Publisher's note: the content of this comment was adjusted on 21 May 2024 after approval of the ACP executive editors since some formulations were inappropriate.

General Comments:

This work reports on the measurements of ice-nucleating particles (INPs) active below -24 degree C in a Finnish boreal forest during the HyICE-2018 campaign (13 March to 11 May 2018). In my impression, the advance of this work is to show the time series of the INP number concentrations measured using a PINE chamber over this campaign period. On the other hand, it seems that the key research approaches and conclusions of this study are very similar to those of earlier studies at the same site and period. For example, Brasseur et al. (2022) already reported the comparison of INPs active at temperatures around -30 degree C measured using several cloud-chamber-type instruments (PINE, SPIN, and PINC) with those active above -25 degree C measured using cold-stage-type instruments (INSEKT and uL-NIPI) during the same campaign, while this study has added some data points derived from the PINE. In addition, Brasseur et al. (2022) already evaluated the performance of INP parameterizations designed by DeMott et al. (2010), Tobo et al. (2013), and Schneider et al. (2021), and concluded that the Tobo et al. (2013) parameterization could better reproduce the number concentrations of INPs active below -24 degree C (-29 to -32 degree C) during this campaign. Schneider et al. (2021) also reported the comparison of the INP schemes designed by DeMott et al. (2010) and Tobo et al. (2013) with the INP data at the same site. This work contains some new results and discussion (e.g., INPs vs. RH and new particle formation (NPF)), but the results are based on the correlation coefficients listed in a table-like figure (Figure 6) only. For these reasons, I don’t think that the current manuscript measures up to ACP’s standards.

Specific Comments:

1) The time series of INP data measured using the PINE chamber are unique and potentially valuable. However, this work appears to repeat essentially the same work as reported by Schneider et al. (2021) and Brasseur et al. (2022). I strongly encourage that the authors reconsider if they can add new topics and findings using their data sets. Then, the authors should significantly update the contents of the manuscript in order to clarify the difference between this study and other earlier studies.

2) In my understanding, cloud-chamber-type instruments other than PINE (e.g., SPIN, PINC) had also been used to measure the time series of INPs active below -24 degree C at this site during the HyICE-2018 campaign. The authors should compare the difference between the PINE and the other instruments in the lower temperature regime and evaluate the strengths (and limitations) of the PINE. For example, to my knowledge, Paramonov et al. (2020) already reported the full INP data sets active around -30 degree C measured using a PINC during the overlapping period.

3) Please explain more details for the WIBS data. It is unclear how the WIBS fluorescent particle data were applied to the Tobo et al. (2013) (2) parameterization. As a result, it is difficult to judge whether the authors’ explanations that “this means that the measured INP concentration ~ from different sources (Lines 250-252)” and “a second parameterization proposed by Tobo et al. (2013), linking ~ but they are not identical (Lines 373-376)” are indeed reasonable. For example, Tobo et al. (2013) used the data on fluorescent particles larger than 0.5 um measured using UV-APS that employes an excitation wavelength of 355 nm and detects laser-induced fluorescence spectra in the range of 420–575 nm). