The paper presents an analysis of optical properties of biomass burning aerosol (BBA) transported over the southeast Atlantic (SEA) in relation to chemical composition. By using aircraft observations from the ORACLES field campaign and connecting results from other field campaigns over SEA, they attribute the low single scattering albedo (SSA) values in BBA plumes to the loss of scattering organics organic aerosol after 4-7 days of transport in the free troposphere through fragmentation induced by photochemistry.

The results presented in the paper are of general interest to the scientific community. Recent airborne and ground-based field campaigns have highlighted that the absorption by BBA over SEA is higher than previously thought. The cause of the low SSA is currently widely debated in the literature due to the importance of this parameter for better understanding the role of BBA on the radiative balance and climate of this region. This paper provides new insights on this hot topic. However, I do have some comments related to the readability of the paper and the interpretation of the results that I feel are critical for the authors to address prior to acceptance.

• The paper is very difficult to read because it contains many mistakes and writing problems. All co-authors must carefully read the manuscript and correct any errors. It is not possible to list here all the errors, but it includes:
  • missing figure number (e.g. L421) and figure number in the wrong sequence (e.g. Fig. S7-S8 before Fig. S6, Fig. S4 before Fig. S3,…).
  • errors (eg. Fig. 9 is colored by flights instead of AAE and MAC values), missing elements (eg. the name of y-axis in Fig. 4b, the unit of y-axis in Fig. 7a, 7d, 12a and 12b) and readability issues (eg. Fig. 12c and 12d) in some figures.
  • inconsistency in the abbreviation/acronym usage (eg. the use of “black carbon”/”BC”, “brown carbon”/”BrC “, “organic carbon”/”OA”, “modified combustion efficiency”/”MCE”… at different places in the text)
• Some parts of the Introduction includes some description of the flights and a brief presentation of the results that are very difficult to follow without reading the entire document. The choice of the flights analyzed in the paper should be moved in Section 2 after the presentation of the ORACLES and CLARIFY field campaigns. The authors could introduce the results sections in a more classical way to make reading easier: “Section 4 presents the chemical composition and age distribution within the six flights”, “Section 5 investigates the link between BBA optical properties and chemical composition”,… The last Section 8 is also difficult to read because it follows very dense discussion sections.  A brief summary of the main results would help the reader.

• The authors interpret the changes in BBA composition and optical properties to fragmentation of oxidized aerosol thought photochemistry. This is based on the observed decrease in OA mass concentration and increase in oxidation with plume age. I am wondering if other processes can’t explain these results. Do you have elements to reject the following assumptions?
  
  
  • Aqueous phase reaction and cloud processing could contribute to the oxidation of OA and decrease in OA mass concentration.
  • The condensation of less oxidized material onto preexisting highly aged organics favored by lower temperature during transport would favorize the evaporation of OA into the gas phase.

Specific comments:

L9: I can’t see the link between the increasing fraction of BC-containing with chemical age, and the processes of vapor condensation and coagulation. Please clarify it.

L12-13: It would be clearer for the reader to add that BBA sampled during CLARIFY have travelled longer distance than those sampled during ORACLES.

L15-17: The reason to focus on inorganic ammonium nitrate is not clear when reading the abstract. There would have been insufficient purely scattering nitrate particles to explain the vertical variation in the SSA?

L17 : Please remove « 2017 »

L103: I am wondering if CO₀ should not decrease with the altitude due to vertical dilution. Why did not you use CO measured outside BBA plumes to obtain the background values as a function of the altitude?

L134: please replace “later” by the corresponding section.

L168: The manufacturers of CPC and SP2 are missing.

L177: Which refractive index did use for UHSAS corrections?

L181: Please add P=1013 mbar

L197: Do you mean that you used the scattering Angtrom exponent to convert scattering coefficients to different wavelengths?

L209: Why did you choice the limit of OA > 20µg m-3? Could not it biases your results? (For instance missing analysis of case studies with lower OA to BC ratio)

L232 : Please replace g m-3 by µg m-3.

L235-236 : Does it mean that BBA were not dominated by OA during CLARIFY ?

L250-252 : The authors could mention that BC was not measured by the same method  (SP2, thermal-optical transmission) in the literature summary in Table 1, which may explain some differences.

L 271: Table 1 instead of Table 2

L177: Could you please remind the reader that the peak at m/z 60 is often associated with levoglucosan from biomass burning?

L285: I don’t understand why O:C values in BBA are expected to be comparable with urban measurements at Mexico ?

L311: Please precise that measurements were performed in the remote South West Africa in Denjean et al (2020) and provide the range of BC-containing particle faction obtained in Taylor et al. (2020).

L335-336: I don’t understand what you mean by “because our data lack highly-scattering aerosol”.

L343: Please provide a reference for the primary emission of BrC.

L353: AAE is in the range 1.1-1.3. Does it mean that BrC is a significant contribution to BBA absorption ?

L383 : Please precise that precursor gases may be more avalaible for nucleation.

L385: Does a constant dBC/dCO mean that BBA had the same source and that there was no wet deposition ?

The manuscript addresses the influence of biomass combustion aerosol on the single scattering albedo over the South Atlantic. The underlying measurements were taken with airborne measurements during the ORACEL campaign. Relationships between optical properties and the organic aerosol within in the context of aerosol aging are shown.

The subject is of high interest and the complexity of this topic requires a high level of understanding in the interaction of physical and chemical processes. Some interesting aspects are raised in the present manuscript. However, there are some deficiencies in content and presentation. These should be addressed before the manuscript can be accepted.

General comments:

The manuscript is difficult to read, as one must read the appendix in parallel with the manuscript to understand it. The description of the aerosol devices and experimental setup are not sufficiently described and difficult to read. In some cases the common abbreviations are not used. Furthermore, an error analysis due to experimental uncertainties and instrumental artefacts is missing. A summary of measurement techniques and derived parameters in tables would increase the readability.

Analyses and discussion on optical properties is driven by correlations of observed mass and number fractions.  A discussion using mixing models and light scattering theory is not mentioned. Even though mixing state data are not available from SP-2 and even though scattered light theory was not the focus of this manuscript, the basic insights derived from light scattering theory, e.g. light absorption enhancement factor, should be considered in the discussion. 

Specific comments:

Line 20: BBA yet not defined

Line 110: Is the limited size range sufficient for the analyses. How large is the fraction of particles not detected? Is this fraction constant or does it change in the different cases?

Line 167: Were the mobility spectra corrected for multiple-charges?

Line 176: What refractive index was used for correcting the UHSAS size spectra?

Line 182 to 184: Why are the problems with PCASP mentioned? If necessary, this would fit better into a chapter on corrections and quality control.

Line 203: ‘Ångström’ throughout the manuscript

Line 204: Should be called rBC when measured with SP2

Lines 310 ff: The authors state, "The mass absorption coefficients (MAC660nm) and SSA values depend to first order on an estimate of the fraction of particles containing black carbon." This may seem to contradict the formula "SSA530nm=0.801+0055*(OA:BC)" highlighted in the abstract (line 19), where a mass fraction is used.  The reviewer finds it critical that number and mass fraction are mentioned in various contexts as a proxy for SSA, while the more precise concept of a physical mixing state is not mentioned. Furthermore, the basic definitions of the quantities such as SSA and Ångström exponents should be presented before showing "first order estimates".

Line 354: What plot shows the correlation between AAE and OA:BC? Is the result significant considering the small range of AAE values and typical uncertainties?  

Figure 12: The reviewer believes that there is an error in the calculation of the volume distribution. The modal diameters of the volume distribution should be larger than those of the corresponding number distribution, and the width of the volume distribution is usually equal or larger. The ratios of the total volumes also appear to be incorrect. The reviewer suspects that the total volumes for cases f44>0.18 and f44 >0.21 should be closer to case f44>0.15 than the figure shows. Accordingly, all statements referring to the figure should be verified.

Line 405: “The heating can be interpreted as a proxy for dilution, as both physical processes increase volatility.” The reviewer does not understand the content.

Supplement:

Line 23: The nephelometer wavelength is 550 nm.

Line 24: The SP2 derives refractory carbon rBC. See also Petzold et al. (2013) to differentiate between BC, eBC and rBC.  

Line 24: What is the uncertainty in total aerosol mass when derived from AMS and SP2. 

Lines 22 to 31: The reviewer can not follow the method of calculating wall losses, especially what is the role of the mass scattering coefficient?

Line 87: Should be Figure S6?

Line 90ff: The Anderson and Ogren (1998) correction is on correcting the nephelometer and not to derive absorption coefficients from PSAP. What method was used for the PSAPs? It is not clear how the Virkkula (2010) correction is used.

The reviewer suggests discussing the nephelometer first, since this instrument is used for PSAP correction and subsequently for the derivation of SSA.

Figure S5: Were UHSAS diameters corrected for refractive index?

Line 55: It is unfortunate to call the combination of instrument (LDMA & CPC) for measuring the particle number size distribution LDMA. The TSI 3934 should be named as SMPS (scanning mobility particle sizer).

Figures S1 and S2: Many acronyms (e.g. UCN, ACN, RCN, RRwet, RRdry, … ) are not explained.

S6,  figure caption: “OA > 20 µg/m3”

S4, figure caption: What is the color scale showing?

References

Petzold, A., Ogren, J. A., Fiebig, M., Laj, P., Li, S.-M., Baltensperger, U., Holzer-Popp, T., Kinne, S., Pappalardo, G., Sugimoto, N., Wehrli, C., Wiedensohler, A., and Zhang, X.-Y.: Recommendations for reporting "black carbon" measurements, Atmos. Chem. Phys., 13, 8365–8379, https://doi.org/10.5194/acp-13-8365-2013, 2013.