What is the sensitivity for the time resolution and number of averages typically used in this study?
Response: See our response to Referee #1 who asked a related question.
Comment: Line 204: What is the cause of the deviation from single exponential behaviour for t < 10 ms in experiments measuring OH and not HO2? There appears to be some growth in signal in the examples given in Figure 2, although the scale makes this difficult to see, is there any growth evident and, if so, what is the cause?
Response: In fact, a rapid increase in the OH signal is observed before the signal decreases slowly exponentially. Several factors play a role in the beginning. Before the photolysis laser pulse, the OH concentration in the detection cell is initially zero. After the photolysis pulse, OH is swept into the detection cell and the OH signal builds up over a period of 1-2 ms. Another effect influencing the initial phase are possible reflections of the photolysis beam at the metallic inlet nozzle. The reflections probably lead to an enhancement of the OH density near the nozzle. Furthermore, inhomogeneities due to the photolysis laser beam profile are conceivable, which are homogenized by subsequent diffusion. However, this does not affect the single exponential behaviour, as this portion of the data is not used for the fitting process and thus does not influence the observed kinetics. We added in Line 204: “due to inhomogeneities in the initial OH concentration”
Comment: Lines 215-218: Do the observed k0 values show agreement with values expected from diffusion?
Response: The displacement of OH radicals is 0.7 cm within the OH lifetime in zero air in the flow tube. This is similar to the mean lateral distance needed to leave the reaction volume. Therefore, there is consistency with the diffusion rate and the observed zero loss rate.
Comment: Line 220: The dynamic range is relatively low compared to those typically used in other studies. While the potential advantages of low reactant/reagent concentrations are discussed, are there any potential disadvantages that are introduced by a low dynamic range?
Response: We agree that the applied reactivities in our work are relatively small compared to values used in other kinetics studies. Our method has one disadvantage that we mentioned in line 167 in the original manuscript. Since the measurements take place on a relatively slow time scale of 1 s, there is a higher possibility of secondary unimolecular reactions becoming more important. This is explained in detail in Section 3.3 with respect to the unimolecular decay of the association reaction product HOONO. We add a corresponding reference in line 167.
Comment: Line 403: While the change in NO2 concentration may be relatively small, is there any potential impact of the NO or O formed by photolysis of NO2?
Response: Any NO formed by phtotolysis would react with HO2 forming OH, but this is not an additional HO2 loss as the OH radical produced reacts back to HO2 in the presence of the excess CO in the flow tube during this experiment. Oxygen atoms rapidly react with oxygen molecules forming ozone in the presence of high oxygen concentrations like in our experiments. As ozone is already present in a much higher concentration, this would not affect our results. We added in Line 403: “Although NO produced in the photolysis of NO2 can react with HO2, this does not affect the observed HO2 decay because the OH produced reacts back to HO2 in the reaction with excess CO.”
Comment: Figure 11: The caption refers to OH reactivity, should this be HO2 reactivity here?
Response: We thank the reviewer for noticing this typo and corrected the text in the caption accordingly.
Citation: https://doi.org/10.5194/egusphere-2024-3550-AC2
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AC2: 'Reply on RC2', Hendrik Fuchs, 27 Jan 2025
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Michael Rolletter
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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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