From the journal Environmental Science: Atmospheres Peer review history

09-Oct-2023

Dear Dr Gorkowski:

Manuscript ID: EA-ART-08-2023-000130
TITLE: Insights into Pyrocumulus Aerosol Composition: Black Carbon Content and Organic Vapor Condensation

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Reviewer 1

Review of “Insights into Pyrocumulus Aerosol Composition: Black Carbon Content and Organic Vapor Condensation” by Gorkowski et al. (2023) submitted to ESA.

Gorkowski et al. (2023) reports the measured BC mass fraction for wildfire, agricultural, urban burning and regional air. The study shows that the BC mass fraction is consistently between 0.5 to 3% above the free troposphere and the mass fraction is decreasing with altitude from surface to the tropopause. The authors think OA condensation is the main driver of decreasing trend of BC mass fraction. The author then uses a LES model to simulate the condensed OA in the plume. Overall, I find the study very interesting, important and likely significantly contribute to our understanding of the climate impacts of the wildfire and pyrocb smoke. The study is trying to explore the fire VOCs’ contributions to the OA budget, which is of great importance and challenging too due to a lack of observations. I am most interested in the LES modeling work of pyroCb. However, there are some unclear points in the manuscript that needs to be clarified before publication.

Gorkowski et al. compares BC mass fraction across various wilfire/pyrocb and burnings and finds that the BC mass fraction consistently is between 0.5-3% above the free troposphere regardless the BC mass fraction near the sources. The authors then claim that VOC-particle partitioning (in equilibrium) drives the change of BC fraction with altitude. While VOC-particle partitioning is expected in fire plume, quantification of the relative contribution from this process relative to other microphysical and chemical processes is of the largest interest. Unfortunately, I couldn’t find the detailed analysis from current manuscript.

The authors claim that the fire BC/OA mass fractions are between 0.5 and 3% (Fig2). It relies on one assumption that the measured particles are inside of the plume without significant contributions from the background conditions. Kaitch et al. measured smoke inside of the plume and find that the BC mass fraction is 1.6+-0.8%. I wonder is the low values above 10 km shown in Fig2a are the results of mixing of smoke (relative high BC fraction, e.g. 2%) and background (with low BC fraction). In the manuscript, please elaborate on the measurements in Fig2 especially regarding the measured air masses inside of plume or not. In another word, more work is needed to justify the decreasing BC mass fraction with altitude (Fig) (is dilution a potential cause of the decreasing BC mass fraction instead of the OA condensation.

The simulated BC mass fraction shown in Fig3b is very concerning. As the authors2 mentioned, it consistently has a 5% overestimation, which is huge. Pyrocb BC mass fraction is like 2%. The authors provide some speculations near Line 264-273. If dust and ash can explain the difference, doesn’t that mean dust/ash contribute to the ~50% of PM1 in smoke plume? Note, modeling studies cited in the manuscript show a mass fraction of 2% without dust and ash.
The authors also provide another speculation that BC coagulation prior to the organics condensation can results to a decreasing BC mass fraction. Note, for pyroCb smoke, which is far from the source fields, BC is almost internal mixed. I am not sure how the process can contribute to the overestimation of BC mass fraction by LES in the manuscript. The authors need to revisit the overestimation from the LES model, like the radiative parameterizations and the radiative properties like refractive indices. Some known issues with LES model? The representativeness of volatile vapor 1-nonen for smoke?

Minor comments:

Line 117-120: can dilution of plume (including organic vapor) decrease vapor concentrations with altitude instead of condensation sink?

Fig3c: Is “OA primary” OA particles emitted at the surface? Why OA primary and OA condensed zero in the troposphere, while lower troposphere shows some condensed OA. It will be very useful and interesting to add a panel comparing condensed OA and primarily emitted OA particles similar to Fig3a.

Line 34-35, modeling studies (Yu et al., 2019, etc) and aircraft observation study (Katich et al., 2023) have suggested the BC mass fraction of ~2%. Please include related references.

