From the journal Environmental Science: Atmospheres Peer review history

19-Oct-2020

Dear Dr Martin:

Manuscript ID: EA-ART-09-2020-000006
TITLE: Near-canopy horizontal concentration heterogeneity of semivolatile oxygenated organic compounds and implications for primary emissions

Thank you for your submission to Environmental Science: Atmospheres, published by the Royal Society of Chemistry. I sent your manuscript to reviewers and I have now received their reports which are copied below.

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Associate Editor, Environmental Science: Atmospheres

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Reviewer 1

Ye et. al present measurements of SV-OVOCs above 2 different canopies located ~700 m apart. Using a series of model scenarios, the authors conclude the observed spatial heterogeneity of 2-methyltetrols indicates significant direct emissions of the species from the forest canopy. More broadly, the authors suggest that the measurement approach could be further developed to provide uniquely useful information about SV-OVOC direct emissions. While the branch enclosure or similar measurements would add significant value to the results here, overall, the analysis is useful and provides new and meaningful insight into OVOC budgets. I have only a few minor questions, comments, and suggestions:
1.) Do changes in T, RH, or any other local parameters influence SV absorption on cartridges?
2.) Is the vertical resolution of the model sufficient to capture any gradients that might result from measuring 149 v 15 m above the canopy?
3.) Could there be other potential sources secondary sources of 2-methyltetrols that would look similar to direct emission (i.e., rapid oxidation from an unmeasured precursor)? In other words, can you differentiate between direct emission and effective emissions?
4.) The sentence “In regard to isoprene, 2-methyl-D-erythritol, which is one of the diastereomers of 2- 76 methyltetrols, is a metabolism byproduct in plants…” is confusing. I suggest removing “In regard to isoprene”.
5.) There seems to be a lack of figure captions.
6.) Figure 2. It appears that only a small fraction of the total available measurements show non-overlapping errorbars for the two locations. Can this be quantified in the text (e.g., 4/12, or whatever it may be?). In general, the error bars are hard to interpret as location P and S overlap. Can one of these be staggered slightly to the right or left, bolded, or otherwise differentiated?

Reviewer 2

This study explores the dynamics of semi volatile compounds in terms of horizontal concentration heterogeneity at an Amazonian forest canopy. I think this study fits in the scope of Environmental Science Atmospheres journal and shows measurements and conclusions highly relevant to the scientific community. I recommend its publication after major revisions.
Major comments:
Introduction: there is no mention to pinic nor pinonic acid in introduction.
There are no clear objectives and hypothesis at the introduction, but then the hypothesis come in results and discussion without no previous mention.
There is no figure notes to the figures
I would like to see more clarity with the 2-methyltetrol terminology. Please clearly state which species is which group (i.e. first mention to 2-methylerythritol is already in methodology. Also explain clearly organosulfate analogues. It would be better if you would use the same terminology throughout the text (clearly name the exact species instead of saying organosulfate analogues).
The simulation details should go into methodology and a summary of the difference between simulations (not only with im changing this values, but rather and explanation why do you choose such conditions for your simulations.
There is no clear explanation of the gas vs particle situation. Please explain when do you measure gas, when do you measure particle and how do you separate both. For instance in line 408, how do you arrive to those measurements?

