This paper presents results on aerosol BrC and various other species measured in smoke plumes from the NCAR C130 research aircraft during the WE-Can study.  Online measurements of only water-soluble BrC were compared to BrC inferred from a PAS that measured particle light absorption at 3 wavelengths.  Comparisons to other smoke species, such as potassium and levoglucosan are also made. Many of the findings of this paper are similar to published results by other investigators, confirming the earlier work and providing a more expansive view.  A common finding of this and other recent studies is the high level of variability in BrC evolution between different smoke plumes.  These findings seem to contrast with a paper published (Palm et al) by some of the same co-authors based on data from the same study, which suggests that the evolution of OA and BrC is largely described by a balance between formation and loss resulting in little change over time.  Why is there (or is there) a difference in the data interpretation within the same study? As noted below, the findings of this paper should be put in context with those of the Palm analysis.  Overall, the results presented here are pertinent to the current interest in BrC from wildfires and suitable for publication in this journal.

Specific comments:

Line 265-268: Might want to discuss issues with ignoring lensing of BC, see Pokhrel. What about the other assumption of no BrC at 660 nm? Does this matter in the following analysis? 

Give the particle size range measured by the UHSAS and what size range was used to calculate overall particle mass concentration.

What is the cause of the large difference in correlations in Figs 2a vs 2b?  The poor correlation in 2b is somewhat surprising and suggests the AAE was highly variable.  Is this because 2a is data from one fire and 2b is from several different fires?  This difference in correlation between the two flight is seen throughout the analysis.  This deserves more investigation.  If it is caused by high variability due to smoke from different fires for RF11, then maybe a more uniform plume should be used, although this flight does provide a contrast.

Slopes could be included in Fig 4 plots, which is the MCE.

Why not write equation in line 348 as: UHSAS/(1.6 WSOC)?

Line 349, what properties of WSOC and WIOC are assumed to be same? It appears the assumption is that the MCE is assumed the same for both, and that the total mass = total OA mass.

Fig 5a and b are somewhat confusing. The x-axis is measured WS absorption and the y axis is the Mie calculate for WS and total.  What data was used as input to the Mie calculation in each case, for water soluble was it the MAE for soluble species and for total the MAE total calculated previously from the WS species?

What would happen if the same calculations were repeated for different wavelengths?

Lines 474 to 484, on the discussion of the evolution of smoke plumes.  The results of this paper seem to contradict a study already published (Palm et al) based on these same (WE-CAN) data and which there are common co-authors to this manuscript. Palm concludes that although changes in the OA may be occurring, there tends to be a balance, so parameters like OA mass and BrC relative to CO remain steady as the smoke plume evolves.  The authors should contrast their findings to this paper, since it seems to be two different interpretations from the same study with common co-authors. Additionally, the Palm paper should be cited and findings discussed in this papers Introduction.  (Palm, B., Q. Peng, C. D. Fredrickson, B. H. Lee, L. A. Garofalo, M. A. Pothier, S. M. Kreidenweis, D. K. Farmer, R. P. Pokhrel, Y. Shen, S. M. Murphy, W. Permar, L. Hu, T. L. Capos, S. R. Hall, K. Ullmann, X. Zhang, F. Flocke, E. V. Fischer, and J. A. Thornton (2020), Quantification of organic aerosol and brown carbon evolution in fresh wildfire plumes, P. Natl. Acad. Sci., 117(47), 29469-29477.)

In Fig 10, trends for different flights can’t be discerned, maybe add regression lines for each flight?  Also, do a regression for PAS total BrC vs dCO to support the claim that there is a consistent drop in the first 2 hours (eg, on what bases was this conclusion reached)?

Maybe look at change in WSOC relative to CO to check specifically for production or loss of WSOC with plume age?

Conclusion 2, last line, give wavelength for the ratio of WS BrC to total.

The manuscript by Sullivan et al. describes aerosol absorption in aircraft-collected samples collected from aircraft of biomass burning plumes. The authors describe particle-into-liquid (PiLS) samples and photoacoustic spectroscopy measurements of brown carbon. They assess the comparability of the data from the two instruments, the relationship between brown carbon and biomass burning tracers, as well as evolution of brown carbon with plume age. This is an interesting and valuable analysis that adds to our understanding of absorbing aerosol. After addressing the comments below, I believe the work will be suitable for publication in Atmospheric Chemistry and Physics.

General comments

There are other studies that have not been mentioned that examined relationships between organics and biomass burning tracers (e.g., Lee et al., (2016) and Di Lorenzo et al., (2018)). The discussion in this work would be stronger if it related these observations to those in these previous works. It would also be interesting to know more about the relationship (or lack thereof) between ammonium and brown carbon and/or other biomass burning trackers. Di Lorenzo et al. (2018) saw a relationship between reduced nitrogen and brown carbon in aged samples.

The discussion of brown carbon absorption with plume age should include discussion of these results in the context of previously published results from the same aircraft campaign (Palm et al., 2020). The section should also discuss recent aircraft PiLS work that has examined similar trends (Washenfelder et al., 2022).

