the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 11 Apr 2022
Radical chemistry at a UK coastal receptor site – Part 1: observations of OH, HO2, RO2, and OH reactivity and comparison to MCM model predictions
Abstract. OH, HO2, total and partially-speciated RO2, and OH reactivity (k’OH) were measured during the July 2015 ICOZA (Integrated Chemistry of OZone in the Atmosphere) project that took place at a coastal site in North Norfolk, UK. Maximum measured daily OH, HO2, and total RO2 radical concentrations were in the range 2.6–17 × 106, 0.75–4.2 × 108, and 2.3–8.0 × 108 molecule cm−3, respectively. k'OH ranged from 1.7 to 17.6 s−1 with a median value of 4.7 s−1. ICOZA data were split by wind direction to assess differences in the radical chemistry between air that had passed over the North Sea (NW–SE sectors) or major urban conurbations such as London (SW sector). A photostationary steady-state (PSS) calculation underpredicted daytime OH in NW–SE air by ~35 %, whereas agreement (~15 %) was found within instrumental uncertainty (~26 % at 2σ) in SW air. A box model using MCMv3.3.1 chemistry was in better agreement with the OH measurements, but it overpredicted HO2 observations in NW–SE air in the afternoon by a factor of ~2–3, although slightly better agreement was found for HO2 in SW air (factor of ~1.4–2.0 underprediction). The box model severely underpredicted total RO2 observations in both NW–SE and SW air by factors of ~8–9 on average. Measured radical and k’OH levels and measurement-to-model ratios displayed strong dependences on NO mixing ratios. The PSS calculation could capture OH observations at high NO but underpredicted the observations at low NO. The box model overpredicted HO2 concentrations at low NO in NW–SE air, whereas in SW air, the measurements and model results were in agreement across the full NO range. The box model underpredicted total RO2 at all NO levels, where the measurement-to-model ratio scaled with NO. This trend has been found in all previous field campaigns in which total RO2 was measured using the ROxLIF technique and suggests that peroxy radical chemistry is not well understood under high NOx conditions.
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Interactive discussion
Status: closed
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RC1: 'Comment on acp-2022-207', Anonymous Referee #1, 01 May 2022
The authors report measurements of radicals at a coastal site in the UK. In general, campaigns with a full set of radical measurements are sparse, so that further exploration of radical concentrations in different chemical conditions are valuable. The paper has a companion paper investigating the chemical budgets of radicals. In this manuscript the authors focus on the description of measurements and model-measurement comparisons. A large part of the manuscript is very descriptive, also in the discussion part, which puts the results into the context of results from other campaigns reported literature. Little new results are shown in the sense of improving the understanding radical chemistry in the atmosphere. Therefore, this manuscript rather fits a measurement report instead of a research article. It should be considered to change the manuscript category.
The authors need to improve the manuscript by a concise writing. It is not clear, if separte papers for the measurements / model reszlts and chemical budget would have been required, if the authors had carefully planned to focus the writing of new findings and had cut on lengthy descriptions of figures that can be easily grasped by seeing the figures. In addition. there are some sections, in which it is not clear, if there is a deeper meaning of the analysis that is shown or if the analysis has only be done, because a similar analysis has been done in other papers and therefore, these sections could have been omitted. The manuscript as it is written now clearly suffers from having the interpretation of model results and of the chemical budgets separated in 2 papers due to the close connection between both. Specifical the PSS calculations for OH shown in this paper is essentially the same as doing a chemical budget presented in the other paper. Merging the 2 papers would clearly be the best to present the insights into radical chemistry and likely also possible, because parts of the papers are similar, since the same data set is analysed and results from each paper is described in the other paper, and descriptive and unnecessary parts can be omitted.
The presentation quality of figures also needs considerable improvements. Font sizes in most of the figures contain are too small to be readable and scaling of axis are not appropriate. Light colours of text as used in the current figures are not suitable for reading (e.g. yellow).
Additional specific comments:
L50: For this type of paper, just showing H-abstraction to form RO2 may oversimplifying the chemistry. Overall, some of the text-book like introduction may not be required.
L84: I assume that you mean “their photolysis can also be important radical sources”
L94: The conclusion in Novelli et al. is not that Criegee intermediates are the reason for interferences observed in the field, because reactant concentrations in their work were much higher than atmospheric concentration.
L144: Are you sure that the purity of NO was 99.95%? Typically the best purity that is available is only 99.5%.
Fig. 2 It may be a good idea to improve visibility by splitting the figure into 2 panels by time.
L330: The statement about HONO is rather short and does not really reflect the high variability that is observed. On some days, values during the day were even higher than during the night.
L384: Looking at the entire time series, the second peak that appears in the median diel profile looks more like an artefact of the median calculation than a real feature of the diel profile as it sounds in this statement.
L411: I would avoid giving information in the text that is repeating what can be seen in the legend of the figure
Fig 7: Here, it may make sense to have the same scale of the y-axis for sRO2 and cRO2.
Fig. 8: Labels of the pie chart are not easy to read. Names may need further explanation in the figure caption. Numbers of fraction could be useful as well as the total median RO2 concentration.
L430 ff: Is the relative abundance of specific RO2 radicals consistent with measured OH reactants? This should be discussed.
Section 3.2: RO2: It looks like an offset between measured and modelled simple RO2. Can you exclude that there is an unaccounted instrumental offset?
Section 3.3: This analysis does not give much insights as it is done here. More discussion and comparison with previous findings with interpretation of different and similar results would be needed.
Section 3.4: Again, there is little interpretation or discussion of the correlation and it is not clear what is learned from this analysis. What is the meaning of the different slopes? What is expected for what reason?
L465: The offset does not necessarily indicate that some RO2 sources (= type of RO2 radicals) do not form HO2 as it sounds in the statement. It can also be that the lifetimes of HO2 and RO2 were much different or RO2 loss channels did not lead to HO2 formation, but the RO2 from all sources may still generally form HO2. If there were mainly RO2 sources in the night but little HO2 present, why would you expect that there is a correlation between RO2 and HO2, when the reaction of RO2+NO as most important pathway to HO2 is not relevant in the night?
Section 3.6 / Figure 15: Would you expect that an exponential behaviour of BVOC emissions is visible for the range of temperature that is experienced in the campaign? Can you make an estimate, how much RO2 concentrations will change, if you assume additional VOCs in the model to account for the gap between measured and modelled OH reactivity?
Section 4.4: It would be good to have some numbers of reactive halogen species that would be required to explain observations.
Citation: https://doi.org/10.5194/acp-2022-207-RC1 - AC1: 'Response to Reviewer 1', Dwayne Heard, 16 Jan 2023
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RC2: 'Comment on acp-2022-207', Anonymous Referee #2, 22 May 2022
This paper presents measurements of OH, HO2, RO2, and total OH reactivity at a coastal site during the 2015 ICOZA (Integrated Chemistry of OZone in the Atmosphere) campaign. The authors compare the measurements to predictions by both a photostationary state model as well as a zero-dimensional model based on the Master Chemical Mechanism (MCM 3.3.1). The authors find that in general the MCM model was able to reproduce the measured OH concentrations during the campaign, but overpredicted the measured concentrations of HO2 under lower NOx conditions when air arrived to the site from the northwest-southeast sectors, while underpredicting the measurements when more polluted air arrived to the site from the southwest sector. The authors also found that the model underpredicted the measured RO2 concentrations for both lower and higher NOx air that arrived from all sectors. The authors also find that the measured total OH reactivity was consistently greater than that calculated by the model.
The measurements described add to a growing dataset that suggest that our understanding of radical chemistry under a range of conditions may be incomplete, and as a result are of interest to the atmospheric chemistry community. The results are consistent with several previous measurements, and the authors suggest several possible reasons for the model discrepancies based on these previous results, including missing halogen chemistry and autooxidation of RO2 radicals reducing the rate of conversion to HO2 radicals. Unfortunately, the impact of these proposals on their model results are not included in this paper, as they are discussed in the companion paper. While the companion paper focuses on the impact of their proposed mechanisms on the radical budgets, this paper would benefit from some additional discussion of the impact of the proposed mechanisms on the modeled radical concentrations.
Specifically, the authors should consider including their model results when they reduced the rate of the RO2 +NO propagation rate as discussed in sections 4.4 and 4.5 as it appears that a reduction in this rate, perhaps due to the competition of RO2 autooxidation with radical propagation, improves the agreement with the measured HO2 and RO2 concentrations. While including an expanded discussion of the model results would add to an already lengthy manuscript, the authors should also consider condensing and or moving some of the discussion of previous measurements into a supplement.
Additional comments:
1) The authors conducted interference measurements during two different periods, finding that unknown interferences contributed less than 20% to the measured OH signal. It appears that these measurements occurred during both NW-SE and SW periods. Did the authors see a significant difference in the interference measurements from the different wind sectors?