Minor comments:
Line 31-32: This sentence reads odd
Line 39-42: This sentence reads odd and is missing references.
Line 75-77: I do not understand why do you say in regard to isoprene and right after that 2-methyl-D-erythritol is a metabolism byproduct. What does this have to do with isoprene… Please rephrase. Also both phosphates are missing pathway at the end.
Line 77-79: there is repetition confusing the reader… what about introducing first the 2methyltetrols family and then the diastereomers. So are they directly emitted, or by produced by isoprene oxidation. Please rephrase.
Line 80: What are isoprene epoxidiols?. Can you introduce this subject better, are we talking about gas or particle phase? What are organosufate analogues? These two sentences are strange, you pass from plant biosynthesis to aerosol phase, and it is confusing. Please rephrase.
Line 85: what do you mean by application to the data set of the principle….
Line 92: what are biomarkers? The sugars or the 2-methyltetrols
Line 97: there has to be a reference here
Line 99: explain what other factors or delete
Line 120: I think this sentence, takes away credit from your study. If you way the technique is nor developed, well tested and understood….then it should not be published.
Line 125: ahaa first mention to monoterpenes.
Line 125-134: I feel this is methodology, and there is nothing about your hypothesis when in results and discussion you mention several hypothesis.
Line 143: what do you mean by local topography?
Line 144: what do you mean by local slope?
Line 145-146: so both canopies had the same height?
Line 151: what do you mean by approximately stable?
Line 156-7: when you say a wide range of organic compounds do you mean both polar and non-polar compounds?
Line 164: At which exact time are SV collected. The graphs should provide with markers that expand for as much as the collection time i.e. 20 minutes.
Line 168: at what flow was sampling at P? And how do you mimic UAV sampling with the operator t the tower?
Line 170: so you sampled for 7 consecutive days in one cartridge and you repeated this process 4 weeks. Is this correct? Please explain better in the text. Also how do you account for decomposition in the cartridge? This can happen not only during the week of sampling but also between sampling and analysis. Why did you decide to use this sampling scheme, where you expecting differences between weeks, (i.e. wet season, central Amazonia….)?
Line 177: how long did it took from collection to analysis?
Line 181: Can you give a sentence explaining why the sample must be derivatized by MSTFA?
Line 191: so this means you cant separate between 2-methylthreitol and 2-methylerythritol?
Line 199: brand for 2-methyltetrols
Line 233: this line comes out of the blue and should be better in intro so you can introduce pinic and pinonic acid
Line 234-238: This information is really interesting, perhaps, just to compare dynamics you could show this data in supplementary, explaining limitations, but this would certainly add to the scientific community.
Line 243-244: for many days and times? What do you mean, specify.
Line 245: -70 to +480% does not tell me anything. It is too big of a difference. The same applies to the ratios. Please find another way to express figure 2A and 2B. Perhaps talk about actual concentrations, or do not average over the 4 weeks.
Line 247: Why and how, please explain. Perhaps this info can go into the intro when you introduce pinic and pinonic acid.
Line 248: why do you choose pinonic and not pinic?
Figure 2b: where are the two S points of 9 and 10 am on week 1?If not there? How is this week representative?
Line 250: do you mean VOC species or plant species?
Line 251: Do you have a reference or explanation for this?
Line 253-254: can you explain why the denominator can serve as a normalization factor?
Line 254: What location?
Line 258: figures 2a and 2b are not similar, week 3 is really different.
Line 261: why should the ratio be banned by -40 to +40%? Please explain
Line 265: I think here you should specify 2 methyltetrols as pinic and pinonic acid do not change.
Line 266-270: what about further chemistry of 2methyltetrols?
Line 278: where do you get the values for E, D, and R? literature?
Line 282-288: this looks more like objectives
Line 289: Please state clearly what is the difference for each simulation, like you say in this sentence, but not only with values (i.e. 200% percent or 300% difference in emission rate, but rather Stronger uneven distribution of isoprene, less strong uneven distribution of isoprene,….for example). Also 200% between the two forests doesn’t indicate to me which forest are you choosing as higher emitter.
Table 1: what is upwind forest? First time this is mentioned.
Line 295: please state the range
Line 300: I think expected is not the word here, but rather supported?
Line 307: what do you mean by mathematical related possibility
Line 320: are you still talking of Butler et al., or your results?
Line 324: explain what averaging kernel is.
Line 327 replace present by presence.
Line 346: I’m missing a reference here.
Line 363: -18% and -235 heterogeneity with respect to what?.
Line 391: how is in good agreement with the atmospheric observations, please explain.

Response to reviews

We thank the reviewers for the detailed and helpful comments that have resulted in improvements to the manuscript. The manuscript has been revised based on the reviewers’ comments.

Author responses to the comments from each reviewer are listed below. The reviewer comments start with “>>” and the author responses start with “**”. Line numbers in the responses correspond to those in the original submission.

>> Reviewer 1

>> Ye et al. present measurements of SV-OVOCs above 2 different canopies located ~700 m apart. Using a series of model scenarios, the authors conclude the observed spatial heterogeneity of 2-methyltetrols indicates significant direct emissions of the species from the forest canopy. More broadly, the authors suggest that the measurement approach could be further developed to provide uniquely useful information about SV-OVOC direct emissions. While the branch enclosure or similar measurements would add significant value to the results here, overall, the analysis is useful and provides new and meaningful insight into OVOC budgets. I have only a few minor questions, comments, and suggestions:

** We thank the reviewer for the insightful comments and helpful feedback for improving the manuscript. The response to each comment is listed below.

>> Do changes in T, RH, or any other local parameters influence SV absorption on cartridges?

** Thank you for the comment. The meteorological parameters were measured during the campaign, as shown in Table S1. The temperature was 27.4 ± 1.6 ℃ (mean ± one-sigma variation) during the sampling time (9:00–13:00) across four weeks. The small variation in temperature is not expected to play an important role in SV-OVOC adsorption on cartridges. In regard to RH, the cartridges were packed with a combination of Tenax TA and Carbograph 5TD. These materials are hydrophobic and suitable for atmospheric sampling at a wide range of RH. Therefore, RH is not a concern. Other meteorological parameters such as wind are also not expected to influence SV-OVOC collection, given that this is an active sampler and the collection efficiency depends on the sampling flow rate and the affinities of SV-OVOCs to the adsorbent materials. The manuscript is updated as follows:

** Lines 149-152: “For the four weeks of measurements, the variability in the meteorological parameters, such as temperature, relative humidity, wind speed, and wind direction, was small during the time periods that samples were obtained (Table S1). The implication is that meteorology did not significantly contribute to the variability of relative SV-OVOC concentrations between locations P and S.”

** Lines 155-156: “The cartridges were packed with a combination of Tenax TA and Carbograph 5TD. These adsorbents were hydrophobic and suitable for atmospheric sampling at a wide range of relative humidity.”

>> Is the vertical resolution of the model sufficient to capture any gradients that might result from measuring 47 vs. 15 m above the canopy?

