|“Ice-nucleating ability of aerosol particles and possible sources at three coastal marine sites” by Meng Si et al, resubmitted to ACPD|
The manuscript has been improved to such an extent that it is now close to merit publication in ACP. However, I still have a few remarks I’ll give below. But I trust that the authors will consider them carefully and will therefore only suggest minor revisions (although I feel that these remarks are still important). Much of this deals with how you treat / mix different concepts of particle size and related surface area (dry / wet; geometric / aerodynamic; mineral dust fraction / total aerosol). I suggest a few estimations that can at least put these different “approaches” into perspective with each other. They should be included in the text.
1.) Typically when measuring particle number size distributions in the atmosphere, the dry particle size is used as a reference (and often also the geometric diameter instead of the aerodynamic one), and typically this is also what should be used for the derivation of other values as e.g., surface area distributions, n_s and such. This is done to be comparable, as otherwise reported results change with the RH at which the measurement was done and cannot be easily compared. I’m sorry that I did not pick that up so clearly in the first round, as I feel this could have been easily corrected. (And you stumbled across that yourself when you compare with n_s derived from DeMott et al. (2016), top line of page 11.)
For reasons of comparability, the authors could consider changing this, although it is clear to me that this is requesting quite a bit as it needs to do a lot of recalculation. This is also why the authors could also settle for the second best solution which would be to deal with this openly. This means, that it should be estimated by how much the “wet n_s related to aerodyn. diameter” you derive underestimates the “n_s based on dry particle diameters” (it would be good to also convert aerodynamic to geometric diameter, for that exercise, too). This is easily done, following the conversion you describe in the SI and deriving the surface area for the different bins under these circumstances (the MOUDI-stage-cut-offs would have to be converted to geometric diameters, too, then, if you used geometric diameters to calculate the surface area).
This could quite quickly give you a measure of how much you might underestimate n_s, and that could be added to the text (in sec. 2.4, for example, where you discuss the conversion, or wherever you feel it fits). This can then also be used in the argument when the comparison with the marine INP concentrations from DeMott et al. (2016) is done, although you quite correctly say that the DeMott-based n_s values are an upper limit. (BTW: are n_s values from DeMott et al. (2016) related to the surface area of the total aerosol or only he sea spray particles? Check that, and if the latter is the case, the same applies as for the comparison with mineral dust that I comment on in the remark 2.)
And a related point is that the hygroscopic growth factors should be explicitly mentioned in the SI (only saying “factors consistent with measurements in the marine boundary layer” makes them less trustworthy than they likely are).
2.) On page 10, lines 18-19, you say:” Since this equation considers the surface area of all aerosol particles, rather than the surface area of just the INPs, the calculated ns values are lower limits to the ns values for the INPs.”
Comparing n_s derived based on mineral dust particles only with n_s derived from the total surface area of the measured aerosol is a bit like comparing apples and oranges. And using the total surface area of the measured aerosol is a good way to go. (This ultimately is what the model output gives you, too, or am I mistaken? As does the parameterization suggested by DeMott et al., 2010.) It just needs to be well defined, and then comparisons of values based on different bases should be avoided.
In that sense, it does not make sense if you talk about a “lower limit”. "Lower limit" connected to what? Make clear that you compare different parameters, where one is a value retrieved from laboratory measurement relating to mineral dust INP only, while the other one relates to the total atmospheric aerosol.
It would be interesting if you could give an order of magnitude by which n_s based on only mineral dust and "all aerosols" deviate. I guess you could at least estimate this number roughly, based on your data. Simply calculate the surface area related to the INP concentrations and compare that to the surface area you show in Fig. 4 (b) - the resulting factor is the one by which n_s related to "all aerosols" differs from that related to "only INP" - and that at least should give an idea about the order of magnitude of the deviation, and could help putting your n_s in relation to that derived from Niemand et al. (2012).
DeMott, P. J., A. J. Prenni, X. Liu, S. M. Kreidenweis, M. D. Petters, C. H. Twohy, M. S. Richardson, T. Eidhammer, and D. C. Rogers (2010), Predicting global atmospheric ice nuclei distributions and their impact on climate, Proc. Natl. Acad. Sci. USA, 107(25), 11217-11222, doi:10.1073/pnas.0910818107.
3) A few specific remarks:
page 11, line 6-7: “sea spray aerosol was … not a contributor to the largest INPs (5.6-10 μm in size) observed at Lancaster Sound.” - You can really only say that it did not contribute the majority of these INP! There could still be some sea spray particles that act as INP in this size range. Please adjust the sentence.
page 11, line 18: I assume when you talk about “measured INP concentration” here, this is the sum of the concentrations in all size ranges? Mention this!
page 12, end of section 3.5: Deviations between measurements and model results can also come from uncertainties in the measurements. You may have losses in the MOUDI (the RH of 50% that you give is comparably low alreaedy, and you’ll get the 50% only, at the described circumstances, as a possible maximum value, anyway (dew point outside 14°C, T in the container 25°C)), and you may overestimate the surface area and underestimate n_s (as you use the wet aerodynamic diameter – my assumption would be that the model used dry geometric diameters). This should be shortly discussed in a new paragraph.
page 12, line 20 ff: This example given here is very arbitrary. Particularly for black carbon, there was a number of indications lately that these do not act as INP, e.g., https://meetingorganizer.copernicus.org/EGU2017/EGU2017-16057.pdf, and (already published) Chen et al. (2018).
Also, this would better be moved to section 3.5 and discussed there, where it is first speculated on the four different types (as I wondered there what you might imagine them to be, anyway). As it was not mentioned before, it is not suited to appear at the “Summary and Conclusion”. And if you leave the choice of the four types, this information that BC might not be a good / no INP should be added, too.
Chen, J., Z. Wu, S. Augustin-Bauditz, S. Grawe, M. Hartmann, X. Pei, Z. Liu, D. Ji, and H. Wex (2018), Ice nucleating particle concentrations unaffected by urban air pollution in Beijing, China, Atmos. Chem. Phys., 18, 3523–3539, doi:10.5194/acp-18-3523-2018.