Referee Comment:

General scientific comment:

The manuscript presents interesting and valid results on aerosol properties measured during cold air outbreaks at the Norwegian measurement station Andenes. In addition, cases are considered when there were nearly connected flow conditions between Zeppelin Observatory and Andenes station making comparisons of aerosol properties possible concluding about the general transformation processes. Only few such studies are available making the dataset valuable especially as input data for modelling studies are scarce and needed to better represent Arctic aerosol conditions in such model simulations. I suggest minor revisions to the manuscript addressing the below mentioned comments and text corrections.

Detailed scientific comments:

Abstract

Page 1, line 8:

Comment: (similar to non-CAO conditions)

Give the number for direct comparison as you have it by your statistics. You also give at other places direct comparison.

Introduction

Page 2, line 26-27:

Comment:

Take also a look at the following papers that might be supportive:

https://acp.copernicus.org/articles/22/3067/2022/

https://iopscience.iop.org/article/10.1088/1748-9326/ac444b

Page 2, line 36-38:

Comment:

There is evidence that even quite small particles can serve as CCN in the Arctic as e.g. here:

https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2021JD036383

https://acp.copernicus.org/articles/15/13803/2015/

Please comment!

2                    Methods

-

2.1                 CAO events

Page 3, line 64-65:

Comment: (surface level; 2 meters above 65 mean sea level; 69.30◦N, 16.15◦E)

The closer you come to the Arctic the more uncertain trajectories will get. This is because meteorological observations needed to run the models are more scarce. This is a general problem in the Arctic. Is it reasonable to use 2m above sea level trajectories having in mind these could have touched the surface before or did you check for this bias?

2.2                 In-situ aerosol measurements

Page 4, line 79-85:

Comment:

Please specify some details about inlets and drying conditions in the instruments. This might be of high relevance if you later want to make statements on changes of the PNSD in the nanometer size range, e.g. the change of Hoppel minimum, etc…. Zeppelin has ACTRIS status, but what about conditions at Andenes station? Please elaborate on this!

Page 4, line 92-98:

Comment:

Also, HTDMA and CCN counter are quite sensitive instruments of which performance need to be checked regularly. Provide information about calibration procedures before, during and after the measurements.

Page 5, line 101-112:

Comment:

You calculated the sea salt mass based on the mass additions given the page before. Can you comment on how well the masses fit in terms of their abundance of number of ions? Also, you calculate here sea salt mass from the fitted mode concentrations. I guess, you transferred the number mode to a volume mode and applied a density. If so, give the details.

I would recommend to give some general quality criteria of the combination of the two particle number size distributions (submicron and supermicron) at both stations in the appendix, so the reader can evaluate how good the procedure was. You combine here particle number size distributions of two different diameters (mobility and optical). Because of all the problems that can arise, ACTRIS is not recommending to use optical particle counters at their stations for data submission. As you do so, you should convince the reader that this is a valid approach in your case.

2.3                 Ancillary variables

-

3                    Greater sea spray particle number concentration

Page 6, line 140:

Comment:

Please check, I feel you exchanged marine and continental wind directions at least in the text. If this is the case, please recheck your data analysis for consistency.

General comment:

The information here on wind directions for CAO events and non-CAO conditions, numbers observed in different modes, significance between variables, etc. could be put in a table for better overview.

4                    Activation of particle to cloud droplets

Page 8, line 159 - 160:

Comment:

I do not get how you derive the Hoppel minimum diameter. You have to elaborate on this. It seems obvious from Figure 5 that it is somewhat smaller for CAO events compared to non-CAO conditions, but you need to explain how you quantitatively come up with a value. Are there some references about the procedure you use?

Page 9, line 174 - 175:

Comment:

How sensitive is effective SS against uncertainties in kappa derived from HTDMA and Hoppel minimum diameter. Also, if you calculate kappa from HTDMA and CCN counter for the same aerosol type, you will receive different results. Put this in perspective to your results and their uncertainties! I request here some kind of sensitivity study. Here are some papers on kappa discrepancies using the two different techniques:

https://acp.copernicus.org/articles/23/4931/2023/

https://pubs.rsc.org/en/content/articlelanding/2020/em/d0em00179a

Page 9, line 177-179:

Comment: Please check this sentence, I think you make the wrong conclusion based on theory.

Page 9, line 179 – 180:

Comment:

I do not think you can say this. This is dependent on the quantitative change of the parameter and here you compare apples and pears!

