The manuscript presented the effect of ammonium sulfate (AS) on ice nucleation ability of a set of non-mineral dust substances via immersion freezing. This study investigated the immersion freezing of bacteria, fungi, sea ice diatom exudates, sea surface microlayer, and humic substances in dilute AS solution. For comparison, the effect of AS on immersion freezing of four types of mineral dust were also investigated. The manuscript showed that there is no significant change for most of the tested non-mineral dust substances, expect for bacteria X. Campestris; for the tested mineral dusts, there is an increase in the median freezing temperatures, ranging from 3 K to 8 K. This study provides additional data sets for the better understanding in the ice nucleation potential of different types of substances and the effect of additional AS. The manuscript is well written and suitable for the publication in this journal. A few issues and comments need to be considered before publication.

Comments:

1, Line 170-200, for mineral dusts, the weight percentages of dust in the solution used here are not the same, can the authors comment on why different concentrations were used for preparing the droplets? What are the potential effects of AS on the freezing temperature of droplets with different concentrations of dust?

2, In Figure 2 to Figure 5, the frozen fraction for the Blank and Blank+AS data presented in different figures are somehow different. Were these blank experiments done at different time periods? In Line 228-231, it was mentioned that only the heterogeneous freezing temperatures have been corrected, have the blank data also been corrected?

3, In Section 3.2, as mentioned in several places that the results presented in this study are consistent with previous studies, e.g., Line 300, 316, 322. I would suggest to summarize these data and present in a table. This table or summary may further support the claim that freezing temperature response of unknown substances to additional AS could be used as a “fingerprint” for the presence of mineral dust.

4, For Figure 2-5 and Figure 7, for comparison purpose, it is great to show the data for different trials and see the variations, but the author may need to consider summarizing the data from different trials, I think that is the final form of data sets which readers may use.

General comments

This paper presents results from experiments investigating the impact of ammonium sulfate additions on the freezing properties observed for suspensions containing dust and non-dust INPs. The paper has a clear structure and is well written. The visual presentation of experimental results is adequate. The findings presented in this study are interesting, but a much larger emphasis should be put on enabling the translation of these findings into atmospheric implications, i.e. through a more thorough analysis delivering a quantification of the measured effects which is not dependent on the experimental parameters chosen for this study (e.g. dust mass).

The following points need to be addressed before this paper can be published:

• To be able to compare against other experimental studies more easily, results should also be reported as ice nucleation active surface site densities (i.e. normalized to mass or surface), so that at least relative changes at a fixed ammonium sulfate concentration could be quantified independently from the INP concentrations. Ideally, I would like to see measurements at different INP and ammonium sulfate concentrations, so that these results could be used more readily in climate models.
• Also, there is no discussion of statistical uncertainties associated with the observed frozen fractions.
• Lastly, I don’t agree with the authors’ conclusion that ammonium sulfate solutions can be used to reliably detect the presence of dust INPs. There are no experimental results showing how sensitive this method would be to variations in concentration (INPs, ammonium sulfate). Along the same lines, it is not clear what the results would be for complex particles, e.g. agricultural soil dusts that are mixtures of mineral dust and organics.

Specific comments:

• l. 90: Reference for selected ammonium sulfate concentration?
• Fig. 2-5: How do you explain the substantial variability in the blank measurements or between different microlayer samples? And how does this variability impact the interpretation of your experimental results?

Technical comment:

• Fig. 5: Please change 'control' to 'blank'.