From the journal Environmental Science: Atmospheres Peer review history

09-Aug-2023

Dear Dr Permar:

Manuscript ID: EA-ART-06-2023-000098
TITLE: Assessing formic and acetic acid emissions and chemistry in western U.S. wildfire smoke: implications for atmospheric modeling

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Associate Editor, Environmental Science: Atmospheres

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

Reviewer 1

This study present a comprehensive investigation of formic and acetic acid emissions, chemistry, and model representation over the western U.S using measurements from the WE-CAN and FIREX-AQ aircraft campaigns. The results provide some references for understanding formic and acetic acid emissions from large wildfires and near-field production and also offer valuable insights for improving future model development. The manuscript is well written and the approach is clear and detailed. In general, I recommend publication of this work, and have some suggestions below.

1) Glycolaldehyde is an important potential interference of acetic acid in PTR measurement, and the quantification of acetic acid needs to be treated with great caution. PTR-ToF m/z 61 was treated as acetic acid in this work, which is not entirely convincing, though the authors have given a certain degree of explanation.

2) As noted by the authors, current models are missing a substantial amount of secondary formic acid production in BB smoke. Based on a recent study, photo-oxidation of aromatic hydrocarbons could contribute to a significant missing source of atmospheric acidity through the formation of HCOOH and other small organic acids (DOI: 10.1021/acs.est.0c00526). I would suggest the authors take aromatics into consideration when modeling chemical productions of FA and AA.

Reviewer 2

Thank you for a very thorough work-up of these lovely data sets. You've deeply explored most of the relevant questions I had on both the thought-provoking measurement inconsistencies (comparing with the literature and across instruments) as well as interpreting the smoke emissions. It is my opinion that your manuscript should be accept without the need for further revisions.

Below are some questions / comments that I developed during my study of your work:
LINE 422: It's a bit tough to square that both the FA and AA measurements are in better agreement with the literature values than the I- CIMS FA ERs. Fig. 1 provides some evidence that the instruments see the same result albeit a bit smeared out over time for the PTR. However, the first two plume passes exhibit poor agreement between the instruments despite similar peak concentrations with the last two plumes. Further, those data come from the Taylor Creek fire, which seems to be biased towards the lower end of the measured concentrations (Fig. 4). Ruling out erroneously high numbers from the I- CIMS remains difficult. However, the authors' thorough analysis and discussion is much appreciated.

LINE 548: Are EFs or ERs available from the other studies for comparison with WE-CAN and FIREX AQ FA EFs or ERs? If so, are the other studies' values much lower? I'm struggling to understand the justification for the authors' hypothesis that rapid production leads to higher FA EFs and ERs.

LINE 628: On the contrary, modeled AA NEMRs appear to agree well with Taylor Creek observations, but not the rest, and seeming majority, of the data to my eye.

LINE 917: Could controlling the IMR humidity also be a viable solution to this problem?

We thank the referees for their very positive feedback. Our responses to the reviewer comments are below. In addition to the changes discussed in our responses, we have made minor grammatical and spelling corrections throughout the manuscript, as well as have added two relevant references that have become available since our initial submission: Gkatzelis et al., 2023 (https://doi.org/10.5194/egusphere-2023-1439) and Jaing et al., 2023 (https://doi.org/10.5194/egusphere-2023-1140)

REVIEWER REPORT(S):
Referee: 1

Comments to the Author
This study present a comprehensive investigation of formic and acetic acid emissions, chemistry, and model representation over the western U.S using measurements from the WE-CAN and FIREX-AQ aircraft campaigns. The results provide some references for understanding formic and acetic acid emissions from large wildfires and near-field production and also offer valuable insights for improving future model development. The manuscript is well written and the approach is clear and detailed. In general, I recommend publication of this work, and have some suggestions below.

1) Glycolaldehyde is an important potential interference of acetic acid in PTR measurement, and the quantification of acetic acid needs to be treated with great caution. PTR-ToF m/z 61 was treated as acetic acid in this work, which is not entirely convincing, though the authors have given a certain degree of explanation.

This is a good point, and we agree that glycolaldehyde is an important potential interference that we are unable to fully resolve with the measurements available for this work. The main conclusion that treating m/z 61 as primarily AA would influence is that the ERs/EFs during WE-CAN and FIREX-AQ could be biased high by ~30 %. Consequently, as discussed in Section 2.2.1, AA in this work should be interpreted as an upper bound. We have added the following sentences to reiterate this point throughout the manuscript and note here that the potential interference from glycolaldehyde does not change the main conclusions of this study, as the model underestimates are much great than the uncertainty in the interference.
450-452: “We note that by treating the PTR-ToF m/z 61 as being primarily AA (Section 2.2.1), the WE-CAN and FIREX-AQ ERs and EFs likely represent an upper bound, though still agreeing well with literature values assuming potentially 30 % glycolaldehyde interference.”
750-751: “Similarly, the model underestimate of AA is much greater than can be explained by the potential interference of glycolaldehyde measured at m/z 61.”