** Yes, the vertical resolution of the model is 20 m. It is sufficiently fine to capture the gradient, if there is any, between the two sampling heights. This information is updated in Text S1 in the Supplementary Materials as follows: “The domain size was 100 × 1.0 × 0.1 km^3 (longitudinal × vertical × transversal), and the corresponding resolution was 0.1 × 0.02 × 0.1 km^3.”

>> Could there be other potential secondary sources of 2-methyltetrols that would look similar to direct emission (i.e., rapid oxidation from an unmeasured precursor)? In other words, can you differentiate between direct emission and effective emissions?

** To our knowledge, this kind of rapid secondary source of 2-methyltetrols is not yet discovered or described in the literature. If there were one, chiral analysis of 2-methyltetrols or correlation analysis between 2-methyltetrols and strong chemical tracers of primary and secondary sources could provide further insights into the sources (i.e., direct emission of secondary products vs. secondary oxidation in the atmosphere). The “Conclusion” section of the original manuscript makes a comment (lines 484-492): “Another recommendation for future efforts that seek to delineate between primary and secondary sources of SV-OVOCs based on near-canopy horizontal concentration heterogeneity is that the concentrations of strong biomarkers of direct forest emissions only, such as sugars and sugar alcohols, and the concentrations of strong chemical tracers of atmospheric secondary processes, such as inorganic nitrate and sulfate salts, could be simultaneously measured. In a combined data set, a correlation analysis between the SV-OVOC concentrations produced by the two mechanisms and the concentrations of species produced dominantly by one mechanism (i.e., primary or secondary) might provide additional robustness and quantification with respect to atmospheric SV-OVOCs sources.”

>> The sentence “In regard to isoprene, 2-methyl-D-erythritol, which is one of the diastereomers of 2-methyltetrols, is a metabolism byproduct in plants…” is confusing. I suggest removing “In regard to isoprene”.

** Thank you for the comment. The sentence is clarified as follows (lines 75-77): “For isoprene, the 2-methyltetrols family is an important class of SV-OVOCs. 2-Methyl-D-erythritol, which is one of the diastereomers of 2-methyltetrols, is an isoprene metabolism byproduct in plants of deoxyxylulose phosphate and methylerythritol phosphate pathways.”

>> There seems to be a lack of figure captions.

** Thank you for pointing out this mistake. The figure captions were inadvertently omitted in the original submission, and they are restored in the revised manuscript, as follows:

** Figure 1. Sampling site at the Adolfo Ducke Forest Reserve. The left panel shows the location of the reserve on the northern outskirts of Manaus city, Amazonas, Brazil. The right panel shows the local topography in the region of the two sampling sites. Location P is over a plateau forest. Location S is over a slope forest.

** Figure 2. SV-OVOC concentrations across the study period. (A) Concentrations of 2-methyltetrols, pinonic acid, and pinic acid over the plateau forest at location P and the slope forest at location S. (B) Ratios of the concentrations of 2-methyltetrols and pinonic acid. Error bars on the data points in Panel A represent 30% experimental uncertainty. Error bars on the points in Panel B are the propagated uncertainties from concentration measurements of 2-methyltetrols and pinonic acid. Data are plotted for each hourly sampling period of each week. Local time was 4 h earlier than UTC. Plotted concentrations are listed in Table S2. The 2-methyltetrols concentration is the sum of 2-methylthreitol concentration and 2-methylerythritol concentration. For clarity of presentation, the data markers for 2-methyltetrols and pinic acid are offset along the abscissa to the left and to the right in Panel A, respectively.

** Figure 3. Simulated vertical concentration profiles. Vertical concentration distribution for simulations 1, 5, and 9 at locations P and S for (A) isoprene (B) 2-methyltetrols.

>> Figure 2. It appears that only a small fraction of the total available measurements shows non-overlapping error bars for the two locations. Can this be quantified in the text (e.g., 4/12, or whatever it may be?). In general, the error bars are hard to interpret as location P and S overlap. Can one of these be staggered slightly to the right or left, bolded, or otherwise differentiated?

** This is a good question. This information is presented in the original manuscript (lines 240-244). The text is further clarified as follows: “Between the two locations, the concentrations of pinonic and pinic acids had no difference within the experimental uncertainty of 30% for the entire data set (Table S2). In contrast, the concentration of the 2-methyltetrols family at location P differed beyond the experimental uncertainty with that of location S for 8 of the 15 samples.”

** In the revised manuscript, in response to the reviewer’s concern, a cross reference to this section is added to the caption of Figure 2.


>> Reviewer 2

>> This study explores the dynamics of semivolatile compounds in terms of horizontal concentration heterogeneity at an Amazonian forest canopy. I think this study fits in the scope of Environmental Science Atmospheres journal and shows measurements and conclusions highly relevant to the scientific community.

** We thank the reviewer for the constructive comments and feedback for improving the manuscript. The response to each comment is listed below.

>> There is no mention to pinic nor pinonic acid in introduction. And there are no clear objectives and hypothesis at the introduction, but then the hypothesis comes in results and discussion without no previous mention.