Fig2, “1000 to 50000 data points per altitude interval”. Not clear. For example, how many datapoints above 10 km vs. 6-8km? Please clarify, suggest adding one line explicitly telling reader the data points for each altitude range (2 km range?)

Fig3e, change OA formation to OA aerosol concentrations; Fig3d OA vapors -> OA vapor concentrations

Fig3c legend is not clear

Reviewer 2

I agree with the goals of this paper, i.e., it is important to assess the BC fraction of total carbonaceous aerosol that makes its way to the upper troposphere. This material is likely to feed the stratosphere, where it may reside for a long time. The BC to OC ratio is important for assessing climate effects. As well, I agree with the hypothesis of the paper that lower temperatures will lead to more extensive condensation of organic vapors, leading to lower BC to OC ratios. The paper: 1. Compiles data from field campaigns, 2. Calculates organic partitioning ratios, 3. Runs an LES simulation that incorporates a simple gas-to-particle partitioning model based on water activity and temperature.

Comments:

1. This is a minor comment but I am surprised by the very high BC ratios in nuclear war scenarios. I realize that references are given but perhaps a sentence indicating that these are estimates without measurements.

2. Equation 1 is critical and the authors should provide a definition of each quantity in it. In particular, my understanding is that the partitioning parameter is the fraction that will be in the particle, not in the gas phase, e.g., see Stolzenburg, JAS, 2022.

3. I have some problems with Figure 1a. Is this the same partitioning coefficient as in Equation 1? I agree that species will be increasingly in the particle phase as temperature goes down (altitude up).

4. In Figure 1b, fire plume particles are bigger than regional particles. Isn’t that just an indication of large primary fire particles? How do these data support condensation?

5. It is claimed that only nonene shows partitioning behavior to the particle at high altitude. Can the other 128 VOCs measured in the canister sampling be added to the SI?

6. The results in Figure 4 from the LES simulation are interesting, such as the chimney effect and horizontal distance scale for the condensation effect; they provide the main value of the paper. However, I am puzzled: In the upper frame of the figure where the two color scales have different upper limits. How can vapors that have an upper limit of 3 (in log units) give rise to OA that has upper limits of 4 (in log units)? It does not seem that mass is conserved.

Overall, the LES simulation is novel and interesting, and the topic is important. I have questions with the data interpretation (Figure 1 and with one aspect of Figure 4). I would need to review this paper again to recommend publication.

Dear Editor and Reviewers,
Thank you for dedicating your time to review our manuscript. In the following sections, we have addressed each comment. The reviewers' remarks are presented in italics, followed by our responses in plain text. Additionally, all modifications made to the manuscript are highlighted in red for ease of identification.
We eagerly await your feedback.


REVIEWER REPORT(S):
Referee: 1

Comments to the Author
Review of “Insights into Pyrocumulus Aerosol Composition: Black Carbon Content and Organic Vapor Condensation” by Gorkowski et al. (2023) submitted to ESA.