Specific comments

Line 178: I think both PiLS sampled from the same inlet—“each” implies they have their own separate inlets. It would improve clarity to say “both PILS systems sampled from…”

Line 230: Other anhydrosugars being below detection is mentioned later on in the manuscript. Suggest giving the more general term at this point in the methods, before later saying that you focused on levoglucosan.

Line 230: Suggest not mentioning anions/organic acids since none of the data or methods are presented.

Line 246: What detector was used here? I assume conductivity.

Line 389: What fraction of the samples was not affected by biomass burning? That will affect the robustness of this claim. Somewhere in the discussion of tracers, it might be worth explicitly stating why you might expect to see differences in correlations with CO versus the other tracers that are discussed.

Lines 397-399: This sentence seems out of place. It is not clear how it connects to the preceding discussion.

Figure 8: I think parts (a) and (b) of this figure could be removed.

Line 444: Could you be more specific about the other types of burning you mean here?

Line 470: There is field data that describes this phenomenon as well that may be of interest for comparison (though the aging times may be too long to be relevant, Di Lorenzo et al. (2017)).

References

Di Lorenzo, R. A., Washenfelder, R. A., Attwood, A. R., Guo, H., Xu, L., Ng, N. L., Weber, R. J., Baumann, K., Edgerton, E., and Young, C. J.: Molecular-Size-Separated Brown Carbon Absorption for Biomass-Burning Aerosol at Multiple Field Sites, Environmental Science and Technology, 51, https://doi.org/10.1021/acs.est.6b06160, 2017.

Di Lorenzo, R. A., Place, B. K., VandenBoer, T. C., and Young, C. J.: Composition of Size-Resolved Aged Boreal Fire Aerosols: Brown Carbon, Biomass Burning Tracers, and Reduced Nitrogen, ACS Earth and Space Chemistry, 2, https://doi.org/10.1021/acsearthspacechem.7b00137, 2018.

Lee, A. K. Y., Willis, M. D., Healy, R. M., Wang, J. M., Jeong, C. H., Wenger, J. C., Evans, G. J., and Abbatt, J. P. D.: Single-particle characterization of biomass burning organic aerosol (BBOA): Evidence for non-uniform mixing of high molecular weight organics and potassium, Atmospheric Chemistry and Physics, 16, 5561–5572, https://doi.org/10.5194/acp-16-5561-2016, 2016.

Palm, B. B., Peng, Q., Fredrickson, C. D., Lee, B. H., Garofalo, L. A., Pothier, M. A., Kreidenweis, S. M., Farmer, D. K., Pokhrel, R. P., Shen, Y., Murphy, S. M., Permar, W., Hu, L., Campos, T. L., Hall, S. R., Ullmann, K., Zhang, X., Flocke, F., Fischer, E. v., and Thornton, J. A.: Quantification of organic aerosol and brown carbon evolution in fresh wildfire plumes, Proceedings of the National Academy of Sciences, 117, 29469–29477, https://doi.org/10.1073/pnas.2012218117, 2020.

Washenfelder, R. A., Azzarello, L., Ball, K., Brown, S. S., Decker, Z. C. J., Franchin, A., Fredrickson, C. D., Hayden, K., Holmes, C. D., Middlebrook, A. M., Palm, B. B., Pierce, R. B., Price, D. J., Roberts, J. M., Robinson, M. A., Thornton, J. A., Womack, C. C., and Young, C. J.: Complexity in the Evolution, Composition, and Spectroscopy of Brown Carbon in Aircraft Measurements of Wildfire Plumes, Geophysical Research Letters, 49, https://doi.org/10.1029/2022GL098951, 2022.

This paper presents the brown carbon measurements during the WE-CAN study, from a Particle-into-Liquid Sampler with a Liquid Waveguide Capillary Cell and a Total Organic Carbon analyzer (PILS-LWCC-TOC) system, and a Photoacoustic Aerosol Absorption Spectrometer (PAS) system. They also collected a number of liquid samples through another PILS system for offline analysis.  As PAS can only measure optical properties at certain wavelengths, the authors mainly compare PAS and PILS absorption at 405 nm. They find that with correction for water insoluble organics and bulk solution, PILS absorption at 405nm is in good agreement with PILS measurement at 405nm. They also find that the photobleaching is not significant within the first 9h after emission. Overall, this is a very interesting study and great addition to literature on brown carbon measured from wildfires. I recommend it to be published with minor revision. A few comments:

1. For Figure 2, it is unclear why PILS Abs 365 has excellent correlation with PILS Abs 405 for research flight 02, but not so great for research flight 11? Was that because RF11 samples a lot more background air? Some explanation on the difference between these two flights would be useful. Also, it would be important to comment on this aspect for other flights.
2. I kept wondering if the conclusion of no significant photobleaching within the first 9h after emission, could be a wavelength-specific problem. Can the authors add PILS Abs 365 to Figure 11? So, it would be clear whether a similar conclusion can be reached for the absorption at 365nm or even shorter wavelength.