the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Investigation of the limonene photooxidation by OH at different NO concentrations in the atmospheric simulation chamber SAPHIR
- 1Institute of Energy and Climate Research, IEK-8: Troposphere, Forschungszentrum Jülich GmbH, Jülich, Germany
- 2Department of Chemistry and Molecular Biology, University of Gothenburg, Gothenburg, Sweden
- anow at: Federal Office of Consumer Protection and Food Safety, Department 5: Method Standardisation, Reference Laboratories, Resistance to Antibiotics, Berlin, Germany
- bnow at: Institute of Nutrition and Food Sciences, Food Science, University of Bonn, Bonn, Germany
- cnow at: Atmospheric Trace Molecule Sensing Laboratory, School of Earth Sciences and Environmental Engineering, Gwangju Institute of Science and Technology, Gwangju, Republic of Korea
- dnow at: State Key Joint Laboratory of Environmental Simulation and Pollution Control, College of Environmental Sciences and Engineering, Peking University, Beijing, China
- enow at: CBS International Business School, Brühl, Germany
- 1Institute of Energy and Climate Research, IEK-8: Troposphere, Forschungszentrum Jülich GmbH, Jülich, Germany
- 2Department of Chemistry and Molecular Biology, University of Gothenburg, Gothenburg, Sweden
- anow at: Federal Office of Consumer Protection and Food Safety, Department 5: Method Standardisation, Reference Laboratories, Resistance to Antibiotics, Berlin, Germany
- bnow at: Institute of Nutrition and Food Sciences, Food Science, University of Bonn, Bonn, Germany
- cnow at: Atmospheric Trace Molecule Sensing Laboratory, School of Earth Sciences and Environmental Engineering, Gwangju Institute of Science and Technology, Gwangju, Republic of Korea
- dnow at: State Key Joint Laboratory of Environmental Simulation and Pollution Control, College of Environmental Sciences and Engineering, Peking University, Beijing, China
- enow at: CBS International Business School, Brühl, Germany
Abstract. The oxidation of limonene by the hydroxyl (OH) radical and ozone (O3) was investigated in the atmospheric simulation chamber SAPHIR in experiments performed at different nitric oxide (NO) mixing ratios from nearly zero up to 10 ppbv. For the experiments dominated by OH oxidation the formaldehyde (HCHO) yield was experimentally determined and found to be (12 ± 3), (13 ± 3), and (32 ± 5) % for experiments with low (~0.1 ppbv), medium (~0.3 ppbv), and high NO (5 to 10 ppbv), respectively. The yield in an ozonolysis-only experiment was (10 ± 1) %, which agrees with previous laboratory studies. The experimental yield of the first generation organic nitrates from limonene-OH oxidation is calculated as (34 ± 5) %, about 11 % higher than the value in the Master Chemical Mechanism (MCM), which is derived from structure-activity-relationships (SAR). Time series of measured radicals, trace-gas concentrations, and OH reactivity are compared to results from zero-dimensional chemical box model calculations applying the MCM v3.3.1. Modelled OH reactivity is 5 to 10 s-1 (25 % to 33 % of the OH reactivity at the start of the experiment) higher than measured values at the end of the experiments at all chemical conditions investigated, suggesting either that there are unaccounted loss processes of limonene oxidation products or that products are less reactive toward OH. In addition, model calculations underestimate measured hydroperoxyl radical (HO2) concentrations by 20 % to 90 % and overestimate organic peroxyl radical (RO2) concentrations by 50 % to 300 %. Largest deviations are found in low-NO experiments and in the ozonolysis experiment. An OH radical budget analysis, which uses only measured quantities, shows that the budget is closed in most of the experiments. A similar budget analysis for RO2 radicals suggests that an additional RO2 loss rate of about (1–6) × 10-2 s-1 for first-generation RO2 is required to match the measured RO2 concentrations in all experiments. Sensitivity model runs indicate that additional reactions converting RO2 to HO2 at a rate of about (1.7–3.0) × 10-2 s-1 would improve the model-measurement agreement of NOx, HO2, RO2 concentrations, and OH reactivity. Reaction pathways that could lead to the production of additional OH and HO2 are discussed, which include isomerisation reactions of RO2 from the oxidation of limonene, different branching ratios for the reaction of RO2 with HO2, and a faster rate constant for RO2 recombination reactions. As the exact chemical mechanisms of the additional HO2 and OH sources could not be identified, further work needs to focus on quantifying organic product species and organic peroxy radicals from limonene oxidation.
