General comments:

This manuscript by Miller et al. observed supermicron aerosol particles over the Atlantic Ocean, ~1000 km from the coastal line. They conclude that these particles are unburned plant material originated from biomass burning. Their finding is interesting and could be important to understanding biomass burning and its regional influence. On the other hand, I found that this manuscript needs to be largely revised and improved to support the conclusion. I also found many technical issues to be revised. 

Major comments:

1: The authors conclude that the supermicron aerosol particles (SAPs) are unburned plant materials (e.g., page 1 line 36). However, I could not find strong evidence to support that the SAPs are unburned plant materials. BC and other particles may not be excluded. In page 2 line 61-66, the authors showed the possibility of large BC. Burned plant materials (ash) may also be possible. I could not find the reference that supports “previously seen in biomass burning smoke close to the source (page 1 line 37)”. Here and relevant discussions (see below) are confusing and contradictive.  

Page 2 line 70-74: Evidence from transmission electron microscopy (TEM) analysis of aerosol particles, BB aerosol composition analysis, particle shape and size, and prevailing atmospheric conditions together demonstrates that SAPs observed during this campaign were not soot, but rather supermicronsized unburnt plant material.

Page9 line 246-248: The particles found on RF 12 most likely were either unburned plant material or supermicron black carbon aggregates that were formed near the fire and transported in the BB plume over the Atlantic Ocean.

Page 9 line 254-256: The filters contained numerous black carbon and organic particles that were captured during the time that the 2D-S observed SAPs. It is therefore likely that the SAPs are unburnt plant material.

Page 19 line 402-405: However, given the source location, presence of rBC, and the TEM-identified carbonaceous particles, it is hypothesized that based in observed particle shapes, the SAPs imaged by the 2D-S are examples of unburned plant material previously seen in biomass burning smoke.

Minor and technical comments:

2. Page 1 line 22-23 (maximum dimension): What does the maximum dimension mean here? Is this Feret diameter?

3. Page 1 line 23 (supermicron-sized aerosol particles (SAPs)): “SAPs” is also defined as “supermicron aerosol particles (Page 2 line 67)”.

4. Page 1 line 27: black carbon (rBC): rBC is defined as refractory black carbon (page 3 line 139), and black carbon is defined as BC. Please make them consistent.  

5. Page 1 line 29: “black carbon”: Here and elsewhere, black carbon is used as either “black carbon” or BC. Please make it consistent.

6. Page 1 line 28-31: “Transmission electron microscopy images of submicron particulates, collected on Holey carbon grid filters, revealed particles with potassium salts, black carbon and organics while energy-dispersive X-ray spectroscopy spectra detected potassium, a tracer for biomass burning, indicating that the submicron particles originated from biomass burning in addition to black carbon.” This sentence is hard to interpret. 

7. Page 2 line 58: “KCL” should be KCl.

8. Page 2 line 62: Please define “Dp”. “D” is defined as “maximum dimension” in abstract. Is this the same?

9. Page3 line 97: “No SAPS were detected during the 2016 IOP.” This sentence may be placed in the result section. “SAPS” should be SAPs.

10. Page 4 line 148: “The inlet size for the AFS allowed aerosols of 2.0 - 3.1 μm diameters” This sentence reads the measured sizes are between 2.0 and 3.1 μm. However, in the next paragraph (2.7), these sizes are 50% transmission efficiency of the inlet. It looks like something is missing. 

11. Page 5 line 200: “mb” may need to be replaced with hPa.

12. Page 5: Figure 1: Flight tracks are difficult to read because of thir color. 

13. Page 6 Figure 2: I am not familiar with the instrument, but is there any artifact caused by aircraft speed for the long, straight particles on the 2D-S prove images?

14. Page 9 line 253-255. “The soot particles showed silicon inclusions and the organic particles contained potassium, both molecular markers for biomass burning emissions (Andreae et al. 1998).” I could not find that silicon inclusions are a marker for biomass burning emission in Andreas et al. (1998) or any other references. I think silicon can originate from various sources, including biomass burning, dust, and plant, and is difficult to be used for the biomass burning tracer. 

15. Page 10 line 259. Here the sentence mentions that “gold grid” was used for the sampling. On the other hand, in Figure 7, I found Cu signal possibly from the grids but no Au signal. Please check it. 

16. Page 11 Figure 7. It is difficult to read the elements, some of which may be under the detection limits (e.g., Na, S, and Cl). It looks N has a large difference between round organic and soot. 