** Thank you for the comment. The manuscript is revised to include the information on pinonic and pinic acids and to clarify the objective of the study in the introduction, as follows (lines 123-125): “To these ends, in this study a hovering UAV was equipped with sorbent cartridges for SV-OVOCs and flown in the roughness layer over a forest. The objective of this study is to investigate and quantify the concentration distribution of SV-OVOCs and provide insights into the relative importance of primary SV-OVOC emissions compared to their secondary production in the atmosphere. In this regard, near-canopy concentrations of 2-methyltetrols (isoprene oxidation products) and pinonic and pinic acids (monoterpene oxidation products) were examined.”

>> There is no figure note to the figures

** Corrected. Please see response to reviewer 1. Reviewer 1 also noted this inadvertent omission by the authors in the submitted manuscript.

>> I would like to see more clarity with the 2-methyltetrol terminology. Please clearly state which species is which group (i.e. first mention to 2-methylerythritol is already in methodology. Also explain clearly organosulfate analogues. It would be better if you would use the same terminology throughout the text (clearly name the exact species instead of saying organosulfate analogues).

** Thank you for the comment. As demonstrated in the original manuscript (lines 79-83), 2-methyltetrols are a mixture that comprises two diastereomers (i.e., 2-methylthreitol and 2-methylerythritol), each of which has two enantiomers (i.e., 2-methyl-D-threitol, 2-methyl-L-threitol, 2-methyl-D-erythritol, and 2-methyl-L-erythritol). 2-methyltetrols and their organosulfate analogues can be produced through oxidation of isoprene epoxydiols in acidic sulfate aerosol particles.

** Based on the reviewer’s comment, the terminology in the revised manuscript is clarified. All instances of “organosulfate analogues” are updated to “2-methyltetrols organosulfates”.

>> The simulation details should go into methodology and a summary of the difference between simulations (not only with I am changing these values, but rather explain why do you choose such conditions for your simulations).

** Thank you for the comment. The details of the gradient transport model are previously published and described in Batista et al. (2019). After careful consideration and co-author discussions and given the length of the manuscript, we decided to keep the methodology of the model in the Supplementary Material. In response to the reviewer’s concern, however, the revised text is clarified to provide an obvious call out to this modeling section as follows (lines 279-280): “The model is described further in the Supplementary Materials as well as in Batista et al.”

** In regard to the validation of conditions used in the simulations, the parameter values and conditions used in the model simulations (Tables 1 and S3), except for the primary emission rates of 2-methyltetrols, were either measured during the sampling or obtained from the literature for this region. The revised text is clarified in the Supplementary Material (Text S1) as follows: “These values, except for the primary emission rates of 2-methyltetrols, were either measured during the sampling or obtained from the literature for the same region (i.e., the central Amazon).”

>> There is no clear explanation of the gas vs particle situation. Please explain when do you measure gas, when do you measure particle and how do you separate both. For instance, in line 408, how do you arrive to those measurements?

** Both gas- and particle-phase SV-OVOCs were collected onto the cartridges (lines 161-162).

** In regard to the calculation, the particle fractions of 2-methyltetrols obtained by Isaacman-VanWertz et al. (2016) were used. This information appears in lines 411-416 of the original manuscript: “A study by Isaacman-VanWertz et al. measured both phases separately in the wet season of the central Amazon and found that 2-methylthreitol partitioned as 69% in the gas phase and 31% in the particle phase, and 2-methylerythritol partitioned as 45% in the gas phase and 55% in the particle phase. Applying these fractions to the results of the present study leads to estimated particle-phase diastereomeric ratios of 0.34 ± 0.06 at location P and 0.32 ± 0.06 at location S. These ratios are in good agreement with the results reported in the literature for the central Amazon during the wet season (Table S4).”

>> Minor comments below:
>> Line 31-32: This sentence reads odd.

** The sentence is modified as follows: “As an example of the latter, ethanol emissions from plant roots greatly increase when flooding induces anoxic conditions and fermentation.”

>> Line 39-42: This sentence reads odd and is missing references.

** The sentence is modified, and references are added: “Isoprene is important both for anabolism in plants and for emission and subsequent chemistry in the atmosphere (Jardine et al., 2020; Yáñez-Serrano et al., 2020).”

>> Line 75-77: I do not understand why you say in regard to isoprene and right after that 2-methyl-D-erythritol is a metabolism byproduct. What does this have to do with isoprene… Please rephrase. Also both phosphates are missing pathway at the end.

** The sentence is clarified as follows: “For isoprene, the 2-methyltetrols family is an important class of SV-OVOCs. 2-Methyl-D-erythritol, which is one of the diastereomers of 2-methyltetrols, is an isoprene metabolism byproduct in plants of deoxyxylulose phosphate and methylerythritol phosphate pathways.”

>> Line 77-79: there is repetition confusing the reader… what about introducing first the 2methyltetrols family and then the diastereomers. So are they directly emitted, or by produced by isoprene oxidation? Please rephrase.

** Thank you for the comment. The text is revised as follows:

** Lines 77-79: “This compound and 2-methylthreitol are diastereomers that together make up the 2-methyltetrols family. The 2-methyltetrols family has four members, including the two aforementioned diastereomers, each of which also has two enantiomers (i.e., 2-methyl-D-threitol, 2-methyl-L-threitol, 2-methyl-D-erythritol, and 2-methyl-L-erythritol). This family is identified as produced by isoprene oxidation in the atmosphere...”