Reviewer:
Gorkowski et al. (2023) reports the measured BC mass fraction for wildfire, agricultural, urban burning and regional air. The study shows that the BC mass fraction is consistently between 0.5 to 3% above the free troposphere and the mass fraction is decreasing with altitude from surface to the tropopause. The authors think OA condensation is the main driver of decreasing trend of BC mass fraction. The author then uses a LES model to simulate the condensed OA in the plume. Overall, I find the study very interesting, important and likely significantly contribute to our understanding of the climate impacts of the wildfire and pyrocb smoke. The study is trying to explore the fire VOCs’ contributions to the OA budget, which is of great importance and challenging too due to a lack of observations. I am most interested in the LES modeling work of pyroCb. However, there are some unclear points in the manuscript that needs to be clarified before publication.
Gorkowski et al. compares BC mass fraction across various wilfire/pyrocb and burnings and finds that the BC mass fraction consistently is between 0.5-3% above the free troposphere regardless the BC mass fraction near the sources. The authors then claim that VOC-particle partitioning (in equilibrium) drives the change of BC fraction with altitude. While VOC-particle partitioning is expected in fire plume, quantification of the relative contribution from this process relative to other microphysical and chemical processes is of the largest interest. Unfortunately, I couldn’t find the detailed analysis from current manuscript.
Response:
We appreciate the reviewer's interest in the detailed analysis of the relative contributions of various processes affecting the BC mass fraction with altitude. Our study indeed focused on assessing the influence of organic aerosol condensation on BC mass fractions in wildfire plumes, supported by existing data, particularly the new ones collected over PyroCbs during FIREX. We acknowledge the significance of a comprehensive quantification of all contributing processes by more detailed analysis, which indeed would provide a more complete understanding. However, such an extensive analysis extends beyond the scope of our current project, which aimed at utilizing available data to evaluate the role of organic vapor condensation. We agree that obtaining measurements from near-source to the upper troposphere would be ideal for this purpose. This would likely involve complex, expensive, and opportunistic experimental setups, such as Lagrangian flights of aircraft measurements that our paper findings will likely stimulate.
To our knowledge, no such datasets currently exist that could facilitate the proposed level of analysis. Nevertheless, we believe our findings contribute valuable insights by highlighting the significance of vapor condensation of organics similar to nonene in altering BC mass fractions, as supported by observational evidence. While our study does not negate the influence of other processes, it emphasizes one plausible mechanism that is supported by observations for the first time within the constraints of available data (e.g. statistical analysis and comparisons of multiple fire regimes).

Reviewer:
The authors claim that the fire BC/OA mass fractions are between 0.5 and 3% (Fig2). It relies on one assumption that the measured particles are inside of the plume without significant contributions from the background conditions. Kaitch et al. measured smoke inside of the plume and find that the BC mass fraction is 1.6+-0.8%. I wonder is the low values above 10 km shown in Fig2a are the results of mixing of smoke (relative high BC fraction, e.g. 2%) and background (with low BC fraction). In the manuscript, please elaborate on the measurements in Fig2 especially regarding the measured air masses inside of plume or not. In another word, more work is needed to justify the decreasing BC mass fraction with altitude (Fig) (is dilution a potential cause of the decreasing BC mass fraction instead of the OA condensation.
Response:
Thank you for your comment regarding the BC/OA mass fractions reported in our study and for raising the point about potential contributions from background conditions. We understand the importance of ensuring that the particles measured are representative of the plume itself, and we have taken steps to address this in our analysis (point to it below in red).
Regarding the comparison with Kaitch et al.'s findings, we note there is a difference between our reported median values and their mean values. When considering the range of error, our reported values at 11 km have a median of 0.76% with an error range (lower error=0.25%, upper error=1.2%) that overlaps with Kaitch et al.'s reported mean of 1.6% ± 0.8%. This overlap within error margins suggests our findings are statistically consistent with existing research.
To substantiate our measurements being in plume, we implemented a CO concentration threshold of 100 ppbv, which aligns with the established 'in smoke' criteria from the FIREX-AQ and WECAN campaign. This information has been added in the supplemental material and referenced in the main manuscript
Text added: "To exclusively analyze in-smoke measurements, we utilized the in-smoke data flags for FIREX-AQ and WECAN campaigns, and for BBOP, a threshold of over 100 ppbv carbon monoxide was employed, consistent with the former's in-smoke flag criteria."
In response to the possibility of dilution as a cause for decreasing BC mass fractions with altitude, that seems less likely to be the main factor. The hypothesis that dilution alone (or be the major factor) could account for observed changes is inconsistent with the characteristics of analyzing the mass fraction. Because BC is a small fraction of the aerosol mass, it is very difficult to dilute it out, because you need to change the denominator which is the whole aerosol mass. Additionally there doesn’t seem to be a mechanism to specifically dilute only the BC mass. If it is direct dilution by entrainment of 'clean air' then the BC mass fraction remains unchanged. Note, this is the case when at cold temperatures (220 K) limiting volatile evaporation of organic aerosol back to the gas phase (due to the decrease in vapor pressure with decreasing temperature).

