From the journal Environmental Science: Atmospheres Peer review history

31-Oct-2021

Dear Dr Langford:

Manuscript ID: EA-ART-07-2021-000057
TITLE: Seasonality of isoprene emissions and oxidation products above the remote Amazon

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Reviewer 1

This paper describes flux measurements of ion signal from a PTR-MS at m/z corresponding to isoprene and a sum of isoprene oxidation products (MACR, MVK, ISOPOOH). If I understand correctly, the authors use these flux measurements to extract a fraction, ISOPOOH/(MVK+MACR) (i.e. a ratio of concentrations), which they then analyze as a function of biomass burning influence (via acetonitrile or BC). They conclude that, in terms of the source of IEPOX, an SOA forming isoprene oxidation product, higher isoprene emissions in the dry season are offset by lower IEPOX production efficiency due to NO from biomass burning.

Overall summary - The paper presents an interesting hypothesis. But, the authors' main conclusions about IEPOX are at the end of a chain of if/then's attached to flux measurements by an instrument known to have chemical interferences. I personally think a paper on the fluxes and their implications for emission potential over the year would be a useful piece of knowledge for the community. The effort to connect all the way to IEPOX SOA seems forced and unable to be sufficiently tested with the evidence at hand.

Each link in the chain is certainly logical, and one could argue, even reasonable based on what is currently known, but to go from a higher flux of signal at one m/z that represents a sum of compounds, a sum perhaps not even limited to MVK, MACR, ISOPOOH in a location like the Amazon, to conclusions about annual IEPOX abundance is indirect and highly uncertain.

All that said, the conclusion that higher NOx would suppress IEPOX production seems a straightforward extension of current mechanisms. My advice would be to have a shorter paper that details the observed fluxes and a short section in the discussion on "one possible explanation for the observed seasonality" would be an increased fraction of ISOPOOH relative to MACR + MVK. Then a conclusion section that has IEPOX implications as a sentence.

Other questions / concerns:
There is a leap from Figure 1 and 2, to the ISOPOOH fraction. Is there no other explanation for a decreased Rc than a higher ISOPOOH fraction? The authors need to make this connection far more concrete for the conclusions about IEPOX to be taken more seriously. At the moment it is simply stated that "This [Fig 1 + Fig 2] can be related to a change in the relative abundance of the three components of iso_ox." What if, e.g., Vd of ISOPOOH isn't a fixed value but depends on canopy conditions that change seasonally?

A Vd of 4.5 cm/s is assumed or ISOPOOH. In the same section, the authors point to a study by Canval et al (to support efficient ISOPOOH uptake) where I believe they argue for a stomatal uptake with a Vd of 0.8 cm/s? Not that 1 enclosure study would represent the Amazon forest, but it seems a significant discrepancy if the authors are assuming 0.45 cm/s for MVK + MACR.

Why GEOS-Chem doesn't find increased NO in the BB period is interesting, but the lifetimes of NO and ISOPOOH are pretty short so comparing to a single tower measurement might be inconclusive not least in part due to isoprene emission parameterization uncertainties. Could satellite observations of NO2 be used as a constraint - probably clouds will be a challenge but if they column had a seasonality that would at least be an observation to use as a counterpoint to the GEOS-Chem simulation.

Why is there such an effort to connect to IEPOX? Has it been shown to be particularly important to OA in the Amazon? Surely it would be most important outside of BB (I would assume BBOA to dominate the OA budget then). But even during the wet season, it might be that IEPOX SOA formation is more a function of sulfate availability than IEPOX availability (Riva et al 2019 ES&T: https://pubs.acs.org/doi/10.1021/acs.est.9b01019)

Reviewer 2

Review of manuscript EA-ART-07-2021-000057 “Seasonality of isoprene emissions and oxidation products above the remote Amazon” by Langford et al.

The manuscript presents unique observations and related analysis of isoprene chemistry and oxidation pathways on seasonal basis from Amazonia. It has a clear “red thread” and delivers a clear message. Methodological and supplementary parts are comprehensive and provide necessary background for main body of the manuscript.
Overall this is a high quality work deserving publication.
My only comment is related to SOA and how its formation is linked to different isoprene oxidation mechanisms presented in the manuscript. In Figure 5 it is shown what is a potential reduction of IEPOX formation during due to increased NO concentrations. Authors several times in manuscript mention influence on SOA formation and related effects on clouds. If there is a clear link between NO and BC, than I believe that there should be also data available on link between BC and aerosol number/mass in PM1 or PM2.5 fraction. Although it is not directly along the lines of main scope of the manuscript, I would like to encourage authors to extend the discussion on comparing the potential change in SOA formation through changes in IEPOX on CCN (one can safely assume that contribution will be in aerosol mass, not number) and relate it to changes in CCN number and/or PM mass associated with advection of biomass burning related pollution. Is this reduction actually important? If so, give estimate on what is magnitude.

We thank both reviewers for their comments on our manuscript and we will address each of their points below. Each Author comment is marked as "AC:"

Referee: 1

Comments to the Author
This paper describes flux measurements of ion signal from a PTR-MS at m/z corresponding to isoprene and a sum of isoprene oxidation products (MACR, MVK, ISOPOOH). If I understand correctly, the authors use these flux measurements to extract a fraction, ISOPOOH/(MVK+MACR) (i.e. a ratio of concentrations), which they then analyze as a function of biomass burning influence (via acetonitrile or BC). They conclude that, in terms of the source of IEPOX, an SOA forming isoprene oxidation product, higher isoprene emissions in the dry season are offset by lower IEPOX production efficiency due to NO from biomass burning.

Overall summary - The paper presents an interesting hypothesis. But, the authors' main conclusions about IEPOX are at the end of a chain of if/then's attached to flux measurements by an instrument known to have chemical interferences. I personally think a paper on the fluxes and their implications for emission potential over the year would be a useful piece of knowledge for the community. The effort to connect all the way to IEPOX SOA seems forced and unable to be sufficiently tested with the evidence at hand.

Each link in the chain is certainly logical, and one could argue, even reasonable based on what is currently known, but to go from a higher flux of signal at one m/z that represents a sum of compounds, a sum perhaps not even limited to MVK, MACR, ISOPOOH in a location like the Amazon, to conclusions about annual IEPOX abundance is indirect and highly uncertain.

AC: The reviewer is right to point out that our work involves a set of assumptions which introduce uncertainty to the results presented. It is for this reason that we devoted an entire section of the paper to discuss in detail all of the assumptions and uncertainties that went into our work (See Section 4 – Assumptions and uncertainties). This includes sensitivity studies exploring the values of Vd(s) used for both ISOPOOH and MVK+MACR, as well as investigating the potential seasonality of OH concentrations used in our model simulations. We also carefully ground truth our results through comparison with the only direct observations of ISOPOOH to MVK+MACR production ratios. We conclude this section by stating that further measurements, with the latest mass spectrometers (capable of measuring ISOPOOH directly), are now required to better quantify the complex role that anthropogenic pollution plays in mediating isoprene oxidation chemistry in the Amazon. We believe this section, gives a full and frank assessment of the uncertainties upon which our results depend. This is presented within the main manuscript, rather than SI, to ensure the assumptions and uncertainties are clearly visible to the reader.

All that said, the conclusion that higher NOx would suppress IEPOX production seems a straightforward extension of current mechanisms. My advice would be to have a shorter paper that details the observed fluxes and a short section in the discussion on "one possible explanation for the observed seasonality" would be an increased fraction of ISOPOOH relative to MACR + MVK. Then a conclusion section that has IEPOX implications as a sentence.

AC: We thank the reviewer for their suggestion. However, we believe that in its current form, the manuscript provides a very clear and robust analysis of the data, accompanied by a comprehensive and upfront assessment of the uncertainties and assumptions. Limiting the scope of the paper would seem a great disservice to a piece of work which Reviewer 2 describes as “high quality” and “deserving of publication”. An argument for limiting the analysis to a discussion of fluxes could be made if our results were either highly controversial or poorly justified, but the reviewer points out several times how our findings appear to be “a straightforward extension of current mechanisms” and that the steps of our analysis appear both “logical” and “reasonable”.
There are currently no seasonal measurements of the ISOPOOH/MACR+MVK branching ratios available in the literature and our analysis, despite being indirectly inferred from the flux behaviour, makes a significant contribution to literature.

Other questions / concerns:
There is a leap from Figure 1 and 2, to the ISOPOOH fraction. Is there no other explanation for a decreased Rc than a higher ISOPOOH fraction? The authors need to make this connection far more concrete for the conclusions about IEPOX to be taken more seriously. At the moment it is simply stated that "This [Fig 1 + Fig 2] can be related to a change in the relative abundance of the three components of iso_ox." What if, e.g., Vd of ISOPOOH isn't a fixed value but depends on canopy conditions that change seasonally?

AC: We agree that it is important to discuss other potential mechanisms for the change in ISOPOOH to MVK+MACR production ratio and would direct the reviewer to Section 3 of the SI where we already discuss in detail the possible explanations for the observed seasonality, including changes in canopy conditions, and why we believe the most plausible explanation is the changing contribution of ISOPOOH to MVK+MACR.

A Vd of 4.5 cm/s is assumed or ISOPOOH. In the same section, the authors point to a study by Canval et al (to support efficient ISOPOOH uptake) where I believe they argue for a stomatal uptake with a Vd of 0.8 cm/s? Not that 1 enclosure study would represent the Amazon forest, but it seems a significant discrepancy if the authors are assuming 0.45 cm/s for MVK + MACR.

AC: We are aware of the data presented by Canaval et al. (2020), and that their measurements do not derive the deposition rate that would be expected in our study. First, we would point out that the figure of 0.8 cm s-1 presented by Canaval et al. (2020) is a deposition velocity (e.g. Vd = 1/(Ra+Rb+Rc)) and our analysis is based on deposition velocities extrapolated to the surface (e.g. Vd (s) = 1/(Rb+Rc)). Although we can assume that the influence of Ra would be minimal, due to the enclosure fan generating turbulence, it would not be zero. Second, the enclosure measurements are made under fairly dry lab conditions (RH≈35%) thus suppressing the non-stomatal deposition pathway to the leaf cuticle compared with the conditions in the Amazon forest. We note that even under these dry conditions, the night-time fluxes of Canaval et al. accounted for 15% of the daytime flux. Third, and possibly most significantly, the enclosure deposition velocities presented by Canaval et al. (2020) are calculated as Vd = –flux/concentration, where the flux is in units of mass per m2 of leaf area per second. The canopy scale flux measurements we present calculate the deposition velocity based on a flux per m2 of land surface area per second. As we discuss in Section 2 of the SI, the leaf area index (LAI) at our site, derived from the MODIS satellite observations, had an average of 4.6 m2/m2 and therefore, a much larger deposition flux and hence Vd(s) would be expected for our site. Across different studies in the literature, canopy resistance has been scaled either by LAI or its square root, which means a leaf-area value of Vd = 0.8 cm s-1 would relate to a canopy value of 3.7 cm s-1, even without taking into account the additional effect of Ra or leaf moisture.
As explained clearly within our manuscript, the Vd(s) of 4.5 cm s-1 used in our analysis is based on the understanding that ISOPOOH deposits with a zero Rc as demonstrated by comparable eddy covariance measurements made above a temperate forest by Nguyen et al. (2015), and Canaval et al. (2020) themselves state that their finding are consistent with those of Nguyen et al. (2015). Within Section 4 of the manuscript we explore the implications for our results when assuming different values of Vd(s) for both ISOPOOH and MVK+MACR.

Why GEOS-Chem doesn't find increased NO in the BB period is interesting, but the lifetimes of NO and ISOPOOH are pretty short so comparing to a single tower measurement might be inconclusive not least in part due to isoprene emission parameterization uncertainties. Could satellite observations of NO2 be used as a constraint - probably clouds will be a challenge but if they column had a seasonality that would at least be an observation to use as a counterpoint to the GEOS-Chem simulation.

AC: We thank the reviewer for their suggestion. We have analysed NO2 satellite retrievals for the period between 2005 and 2014 for the grid square coincident with the ZF2 measurement site. This shows a clear seasonal cycle in NO2, with higher concentrations during the dry season and lower during the wet season, which is consistent with our derived NO concentrations. We have added both the average (2005 to 2014) and current (2013/14) NO2 column concentration to figure S9 of the SI.





Why is there such an effort to connect to IEPOX? Has it been shown to be particularly important to OA in the Amazon? Surely it would be most important outside of BB (I would assume BBOA to dominate the OA budget then). But even during the wet season, it might be that IEPOX SOA formation is more a function of sulfate availability than IEPOX availability (Riva et al 2019 ES&T: https://pubs.acs.org/doi/10.1021/acs.est.9b01019)


AC: Our objective is simply to better understand the seasonality of isoprene oxidation chemistry within the Amazon. At present the study published in PNAS by Liu et al. (2016) is the only data set where the production ratios of ISOPOOH to MVK+MACR have been measured, but this data seemingly only covers two days of measurements. We note that Liu et al. (2016) went through the same “effort” to connect their measurements with the isoprene oxidation pathways and we simply replicate their approach. The major difference is that we employ an innovative analysis of the measured deposition velocities to derive the fraction of ISOPOOH and MVK+MACR. While this approach has larger uncertainties associated with it, our observations cover an 11 month period which provides a unique opportunity to explore how isoprene oxidation chemistry changes with season and how these changes relate to current understanding.
The reviewer is of course correct to point out that the formation of IEPOX SOA is dependent on the availability of sulphate. This is why we were careful to limit our modelling to that of IEPOX (the SOA precursor) rather than the end product IEPOX SOA. But both of these influences, e.g. IEPOX production and sulphate availability, need to be understood to provide the mechanistic understanding to predict IEPOX SOA production in the Amazon.


Referee: 2

Comments to the Author
Review of manuscript EA-ART-07-2021-000057 “Seasonality of isoprene emissions and oxidation products above the remote Amazon” by Langford et al.

The manuscript presents unique observations and related analysis of isoprene chemistry and oxidation pathways on seasonal basis from Amazonia. It has a clear “red thread” and delivers a clear message. Methodological and supplementary parts are comprehensive and provide necessary background for main body of the manuscript.
Overall this is a high quality work deserving publication.
My only comment is related to SOA and how its formation is linked to different isoprene oxidation mechanisms presented in the manuscript. In Figure 5 it is shown what is a potential reduction of IEPOX formation during due to increased NO concentrations. Authors several times in manuscript mention influence on SOA formation and related effects on clouds. If there is a clear link between NO and BC, than I believe that there should be also data available on link between BC and aerosol number/mass in PM1 or PM2.5 fraction. Although it is not directly along the lines of main scope of the manuscript, I would like to encourage authors to extend the discussion on comparing the potential change in SOA formation through changes in IEPOX on CCN (one can safely assume that contribution will be in aerosol mass, not number) and relate it to changes in CCN number and/or PM mass associated with advection of biomass burning related pollution. Is this reduction actually important? If so, give estimate on what is magnitude.

AC: We thank reviewer two for their positive comments. As discussed in relation to the last point raised by Reviewer 1, the relationship between isoprene oxidation pathways and SOA production is modulated by the amount of sulphate and direct relationships are not necessarily expected. Unfortunately, we do not have the requisite PM data to extend our analysis beyond its current scope. With a lack of PM measurements, we made a conscious effort not to look beyond the formation of the SOA precursor IEPOX and deliberately avoided attempting to estimate effects on actual aerosol abundance or CCN.

08-Jan-2022

Dear Dr Langford:

Manuscript ID: EA-ART-07-2021-000057.R1
TITLE: Seasonality of isoprene emissions and oxidation products above the remote Amazon

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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