Comments on: Contribution of fluorescent primary biological aerosol particles to low-level Arctic cloud residuals

Authors: Gabriel Pereira Freitas, Ben Kopec, Kouji Adachi, Radovan Krejci, Dominic Heslin-Rees, Karl Espen Yttri, Alun Hubbard, Jeffrey M. Welker, and Paul Zieger

Summary:  This paper provides an overview of efforts to collect information on fluorescent primary biological aerosol particles on low-level Arctic clouds, using observations collected on Svalbard over two years.  In general, the paper provides new perspectives on an important and timely topic, and offers new information to the community.  While I was initially very excited to read the article and learn about the results, in the end I found the paper to be a bit lacking.  There is a lot of focus on methods, an overview of some (very limited and not very well connected) case studies, and then some information on broader statistics.  However, after reading all of this, the primary (only?) takeaway that I had was that fPBAPs exist and are present in the vicinity of clouds.  Clearly there should have been more to it than that?  If so, I don’t believe that the authors did a great job of highlighting their outcomes and main take-home messages.

Given all of this, I recommend that this paper be considered for publication after minor revisions.  I say minor not because the recommended revisions are easy, but because they don’t require significant additional analysis and/or a total re-structuring of the manuscript.  I really do think that this work represents a meaningful contribution and that the results should be shared with the community.  However, I believe that the paper should be carefully refined to ensure that the results are clearly communicated and that the primary messages are highlighted and come with some broader discussion and context.  Beyond this, please see additional comments below.

Major Comments:

• Case III (Ice Cloud): I didn’t get much useful information out of this case study. It almost feels as though it was tacked on just to say that an ice cloud was looked at.  As a general comment it would have been good to include some language that highlights the unique elements of each of these cases and perhaps contrasts those elements with the other cases.  From all three, I got the message that fPBAP are present – what else should I have learned?
• Given the potential for the ocean surface to be a significant source of PBAP and fPBAP, it seems as though the paper could use some additional discussion on the general state of regional sea ice. It is a more complicated scene than in many other parts of the Arctic due to the highly dynamic region that these samples are collected in.
• In general, I found section 3.6 (Annual cycle of cloud parameters) sloppy and rambling. This section seems like it should be one of the most important ones in the text, yet there are no clear messages.  Please take time to rework and expand on this section to ensure that you clearly communicate the most important points.
• Section 3.7: This section feels like an afterthought. Should there be relationships between deuterium excess and fPBAP concentrations?  What is a “reasonable contribution” (e.g., line 345)?  Please be more quantitative and help the reader understand what might be expected and why.  To me it doesn’t necessarily make sense to segregate this by liquid, mixed-phase and ice clouds.  Why?  Are there differences expected?  Or are those just seasonal?  More detail and information is required in this section.
• The “Conclusions” section reads more like a summary, essentially just repeating the findings laid out in the rest of the paper. I would recommend that the authors consider highlighting these points within the actual text, and then use section 4 to offer more discussion and put these observations into broader context. 

Minor Comments:

• Line 2: This is true, but it also depends on the size of the hydrometeors, the thickness of a given layer, the temperature of the layer, etc.This might be a bit over-simplified.
• Lines 34 and 35: For an earlier reference, also consider Creamean et al., 2018 (Creamean, J.M., R.M. Kirpes, K.A. Pratt, N.J. Spada, M. Maahn, G. de Boer, R.C. Schnell and S. China (2018):Marine and terrestrial influences on ice nucleating particles during continuous springtime measurements in an Arctic oilfield location,  Chem. Phys., 18, 18023-18042, https://doi.org/10.5194/acp-18-18023-2018)
• Lines 35-37: Given that phase in high-latitude clouds and INP concentrations are both notoriously difficult to sample with satellite-based sensors, it seems that this sentence could use some more explanation (even if supported by a reference).
• Lines 168-169: “Here, we focus on an altitude of 400 to 600 meters.” – presumably this is because that is the height of the Zeppelin Observatory, though it might be worth stating this directly.
• Line 176: It isn’t clear what is meant by “mixed ratio”, and the term sounds a lot like “mixing ratio”.I would consider rewording.
• Line 194: “within the mixed layer (as defined by the model/HYSPLIT output)”:How confident are we in the fact that HYSPLIT and/or “the model” can do this correctly?  Is there any evaluation of this for the Arctic?  Please consider adding some language on uncertainties or potential challenges.
• Line 207: Consider rewording “the last part” to “finally” or similar to maintain some level of formality in the text.
• Line 274: “Close” should be “Closely”.
• Line 303: I think that this is debatable.Shupe et al (https://psl.noaa.gov/people/matthew.shupe/publications/Shupeetal.JAMC2011.pdf) shows that the fall is generally cloudier.
• Line 306: There is an increased contribution of sea spray in winter months (over summer months)?Is this due to more storms?  More explanation would be good.
• Line 319: “(1-5 and 9-12)” Is this supposed to be month numbers?If so, I would recommend just using the names of the months to avoid confusion.
• Line 321: “as such were most-likely MPC”—no need for a hyphen between most and likely.Also, don’t you have measurements to confirm this?  If so, why say “most likely”?
• The paragraph starting on line 324 should be cleaned up, language-wise.I found it challenging to read and there were things like open parentheses, lots of commas, and unclear language.  For example, the sentence structure “the altitude in which temperature reached -15 C sat above 2500 m” is unnecessarily complicated.  Please re-read and re-word this section.

General overview

The paper comprises three case studies of an experiment to investigate the abundance of fluorescent primary biological aerosol particles, which may act as ice nuclei.  The experiment was run over one and a half years, in the Svalbard archipelago, at an observatory atop a 500m mountain.  Data was recorded from a Multiparameter Bioaerosol Spectrometer (MBS, to detect fPBAP), and a FIDAS 200S to measure size distribution, a Picarro L2130-i to measure isotopic ratio of oxygen and hydrogen.

The description of the data and the experiment is extremely useful, and the data provided is novel, in that the size distribution and concentration of fPBAP which may act as ice nuclei is timely and of interest to cloud formation studies.  However, the paper itself I found difficult to interpret at times, lacking in flow, and descriptions of some key elements.  It is perhaps lengthier than necessary, and could be more concise without losing the key elements.

Major comments

• Section 2.2 doesn’t make it clear that the CVI is the component responsible for delivering samples to the MBS, TEM, and FIDAS 200S. The authors refer to the CVI as though it is measuring parameters of particles. This could be made clearer throughout.
• The case studies are all short-duration (days) and spaced by months. It would be useful to know the reason for choosing these CE specifically, and also to contextualise them with some long-term data.  Including outside of CE.
• The conclusions section describes the experiment, and provides a brief description of the results. There are few impactful conclusions drawn from the data.

Minor comments

• Line 168, “we focus on” suggests optical focussing of the remote sensing devices. Is that as intended, or are you extracting data for this region?
• Line 213, missing unit on concentrations
• The MBS includes shape classification capabilities, which may help with discriminating cloud phase. It would be useful to explain why these weren’t employed (i.e., in order to measure the nuclei, the sample was dried)