From the journal Environmental Science: Atmospheres Peer review history

14-Jul-2021

Dear Dr Lee:

Manuscript ID: EA-ART-05-2021-000041
TITLE: Ozone production and precursor emission from wildfires in Africa

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

************

Reviewer 1

This paper describes measurements of NOx, CO, O₃, HCN, and a few selected VOCs in wildfire plumes during several aircraft campaigns in Africa and off the west coast of Africa. The data and analysis are useful, particularly since there are not many studies of wildfire chemistry in this environment. There are a number of things about the analysis and presentation that need to be attended to before the paper is acceptable for publication. I have the following general and specific comments.

General Comments

There are numerous places where the authors have neglected to include units. The emission factors (EFs) are quoted without units in the text and in Table 1. Estimated air mass ages are given in Tables 2, S1a, and S1b without units. The expert reader might reasonably assume what they are, but we should not have to.

It should be noted that NOx and CO come from completely different fire regimes (see Roberts et al., 2020 for a recent reference): CO from low temperature pyrolysis and NOx from high temperature combustion. As a result, while NOx and CO will be correlated in a given plume once it’s been emitted, there can be very different NOx/CO ratios for different fires.

The authors should strongly consider using Ox (O₃ + NO₂) for their analysis of O3 production, especially close-in to fires where NO titration could be happening. Ox should give a much clearer signal of O₃ production.

Specific Comments

Abstract: Putting a sentence like “It is estimated that surface emissions from biomass burning contribute ~24% to boundary layer ozone over Africa” in the abstract makes it sound like a result from this paper. Actually, it is from the literature and appears in the introduction (and nowhere else). This should not appear in this abstract in this form. If the authors want to allude to O₃ as a reason for doing this study they need to completely change the way they phrase this.

Line 26: you report EFs for 9 VOCs in Table 1. Mole fractions of what?

Lines 27&28: What are the units? What is the basis of the NOx EF (NO, NO₂?).

Line 38: “is” should be ‘of’.

Line 200: Complementary to what?

Line 249: “shortly” should be ‘short’.

Line 266: How was EFCO calculated?

Line 281. Is it really large error, or just large natural variability due to the fact that NOx and CO come from different fire regimes?

Line 295: should be ‘butane’.

Line 303. How do you know your HCHO is all from primary emissions? There are a lot of highly reactive VOCs emitted by fires that from HCHO readily.

Line 305. Again, we must make the point that NOx and VOCs come from completely different fire regimes, so we would not necessarily expect consistent numbers from different fires.

Lines 338-339: The use of the Toluene/ Benzene ratio is an interesting approach. Perhaps you could reference the original work on this (Roberts et al., 1984)?

Eq 2&3: What did you use for the ratios at t=0?

Line 395-399: It is not clear that all the NMBs you are quoting are for outside the plumes.

Line 404-405: Given that you have some information about the extent of the plumes, can you estimate what the impact of diluting plumes into model cells would be?

Section 3.5: Another thing to consider with regards to model resolution is that the O₃ formation chemistry is not necessarily linear with respect to concentrations of precursors.

Line 446: The authors mention furans specifically, perhaps they could reference Coggon, et al., 2019, who quantified how furans contribute to WF reactivity?

Line 462: Using Ox instead of O₃ might change your conclusions regarding O3-production close in to fires.

Line 470: Undoubtedly there is a lot of VOC chemistry missing from GEOS-Chem that is specific to biomass burning and wildfires.

Figure 3: I have a hard time reading the numbers on the axes of these graphs, and the points for HCN are hard to see.

Figure 4: I have a hard time seeing the data points in these figures. Would it be better to use a log scale for the NOx data since some many points end up off the edge of the graph?

Table 1: What were the MCE ranges for the fires used to generate these EFs?

Figures S1&S2. The individual points are hard to see on these graphs.

References for this review:

Coggon, M.M. et al., OH-chemistry of non-methane organic gases (NMOG) emitted from laboratory and ambient biomass burning smoke: evaluating the influence of furans and oxygenated aromatics on ozone and secondary NMOG formation, Atmos. Chem. Phys., 19, 14875-14899, 10.5194/acp-19-14875-2019, 2019.

