Review of “Measurement report: New particle formation and aerosol properties at a newly founded atmospheric observatory at the Finnish Baltic Sea coast” by Peltola et al.

This study reports on new particle formation at a coastal site, specifically the Finnish Baltic Sea coast. An observatory was formed in 2022 at the Tvarminne Zoological Station (TZS) on the southern coast of Finland which was leveraged in this work for measurements pointing to NPF being just as often as at the well-known boreal forest site in Hyytiala more than 200 km to the north/northeast where many publications have focused on NPF. Interestingly only one third of NPF events occurred at both sites on the same day. NPF was favorable when clean air masses from the Norwegian Sea passed over the boreal forest environment. A nice feature of this study was looking into local intermediate ion formation (LIIF), which informs about NPF potential near the measurement site. Their results suggest that LIIF was more frequent over land versus marine areas due to low condensation sinks and sunny/dry conditions.

The topic of this paper is certainly of interest to this journal. It is a Measurement Report, which lessens the pressure in a way to show very novel results with broad implications. Instead, this paper reports on high quality data at a fairly new site and put the results into context with comparisons to other regions. The paper adds to the growing literature about NPF. The draft is well written with good figures and tables.

The general recommendation is to publish this work after considering minor comments below.

Specific Comments:

Line 77-78: The authors mention that the location of the NAIS changed two months into the data collection period. Did this change in location affect any of the results, and was there a specific reason for the location change?

Line 95: “based on the”

Line 131-132: “This is why we focus our analysis…”

Line 133: “and larger ions (7-25 nm) that are still…”

Line 139-140: Stitching together size distributions from independent instruments is not trivial. Can the authors provide more details about special considerations and/or uncertainties in their stitching process?

Section 3.3.2: this is an important section as this is where identified relationships have broader implications to other regions in contrast to previous sections that are very locally dependent.

“Measurement report: New particle formation and aerosol properties at a newly founded atmospheric observatory at the Finnish Baltic Sea coast” by Peltola, et al.

This study presents data from a new aerosol measurement station on the Finnish Baltic Sea coast and is particularly focused on new particle formation events and the factors related to them. The dataset is large, with many variables measured, and it was well analyzed and put into context with the other Finnish measurement site at Hyytiälä. The writing and explanations could be improved, but overall the paper is good and I recommend it to be accepted for publication as a measurement report in ACP, after addressing the specific comments below.

General comments:

1. The introduction could have more background on what NPF is and what the known factors are that lead to NPF. The introduction now is a bit too heavily focused on why coastal research is important – this is good, but introductions are also important for explaining concepts and background knowledge needed to understand the study.
2. The differences between “marine” vs. “continental” air masses are a big point of the paper. However, your “marine” sector comes from the south which is immediately marine but preceeded by the entirety of continental Europe. Does this not influence the air? And the “continental” sector is from the north, which is immediately land, but further north is all sea and the Arctic, which I would think also influences the conditions of the air mass. Your coast is different than say, the coast of California where west is truly marine and east is truly continental. Although NPF is a more local phenomen, I wonder if the longer history of the air masses matter. I think it would be good to this address this further.
3. Which wind direction provides cleaner air needs to be more clearly and consistently explained. What I understand is a) that NPF is more likely in cleaner (less aerosol) conditions, and b) the marine sector is south/southeast and the continental sector is north/northwest/northeast. However – Line 126 says that the marine sector is less polluted than the continental sector, but then Line 158-162 (and again later) says that most NPFE days were from north/northwest wind direction (continental) because it is “clean compared to other directions”, which is a contradiction to Line 126. I was left a bit confused throughout the results & discussion because it was not clear to me which direction is clean and which is polluted (especially because typically we think of continental as being more polluted than marine due to anthropogenic influences – why is your “continental” not like that?). It may help to be specific in each case about what “clean” or “polluted” refers to, i.e., refer to specific species (less CO in marine than continental, or more particles >25nm in marine than continental).
4. You use Hyytiälä as a comparison for NPFE, but why don’t you compare other data to that site, like ions and larger aerosol concentrations and trends?
5. Can you compare your results to other coastal or marine sites, in addition to Hyytiälä?
6. Appendix B should come before Appendix A because it is mentioned first. Figures should always be in order of when they are referenced in the main text.
7. The language and sentence structure is very repetitive at times. Try to vary it more. For example, many (41) sentences start with “This..” (and ‘this’ should always be followed by a noun, otherwise it can be confusing). “This is in line with”, “this is expected / as can be expected”, “this explains”… are used quite a lot.