2) The authors should consider highlighting the NW–SE and SW periods on Figure 5 to help illustrate the impact of the different air masses on the radical measurements.
3) Given that the main focus of the paper is on the measurement/model discrepancy of the radical concentrations, there are several sections and figures in the paper that could be moved to a supplement to improve readability. In particular, sections 3.3 and 3.4 along with figures 9 and 10 could be moved to a supplement.
4) The authors could also condense much of the discussion of previous measurements by including a table summarizing the previous measurements/model agreement under the different NO conditions and referencing the table in the discussion.
5) As mentioned above, the authors should consider adding the reduced RO2+NO model results to Figures 5-7 to illustrate how this model improves the agreement with the measurements. This illustration of the impact of the reduced rate is not included in the companion paper.
Citation: https://doi.org/10.5194/acp-2022-207-RC2 - AC2: 'Reply on RC2', Dwayne Heard, 16 Jan 2023
Interactive discussion
Status: closed
-
RC1: 'Comment on acp-2022-207', Anonymous Referee #1, 01 May 2022
The authors report measurements of radicals at a coastal site in the UK. In general, campaigns with a full set of radical measurements are sparse, so that further exploration of radical concentrations in different chemical conditions are valuable. The paper has a companion paper investigating the chemical budgets of radicals. In this manuscript the authors focus on the description of measurements and model-measurement comparisons. A large part of the manuscript is very descriptive, also in the discussion part, which puts the results into the context of results from other campaigns reported literature. Little new results are shown in the sense of improving the understanding radical chemistry in the atmosphere. Therefore, this manuscript rather fits a measurement report instead of a research article. It should be considered to change the manuscript category.
The authors need to improve the manuscript by a concise writing. It is not clear, if separte papers for the measurements / model reszlts and chemical budget would have been required, if the authors had carefully planned to focus the writing of new findings and had cut on lengthy descriptions of figures that can be easily grasped by seeing the figures. In addition. there are some sections, in which it is not clear, if there is a deeper meaning of the analysis that is shown or if the analysis has only be done, because a similar analysis has been done in other papers and therefore, these sections could have been omitted. The manuscript as it is written now clearly suffers from having the interpretation of model results and of the chemical budgets separated in 2 papers due to the close connection between both. Specifical the PSS calculations for OH shown in this paper is essentially the same as doing a chemical budget presented in the other paper. Merging the 2 papers would clearly be the best to present the insights into radical chemistry and likely also possible, because parts of the papers are similar, since the same data set is analysed and results from each paper is described in the other paper, and descriptive and unnecessary parts can be omitted.
The presentation quality of figures also needs considerable improvements. Font sizes in most of the figures contain are too small to be readable and scaling of axis are not appropriate. Light colours of text as used in the current figures are not suitable for reading (e.g. yellow).
Additional specific comments:
L50: For this type of paper, just showing H-abstraction to form RO2 may oversimplifying the chemistry. Overall, some of the text-book like introduction may not be required.
L84: I assume that you mean “their photolysis can also be important radical sources”
L94: The conclusion in Novelli et al. is not that Criegee intermediates are the reason for interferences observed in the field, because reactant concentrations in their work were much higher than atmospheric concentration.
L144: Are you sure that the purity of NO was 99.95%? Typically the best purity that is available is only 99.5%.
Fig. 2 It may be a good idea to improve visibility by splitting the figure into 2 panels by time.
L330: The statement about HONO is rather short and does not really reflect the high variability that is observed. On some days, values during the day were even higher than during the night.
L384: Looking at the entire time series, the second peak that appears in the median diel profile looks more like an artefact of the median calculation than a real feature of the diel profile as it sounds in this statement.
L411: I would avoid giving information in the text that is repeating what can be seen in the legend of the figure
Fig 7: Here, it may make sense to have the same scale of the y-axis for sRO2 and cRO2.
Fig. 8: Labels of the pie chart are not easy to read. Names may need further explanation in the figure caption. Numbers of fraction could be useful as well as the total median RO2 concentration.
L430 ff: Is the relative abundance of specific RO2 radicals consistent with measured OH reactants? This should be discussed.
Section 3.2: RO2: It looks like an offset between measured and modelled simple RO2. Can you exclude that there is an unaccounted instrumental offset?
Section 3.3: This analysis does not give much insights as it is done here. More discussion and comparison with previous findings with interpretation of different and similar results would be needed.
