|The manuscript by Tauber and coworkers present an extensive set of experiments probing the heterogeneous nucleation of butanol with NaCl and silver seeds, and influence of water vapor on the process. The used nucleation reactors are the SANC and a commercial TSI 3776 CPCs. Wide range of conclusions are made on the effect of particle charge, composition and hygroscopicity on the nucleation probability. Interesting findings, among many others are that increasing sample flow RH decreases the CPC cutoff for NaCl seeds while not for Ag seeds, which is explained by water vapor uptake by NaCl seeds but not Ag. Contradicting results are presented for the role of charge measured with the SANC (enhancement) and 3776 (suppression) on the nucleation probability of NaCl particles with butanol, which is not satisfactorily explained. I have also other concerns on some interpretations of the data that are specified below. What distracts me also is that the manuscript is full of comparative statements (reduces, increases, lowers etc) without specifying what is compared to what, or otherwise logically incomplete statements so that they could be well understood. Some examples are picked below. It makes the text hard to read and partly ambiguous. Overall, I think the presented data and results are valuable and publishable, but at current state, the manuscript requires improvements before publication. |
Below are my detailed concerns.
- P2 l12-13, lower the energy barrier compared to what?
- P2 l12-17, the particle chemical structure plays major role, as shown for example in each of the three cited references in the paragraph and this study. Also, it is not limited only to sub-3 nm size range as shown for example by this current study (fig6) and many previous water CPC studies
- P2 l24, “indicated a decrease of the activation barrier”, because of what?
- P2 l25-26, the high relevance is coming from that many CPCs operate with butanol, and not because butanol is similar to propanol
- P2 l28, “revealed a strong dependence on chemical composition and water vapor”, what is dependent on chemical composition and water vapor?
- P2 l30, affect physicochemical properties of what?
- P3 l3-4, in the scope of this study, at least the work of Barmpounis et al. (2018) and Sem (2002) should be relevant for their observations of detection efficiency as a function of condenser temperature and effect of RH, respectively. Possibly also relevant references are Iida et al. (2009) and Kangasluoma et al. (2013) who have studied RH dependence of DEG based CPCs
- P3 l6 “we focus on the deviating activation behavior of butanol-based CPCs in this work.” What does this mean?
- P3 l20, what kind of ion trap was used?
- P3, how it was taken care that particle losses in the lines due to diffusion do not affect the results? Are the particle losses the same for the measurements of charged and neutral experiments?
- P4 l10-19, if you neutralize the size selected particles, what is the concentration reference for the neutral particles? It cannot be the FCE as in eq1.
- P5 l21-22, can you speculate why is this? If silver is generally considered insoluble, why it is even harder to activate sodium chloride than silver? Is it related to the NaCl particle shrinkage due to water (see next comment)? Also, can you elaborate somewhere briefly in the manuscript why Kelvin predicts increasing onset S for decreasing Tnuc? To me it is not obvious
- P9 l10-16, does this speculation mean that the NaCl-butanol measurements with RH < 3.5% and NaCl-propanol measurements of Schobesberger et al. (2010) are also contaminated with water? And that for < 3.5 nm sizes the seed is just too small to uptake water at lower RH, and the unusual trend vanishes or is not observed at all (2.5 nm)? Is this also why at RH 0% in fig4 the normalized mean diameter is never 1? Due to the previous, to me it looks that the RH < 3.5% data is also affected by water vapor, and the discussion should be modified accordingly.
- P11 l1, there is structural change according to fig4 even at RH 3.5%
- P11 l8, is the number 8.1% for a single detection efficiency value or cutoff or something else?
- P11 l12, this seems to be true around RH > 20% according to figs 6-7, while not true according to fig2 where you have RH < 10%
- P11 l15, “decrease”, compared to what? To RH 3.5% data? Based on fig3 I would say increase
- P11 l22, “charge history”, with respect to this study I think this term is misleading. In an ideal experiment the generated positively and negatively charged particles are identical except for the charge sign. When they are neutralized, ideally the resulting neutral particle is the same, and thus also its activation related properties, regardless of the initial charging state. The reason why different results are obtained with different “charge histories” can be either that the particles are different from the beginning, or that their properties change during the neutralization process. Kangasluoma and coworkers have shown some evidence for the former, that the composition of the generated particles are different in different polarities (and also that it affects the cutoffs of some CPCs) e.g. in the paper referred below, and in the paper cited in the manuscript. For the latter Alonso and Alguacil (2008) have provided measurements. Therefore, I suggest removing the term “charge history”, and add some other speculation to explain the observations, or at minimum elaborate clearly the mechanism how “charge history” can affect the experimental results.
- P11 l27-31, to me this speculation is contradictory to previous. First it is speculated that solvation of NaCl to water decreases the onset S of butanol, while here it is speculated that increased water uptake of charged seeds increases onset S of butanol (cutoff)
- P11 l35, polarity of the cluster, or polarity of the vapor molecule?
- P12 l2-9, These statements need more elaboration. With SANC charge enhancement is observed at <= 3.5 nm, while with CPCs neutrals are activated more easily more or less at all measured sizes (1.5-4.5 nm). So I think the data does not support that the particle size can explain the observed differences between the SANC and CPC. Also, I don’t understand how the temperature trend affects the charge enhancements? The processes should be the same for both instruments? L5-9 are hard to understand
- P13 l6-8, why is this? It is exactly the opposite what is reported by Winkler et al. (2008). Also, why saturation vapor pressure would affect charge enhancements, instead of e.g. supersaturation?
- P14 l1-3, what do you mean? Why solubility has something to do with high concentration?
- P14 l8-9, can you recommend a good RH threshold? If I remember right ACTRIS suggests RH < 40%, is it good?
- P14-15 l10-5, are you suggesting sub-10 nm particles in the coastlines are NaCl from sea spray? Can you provide some references? I think so far iodine and sulfuric acid have been shown to be responsible for sub-10 nm particles in the coastlines (Jokinen et al. 2018; Sipilä et al. 2016). What are hygroscopicities of these particles? Also, how would you suggest to correct for this? Adjust the CPC cutoff, or also somehow correct the measured concentration?
- P15 l9-10, is it (Kangasluoma et al. 2013)?
- P15 l10-12, DEG has been shown to be good in activating 1 nm particles in a CPC modified from 3776 (there are many papers on DEG based ultrafine CPC modifications), so it is not the “highly dynamic environment” that makes PSMs good down to 1 nm.
- P15 l18, which size change?
- P15 l18-19, I don’t understand this sentence
- P15 l19, which influence?
- P15 l29-30, this is not true according to figs 6 and 7
- P16 l1-2, which polarity? There are several other similar studies on the topic. It is hard to follow the logic of this and the following sentence
- P16 l14-16, this is contradictory to fig6 where neutrals are activated more easily than charged
Alonso, M. and Alguacil, F. J. (2008). Particle Size Distribution Modification During and After Electrical Charging: Comparison between a Corona Ionizer and a Radioactive Neutralizer. Aerosol Air Qual Res 8:366-380.
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Iida, K., Stolzenburg, M. R., McMurry, P. H. (2009). Effect of Working Fluid on Sub-2 nm Particle Detection with a Laminar Flow Ultrafine Condensation Particle Counter. Aerosol Sci Tech 43:81-96.
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