17. Page 12 line 282. Add a period. 

18. Page 13 line 308-310: “Higher intensity fires with active flaming with high combustion temperatures most likely produce more SAPs than smaller smoldering fires as higher intensity fires inject more aerosols and unburned plant material into the atmosphere.” It is possible that active flaming emits not only unburned plant materials but also various SAPs (e.g., BC). 

19. Page 14 Figure 10. What do the colors indicate? 

20. Page 15 Table 1. The decimal places are inconsistent. An example is soot mass concentration (1.75, 1.1, and 5).

21. Page 18 Figure 12: The bin sizes are different in the two panels. Please consider using the same bin size for both panels so that both panels are directly comparable. 

22. Page 18 line 364-366: “The bounds for the fall speed times were the longest and shortest times from the HYSPLIT back trajectories, which ranged from 48 hrs and 144 hrs. The range of fall velocities were then calculated to be at most, between 24.7 cm s-1 and 0.4 cm s-1.” This estimation could be interesting. I suggest considering if the estimated fall velocities are reasonable for the measured SAPs by assuming their sizes and densities.

23. Page 18 line 369-371. “There were too few days without SAPs during the 2018 campaign to determine whether chemical composition, plume age, or temperature was related to the presence of SAPs in the BB plume.” This sentence seems strange. Does it mean “too few days with SAPs”? If not, it may be discussed using airmasses with SAPs. 

24. Page 18 line 391-and Page 19 404-405 “Similar particles emitted from biomass burning have been observed previously in localized field studies as well as laboratory experiments” and “the SAPs imaged by the 2D-S are examples of unburned plant material previously seen in biomass burning smoke” Where do these sentences indicate in the main text (result or discussion)? I could not find the previous study in the reference.

25. Page 20 References: The reference order is not consistent. An example is those on page 20 line 466-474.

Suggestion: Many references were published more than 20 years ago, and some can be replaced with recent ones. An example is in page 1 Line 41:  “Particulates generated by BB scatter and absorb solar radiation, affect the properties and lifetime of clouds (Andreae 1991, Penner et al. 1992, Ackerman et al. 2000, Bond et al. 2013), and influence regional and global climate (Crutzen & Andreae 1990, Andreae 1991; Bond et al., 2013).”

This work reports observations of supermicron-sized aerosol particles (SAPs) using a wing-mounted imaging probe during the ORACLES airborne measurement campaign. Coincident measurements of aerosol composition and corroborating back trajectories support a biomass burning source for these SAPs. The authors hypothesize that the SAPs are unburned vegetative matter (i.e., grass) that is convectively lifted and transported by fires in south-central Africa.  

Major Critiques

The number of individual SAP particles identified during extensive smoke-plume sampling is extremely low, and the authors opt to not even calculate their concentrations and rather simply report counts. Thus, it feels that the paper lacks sufficient 1) presentation of the actual data, 2) second-level analysis of SAP properties, and 3) justification for the vegetative source.

1. With so few of the SAP particles identified, it would be very informative to show more 2DS images in the paper. My suggestion would be to show a full-page figure of all SAPs for each of the case studies. This will allow readers to get a better sense of the variation in particle shape and would help to corroborate the unburned-grass hypothesis. Second, the paper is focused solely on observations in the African smoke plume, but are there any SAPs that were identified outside of the plume? A comparison of the BB data with data in background boundary layer or for free-tropospheric conditions would strengthen the argument that the SAPs are truly being emitted with smoke.
2. There is some qualitative discussion of the SAP particle shape in the text, but a more systematic and quantitative analysis strengthen the paper. First, it is not clear how many of the SAPs are identified in only one 10-um pixel and whether those single pixel detections can be confidently counted as real particles and not a sampling artifact biasing the results. The number of single-pixel counts need to be explicitly presented in the text. Second, can the SAPs be grouped into similar shapes to quantify those that are grass-like (i.e., rods, elongated in one dimension) compared to other more spherical shapes? With so few particles, this grouping could be done manually and likely would not necessitate a mathematical clustering approach similar to cloud morphology studies. Some form of shape clustering would be helpful, especially to justify the grass source theory.
3. The source of the SAPs is purely speculative and needs some further justification. Shape analysis (above) would help, but at least providing a more rigorous literature feasibility study is necessary. Has vegetative material been identified using cloud imaging probes before? Are there images of these particles in the literature that could be compared to the measurements presented here? Can any of the variability in counts flight-to-flight be related to the presence or type of vegetation in the source area?

