A relative rate study is reported, which considers the reactions of NO₃ with a series of furans and related compounds, which are known to be important components of biomass burning emissions. The experiments were carried out in the large CNRS-ICARE chamber, using N₂O₅ decomposition as the source of NO₃, with the rate coefficients being mainly determined relative to those of a series of alkenes of comparable reactivities. The target compounds include furan, 2-methylfuran, 2,5-dimethylfuran, furan-2-aldehyde, 5-methyl-2(3H)-furanone, 2(5H)-furanone and pyrrole. In the case of 5-methyl-2(3H)-furanone, this is the first reported determination. A rate coefficient for the reaction of NO₃ with the reactive monoterpene, alpha-terpinene, is also reported which is higher than previous absolute and relative rate determinations.

This paper considers an important topic, where new and confirmatory kinetic data are required, and the paper is clearly presented and written. I have one major comment on the experimental set-up and its interpretation, and some minor comments, which are outlined below. The major comment relates to the possible impact of NO₂ chemistry in the system, particularly for the α-terpinene rate coefficient determination, but which should probably also be checked in other cases where the current and previous results differ. I have submitted this review promptly, and I hope this will give the authors time to consider this and make adjustments, if required.

Major comment

The NO₃ source employed (N₂O₅ decomposition) produces an equivalent amount of NO₂, and additional NO₂ is likely formed as a product of the NO₃-initiated organic chemistry (i.e., from decomposition of nitro-oxy oxy radicals). Given the timescale of the experiments (0.5 – 2 hours), and the lack of reaction partners for NO₂, the potential reaction of NO₂ with the unsaturated organics needs to be considered and assessed, to confirm that the assumption of NO₃ being the only reagent is valid (i.e., Eq. (E1)).

NO₂ has previously been shown to react only very slowly with monoalkenes, but systematically more rapidly with conjugated dienes – and with further increases in reactivity resulting from alkyl substitution and in cyclohexadiene rings (Atkinson et al., 1984; Niki et al., 1986; Ohta et al., 1986; Jenkin et al., 2005; Bernard et al., 2013). The rate coefficient reported for α-terpinene (1-isopropyl-4-methyl-cyclohexa-1,3-diene: 6.5 x 10^-18 cm³ molecule^-1 s^-1) is therefore one of the highest measured to date (Atkinson et al., 1984). The lifetime of NO₂ with respect to reaction with 3 ppm alpha-terpinene is therefore about 0.5 hours, such that some supplementary removal of α-terpinene by this reaction is likely to have occurred under the conditions employed in the present work. In contrast, the alkene reference compound, 2,3-dimethyl-2-butene, reacts much more slowly with NO₂ (1.5 x 10^-20 cm³ molecule^-1 s^-1). As a result, this interference may contribute to the present k(NO₃) determination for α-terpinene being higher than all previous determinations. The previous relative rate studies either corrected for NO₂ reaction (Atkinson et al., 1985) or employed flowing systems with short residence times (Berndt et al., 1996). The authors therefore need to check for this potential interference (which presumably would be a very straightforward experiment) and make corrections if required.

For completeness, the possible reaction of NO₂ with the furans etc. also ideally needs to be checked, although these are likely slower reactions than for α-terpinene. Atkinson et al. (1985) verified that the NO₂ reaction was unimportant for furan under their NO₃ + furan experimental conditions.

The α-terpinene k(NO₃) determinations using 2,5-dimethylfuran and pyrrole as references yield results that are similar (but not identical) to that obtained using 2,3-dimethyl-2-butene as the reference, suggesting that they also probably do not react rapidly with NO₂. However, the observed differences between the systems could also result from the differences in NO₂ generation from the NO₃ + reference compound chemistry and the secondary effect on α-terpinene decay.

Minor comments

Abstract: The common names furfural, α-angelicalactone and γ-crotonolactone are generally used for furan-2-aldehyde, 5-methyl-2(3H)-furanone and 2(5H)-furanone throughout the manuscript. It might be useful to include the common names in the abstract summary (i.e., as done on line 54).

Line 70: Please further clarify whether the N₂O₅ sample was introduced “continually” (i.e., repeatedly in aliquots) or “continuously” (i.e., without interruption in a constant flow throughout the experiment).