Roberts, J.M. et al., Measurements of aromatic hydrocarbon ratios and NOx concentrations in the rural troposphere: Estimates of air mass photochemical age and NOx removal rate, Atmos. Environ., 18, 2421-2432, 1984.

Roberts, J.M. et al., The nitrogen budget of laboratory-simulated western US wildfires during the FIREX 2016 FireLab study, Atmos. Chem. Phys., 20, 8807-8826, 2020, 10.5194/acp-20-8807-2020, 2020.

Reviewer 2

This paper presents results from multiple flight measurements sampling wildfires in West and Central African savannah regions. The authors calculated EFs for major products and O3 enhancement, and then compared the measurements with model predictions. This study provides useful information about biomass burning emissions in this region in a well-structured manner. I recommend publication after the following minor comments are addressed.
Specific comment:
1. Line 158: “Specifics about the principles of operation for this instrument are provided by…” This sentence is incomplete.
2. Line 249: Change “shortly” to “short”.
3. Figure 7: Can the authors add correspondence of data points to the flight number (if it can be achieved without adding complexity to the graph)?
4. Figure 9: The differences between inside and outside the plume NMB were noticeable for CO, NOx, and O3. Can the authors elaborate more about it? In addition, can the authors suggest what could be the missing sources for formaldehyde, which was significantly underestimated in the model?

The authors thank the reviewers for their helpful comments. We answer each one in turn here.


Referee: 1

Comments to the Author
This paper describes measurements of NOx, CO, O₃, HCN, and a few selected VOCs in wildfire plumes during several aircraft campaigns in Africa and off the west coast of Africa. The data and analysis are useful, particularly since there are not many studies of wildfire chemistry in this environment. There are a number of things about the analysis and presentation that need to be attended to before the paper is acceptable for publication. I have the following general and specific comments.

General Comments

There are numerous places where the authors have neglected to include units. The emission factors (EFs) are quoted without units in the text and in Table 1. Estimated air mass ages are given in Tables 2, S1a, and S1b without units. The expert reader might reasonably assume what they are, but we should not have to.

We apologise for not including units. All emission factors are reported in g / kg of dry fuel burned. This has now been added to the text (line 269) and the table and figure captions. Age is in hours and this has also now been added to the caption of table 2 and S1a and S1b.


It should be noted that NOx and CO come from completely different fire regimes (see Roberts et al., 2020 for a recent reference): CO from low temperature pyrolysis and NOx from high temperature combustion. As a result, while NOx and CO will be correlated in a given plume once it’s been emitted, there can be very different NOx/CO ratios for different fires.

We thank the reviewer for pointing this out. We believe the fires sampled here to be mixed-phase and so it would be difficult to separate low and high temperature burning. We have changed the text in this section so it now reads:

It should be noted that NOx and CO are emitted from completely different fire regimes: CO from low temperature pyrolysis and NOx from high temperature combustion. We believe the fires sampled are a combination of the different types of burning, reflected in the relatively large error on the emission factor calculations. The NOx emission factors calculated here should thus be treated with caution and not attributed to any particular burning type.


The authors should strongly consider using Ox (O₃ + NO₂) for their analysis of O3 production, especially close-in to fires where NO titration could be happening. Ox should give a much clearer signal of O₃ production.

This is a very good suggestion and we have repeated the analysis using Ox vs CO as well as O3 vs CO. However, we see very little difference in the enhancements and all Ox vs CO slopes are within error of the corresponding O3 vs CO. We have added a note in the text describing this (lines 337 – 341) and a figure in the supplementary information showing the plots. We do not feel it is necessary to include this figure in the main manuscript.


Specific Comments

Abstract: Putting a sentence like “It is estimated that surface emissions from biomass burning contribute ~24% to boundary layer ozone over Africa” in the abstract makes it sound like a result from this paper. Actually, it is from the literature and appears in the introduction (and nowhere else). This should not appear in this abstract in this form. If the authors want to allude to O₃ as a reason for doing this study they need to completely change the way they phrase this.