Minor Edits

Page-Line

2-50       Please clarify this sentence.  I’m not sure what you mean by “still contributes to uncertainty estimates of RF”.

2-56       A more recent reference is Shingler et al. [2016] for soot restructuring (https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2015JD024498).

2-58       Please provide a reference for the KCl age statement.  Also this should be “KCl” not “KCL”.

2-76       Can you further comment here, or more appropriately in the discussion section, about why no SAPs were identified in 2016?

2-85       change “difference” to “differences”

3-104     Can you comment in the 2DS section about instrument noise?  What is the false-count rate in clear air? Given that the number of SAPs is so low, I think it’s worth the effort to quantify the false-count rate in order to trust the SAP observations. Also for this section, can you include a sentence or two on calibration of the probe?

3-126     The AMS vaporizer is typically operated at a nominal 600C temperature, see the cited DeCarlo [2006] reference.  Can you comment on why it was run at 650C for these flights and if that affects the AMS data presented?

3-129     Since you state that the AMS measurements are quantitative, please include a statement about the collection efficiency that was used to calculate final mass concentrations.

4-177     I don’t think the Eloranta [2008] reference is appropriate for the HSRL-2 instrument. I suggest Hair et al. [2008] for a general reference for the instrument (https://www.osapublishing.org/ao/fulltext.cfm?uri=ao-47-36-6734&id=175351) or just use Burton et al. [2018] (https://www.osapublishing.org/ao/fulltext.cfm?uri=ao-57-21-6061&id=395340)

5-199     For this section, I highly suggest adding time-series plots for the case study. This should show, at a minimum, altitude, 2DS counts for SAPs, LWC to confirm a lack of cloud, CO and BC to illustrate the location of the smoke plume. Also, please comment on the following: Are the SAP detections randomly observed or do they cluster in time?  Are there SAP detections outside of the smoke plume? Are the SAPs more frequently observed at a specific altitude?  As stated above, I recommend that all 72 + 12 SAP images should be shown and discussed for this section.

6-209     The text states that RF11 had 71 SAPs measured but TABLE-1 shows only 12.  TABLE-1 also does not have RF12.  Please check the text and table and confirm this discrepancy.

8-230     I do not understand the following sentence, “The CAS did not report…”.  Please clarify.

8-232     What is the conclusion drawn from the CAS data? There seems to be two orders of magnitude difference in concentrations of particles > 10um diameter for RF11 and RF1, but these flights have nearly the same number of SAPs observed (12 and 15 SAPs, respectively, from Table-1). Please comment on this discrepancy. Also, why were these 5 flights chosen for the plot, and what about 2018 flights? In general, I suggest inclusion of a more succinct description of the utility of the CAS data for this plot to be useful and included in the final paper.

9-256     Are the soot, organic, and dust particles that you reference here shown in Figure-6?

9-255     “It is therefore likely that the SAPs are unburnt plant material.”  I do not follow this argument.  Please provide justification.

11-263   As with the first case study, I highly suggest addition of a time series to address the questions posed above. Even though there are more SAP observations, showing each image would be beneficial.

12-278   “SAPs could be correlated with fire intensity”.  This statement is very speculative and needs to be justified or removed. I do not know what “copious amount” of material means, please explain.

13-292   “… were always observed within the BB aerosol plume…” As written, there is no evidence for this statement.  Time series of the case studies showing both BB and non-BB sampling would be helpful. Statistics for BB and non-BB sampling would also help.

13-303   “This could have resulted from an increase in fire intensity.”  This statement is very speculative and needs to be justified or removed.

14-322   “Aerosol plumes where SAPs were observed…”.  I do not understand this statement.  Why is the “spread” in the AMS data indicative of more intense fires?  Please explain.

15-325   The caption states that data are shown for “5 minute average when SAPs are observed within the BB plume”, but for many of the flights the SAP count was zero and data are still shown?  I’m confused.  Please clarify. Also, RF5 (2017) has more sulfate than organics and therefore does not appear to be from BB.  Please comment. Same argument for RF4 (2018).

18-355   What is the bin width of the histograms?  How many of the counts are for 10um particles (i.e., for a single pixel).  Please comment on if you think these are real or a possible artifact.