Reviewer:
The simulated BC mass fraction shown in Fig3b is very concerning. As the authors2 mentioned, it consistently has a 5% overestimation, which is huge. Pyrocb BC mass fraction is like 2%. The authors provide some speculations near Line 264-273. If dust and ash can explain the difference, doesn’t that mean dust/ash contribute to the ~50% of PM1 in smoke plume? Note, modeling studies cited in the manuscript show a mass fraction of 2% without dust and ash.
Response:
Yes, the reviewer is correct in that if dust and ash drive the BC fraction lower, then they would need to be 50% of the PM1 in the smoke. We don't know of good observational evidence in for or against that argument, other than given the surface winds at these fires dust pickup is possible which is how the Reisner et al JGR2023 incorporated it into their analysis.

Reviewer:
The authors also provide another speculation that BC coagulation prior to the organics condensation can results to a decreasing BC mass fraction. Note, for pyroCb smoke, which is far from the source fields, BC is almost internal mixed. I am not sure how the process can contribute to the overestimation of BC mass fraction by LES in the manuscript. The authors need to revisit the overestimation from the LES model, like the radiative parameterizations and the radiative properties like refractive indices. Some known issues with LES model? The representativeness of volatile vapor 1-nonen for smoke?
Response:
We appreciate the reviewer’s attention to the details regarding BC coagulation and organic condensation processes discussed in our manuscript. Our mention of BC coagulation as a potential contributor to the observed BC mass fraction trends was indeed an attempt to account for processes occurring in proximity to the fire source. We hypothesized that coagulation events, which may involve BC and ash particles before organic condensation has significantly progressed, could influence the initial BC mass fraction. We recognize that such events would primarily be relevant within a short range from the source where ash deposition is more likely.
In line with your suggestion, we have revised the manuscript to clarify that this coagulation hypothesis that pertains to near-field scenarios and is unlikely to have a significant impact on the BC mass fraction in the far-field smoke plume, where particles are more internally mixed.
Modified text: Additionally, as wildfire plumes evolve, coagulation scavenging near the source can also lead to the removal of BC prior to the condensation of organic vapors. This type of near-field scavenging would improve the agreement between our simulated BC mass fraction and observations.
As for revisiting the overestimation from the LES model, there are no parameterizations for radiative properties. It emits aerosol and vapors at the ground and then tracks them as they rise. The source term could be wrong, but we would need a specific suite of measurements to compare which are limited as first response discussed.
As for representativeness of 1-nonene, we agree that it may not be representative VOC for smoke. However, it is the only vapor that we have measurements for in a pyroCb.

Minor comments:

Reviewer:
Line 117-120: can dilution of plume (including organic vapor) decrease vapor concentrations with altitude instead of condensation sink?
Response:
You are correct in noting that dilution is a factor that likely influences vapor concentrations. To account for this, we have utilized partitioning coefficients rather than relying solely on mass concentrations of vapors in our analysis. This approach allows us to consider the relative abundance of vapors, providing a more robust understanding of concentration changes that may occur due to dilution or other processes with altitude.

Reviewer:
Fig3c: Is “OA primary” OA particles emitted at the surface? Why OA primary and OA condensed zero in the troposphere, while lower troposphere shows some condensed OA. It will be very useful and interesting to add a panel comparing condensed OA and primarily emitted OA particles similar to Fig3a.
Response:
Thank you for your inquiry about Figure 3c. This figure represents the total aerosol concentration after a 12-hour simulation period. Our intention was to emphasize the accumulation of smoke in the stratosphere, which is a key focus of our study. There are continuous emission from the ground source during the simulation. To provide a view at ground level, we have included a log-scale plot in the supplemental material, which illustrates the ground-level source emissions more distinctly.

