Immersion Freezing Ability of Freshly Emitted Soot with Various Physico-Chemical Characteristics

Round 1

Reviewer 1 Report

Comments on "Immersion freezing ability of freshly emitted soot with various physico-chemical characteristics" by Falk et al.

This study investigates the immersion freezing ability of freshly emitted soot particles with various physico-chemical characteristics. Eight freshly emitted soot samples are generated in a soot generator under various conditions by altering the air to fuel ratio or by a combination of reducing the oxidation airflow and dilution. The chemical and physical properties of those soot particles as well as their immersion ice nucleation capability are characterized. Results show that partially matured to mature soot particles have low-to-insignificant immersion freezing ice-nucleating ability. But the link between ice-activity and the studied physico-chemical soot particles is not obvious and universal. The manuscript is well written and easy to read. I only have some minor comments here:

1. SI: Only captions of supplementary materials are provided at the end of the main text. I can not find the supplemental materials (Figures and Table) in the submission system.
2. Table 1: It is not clear to me why dilution air is set to be 4 L/min for OP2 and OP4, but 0 for other OP cases. Is there a reason for this non-linear setup?
3. Can deposition ice nucleation occur in the SPIN instrument? If so, will it affect your interpolations of, e.g., the immersion freezing ability?
4. What is the resident time of particles in the SPIN main chamber? What about in the evaporation section? 

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

  This paper is the results from the experimental study on immersion freezing of soot particles. The study focuses on the measurements of ice-nucleating ability of various soot particle types generated with a miniCAST burner by changing the combustion conditions. In terms of attempts to link the immersion freezing ability to the physico-chemical particle properties, the authors extended the discussion on the relationship based on not only the current study results but also similar previous study ones. Although the readers would like to get some insight about the difference between the generated particles in this study and the biomass burning particles in real nature, they realize that the physico-chemical parameters of soot particles are not critical to determine the immersion freezing ability from this study. In the original manuscripts, the methods, results and discussion were mostly described in an appropriate way although some minor points are required to be clarified or modified.

  I am ready to recommend the publication after the authors revise the manuscript by considering the following comments.

 

[Specific comments]

- Page 4, lines 114-115:

  The potential impact of different particle diameter between experiments was discussed in section 3, however, that of different particle number concentration did not seem to be discussed in the manuscript. 

 

- Page 5, lines 163-165:

  As for the ammonium sulfate (AS) experiment, could you explain it in detail? Do you use the pure (solid) crystalline AS particles? How about the size information (mono-disperse or poly-disperse, on average)?

 

- Page 8, line 300:

  The variable, T_onset, was firstly appeared here without any explanation. It was explained explicitly in Page 12. So it needs revision.

 

- Page 10, lines 329, 333, and 341:

  The case of OP6 sample had the similar trend to that of OP2-5 samples. If the OP6 was excluded by intent, further explanation should be included in the text, since the OP6 case was also corrected in the same manner.

 

- Page 11, lines 346-349, Figure 3:

  As for the homogeneous freezing reference, could you compare the results of previous laboratory studies or estimate it theoretically in order to confirm the validity? In general, the homogeneous freezing temperature of pure water droplets is determined as a function of drop size and cooling rate. The water activity (effective mole fraction of solute) is also affected in case of solution droplets.

  In Figure 3, there was an inflection point at around -38.5C for the case of the AS. What do you think of the cause?

 

- Page 11, lines 352-353:

  The statement "with the largest relative importance for the lowest AFs" is not clear to me. Could you explain it?

 

- Page 11, lines 363-364:

  Could you explain the statement "a superposition of multiple ice-activation modes"? Do you identify it by the inflection point at around -38.5C for the case of SP3? If so, can you insist that only by that feature?

Author Response

Please see the attachment.

Author Response File: Author Response.pdf