Review of “Seasonal variations in fire conditions are important drivers to the trend of

aerosol optical properties over the south-eastern Atlantic”

Che et al. investigated the effects of seasonal biomass burning in south central Africa on optical properties observed downwind at Ascension Island using observations from the LASIC campaign during two biomass burning seasons (June – October 2016, 2017). The authors leveraged the comprehensive set of LASIC optical measurements in combination with satellite products and reanalysis to show that changes in cloud processing, fuel source and type, and regional circulation influence the absorbance properties of the aerosol that reach Ascension Island. The paper highlights that the observations show a much more absorbing Southeast Atlantic boundary layer than is simulated by most climate models. This is an exceptionally thorough and convincing paper that is well written and presented. I only have a few minor comments that need to be addressed. 

Minor Comments:

Line 57: add “the”; “during [the] CLARIFY (…) campaign”

Line 127: (Pennypacker et al., 2020) described an Ascension Island biomass burning background BC median concentration of 20 ng m-3 (ultra clean median of 51 ng m-3) with some monthly variability during the LASIC campaign. Was this/these value(s) also considered for the background calculation of BC/dCO as was done for dCO? If not, can you speak to how you think these might impact the calculation and subsequent interpretation of results?

Line 208: delete “the”; “the July-September averaged MACBC measured in 2016 on [the] ASI”

Line 235: change “consisting” to “consistent”

Meteorological factors (Section 4.3.2): Although this section discuss the impact of winds on combustion conditions, the authors should also make note of the effects of the Southern African easterly jet and it’s potential influence on the seasonal optical properties at Ascension. Specifically, (Adebiyi and Zuidema, 2018) described a peak in the AEJ-S during the September – October period when the transport of BB aerosol can be very efficient. Were jet-level winds/trajectories (~600 hPa) also examined as a part of this study?

References

Adebiyi, A. and Zuidema, P.: Low Cloud Cover Sensitivity to Biomass-Burning Aerosols and Meteorology over the Southeast Atlantic, Journal of Climate, 31, 4329-4346, 10.1175/JCLI-D-17-0406.1, 2018.

Pennypacker, S., Diamond, M., and Wood, R.: Ultra-clean and smoky marine boundary layers frequently occur in the same season over the southeast Atlantic, Atmospheric Chemistry and Physics, 20, 2341-2351, 10.5194/acp-20-2341-2020, 2020.

Review of “Seasonal variations in fire conditions are important drivers to the trend of aerosol optical properties over the south-eastern Atlantic”

The authors analysed the seasonal variations in the optical properties of aerosols observed at Ascension island as well as the causes of these seasonal variations. One significant contribution is that they found the lower aerosol single scattering albedo during biomass burning season in Africa and suggested that the burning conditions are the major factor in the variation of aerosol optical properties. The paper is well written with comprehensive analysis and interesting findings, which should be published. However, I believe that some definitions and explanations need to be clearer and I therefore recommend that this paper be accepted with minor revision.

Specific comments

Line 24: change ‘The absorption enhancement Eabs’ to ‘The absorption enhancement (Eabs)’

Line 23 to 25 and other places: Please be consistent with if there is a space needed between the symbol ‘~’ and numbers.

Line 27: ‘(BC/ΔCO)’ looks like should be ‘the enhanced ratio of BC to changes in CO (BC/ΔCO)’.

Line 29: ‘better capture’ compared to what?

Line 34: Maybe specify ‘these period’ refers to which period. Probably also in line 18 specify the month-year period of the 17-month campaign.

Line 44 and other places: ‘the savannah region’ to ‘the Savannah region’

Line 71: any explanations to why the simulation of absorption in this region is bad in climate models? Are the simulations in other regions, by any chance, better?

Line 72: maybe worth mention which months are the BB season here.

Line 88: ‘MACBC’ should be MAC_BC

Line 95: latitude and longitude should be in degree and indicating which hemisphere, e.g. 7.97 °N.

Line 101: either be ‘464nm, 529 nm, and 648 nm’ or ‘464, 529, and 648 nm’. 

Line 101 to 105 and hereafter: why switch to past tense in these sentences, while you used present tense in most other places.

Line 103: change 5 Mm^-1 to 5 M m^-1.

Line 107: ‘represent’ should be represents

Line 137 and Figure 3 caption: ‘9:43-9:49’ refers to am or pm?

Figure 1. The 10th percentile for aerosol absorption in July 2017 is less than 0. Aerosol absorption should not be less than 0. The authors might need to double check.

Line 205. ‘Taylor et al. (2020) found an averaged Eabs around 1.85..’. Is this average for the whole campaign or is it an average at a certain height? Is it within a boundary layer?

Line 207: ‘Zuidema et al. (2018) reported the July-September averaged MACBC measured in 2016 on the ASI as 15.1, 13.3, and 10.7 m2 g−1 at 464, 529 and 648 nm, which corresponds to Eabs as 1.70, 1.71, and 1.68 respectively”. The authors state earlier that the errors in the Eabs for 464 and 648 nm are large (because of the uncertainty in the AAE), and only the Eabs for 529 nm are compared. Then why caculate and compare the Eabs at 464 and 648 nm from Zuidema’ results?

Line 218. ‘suggesting BrC have a minimal influence on the ASI’. Not sure I understand what does it mean here.

Line 235: ‘consisting’ should be consistent.

Line 261: Do the authors have data  (or other literatures) from actual measurements of coating thickness to support their conclusions?

Line 276: ‘curves fitting’ shoud be curve fittings

Line 330: ‘Our measured BC/ΔCO values are generally smaller than 0.012, suggesting this linear relation is applicable to our data’. For fresh plume, MEC and BC/ΔCO have a linear relationship when BC/ΔCO is less than 0.015. But the BC/ΔCO observed by the authors are for aged smoke, does the threshold of 0.015 still be applicable?

Line 347: ‘with the relationship between BC/CO and SSA’. Do the authers mean BC/ΔCO?