Lines 79-83: Cyclohexane (used as the reference for γ-crotonolactone) needs to be included in the materials list. Its rate coefficient should also be included in Table 2.

Line 101: It would be helpful to have some explanation of why the rate of alkene and furan loss rates increase with time during the experiment (Fig. 1). It is not clear from the information given. At what point was the N₂O₅ flow added? It also would be good to have similar plots for other studied compounds in the Supplement.

Line 113: Terminology: The term “rate(s)” is used incorrectly in many places where “rate coefficient(s)” should be used. As the authors are of course aware, a reaction rate (dimension: concentration/time) is the product of the rate coefficient and reagent concentration(s) – i.e., “k” is a rate coefficient, not a rate.

Line 138: Note that there is also a direct source of HO₂ from the reaction of NO₃ with α-terpinene. Like other conjugated cyclohexadienes, a minor H abstraction channel occurs to form a substituted cyclohexadienyl radical, followed by its reaction with O₂ to form p-cymene and HO₂ (Berndt et al., 1996).

Lines 134-150. Because the NO₃ + HO₂ reaction also forms OH, it might be worth mentioning that this is (presumably) uncompetitive under the experimental conditions.

Line 206, Table 3: a-terpinene entries should be “α-terpinene”.

Line 215: The derived values of k(NO₃) for 2,5-dimethylfuran and pyrrole are highlighted as being significantly smaller using α-terpinene as the reference. But, inspection of Table 3 suggests that the values obtained using 2,3-dimethyl-2-butene are further from the average in each case, the highest and lowest of the sets respectively.

Line 224. Replace “TME” by “2,3-dimethyl-2-butene”.

Line 227, Table 4: “Kind et al. (2006)” or “Kind et al. (1996)”?

Line 347: Berndt et al. (1996) reference is missing.

References

Atkinson, R., Aschmann, S. M., Winer, A. M. and Pitts, J. N.: Int. J. Chem. Kinet., 16, 697, 1984.

Atkinson, R., Aschmann, S. M., Winer, A. M. and Carter, W. P. L.: Environ. Sci. Technol., 19, 87, 1985.

Bernard, F., Cazaunau, M., Mu, Y., Wang, X, Daële, V., Chen,J. and Mellouki, A.: J. Phys. Chem., 117 (51), 14132, doi: 10.1021/jp408771r, 2013.

Berndt, T., Böge, O., Kind, I and Rolle, W.: Ber. Bunsenges. Phys. Chem., 100(4), https://doi.org/10.1002/bbpc.19961000410, 1996.

Jenkin, M. E., Sulbaek Andersen, M. P., Hurley, M. D., Wallington, T. J., Taketani, F., and Matsumi, Y.: Phys. Chem. Chem. Phys., 7, 1194, 2005.

Niki, H., Maker, P. D., Savage, C. M., Breitenbach, L. P. and Hurley, M. D.: Int. J. Chem. Kinet., 18, 1235, 1986.

Ohta, T., Nagura, H. and Suzuki, S.: Int. J. Chem. Kinet., 18, 1, 1986.

General comments

The manuscript presents a relative kinetic study in chamber simulation of a series of furans and relative compounds with NO₃ radical, major oxidant in the atmosphere by night. The studied compounds are furan, 2-methyl furan, 2,5-dimethylfuran, furan-2-aldehyde, 5-methyl-2(3H)-furanone, 2(5H)-furanone and pyrrole which are known to be emitted in the atmosphere during biomass burning.

This study reports for the first time rate coefficients for two furanones (α-angelicalactone and γ-crotonolactone) and investigate rate coefficients for the others compounds witch present few rate constant determinations in the literature. Although new kinetic studies are mandatory to complete and improve kinetic data bases the manuscript needs significant improvements before being published in ACP. The recommendations listed here after must absolutely be taken into consideration.

Major comments

1) As the paper includes a relative kinetic study I would expect a detailed presentation of the experimental conditions, analysis of data, results and discussion. However, there is a certain number of information missing from the manuscript (and/or SI) to allow a proper evaluation of the quality of the data. My main comment is that relative rate plots and time series of concentrations are missing for the majority of the compounds. The plots are needed for example to investigate the presence of an eventual secondary chemistry or products interference. This is of particular interest for the compounds for which the rate constants obtained are not in agreement with the literature.