>> Line 80: What are isoprene epoxidiols? Can you introduce this subject better, are we talking about gas or particle phase? What are organosufate analogues? These two sentences are strange, you pass from plant biosynthesis to aerosol phase, and it is confusing. Please rephrase.

** Isoprene epoxydiols (IEPOX) are the first-generation oxidation products of isoprene in the atmosphere. The reactions of IEPOX and acidic sulfate aerosol particles are heterogeneous reactions that occur between the gas and the particle phases. Organosulfate analogues refer to 2-methyltetrols organosulfates. In response to the reviewer’s comment, the sentence is revised as follows (lines 79-81): “Specifically, the heterogeneous chemical reactions of isoprene epoxydiols (IEPOX, first-generation oxidation products of isoprene) in acidic sulfate aerosol particles can produce 2-methyltetrols and the 2-methyltetrols organosulfates.”

>> Line 85: what do you mean by application to the data set of the principle….

** Atmospheric oxidation of isoprene mainly produces racemic mixtures whereas biological metabolism can produce non-racemic mixtures. This principle was applied to the data set collected by González et al. to examine the role of primary emission of 2-methyltetrols in the atmosphere. The text is clarified as follows (lines 85-89): “Non-biological atmospheric oxidation of isoprene mainly produces racemic mixtures whereas biological metabolism produces non-racemic mixtures. This principle allowed the authors to conclude that direct emissions of 2-methyltetrols from the forest accounted under some conditions for more than half of the atmospheric PM concentrations of this species.”

>> Line 92: what are biomarkers? The sugars or the 2-methyltetrols

** Sugars are the biomarkers, as demonstrated by the sentence.

>> Line 97: there has to be a reference here

** References are added to the revised manuscript.

>> Line 99: explain what other factors or delete

** The sentence is clarified as follows (lines 97-99): “Despite this success, scaling from leaf, to plant, and further to forest has large uncertainties because of immense forest biodiversity, especially in tropical regions. Shifting environmental stressors with seasons is a further complicating factor.”

>> Line 120: I think this sentence, takes away credit from your study. If you say the technique is not developed, well tested and understood…. then it should not be published.

** We mentioned in the manuscript that there are limitations for the current UAV techniques such as battery capacity and flight time. The purpose of this sentence is to provide insights for future improvement and development of UAV techniques. It does not mean, however, that the data obtained using the UAV techniques are of poor quality and should not be published. In response to the reviewer’s concern, the sentence regarding the limitations of the current UAV techniques is removed to avoid confusion.

>> Line 125: ahaa first mention to monoterpenes.

** The text is revised. Please refer to comment above concerning monoterpenes.

>> Line 125-134: I feel this is methodology, and there is nothing about your hypothesis when in results and discussion you mention several hypotheses.

** This paragraph provides a general description of the objectives and background of the sampling. The detailed methodology appears in the “Methods” section.

>> Line 143: what do you mean by local topography?

** Local topography refers to the topography in the sampling region. The revised text is clarified as follows: “The forest under location P resided in a plateau region with well-drained soils.”

>> Line 144: what do you mean by local slope?

** Local slope refers to the slope area in the sampling region. The revised text is clarified as follows: “The forest under location S populated a slope between plateau and valley regions.”

>> Line 145-146: so both canopies had the same height?

** As demonstrated in the original manuscript (line 145), the canopy height varied from 25 to 35 m at both locations.

>> Line 151: what do you mean by approximately stable?

** As shown in Table S1, the variability in the meteorological parameters was small during the periods of measurement. The revised text is clarified as follows: “For the four weeks of measurements, the variability in the meteorological parameters, such as temperature, relative humidity, wind speed, and wind direction, was small during the time periods that samples were obtained (Table S1). The implication is that meteorology did not significantly contribute to the variability of relative SV-OVOC concentrations between locations P and S.”

>> Line 156-7: when you say a wide range of organic compounds do you mean both polar and non-polar compounds?

** The range depends on the interactions between the species and the adsorbent materials (i.e., species affinity to adsorbents). These species can be polar or non-polar compounds. The text is clarified as follows (lines 156-157): “They collected a wide range of organic compounds from carbon numbers of C3 through C30 based on species affinity to the adsorbents.”

>> Line 164: At which exact time are SV collected. The graphs should provide with markers that expand for as much as the collection time i.e. 20 minutes.

** The sample collections were categorized in four sequential time periods of a day, as follows: 09:00–10:00, 10:10–11:10, 11:20–12:20, and 12:30–13:30. Detailed information of the collection time of each sample appears in Table S2 in Batista et al. (2019), which focused on VOC analysis. The exact time of sample collection, although all within the above mentioned four time periods, varied slightly from day to day. After careful consideration, we think that the current presentation of the figures conveys the message without leading to confusion, and we decide to keep the original form of the figures. The revised text, however, is clarified, as follows: “UAV-facilitated samples were collected by hovering at location S in four sequential time periods of a day, as follows: 09:00-10:00, 10:10-11:10, 11:20-12:20, and 12:30-13:30, all in local time (4 h earlier than UTC). Two flights were carried out in each time period. Within a flight, a sampling duration of 2.5 min was used for VOCs for one set of cartridges (work reported in Batista et al.) and 17.5 min for SV-OVOCs for another set of cartridges. Specific sampling information, including time period of collection for each sample, appeared in Batista et al., which focused on VOC analysis from the first set of cartridges.”

>> Line 168: at what flow was sampling at P? And how do you mimic UAV sampling with the operator at the tower?

** At the tower site (location P), the sampling was performed simultaneously by using an air sampling pump. The sampling flow rate at location P was similar to that at location S. The information of sampling flow rate is updated in the text as follows:

** Lines 167-169: “SV-OVOC collection was simultaneously performed at location P by an operator using an air sampling pump (GilAir Plus, Gilian; at 0.20 L min^-1) at the top of the tower.”

>> Line 170: so you sampled for 7 consecutive days in one cartridge and you repeated this process 4 weeks. Is this correct? Please explain better in the text. Also how long did it take from collection to analysis? how do you account for decomposition in the cartridge? This can happen not only during the week of sampling but also between sampling and analysis. Why did you decide to use this sampling scheme, where you expecting differences between weeks, (i.e. wet season, central Amazonia….)?

** Collection of SV-OVOC at each time period (e.g., 09:00–10:00) was based on sampling volume. As mentioned in the original manuscript (lines 167-171), for both locations, the same set of SV-OVOC cartridges employed in the first day in a specific time window was used repeatedly in the same time window for other days of the week until a total sample collection of >100 min (i.e., >15 L) was achieved for each cartridge. It usually took less than 7 days to achieve >15 L air. Please refer to Table S1 in Batista et al. (2019) for details of the sampling time.

** This sampling approach ensured that sufficient SV-OVOC mass was collected on a cartridge so that off-line chromatographic analysis was possible (lines 171-173).

** Some reactive SV-OVOCs may undergo decomposition during storage. However, this possibility is not of large concern for this study because (1) the targeted SV-OVOC compounds are highly oxidized and relatively stable and (2) the SV-OVOCs were collected simultaneously at locations P and S with similar sample collection amount. Any decomposition would be similar for samples collected at both locations, and the scientific interpretation is based on relative concentrations rather than absolute concentrations. The revised text is clarified, as follows (lines 175-177): “After sample collection, the cartridges were sealed with Swagelok fittings and kept at room temperature before chromatographic analysis in the laboratory. Although some reactive SV-OVOCs can decompose at least in part during storage and prior to analysis, the highly oxidized SV-OVOC compounds targeted in the present study were stable, as confirmed by laboratory standards (section 2.3). Moreover, the analysis herein is based on the relative concentrations at locations S and P, and any decomposition would be similar for the samples collected at both locations and thus canceling out in the relative analysis.”

>> Line 181: Can you give a sentence explaining why the sample must be derivatized by MSTFA?

** MSTFA converts polar functional groups (e.g., OH and COOH) to a less polar trimethylsilyl group and improves the volatility and thermal stability of the targeted compounds, allowing efficient separation in the GC of analytes from non-target components. The revised text is updated, as follows (lines 188-189): “MSTFA derivatization improved the volatility and thermal stability of the targeted compounds, allowing efficient separation along the GC column of the analytes.”

>> Line 191: so this means you cannot separate between 2-methylthreitol and 2-methylerythritol?

** The reviewer’s comment indicates that the original text was unfortunately confusing. The thermal desorption process in the GC-MS analysis may convert 2-methyltetrols organosulfates into 2-methyltetrols. Therefore, separation between 2-methyltetrols organosulfates and 2-methyltetrols is difficult. This behavior, however, does not affect the detection of 2-methylthreitol and 2-methylerythritol, which are two diastereomers of 2-methyltetrols. The revised text is clarified, as follows: “This step also converted 2-methyltetrols organosulfates into 2-methyltetrols, meaning that the 2-methyltetrols family reported herein is inclusive of both atmospheric 2-methyltetrols and atmospheric 2-methyltetrols organosulfates.”

>> Line 199: brand for 2-methyltetrols

** The 2-methyltetrols standards were synthesized in the laboratory (line 201).

>> Line 233: this line comes out of the blue and should be better in intro so you can introduce pinic and pinonic acid

** The manuscript is revised. Please see earlier comment regarding pinonic and pinic acids.

>> Line 234-238: This information is really interesting, perhaps, just to compare dynamics you could show this data in supplementary, explaining limitations, but this would certainly add to the scientific community.

** The sensitivity of the GC-MS to each chemical compound varies over time. Therefore, regular calibration of GC-MS with authentic standards is required during the chemical analysis. Without authentic standards, the data quality cannot be assured.

>> Line 243-244: for many days and times? What do you mean, specify?

** We counted the number of measurements in which the concentrations of the 2-methyltetrols family differed beyond experimental uncertainty (i.e., >30%). This statement holds for 8 of the 15 samples, and it holds for many samples collected at different weeks and different periods of time. The revised text is clarified, as follows: “Between the two locations, the concentrations of pinonic and pinic acids had no difference within the experimental uncertainty of 30% for the entire data set (Table S2). In contrast, the concentration of the 2-methyltetrols family at location P differed beyond the experimental uncertainty with that of location S for 8 of the 15 samples.”

>> Line 245: -70 to +480% does not tell me anything. It is too big of a difference. The same applies to the ratios. Please find another way to express figure 2A and 2B. Perhaps talk about actual concentrations, or do not average over the 4 weeks.

** We agree with the reviewer that the actual concentrations are important. They are presented in Figure 2A and given in Table S2. The purpose of this paragraph, then, is to compare the difference of SV-OVOCs between the two locations. The large range of -70 to 480% reflects the observations, meaning atmospheric behavior, rather than uncertainty of the measurements. Although actual concentrations are important, the ratios of the concentrations measured at the two locations convey a more insightful message regarding questions such as, “Were the concentrations of SV-OVOCs at location P greater, equal, or smaller than those at location S?” and “To what extent, if they were different?”.

>> Line 247: Why and how, please explain. Perhaps this info can go into the intro when you introduce pinic and pinonic acid.

** The reason is presented in the original manuscript as follows (lines 248-254): “The ratio to pinonic acid is plotted in Figure 2B, and similar results are obtained for the ratio of pinic acid (not shown). In an ideal scenario of constant chemical conditions and emission rates in constant proportion between the species, this ratio should remain unchanged over time and have the same value over both locations P and S. The denominator (i.e., the concentration of pinonic acid) is observed as the same over both locations even as the actual concentration varies over time (Figure 2A), and it can thus serve as a normalization factor for effects of shifting meteorology (i.e., dilution) on the concentrations.”

>> Line 248: why do you choose pinonic and not pinic?

** Both pinonic acid and pinic acid had no difference between the two locations. We chose to use concentration ratio of 2-methyltetrols to pinonic acid as an example to interpret the data. However, the conclusion remains the same if pinic acid is used.

>> Figure 2b: where are the two S points of 9 and 10 am on week 1? If not there? How is this week representative?

** Omitted data typically arose for one of two reasons. Sometimes sampling was not conducted because of inclement weather. Sometimes the sample concentration was below the analytical detection limit (Table S2).

>> Line 250: do you mean VOC species or plant species?

** We meant the chemical species. The text is clarified as follows (lines 249-251): “In an ideal scenario of constant chemical conditions and emission rates in constant proportion between the chemical species...”

>> Line 251: Do you have a reference or explanation for this?

** Unfortunately, there is no reference for this statement. However, as a mathematical inference from the equations, as explained in the manuscript, in an ideal scenario of constant chemical conditions and emission rates in constant proportion between the chemical species (e.g., isoprene vs. monoterpene), the concentration ratios of 2-methyltetrols to pinonic acid from atmospheric oxidation would remain unchanged over time and have the same value over the two locations.

>> Line 253-254: can you explain why the denominator can serve as a normalization factor?

** The concentration of pinonic acid (i.e., the denominator) is observed as the same over both locations, even as the actual concentrations vary over time (lines 251-253).

>> Line 254: What location?

** The text is clarified as follows (lines 254-255): “At a single location, such as location P, the ratio varied from 2 to 51 across the different weeks.”

>> Line 258: figures 2a and 2b are not similar, week 3 is really different.

** The author team has consulted with one another on the topic, but we do not understand why the reviewer said that the message in week 3 is different between Figure 2A and Figure 2B. They are actually quite similar. For example, as shown in Figure 2A, there was no concentration difference observed for 2-methyltetrols in week 3 at time periods of 09:00–10:00, 10:10–11:10, and 11:20–12:20, whereas the concentrations of 2-methyltetrols were significantly different at 12:30–13:30. A similar observation can be found in Figure 2B.

>> Line 261: why should the ratio be banded by -40 to +40%? Please explain

** The experimental uncertainty of 30% was used to examine if the concentrations measured between the two locations were significantly different from each other. For example, the concentrations of pinonic and pinic acids had no difference between the two locations within experimental uncertainty (i.e., <30%). By comparison, the concentrations of the 2-methyltetrols family differed beyond experimental uncertainty (i.e., >30%) for many samples (i.e., 8 of 15 samples) collected in different weeks and at different time periods over locations P and S. In this error propagation for a maximum uncertainty of 30% in the numerator and denominator leads to an overall uncertainty of 40%. That is why the ratio should be banded by -40 to +40%. This explanation appears in lines 260-261 of the original manuscript.

>> Line 265: I think here you should specify 2-methyltetrols as pinic and pinonic acid do not change.

** The text is clarified as follows (lines 265-266): “Several factors could contribute to the heterogeneity of 2-methyltetrols concentrations between locations P and S.” The title of section 3.2 is also modified as: “Possible explanations of SV-OVOC 2-methyltetrols heterogeneity”

>> Line 266-270: what about further chemistry of 2methyltetrols?

** Oxidation of 2-methyltetrols was also considered in the model simulation. However, it plays a negligible role in the heterogeneity of 2-methyltetrols observed, given that the lifetime of 2-methyltetrols is long in the atmosphere (i.e., 4 to 230 days, Table S3). Please refer to section 3.2.1 of the manuscript.

>> Line 278: where do you get the values for E, D, and R? literature?