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Jacky Yat Sing Pang et al.
Status: final response (author comments only)
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RC1: 'Comment on acp-2022-239', Anonymous Referee #1, 03 May 2022
Review of:
Investigation of the limonene photooxidation by OH at different NO concentrations in the atmospheric simulation chamber SAPHIR
Pang et al. 2022
This manuscript presents an experimental study of the OH initiated oxidation of limonene under different NOx concentrations, and the O3initiated oxidation of limonene at ‘zero’ NOx. The experiments are performed in the atmospheric simulation chamber SAPHIR. The first generation stable product formaldehyde is measured, as well as the radicals OH, HO2, RO2, NO and NO2. These measurements are used in conjunction with a box model and the MCM mechanism to interpret the results. Discrepancies between the modelled chemistry and the measurements highlight gaps in our understanding of the oxidation mechanisms. The largest discrepancy between measurements and model is with measured total RO2, particularly at low NOx concentrations, where the model greatly over-predicts measured RO2. This is interpreted as there being a missing RO2 sink(s) in the model, either bimolecular, e.g. RO2+RO2, RO2+HO2, or unimolecular. A number of model scenarios are run to try to further elucidate the nature of the missing reaction(s).
The paper is generally well written, particularly the detailed but concise introduction. There are in fact a rather limited number of experiments, though each has a large amount of data, and few conclusions can be drawn beyond that there are missing RO2 loss processes. This conclusion is of course a useful conclusion to draw, and I presume that the experiments were not performed with the express goal of investigating specific RO2 processes since they were not a coherent campaign but drawn together from various times. The work serves to further highlight that the behaviour of RO2, particularly complex RO2, at low NOx is one of the largest uncertainties in our current understanding of gas phase chemistry. It also highlights that there are a number of possible fates of these RO2 and details the recent body of theoretical work which is attempting to elucidate the most important unimolecular pathways, which are particularly challenging to investigate experimentally. It seems to me that two processes that could be explored more easily experimentally are the possibility of cyclic OH-RO2self/cross reactions being particularly fast, through experiments with cyclohexene/pentene etc. And the reaction of these RO2and/or large acyl peroxy radicals with HO2. A better understanding of these processes, gained through systematic experimental work would greatly aid the interpretation of the oxidation mechanisms of the more complex monoterpenes.
Specific comments
Line 58: no comma needed here
Line 75: should ‘drawing’ be ‘withdrawing’ ?
Line 207/8: Could the authors specify that these are (I presume) assumed to be from processes occurring on the chamber film surface?
Line 205: This could do with a little more discussion highlighting that the majority of previous experiments will have been done at low humidity. The conditions employed here are clearly more relevant to the atmosphere, but do you have thoughts on whether this may affect the major oxidation pathways. Also, does the high humidity have any negative effect on the instrumentation?
Table 2: Is this the correct NO concentration for the high NO experiment? Is it lower than expected because it’s an average? It then doesn’t agree with the value used for NO in Table 3.
Line 361: Highlight that this is as would be expected based on your OH+limonene experiments at low NO, which have a similar OH yield to the ozonolysis experiment.
Line 368: I’m not sure that this is worth noting without some further explanation of what you mean. Which experiments of Gong et al. does this refer to? All of them? This fact could mean different things based on the experiment. Is it because O3is so high that the O3+ limonene reaction is still dominant over the OH reaction? Or because, as in your experiments, OH is reacting with limonene, but, at low NO, the HCHO yield is similar to the ozonolysis?
SECTION 3.1.2: This seems like a rather convoluted process to calculate the organic nitrate yield and I would suggest that, based on this, the stated uncertainty is rather low!
Line 679: This seems like the more likely explanation. You will be forming very different RO2.
Figure 9 and 10: Which experiment is which? Can these be labelled a and b.
