|The revised version of the manuscript includes several improvements. Overall, the authors have addressed almost all of my previous concerns in a pertinent manner. I have found that the quality of the manuscript has improved considerably. I agree that the manuscript contains a number of important new insights for ACP readers. However, I still have some points that need to be taken into account before publication:|
1) VOC identification: monoterpene and isoprene identifications are convincing, unfortunately not yet for the DMS. All the arguments presented by the authors are technically valid. However, the DMS has not been sufficiently validated. Since the main objective of the article is the "discovery of a missing DMS source in the atmospheric environment from mahogany trees", I consider extremely important to provide additional data on the identification of the DMS. From the mass scan data, it is unclear the isotopic distributions of DMS (peaks are too small). What is the percentage of m/z 64 and 65 with respect to 63? Are those peaks reflecting the natural isotopic distribution of 13C, 33S, and 34S? Can you please clearly show it? Another and more robust way to validate DMS is the use of a pure DMS standard and gas chromatography, especially when coupled with mass spectrometry. Honestly, I don't understand why the authors didn't validate the DMS with at least TD-GC-FID in a similar way as they did with isoprene. In fact, the authors possess a standard VOC mixture of Apel-Riemel containing DMS and have (or have access to) a GC-FID instrument. I recommend showing in the supplementary, statistically meaningful GC data (either coupled with FID or and better with MS) of cuvette-enclosed Mahogany trees, compared to background measurements (empty cuvette) and to pure DMS standard measurements.
2) The DMS emissions from Mahogany trees are reported as “high” in the title. The summer emissions rates calculated are in the range of 19.2 ng g-1 hr-1 (+/-19 sd, Table2), or max ~15 pmol m-2 s-1 (Figure1). Despite the large uncertainty, the average emission rate is thousandfold lower than what is known for a strong VOC emitter. For comparison, the isoprene emission capacity of oaks and poplars are in the range of 20-80 nmol m-2 s-1. Even when compared to VOCs emitted by the same plant species (Figure1), DMS emissions are the lowest, i.e. 100 folds lower than monoterpenes and 10 folds lower than isoprene. From the text, I understand that the “high” is just relative to general plant DMS emissions. However, the use of “high” in the text should be use with care, but it is inappropriate in the title and therefore should be removed. It would be more appropriate to say something like this: “Big-leaf Mahogany trees are significant sources of DMS and monoterpene emitted into the atmosphere”.
3) I appreciate the efforts to include the means of biological replicates and the variability of the emissions rates. I understand the willingness to use the data collected form Tree 1 to roughly show seasonal changes of VOCs. However, it is unclear how a seasonal study of VOC can be based on a unique individual of a population and how this should be representative and statistically meaningful. Even when the authors use the data collected in winter to compare Tree 1 to the mean value of Tree 2-4 (Fig.1), there is no evidence on the emission variability among Tree 1 and the other trees in other seasons. Because seasonal changes of VOCs might be strongly plant-specific (in particular those controlled enzymatically), the relationship between Tree 1 and Tress 2-4 seen in winter might not hold e.g., in summer. The uncertainties on seasonal emissions derived from the biological plant-to-plant variability should be better acknowledged. As a remark, I don’t agree to publish works without an appropriate number of replicates, even when poor and scientifically unacceptable studies conducted without adequate repetitions can be found (unfortunately) in literature.
4) In statistics, “n” refers to the sample size or elements in a sample. In P4L6-8 “n” is not clear if the “n” refers to the elements in a sample that are used for the correlation study of modelled vs measured fluxes (Fig3).
5) In methods, the plant material should be clearly described. Growing conditions, soil propriety, age of the plants, number of leaves enclosed in the bag and their stage of development, position of the leaves in respect to the tree and sun exposition, and any other useful information.
6) The statistic paragraph in the method section is missing. In this section, authors should describe the number of biological replicates (“n” as sample size) and “n” as elements in a sample, all statistical tests, levels of significance, and software packages used to perform the statistical analysis. It is important to provide here the justification of the statistical method used in the study.