Specific comments (line numbers):

15-19: The introduction starts rather abruptly and requires prior knowledge of CCN and NPF. I would recommend giving a very basic introduction to NPF and then explain why it is important for our atmosphere and climate, relating it to cloud formation, etc. Then, after the basics, you can go into the marine vs. continental topic.

23: “chemical mechanisms” -> “chemical species”

29: add references after “open ocean” and “polar conditions”

41: “since aerosols can have a cooling effect on climate” -> Aerosols can have a cooling or warming effect on climate (direct radiative effects vs. indirect aerosol-cloud interaction effects), and the uncertainty in the overall direction of cooling/warming is one of the big reasons why aerosols are so important to study. You should therefore rephrase this.  

42: What is CarbonSink+? Is it a model? If it’s relevant to your paper, then you should explain it, otherwise remove it.

74: How high up are the wind measurements?

82: How far from the sea/coast is the Hyytiälä station?

82: Reference Fig. 1 here.

Fig 1: The map scale needs to be bigger to be legible.

84-85: Where were the DMPS and APS located (which trailer)?

90: “for the other instruments” -> which ones?

98: “and that particle mode needs to grow in size” -> “and the growth of that mode”.

99-103: Move the sentence “The difference between Class I and Class II…” to before the sentence “Observing this requires…”

105: Paragraph break before “For the event analysis…”

106: “For the event analysis, we used NAIS size distribution data from both Tvärminne and Hyytiälä” -> do you mean that in order to determine NPFE at each site, you used NAIS data from each site? I would rephrase to: “To determine NPFEs at Tvärminne and Hyytiälä, we used the NAIS size distribution data (total particle concentration measured with the negative polarity) from each respective site.”

107: “where both stations had complete data” -> “where both stations simultaneously had complete data”

126: “the marine sector is less polluted than the continental sector” -> see general comment above.

138: please define “condensation sink” here.

144: delete “located 230 km north-north-east from Tvärminne”

146: “On average, Tvärminne had 9.3% clear NPFE days …” -> What do you mean with “on average” here? Do you mean overall in the year, 9.3% of days had NPFE? Or overall in the whole dataset?

150: Rephrase the sentence “Although the average event frequences…” to “Although the average event frequences we report here are lower than, e.g., earlier Hyytiälä measurements by Nieminen et al. (2014), the spring and early autumn maxima were similarly observed.”

154: Move point (1) to the end and rephrase “classifying events manually can be subjective”

155-162: Rather confusing paragraph. In the sentence “During these clear regional NPFE days…” (158), which site are you referring to? Both of them? It is not clear then, why NPFE were observed so rarely on the same day at both sites… The sentence “This is also in line with previous results from Hyytiälä” also needs rephrasing -> “Similarly, previous results from Hyytiälä show that NPFE typically occur when air masses come from the north-northwest (280°-30°) (Nieminen et al, 2014).”

166: “When comparing…” -> “When comparing the median ion concentrations of the two wind sectors, sub-2 nm ions had 39% and 2-2.3 nm ions had 22% higher concentration in continental air.”

174: Why is there no diurnal pattern in marine air? Do you have any theories?

195-197: Why is the seasonal trend of sub-2nm ions different than the larger ions?

198: Start with “Overall, …”

Section 3.2: Perhaps it would be good to provide some background on the basics of ion formation and the differences between sub-2nm and larger ions (how they are formed, the relation to NPF in general, etc), either in the introduction or in this section.

205: This background info on condensation sink is great, but it should also (or only) be in the introduction.