Line 794: ‘optimised'
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RC2: 'Comment on acp-2022-239', Anonymous Referee #2, 11 May 2022
This manuscript presents a series of chamber studies that investigate the OH initiated oxidation of limonene at various NOx mixing ratios and also presents an additional experiment that investigates the ozonolysis of limonene in a dark chamber with near-zero NOx mixing ratios. Although only a handful of experiments were performed, the manuscript is built around an extensive dataset including measurements of limonene, formaldehyde, HONO, NOx, O3, and photolysis frequencies in addition to the important and technically challenging measurements of OH, HO2, RO2, and OH reactivity. The measured radical concentrations are compared to results from a box model featuring MCM v3.3.1 chemistry which typically suggests RO2 concentrations that are much higher, and HO2 concentrations that are lower than measured values. As suggested by reviewer 1, the discrepancies between measured and modeled RO2 concentrations, particularly during the low-NOx and zero-NOx experiments, are significant results that highlight a gap in our understanding of this type of oxidation chemistry. The paper also examines the formaldehyde and organic nitrate yield to further aid in understanding the fate of limonene RO2 species.
Overall, the paper is well written and is particularly effective at merging the results and discussion of several different experiments, which were performed across a number of years, into a cohesive manuscript with a unified conclusion.
Specific comments:
Line 138: Is there an estimate for the “small” fraction of limonene-RO2 that is converted and measured as HO2 in the LIF detection cell during these experiments? Has this fraction been determined specifically for limonene-RO2 and the NO concentrations used in detection cell or is it possible that this RO2 interference is more significant than anticipated? If so, could this at least partially explain the discrepancies between measured and modeled HO2 concentrations, especially during the ozonolysis experiment when measured RO2 concentrations were highest?
Line 143: Are the RO2 concentrations reported from all experiments derived from calibrations with methylperoxy radicals? If so, does this imply that the reported RO2 concentrations, which are largely due to limonene-RO2, represent a lower limit? Or have adjustments been made that take the ROx-LIF system’s reduced sensitivity to limonene-RO2 into consideration?
line 207: Are the fluctuations in NO mixing ratios (and ultimately measured and modeled radical concentrations) during the low and medium NO experiments (Figures 3 and 4) caused by changes in HONO production from the chamber source that are driven by changes in solar radiation? If so, these fluctuations may be easier for readers to interpret if measured or parameterized HONO mixing ratios or measurements of photolysis frequencies were shown.
Figure S3: This figure is not discussed in the context of the low NO experiments. This is understandable since only a small portion of this experiment involves limonene oxidation, but since the figure is shown – are the observed RO2 concentrations prior to the CH4 addition likely due to the oxidation of some VOC produced in the chamber? It is interesting that, after the CH4 injection, the measured RO2 concentration increases as expected (at least relative to the established background), but the measurement/model agreement quickly reverses after limonene addition. Could this difference in measurement/model response to the different VOCs be related to the previously mentioned ROxLIF sensitivities to CH3O2 and limonene-RO2? Similarly, the model agrees with the HO2 measurements during the CH4 injection but underpredicts the measurements after the limonene injection. While these trends could again indicate a limonene-RO2 interference in the HO2 measurement, they could also support the later claims of missing RO2 loss processes, whether isomerization or RO2 + RO2 recombination reactions, that are much faster for large complex monoterpene peroxy radicals (and produce HO2), but do not occur for smaller RO2 species like CH3O2. A short discussion on this particular experiment could be useful but is not absolutely necessary.
Line 533: “concentration” can be removed, or this sentence should be otherwise rephrased.
Line 619: This sentence is a bit awkward. Perhaps “In the ozonolysis experiment, prior to the addition of CO as an OH scavenger (Fig. 8d) OH is only produced by the ozonolysis of limonene.”
Line 659: Delete “-“ after OH
Figures 9, 12, and others in supplement: When data from multiple experiments are presented in one figure it would be useful to also label each panel (or group of panels) with “low NO” or “ozonolysis” instead of just the date. Figures 8 and S6 are good examples.
Figures 9 and S8: The caption in Figure 9 suggests that CH3O2 is mainly produced from the oxidation of HCHO while the caption in Figure S8 suggests that CH3O2 is mainly produced from the oxidation of limonene.
Lines 716, 720, 731, 1003: Some commas are unnecessary.
Line 764/765: This sentence is a bit awkward. Consider “These reactions could involve an unknown reaction partner X, as used in Hofzumahaus et al. (2009), or could be unimolecular reactions.” Also, this reference may be missing from the reference list.
Line 893: One example instead of one examples.
Line 1018: Second “in the model” is unnecessary.
Jacky Yat Sing Pang et al.
Jacky Yat Sing Pang et al.
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