Review of “Airborne investigation of black carbon interaction with low-level, persistent, mixed-phase clouds in the Arctic summer“ by Marco Zanatta & al

General Comments:

The topic of the manuscript is aerosol-cloud interactions with a particular focus on the Arctic and refractory black carbon (rBC) aerosol particles. There considerable uncertainties in aerosol-cloud interactions that the scientific community needs to address in order to understand how aerosol particles affect the Earth’s radiative balance. The manuscript focuses on airborne measurements of rBC aerosol particles in the Arctic with concurrent measurements of cloud properties. Black carbon aerosol particles are particular detrimental to the Arctic climate in a number of ways, as the authors point out on several occasions. The manuscript presents a unique data set in an under-sampled part of the world using state-of-the-art instrumentation on research air crafts. The design of the measurement campaign allows for the determination cloud properties such as cloud droplet and ice particle size distributions in combination with measurement cloud activated aerosol particles. As stated in the abstract, the goal of the manuscript is to better understand the role of black carbon aerosol particles as cloud condensation nuclei (CCN) and how black carbon is vertically distributed in Arctic clouds. The quest to understand the role of rBC in Arctic clouds is a bold endeavor given that (from the manuscript) less than 1 % of the CCN contain rBC. I would like to see the authors to be more constrained with the ambition and focus on what can be said about rBC in Arctic could drops given the present data. I encourage the authors to leave out the sometimes lengthy speculative parts of the manuscript; i.e. lengthy speculations about processes that cannot be adressed with these data.

The manuscript could use more cohesion. It feels like the introduction is lists studies where particular phenomena have been studied and those studies have come to the listed conclusions. I would expect the authors to connect these studies in an easier way so that it is easy to follow and would subsequently open up to the reader why the authors chose to cite those articles; and why the chosen articles are relevant for the present study. Throughout the manuscript, references to previous work is done in a fashion that the authors expect the reader to know them by hearth, and sometimes mid-sentence without an obvious connection to the present work. I would encourage the authors to consider which references they want to refer to, and only refer to previous work when they are obviously relevant. That would draw more attention to your work and why it is important instead of stating that someone already did something similar in another part of the world.

The nomenclature is not always consistent in the manuscript which sometimes makes the manuscript unnecessarily hard to read. Please make sure that the same terminology is used throughout the manuscript. E.g. use either CCN or cloud drop residual, not both. Use wet scavenging or cloud scavenging, or make a clear distinction what you mean if you refer to below-cloud or in-cloud scavenging or something else.

The figures are clear and well presented!

Please check the use of “this”, “those” and “these” in the text. It can be hard to follow to what the word is referring to. Please, repeat the statement or the results (in brief) instead of referring loosely to what was said before. That would make for easier reading. Examples P11L343 “Under this cloud regime…”, P14L442 “Under this very complex...”

For the above stated reasons, I suggest that the manuscript bee distilled down to comprise results and discussion on Figures 1-5, 7. The conclusions section summarizes the present study well and is a good reference for how the work should be shortened.

Specific comments

Section 2: It is never mentioned how one single SP2 measures both “rBC cloud residuals” and rBC particles outside of clouds behind the CVI. Two inlets are mentioned but only one inlet is described in detail.

Section 3.5 and Figure 8: Different low level clouds have been scaled to the same thickness (rage 0 – 1 where 0 is the cloud bottom and 1 is the cloud top). These data comprise low exclusively level clouds, right? I think it is misleading to talk about cloud layers inside the “ensemble” cloud. A cloud layer is a set of clouds at some altitude and makes a cloud layer when it is distinguishable from another cloud layer with cloud free skies in between. To talk about a cloud layer inside a cloud does not make sense to me. How about talking about e.g. the lowermost 20% of the “ensemble cloud” or uppermost 60% etc? Or something better that you come up with

I would suggest removing the Discussion section (section 4) and compress the results contents to one short paragraph and leave out the speculation.

When talking about the size of the rBC particles in the text, make sure you state if you are talking about the mode of the size distribution, mass or geometric mean diameter or something else. It is not always clear what size is meant.

P1L25 “might suggest” is not that intriguing. Rephrase to raise interest.

P1L26 It would be interesting to give the reader a hint of the evidence for this kind of processing already here.

P2L46-47 rBC after carbon dioxide and methane makes rBC 3^rd. Please rephrase or clarify.

P2L49-50: “Precipitation occurring during long-range transport influences the seasonal cycle of BC” I suggest saying something about the sources of rBC in the Arctic somewhere close to this text. Otherwise it is not that clear why deposition during long range transport has such an impact on rBC concentrations in the Artic.

P2L54-55 “...might easily be the limiting process…” is too vague.

P2L62-P3L65 Unclear and too long sentence. Too many references to too many studies makes for a difficult sentence to follow. “No observations… aforementioned studies … indicating … size distribution of (cloud drop size? Aerosol size?) … degree of internal mixing...”

P3L65 A bit unclear sentence and could be made easier to follow. As it is now, the reader must know that CCN and hygroscopicity are connected, that fresh soot is small in size.

P3L78 Suggestion to change order of words to “unprecedented vertically resolved airborne measurements”

I left out my the specific comment for the results section as it needs to be considerably shortened.

P16L484 NrBC-res is not in the figure. Did you mean NDro?

Figures

Figure 2b: The contribution to total mass of the largest size bins is not commented on in the text. Is this rBC or something else and how much of the mass do these particles represent? State in the figure caption that the in-cloud measurements were excluded.

Figure 3: Add to the text that this figure comprise “warm period” data only.

Figure 4. rBC size range does not match Table 1.

Figure 5. N_rBC-res is likely to be much higher since the SP2 does not measure particles below ~75? nm. Showing the mean/median number size distribution would help understand the degree of underestimation.

Figure 6. Although this is a nice picture, I would remove it since it is related to the speculative parts of the manuscript which I would like to see shortened significantly or removed altogether.