206-207: Make a better/clearer connection between condensation sink and particle size distributions. (e.g., that total surface area of particles is estimated from particle size distributions assuming spherical particles)

213: Replace “thing” with “aspect”. (“thing” is very colloquial and vague, and should not be in scientific papers, in my opinion)

214-215: Why is the NAIS known to show higher concentrations than the DMPS? Is there a physical reason or is it just instrument bias?

217: sentence end after “coarse modes”. New sentence “The differences seen…”

219: “In the nucleation mode…” ->  “In the nucleation mode, the continental air has clearly higher concentrations than marine air during the day (8.00-17.30 h), like the negative ions.”

221: “A similar but stronger pattern…”

226: “This has been shown to favor NPF…” -> What is ‘this’ in this case? The free troposphere mixing in and lower the aerosol concentration? You have two sentences in a row starting with ‘this’ and it starts to become unclear what specific aspects you are referring to. Suggested rephrase here to: “The increase in the mixed layer height has been shown to favor NPF (e.g., Wu et al., 2021) and may explain why we see the highest nucleation mode particle concentrations around the same time”.

227: “As can be expected in a location far from major pollution sources…” -> The fact that your measurement station is far from major pollution sources is important! It should be said in the introduction and methods, because it’s an important advantage to your measurement station that not everyone will know already.

227-228: “As can be expected in a location far from major pollution sources, coarse mode particle concentrations were low” -> Low compared to what? Compared to other particle sizes? Compared to polluted regions? Coarse mode particle concentrations are always lower than other sizes, not just in clean regions.

231: “it is no surprise” -> very colloquial.

231: “CS” has not yet been defined as condensation sink.

230-231: “As the surface area of the particles is dominated by accumulation mode particles, it is no surprise that the CS was higher in marine air.” -> Needs more explanation, a step or two were jumped. Suggestion: “Condensation sink was unsurprisingly higher in marine air (particularly between 6.00 and 20.00h) because a) condensation sink depends on total particle surface area, b) total aerosol surface area is dominated by the accumulation mode (having both high number and large size), c)  marine air had higher concentrations of accumulation mode particles (Fig. 6c), and d) we have already seen that there were fewer NPFE with marine air which means that condensation sink should be higher.”

233-235: “Similarly to accumulation mode particles…” -> rephrase: “The difference in condensation sink between the two air masses is clearly during the day (8-17.30h), which is likely due to boundary layer dynamics, i.e.,…” and then explain specifically which dynamics you mean.

250: “Figure 8 shows the average 2-2.3 nm ion concentration (chosen as a proxy for NPF) binned with mean wind direction, together with wind speed and relative humidity.”

251: “All the plots show that there are more ions when the wind direction is from the northwest (X°), as we already showed that NPF is most likely to occur in air masses from this direction (Fig. X).”

253-256: starting with “Sunny conditions”, rephrase:  “Since sunny conditions favor photochemistry and drive NPF, and relative humidity is often anti-correlated with radiation due to potential cloudy or rainy conditions, we see an anti-correlation of relative humidity with 2-2.3 nm ion concentrations.”

Section 3.3.2: What is the explanation for the correlation with higher wind speeds?

268: “as can be expected from previous results” -> “as we saw in Fig. 8”

269: “the previous results from the wind direction bin plots” -> “Fig. 8”

273:” With this limitation..:” -> “With this filter, the 2-2.3 nm ions and temperature are negatively correlated.”

274: Why are warmer air masses more aged and why do they have higher condensation sink?

278: “no significant correlation” -> how are you defining “significant”?

279: “limited data” -> “filtered data”

Figure 9: I would suggest trying to plot this data as 2D histograms (with matplotlib: hist2d) so that you can better see where the data lies. As it is here, all the points are overlapping so it’s impossible to see how much data is in the center and where.

285: “sub-2nm and 7-25 nm ions which correlate negatively with each other” -> their correlation coefficient is -0.053, which is extremely weak. I would say there is no correlation there. In general, when you describe correlations, you should always provide the R value in parentheses.

Conclusions last paragraph: Add how your new station specifically can help address these new research questions in the future.