- please present relative rate plots and time series of concentrations for compounds of interest (VOC, reference compound and NO₂) for 2,5-dimethyl furan, pyrrole, furfural, γ-crotonolactone, α-terpinene;

- please present relative rate plots and time series of concentrations for reference compound and NO₂ for α-angelicalactone;

Although there is no need to include all the time series and plots in the manuscript I suggest to complete the manuscript by including time series of concentration and relative rate plots for compounds for which this is the first rate constant determination (α-angelicalactone) or/and those experiencing fast reactivity (e.g 2,5-dimethylfuran or pyrrole). Representative plots that are not shown in the manuscript must be included in SI for all compounds.

2) The technique used for monitor VOCs is an in situ IRTF.

- The absorption bands used for reference compounds are missing. Please add a table with the missing information.

- The technique is not selective and reactants as well as the products of reaction are expected to absorb IR light and be presented on the absorption spectra at the level of concentration used here. Some of the products have absorption bands that may interfere with the reactant bands and thus perturb the data analysis. Is this the case? Can you comment on that?

- Generation of NO₃ via the thermal dissociation of N₂O₅ implies the presence of N₂O₅, NO₂ and HNO₃ (depending of the purity of the N₂O₅ synthesis) in the chamber. IRTF allows also to monitor these species. Please add information about the levels of these species in the experiments.

3) Although α-terpinene is a compound of interest in this work, there is very few information regarding this compound in the manuscript:

- The title and introduction of the paper are focused mainly on furans

- No information regarding the data analysis is presented (absorption bands in Table 1, absorption spectra in SI). Please complete

- The compound is used as reference compound in two experiments (2,5-dimethylfuran, pyrrole) but the recommended rate and uncertainties are not present in Table 2. Please complete.

- No concentration profiles, no relative rate plots are presented. Please complete.

- Not included in Figure 3 neither in table S1. Please complete.

- No discussion or explanation is given on the faster rate constant regarding the previous rate constant determinations. I would expect further discussion as previous values are in good agreement within the uncertainties.

-α-terpinene was used with 90% of purity without further purification. Can you specify the nature of the impurity and discuss impact on the rate constant?

In my opinion the data regarding this compound should be further presented and discuss or the authors should take in consideration to remove it from the manuscript.

4) While authors have investigated a possible OH generation and thus reactions of VOC of interest with OH no other sinks in the experiments were considered or discuss. The generation of NO₃ radicals by thermal dissociation of N₂O₅ has indeed the advantage to work in a O₃ free environment but implies large amounts of NO₂. I agree with the first referee, NO₂ may be possible sink for the studied compounds in the experimental conditions presented here (large concentrations of VOC, and N₂O₅). Authors should investigate this possible path for all compounds but especially for rate constants found higher that the already published ones (pyrrole, 2,5-dimethylfuran and α-terpinene).

Minor comments

Furan -2-aldehyde, 5 methyl-2(3H)-furanone and 2(H)-furanone are used either by their scientific names (eg. abstract, table 1) or by their common names (furfural, α-angelicalactone, γ-crotonolactone) ( eg. Materiels, Table 3) which is difficult to follow. For clarity, please homogenize.

Line 43: The sentence “Furan type compounds are removed from the atmosphere by reaction with the major oxidants OH, NO₃ and O₃” should be introduce earlier (line 34) for clarity.

Experimental

More information about the experiments is needed in my opinion.

Line 62: The authors mention that the experiments were conducted with the chamber operated at a slight overpressure to compensate from” removal of air sampling and to prevent ingress of outside air to the chamber”. The reason for “air sampling” is unclear as the only instrument mentioned for these experiments is an in situ IRTF. Please clarify.

Line 67:  Specify the order of introduction of VOCs (VOC of interest followed by Reference VOC?), continuous injection of N₂O₅, …

Line 74: Please specify the time and spectral resolution for the IR spectra

Materials

For clarity, please differentiate interest VOC from reference VOC.

Results and discussion

Table 3: A number of experiments are missing. Please complete.

Table 4: For α-terpinene: information are missing regarding previous studies: i) for Atkinson et al., et Berndt et al. please specify the type of study. For Fouqueau et al., 2020 please specify the study and method used.

SI

Table S1: for clarity: replace “NA” by “-“; reorganize the table (e.g. by Compound 1 or Reference)