Section 3.4: Again, there is little interpretation or discussion of the correlation and it is not clear what is learned from this analysis. What is the meaning of the different slopes? What is expected for what reason?
L465: The offset does not necessarily indicate that some RO2 sources (= type of RO2 radicals) do not form HO2 as it sounds in the statement. It can also be that the lifetimes of HO2 and RO2 were much different or RO2 loss channels did not lead to HO2 formation, but the RO2 from all sources may still generally form HO2. If there were mainly RO2 sources in the night but little HO2 present, why would you expect that there is a correlation between RO2 and HO2, when the reaction of RO2+NO as most important pathway to HO2 is not relevant in the night?
Section 3.6 / Figure 15: Would you expect that an exponential behaviour of BVOC emissions is visible for the range of temperature that is experienced in the campaign? Can you make an estimate, how much RO2 concentrations will change, if you assume additional VOCs in the model to account for the gap between measured and modelled OH reactivity?
Section 4.4: It would be good to have some numbers of reactive halogen species that would be required to explain observations.
Citation: https://doi.org/10.5194/acp-2022-207-RC1 - AC1: 'Response to Reviewer 1', Dwayne Heard, 16 Jan 2023
-
RC2: 'Comment on acp-2022-207', Anonymous Referee #2, 22 May 2022
This paper presents measurements of OH, HO2, RO2, and total OH reactivity at a coastal site during the 2015 ICOZA (Integrated Chemistry of OZone in the Atmosphere) campaign. The authors compare the measurements to predictions by both a photostationary state model as well as a zero-dimensional model based on the Master Chemical Mechanism (MCM 3.3.1). The authors find that in general the MCM model was able to reproduce the measured OH concentrations during the campaign, but overpredicted the measured concentrations of HO2 under lower NOx conditions when air arrived to the site from the northwest-southeast sectors, while underpredicting the measurements when more polluted air arrived to the site from the southwest sector. The authors also found that the model underpredicted the measured RO2 concentrations for both lower and higher NOx air that arrived from all sectors. The authors also find that the measured total OH reactivity was consistently greater than that calculated by the model.
The measurements described add to a growing dataset that suggest that our understanding of radical chemistry under a range of conditions may be incomplete, and as a result are of interest to the atmospheric chemistry community. The results are consistent with several previous measurements, and the authors suggest several possible reasons for the model discrepancies based on these previous results, including missing halogen chemistry and autooxidation of RO2 radicals reducing the rate of conversion to HO2 radicals. Unfortunately, the impact of these proposals on their model results are not included in this paper, as they are discussed in the companion paper. While the companion paper focuses on the impact of their proposed mechanisms on the radical budgets, this paper would benefit from some additional discussion of the impact of the proposed mechanisms on the modeled radical concentrations.
Specifically, the authors should consider including their model results when they reduced the rate of the RO2 +NO propagation rate as discussed in sections 4.4 and 4.5 as it appears that a reduction in this rate, perhaps due to the competition of RO2 autooxidation with radical propagation, improves the agreement with the measured HO2 and RO2 concentrations. While including an expanded discussion of the model results would add to an already lengthy manuscript, the authors should also consider condensing and or moving some of the discussion of previous measurements into a supplement.
Additional comments:
1) The authors conducted interference measurements during two different periods, finding that unknown interferences contributed less than 20% to the measured OH signal. It appears that these measurements occurred during both NW-SE and SW periods. Did the authors see a significant difference in the interference measurements from the different wind sectors?
2) The authors should consider highlighting the NW–SE and SW periods on Figure 5 to help illustrate the impact of the different air masses on the radical measurements.
3) Given that the main focus of the paper is on the measurement/model discrepancy of the radical concentrations, there are several sections and figures in the paper that could be moved to a supplement to improve readability. In particular, sections 3.3 and 3.4 along with figures 9 and 10 could be moved to a supplement.
4) The authors could also condense much of the discussion of previous measurements by including a table summarizing the previous measurements/model agreement under the different NO conditions and referencing the table in the discussion.
5) As mentioned above, the authors should consider adding the reduced RO2+NO model results to Figures 5-7 to illustrate how this model improves the agreement with the measurements. This illustration of the impact of the reduced rate is not included in the companion paper.
Citation: https://doi.org/10.5194/acp-2022-207-RC2 - AC2: 'Reply on RC2', Dwayne Heard, 16 Jan 2023
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