Page 10, line 188 – 189:

Comment:

Please see also the following paper where the CCN counts are listed depending on season:

https://acp.copernicus.org/articles/23/4931/2023/

Page 10, line 193 – 195:

Comment:

I do not see the difference in this ratio as a consequence of lower CN concentrations during CAO events in general. Can you explain why that is? If the pure fact that there are less particles in this air mass has something to do with the fraction activated, then please give the scientific reasons for this.

5                    Scavenging by precipitation

Page 11, line 203 - 204:

Comment:

I feel this is not fully correct. You can only give a decrease in total particle number for a given size range or a mode. When you refer to a specific size (in this case 142 nm) you can only talk about a change in dN/dlogDp. Also in Figure 8a, you label the y axis as delta N, should it not be delta dN/dlogDp for the size distribution, and similarly it needs to be delta N only for the number concentration observed in modes.

Page 12, Figure 8a:

Comment:

It seems that other studies have not done that analysis with a similar size resolution compared to this study. Please add their approach shortly in the Figure caption.

Page 12, Figure 8bcd:

Comment:

Are you sure the means are right? Please recheck. In Figure 8b there seems to be an artifact around 100 #/ccm, similar to Figure 8c at 200 #/ccm.

6                    Conclusions

Page 13, line 239-240:

Comment: Was it 15 air mass trajectories or better eight events with connected flow conditions?

Table S2:

Comment: How can Paramanov measure up to 80% more CCN compared to CN?

General language comment:

I prefer always a detailed description/wording of aerosol size distributions. You can look at number, surface, volume, mass, etc…. I made a number of text suggestions below as you talk about particle number size distributions here, but then also say so.

Specific language comments:

Abstract

Page 1, line 1:

… aerosol particles serving as …

Page 1, line 3:

… aerosol number size distribution modes …

Page 1, line 4:

… upwind from Andenes in …

Page 1, line 11:

… aerosol number size distribution …

Introduction

Page 2, line 34:

… aerosol number size distribution …

Page 2, line 46:

… aerosol number size distribution …

Page 2, line 51:

… aerosol number size distribution …

Page 2, line 54:

… droplets …

2                    Methods

-

2.1                 CAO events

Page 4, line 69:

… aerosol number size distribution …

2.2                 In-situ aerosol measurements

Page 4, line 74:

Particle number size distribution …

Page 4, line 76/77:

… to size and count …

Page 4, line 79:

… particle number size distributions …

Page 4, line 99:

Particle number size distribution …

2.3                 Ancillary variables

-

3                    Greater sea spray particle number concentration

Page 6, line 127:

… aerosol particle number size distribution …

Page 7, Figure 3:

Median particle number size distribution ...

Page 7, line 150:

… at Andenes …

4                    Activation of particle to cloud droplets

Page 7, line 155:

… aerosol particle number size distribution …

Page 8, line 162:

… aerosol number size distributions …

Page 8, line 164:

… or aerosol microphysical properties …

Page 9, Figure 5:

… lower and upper edges …

Page 10, line 195:

… are activated to cloud droplets …

5                    Scavenging by precipitation

Page 11, line 200:

… particle number size distributions …

Page 11, line 201:

… passed …

Page 11, line 207:

… in accumulation mode particles during our …

Page 11, line 217:

… of trajectory arrival at Andenes …

Page 12, line 229:

… from Zeppelin Observatory to Andenes …

6                    Conclusions

Page 13, line 238:

… aerosol number size distributions …

Page 13, line 240:

… aerosol number size distributions …

Page 13, line 241:

… compared to non-CAO events …

Figure caption S1:

… aerosol number size distribution …

Congratulation to the authors, a good contribution to the field, adding important Cold air outbreaks CAO information.

Particularly appreciated is the comparison among two monitoring sites and the good number (49) of event reported. 

Overall, it is a good contribution to the field following the important papers well cited of Abel 2017, Sanchez 2022 and Llloyd 2018.

Two main small comments:

1. Please expand the introduction and explain better what the COA events are and why they are important, perhaps a conceptual diagram or a figure explaining better the event, poorly explianed in the current literature.

2 I was surprised by the sea spray mode at 450nm, sensibly larger to what I understand being the "referement" mode at 160nm (Prather et al 2013, etc). Perhaps a discussion on this and an appropriate small summary of the art on the typical sea spray aerosol mode may be necessary. 

Congratulation, it is a well described paper with a very good dataset collected at two stations.