** These values were obtained from the literature. Please refer to the caption of Tables 1 and S3 for details.

>> Line 282-288: this looks more like objectives

** This paragraph introduces the reason why non-uniform VOC emission rates may lead to uneven distribution of SV-OVOCs. The paragraph after it explores this hypothesis using model simulations. Results conclude that observations of 2-methyltetrols heterogeneity cannot be explained by a non-uniform emission rate of isoprene. The discussion of the relative importance of primary SV-OVOC emissions compared to secondary production in the atmosphere is also presented as objectives in the introduction of the original manuscript in lines 94-122.

>> Line 289: Please state clearly what is the difference for each simulation, like you say in this sentence, but not only with values (i.e. 200% percent or 300% difference in emission rate, but rather Stronger uneven distribution of isoprene, less strong uneven distribution of isoprene,….for example). Also 200% between the two forests doesn’t indicate to me which forest are you choosing as higher emitter.

** The sentence is clarified, as follows (lines 289-290): “Simulation 1 is performed for a difference in emission rate of 200% between the two forests (i.e., Ep = 200% Es) and simulation 2 for a difference of 300%.”

>> Table 1: what is upwind forest? First time this is mentioned.

** Upwind forest refers to the forest located at the upwind direction of location P or S. The caption of Table 1 is clarified, as follows: “U refers to the forests located at the upwind direction which contribute to the concentrations measured at locations P and S.”

>> Line 295: please state the range

** The range is added, as follows (lines 294-295): “…the concentrations and the differences are within the range of concurrent observations at these locations (i.e., 4.4 ± 1.9 ppb at location P and 2.4 ± 2.0 ppb at location S, as presented in Batista et al.).”

>> Line 300: I think expected is not the word here, but rather supported?

** The text is modified as follows: “The result can be understood by consideration of the atmospheric lifetime of 2-methyltetrols.”

>> Line 307: what do you mean by mathematically related possibility

** The heterogeneous distribution of oxidant concentration (e.g., OH) is mathematically related to the heterogeneous distribution of VOC concentration. Specifically, both lead to a heterogeneous distribution of reaction rate in the atmosphere. The revised text is clarified, as follows: “A separate possibility, but one that is mathematically related and therefore also explored in this section, is that a heterogeneous distribution of oxidant concentration (e.g., OH concentration) leads to an uneven distribution of reaction rates and hence 2-methyltetrols concentration in the atmosphere (see Text S2 in Supplementary Materials). The simulations for the effects of non-uniform isoprene emission rates are mathematically similar to the effects of the possible spatial heterogeneity in oxidation rates.”

>> Line 320: are you still talking of Butler et al., or your results?

** It refers to this study. The text is moved to the Supplementary Materials (Text S2) to avoid confusion.

>> Line 324: explain what averaging kernel is.

** Upon formation, 2-methyltetrols molecules are subjected to atmospheric transport and dispersion. The averaging kernel represents an area that is of the same 2-methyltetrols concentration with respect to their lifetime upon formation in the atmosphere. The revised text is clarified, as follows: “In both cases, the multi-day lifetime of 2-methyltetrols implies that atmospheric turbulence evens out the 2-methyltetrols concentration across a spatial scale that is much larger than the observed heterogeneity of <1000 m.”

>> Line 327 replace present by presence.

** Done.

>> Line 346: I’m missing a reference here.

** A reference is added to the manuscript.

>> Line 363: -18% and -23% heterogeneity with respect to what?

** The values of -18% and -23% are percent differences in the concentration at location S relative to that at location P. Please refer to the caption in Table 1.

>> Line 391: how is in good agreement with the atmospheric observations, please explain.

** The concentration of 2-methyltetrols is +8%, to +51%, to +157% for location S compared to location P, for respective direct emission rates of 0.001, 0.005, and 0.015 ppb m s^-1 underlying location S while maintaining no emission underlying location P (simulations 7, 8, and 9). This result is in good agreement with the atmospheric observations with a weekly-average concentration difference of up to +176% (Table S2). The manuscript is revised, as follows (lines 390-392): “The simulated heterogeneity (e.g., +157% in simulation 9) is in good agreement with the atmospheric observations (e.g., weekly-average differences of up to +176% in Table S2).”


References

Batista, C. E. et al. Intermediate-scale horizontal isoprene concentrations in the near-canopy forest atmosphere and implications for emission heterogeneity. Proceedings of the National Academy of Sciences of the United States of America 116, 19318-19323

Isaacman-VanWertz, G. et al. Ambient gas-particle partitioning of tracers for biogenic oxidation. Environmental Science & Technology 50, 9952-9962

17-Nov-2020

Dear Dr Martin:

Manuscript ID: EA-ART-09-2020-000006.R1
TITLE: Near-canopy horizontal concentration heterogeneity of semivolatile oxygenated organic compounds and implications for 2-methyltetrols primary emissions

Thank you for submitting your revised manuscript to Environmental Science: Atmospheres. After considering the changes you have made, I am pleased to accept your manuscript for publication in its current form. I have copied any final comments from the reviewer(s) below.

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Reviewer 2

As the authors have address all my concerns properly i do recommend the articicle for publication.
As my only comment i suggest that for Table 1 you decribe what is 1-9 (i.e. 1 stands for reference conditions, 2 stands for higher isoprene at P) so the reader doesnt have to spend so much time at the table.