2) As noted by the authors, current models are missing a substantial amount of secondary formic acid production in BB smoke. Based on a recent study, photo-oxidation of aromatic hydrocarbons could contribute to a significant missing source of atmospheric acidity through the formation of HCOOH and other small organic acids (DOI: 10.1021/acs.est.0c00526). I would suggest the authors take aromatics into consideration when modeling chemical productions of FA and AA.

Thank you for bringing this reference to our attention. FA production from aromatics is implemented in the current version of GEOS-Chem, though the mechanism likely does not simulate the ketene-enol chemistry in detail. For the Taylor Creek Fire discussed in the manuscript, ~25 % of the FA production in the GEOS-Chem mechanism is from aromatic compounds. However, the MCM does not include this potential pathway, giving negligible FA production from aromatics. We have noted this knowledge gap and added discussion of the mechanism proposed by Wang et al., 2020 to lines 660–668.

Referee: 2

Comments to the Author
Thank you for a very thorough work-up of these lovely data sets. You've deeply explored most of the relevant questions I had on both the thought-provoking measurement inconsistencies (comparing with the literature and across instruments) as well as interpreting the smoke emissions. It is my opinion that your manuscript should be accept without the need for further revisions.

Below are some questions / comments that I developed during my study of your work:
LINE 422: It's a bit tough to square that both the FA and AA measurements are in better agreement with the literature values than the I- CIMS FA ERs. Fig. 1 provides some evidence that the instruments see the same result albeit a bit smeared out over time for the PTR. However, the first two plume passes exhibit poor agreement between the instruments despite similar peak concentrations with the last two plumes. Further, those data come from the Taylor Creek fire, which seems to be biased towards the lower end of the measured concentrations (Fig. 4). Ruling out erroneously high numbers from the I- CIMS remains difficult. However, the authors' thorough analysis and discussion is much appreciated.

We agree that we cannot adequately rule out the I- CIMS FA measurement during WE-CAN being erroneously high and point to the need for future work assessing FA measured by CIMS.
As per Fig. 1 and the Taylor Creek fire, we speculate that part of the reason why that the first two plume intercepts show poor agreement relative to the last two is that these were the first intercepts of smoke after flying to the fire, resulting in the ‘clean’ inlet tubing being able to retain more FA. This initial retention can be seen through transect 3, where the cumulative PTR FA is higher than the I- CIMS indicating FA still washing out of the system. By the final two transects the inlet may have effectively been passivated resulting in overall less FA retention. However, as mentioned above, this does not preclude the possibility of instrument performance issues in high concentrations samples and is not something we can constrain at this time.

LINE 548: Are EFs or ERs available from the other studies for comparison with WE-CAN and FIREX AQ FA EFs or ERs? If so, are the other studies' values much lower? I'm struggling to understand the justification for the authors' hypothesis that rapid production leads to higher FA EFs and ERs.

This is a good question and we have clarified our logic for this hypothesis on lines 549–555, pasted below.
“Given that a majority of the EFs/ERs in our literature review are from laboratory burns, with the few field sampled fires being small enough that aircraft could often sample directly over the source, they represent smoke with little to no aging. As WE-CAN and FIREX-AQ emissions are estimated to have been sampled 10–150 minutes downwind from the fires, we hypothesize that the higher FA EFs and ERs discussed in Section 3 are partially due to the rapid FA production observed during the campaigns.”

LINE 628: On the contrary, modeled AA NEMRs appear to agree well with Taylor Creek observations, but not the rest, and seeming majority, of the data to my eye.

This is a good point and what we had intended to communicate here. We have clarified the sentence to reflect that the model does well simulating the Taylor Creek fire but misses the overall trend of the other 4 fires.

LINE 917: Could controlling the IMR humidity also be a viable solution to this problem?

Controlling the IMR humidity is certainly important for reducing the uncertainty in I- CIMS measurements and has been added this to this sentence. We note here that both I- CIMS instruments controlled their IMR humidity during the WE-CAN and FIREX-AQ campaigns (Section 2.2), making that particular problem unlikely to influence the results discussed in this work.

01-Sep-2023

Dear Dr Permar:

Manuscript ID: EA-ART-06-2023-000098.R1
TITLE: Assessing formic and acetic acid emissions and chemistry in western U.S. wildfire smoke: implications for atmospheric modeling

Thank you for submitting your revised manuscript to Environmental Science: Atmospheres. 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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