We do not think the sentence that the reviewer quotes is in the abstract so we have left it unchanged.


Line 26: you report EFs for 9 VOCs in Table 1. Mole fractions of what?

We have added g kg-1 to the text.


Lines 27&28: What are the units? What is the basis of the NOx EF (NO, NO₂?).

We have added g kg-1 to the text and stated that the NOx EF is calculated as NO.


Line 38: “is” should be ‘of’.

Changed.


Line 200: Complementary to what?

We have removed the word ‘complementary’.


Line 249: “shortly” should be ‘short’.

Changed.


Line 266: How was EFCO calculated?

We used the values calculated in Barker et al 2020 and state this later in the section. We have now moved this statement to line 27 so it appears directly after the equation.


Line 281. Is it really large error, or just large natural variability due to the fact that NOx and CO come from different fire regimes?

We have reworded this section in line with the earlier comment from the reviewer.


Line 295: should be ‘butane’.

Changed.


Line 303. How do you know your HCHO is all from primary emissions? There are a lot of highly reactive VOCs emitted by fires that from HCHO readily.

This is a good point and one we cannot really answer we have added the following sentence to explain this:

We cannot separate directly emitted HCHO with any that may have been formed in the time from emission to our measurements (1 – 2.4 hours) so our emission factor should be treated as an upper limit.


Line 305. Again, we must make the point that NOx and VOCs come from completely different fire regimes, so we would not necessarily expect consistent numbers from different fires.

We have added the following sentence to the end of the paragraph.

Again it should be noted that as the fires sampled are mixed in nature, and VOCs and NOx are emitted from different types of fires, we would not necessarily expect these numbers to be consistent.


Lines 338-339: The use of the Toluene/ Benzene ratio is an interesting approach. Perhaps you could reference the original work on this (Roberts et al., 1984)?

We thank the reviewer for pointing this reference out and have now added it.


Eq 2&3: What did you use for the ratios at t=0?

The initial ratio was taken from the measured data sampled very close to the fires which has been removed from the O3 enhancement analysis as described earlier in the text. We have added a sentence to state this:

The toluene:benzene ratio at t=0 was determined using the measurements made very close to the fires which was removed from the O3 enhancement analysis as described above.


Line 395-399: It is not clear that all the NMBs you are quoting are for outside the plumes.

We have added some text to make this clearer. The section now reads:

For CO, the model under-predicts the observations by between 2 and 13% outside of the plumes, with a total NMB across all flights of -4.6%. O3 shows a similar agreement with NMB between -5.0 and 11.2% and a total of 0.7%. In both cases the comparison between modelled and measured data is within the standard deviation of the data. For NOx the agreement outside of the plumes is poorer, with NMB between -20.1 and 3.2% for the flights and a total across all flights of -9.9%.


Line 404-405: Given that you have some information about the extent of the plumes, can you estimate what the impact of diluting plumes into model cells would be?

The ability to capture diluting plumes within a global Eulerian model has been explored elsewhere, for instance by Eastham et al (2017) cited in this manuscript. The ARNA campaign did not seek to map the 3D extent of the biomass plumes, as it was primarily focused on sampling within dust plumes, and the entry and exit points of the aircraft into to plumes would not provide sufficient 3D information.


Section 3.5: Another thing to consider with regards to model resolution is that the O₃ formation chemistry is not necessarily linear with respect to concentrations of precursors.

Updated sentence on model resolution to explicitly cover this point. The sentence now reads:

It is known too that Eulerian models will struggle to represent individual plumes on the scales of these fires, as least in part due to their limited vertical resolution, and coarser model resolution can fail to capture non-linearity of O3 formation chemistry.


Line 446: The authors mention furans specifically, perhaps they could reference Coggon, et al., 2019, who quantified how furans contribute to WF reactivity?

We have added this reference.


Line 462: Using Ox instead of O₃ might change your conclusions regarding O3-production close in to fires.

As stated to the initial comment about this we have now used Ox in our analysis and it makes virtually no difference to the enhancement and hence the conclusions. We thank the reviewer for the suggestion though!