Reviewer:
Line 34-35, modeling studies (Yu et al., 2019, etc) and aircraft observation study (Katich et al., 2023) have suggested the BC mass fraction of ~2%. Please include related references.
Response:
Changed text: For megafires, modeling studies {DAngelo2022, Yu2019, Torres2020} and observations {Katich2023, Peterson2022} have suggested a BC mass fraction of 2% BC. However, in estimating worst case scenarios and maximizing impacts of nuclear explosion fires, the BC mass fraction used can be up-to 100% BC \{Mills2014, Jagermeyr2020}.

Reviewer:
Fig2, “1000 to 50000 data points per altitude interval”. Not clear. For example, how many datapoints above 10 km vs. 6-8km? Please clarify, suggest adding one line explicitly telling reader the data points for each altitude range (2 km range?)
Response: We added a data points per altitude plot to the supplemental.
Text changed: Medians with 25% and 75% percentiles shaded (see ESI for number of data points per altitude interval).
Reviewer:
Fig3e, change OA formation to OA aerosol concentrations; Fig3d OA vapors -> OA vapor concentrations
Response:
See updated figure below:

Reviewer:
Fig3c legend is not clear
Response:
We revised the legend.
Updated figure:

 
Referee: 2
Comments to the Author
I agree with the goals of this paper, i.e., it is important to assess the BC fraction of total carbonaceous aerosol that makes its way to the upper troposphere. This material is likely to feed the stratosphere, where it may reside for a long time. The BC to OC ratio is important for assessing climate effects. As well, I agree with the hypothesis of the paper that lower temperatures will lead to more extensive condensation of organic vapors, leading to lower BC to OC ratios. The paper: 1. Compiles data from field campaigns, 2. Calculates organic partitioning ratios, 3. Runs an LES simulation that incorporates a simple gas-to-particle partitioning model based on water activity and temperature.

Comments:
Reviewer:
1. This is a minor comment but I am surprised by the very high BC ratios in nuclear war scenarios. I realize that references are given but perhaps a sentence indicating that these are estimates without measurements.
Response:
We added a sentence to the text to clarify that these are estimates, with a focus on worst case climate impacts.
Changed text: For megafires, modeling studies {DAngelo2022, Yu2019, Torres2020} and observations {Katich2023, Peterson2022} have suggested a BC mass fraction of 2% BC. However, in estimating worst case scenarios and maximizing impacts of nuclear explosion fires, the BC mass fraction used can be up-to 100% BC {Mills2014, Jagermeyr2020}.



Reviewer:
2. Equation 1 is critical and the authors should provide a definition of each quantity in it. In particular, my understanding is that the partitioning parameter is the fraction that will be in the particle, not in the gas phase, e.g., see Stolzenburg, JAS, 2022.
Response:
We have now provided a detailed definition of each term within the equation in the revised manuscript. This includes a precise definition of the partitioning parameter, aligning with the convention that it represents the fraction of a species in the particle phase.
text changed: For each of the 128 compounds measured in the gas phase, we calculated the aerosol partitioning coefficient as


\begin{equation}
\xi_{j}=\Bigg(1+\dfrac{C^{sat.}_j}{C_{total}}\Bigg)^{-1}.
\label{EQ:VBS_def_Xi}
\end{equation}

Since the organic aerosol chemical composition in the condensed phase was unknown, we approximated the saturation concentration \(C^{sat.}_j\) from the compound's pure vapor pressure {Compernolle2011}, recognizing that this would likely result in underestimation. The total aerosol mass \(C_{total}\) was calculated from integrating the aerosol size distribution.


Reviewer:
3. I have some problems with Figure 1a. Is this the same partitioning coefficient as in Equation 1? I agree that species will be increasingly in the particle phase as temperature goes down (altitude up).
Response:
Thank you for pointing out the inconsistency in Figure 1a. Upon re-evaluation, we have recognized that there was a discrepancy in the way we defined the partitioning coefficient. Originally, the figure illustrated the ratio of gas to total mass (gas + particle phases), which deviated from our intended definition of particle / total mass. We have since corrected this in both the figure's axis label and the accompanying text (see previous response) to reflect our intended meaning accurately. We appreciate your attention to detail, which has helped us improve the precision of our manuscript.
changed figure:


Reviewer:
4. In Figure 1b, fire plume particles are bigger than regional particles. Isn’t that just an indication of large primary fire particles? How do these data support condensation?
Response:
Thank you for your observation on Figure 1b and the opportunity to clarify the relationship between particle size and the evidence for condensation. We understand that the larger size of fire plume particles compared to regional particles could initially suggest primary particle emissions. However, if primary emissions were the sole factor, we would expect consistently larger particle sizes near the source at ground level and a similar pattern as altitude increases.
The altitude-dependent variation in particle size that we observe supports the hypothesis that secondary processes, such as condensation, contribute to particle growth as the plume rises and cools.
text added: If there was no condensation, coagulation, or entertainment the altitude profile of the mean volume diameter would remain constant.

Reviewer:
5. It is claimed that only nonene shows partitioning behavior to the particle at high altitude. Can the other 128 VOCs measured in the canister sampling be added to the SI?
Response:
We added the list and vapor pressures of the other 125 VOCs to the supplemental. Also we noticed 3 compounds were not used due to no vapor pressure information, so it is 125 not 128 compounds.
Text added: section: Vapor Pressures
Table of vapor pressures \cite{Compernolle2011} used in the partitioning calculations for the measured organic vapors.


Reviewer:
6. The results in Figure 4 from the LES simulation are interesting, such as the chimney effect and horizontal distance scale for the condensation effect; they provide the main value of the paper. However, I am puzzled: In the upper frame of the figure where the two color scales have different upper limits. How can vapors that have an upper limit of 3 (in log units) give rise to OA that has upper limits of 4 (in log units)? It does not seem that mass is conserved.
Response:
Thank you for your interest in the LES simulation results presented in Figure 4 and for the question regarding mass conservation between vapors and condensed organic aerosol (OA). The color scales in the upper frame of Figure 4 are indicative of the concentration levels of vapors and OA at a particular snapshot in time. During the simulation, organic compounds (OC) vapors condense onto particles over time, leading to an increase in 'OA condensed.' Therefore, the snapshot shows higher mass units for 'OA condensed' due to this accumulation, while 'OC vapors' are being lost to their condensation to OA. The mass conservation approach would be the total mass of 'OA condensed' plus 'OC vapors'.
This difference in scales reflects the transformation and distribution of mass from vapors to condensed phase, which is a dynamic and ongoing aspect of the simulation, resulting in 'OA condensed' being more significant than 'OC vapors' at the end of the 12-hour period.
We have added a figure to the supplemental that was previously removed from the manuscript based on a reviewer's comments.
Text Changed: For additional OA and BC altitude profiles see the ESI Section 6 Additional Simulations.
ESI Text Changed:
Additional Simulations
This section presents two supplementary LES experiments. Figure 2 illustrates the results of these simulations. The first simulation operates at a reduced wind speed of $1~m/s$, while the second replicates the conditions described in the main manuscript. These simulations primarily investigate the dynamics of Organic Aerosol (OA) condensation.


Figure 2 Altitude profiles from 12.5-hour LES runs are depicted, contrasting wind speeds of 1 m/s (gold line) and 3 m/s (red line). (a) Displays the altitude profile for total aerosol concentration. (b) Shows the condensed vapor fraction of the total OA, calculated as the ratio of vapor condensed to the sum of OA primary and vapor condensed. (c) Presents the Black Carbon (BC) mass fraction, comparing results from simulations with and without active organic vapor condensation

Reviewer:
Overall, the LES simulation is novel and interesting, and the topic is important. I have questions with the data interpretation (Figure 1 and with one aspect of Figure 4). I would need to review this paper again to recommend publication.

27-Nov-2023

Dear Dr Gorkowski:

Manuscript ID: EA-ART-08-2023-000130.R1
TITLE: Insights into Pyrocumulus Aerosol Composition: Black Carbon Content and Organic Vapor Condensation

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Reviewer 2

The authors have addressed the points raised in my initial review.