Line 470: Undoubtedly there is a lot of VOC chemistry missing from GEOS-Chem that is specific to biomass burning and wildfires.

We agree and have added ‘VOCs’ to the text to better illustrate this point.


Figure 3: I have a hard time reading the numbers on the axes of these graphs, and the points for HCN are hard to see.

Changed


Figure 4: I have a hard time seeing the data points in these figures. Would it be better to use a log scale for the NOx data since some many points end up off the edge of the graph?

Changed


Table 1: What were the MCE ranges for the fires used to generate these EFs?

We used 0.95 for Senegal and 0.94 for Uganda, as presented in Barker et al 2020. This has now been added to the figure caption and the text.


Figures S1&S2. The individual points are hard to see on these graphs.

We have increased the size of the points and made them open circles which makes them easier to see.


References for this review:

Coggon, M.M. et al., OH-chemistry of non-methane organic gases (NMOG) emitted from laboratory and ambient biomass burning smoke: evaluating the influence of furans and oxygenated aromatics on ozone and secondary NMOG formation, Atmos. Chem. Phys., 19, 14875-14899, 10.5194/acp-19-14875-2019, 2019.

Roberts, J.M. et al., Measurements of aromatic hydrocarbon ratios and NOx concentrations in the rural troposphere: Estimates of air mass photochemical age and NOx removal rate, Atmos. Environ., 18, 2421-2432, 1984.

Roberts, J.M. et al., The nitrogen budget of laboratory-simulated western US wildfires during the FIREX 2016 FireLab study, Atmos. Chem. Phys., 20, 8807-8826, 2020, 10.5194/acp-20-8807-2020, 2020.



Referee: 2

Comments to the Author
This paper presents results from multiple flight measurements sampling wildfires in West and Central African savannah regions. The authors calculated EFs for major products and O3 enhancement, and then compared the measurements with model predictions. This study provides useful information about biomass burning emissions in this region in a well-structured manner. I recommend publication after the following minor comments are addressed.
Specific comment:

1. Line 158: “Specifics about the principles of operation for this instrument are provided by…” This sentence is incomplete.

Gerbig et al. now added to the end of the sentence.


2. Line 249: Change “shortly” to “short”.

Changed.


3. Figure 7: Can the authors add correspondence of data points to the flight number (if it can be achieved without adding complexity to the graph)?

This has now been done in an updated figure.


4. Figure 9: The differences between inside and outside the plume NMB were noticeable for CO, NOx, and O3. Can the authors elaborate more about it? In addition, can the authors suggest what could be the missing sources for formaldehyde, which was significantly underestimated in the model?

As explored in section 3.5, the 25x25km global model would be better expected to capture background (outside of plume) concentrations. Although the coarser model often captures the qualification enhancements within plumes, we would not expect the model to be able capture the quantitative values of the enhancement. A sentence and reference has been added to Section 3.4, that gives some context for the biomass burning emissions inventory used here (QFED) and how this compares to other inventories commonly used and how this could contribute to an underestimate of reactive VOCs that could explain the low bias seen for formaldehyde.

It is worth noting too, that the QFED emission inventory used here prescribes higher fluxes of CO (as well as organic carbon and black carbon) than other inventories for this region (Carter et al 2020) and some of the missing sources of formaldehyde could be due to carbon being emitted as CO within the inventory when it in fact it should be emitted as more reactive VOCs.

Carter, T. S., Heald, C. L., Jimenez, J. L., Campuzano-Jost, P., Kondo, Y., Moteki, N., Schwarz, J. P., Wiedinmyer, C., Darmenov, A. S., da Silva, A. M., and Kaiser, J. W.: How emissions uncertainty influences the distribution and radiative impacts of smoke from fires in North America, Atmos. Chem. Phys., 20, 2073–2097, https://doi.org/10.5194/acp-20-2073-2020, 2020.




 

29-Aug-2021

Dear Dr Lee:

Manuscript ID: EA-ART-05-2021-000041.R1
TITLE: Ozone production and precursor emission from wildfires in Africa

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 1

The Authors have dealt with all of the original review comments

Reviewer 2

N/A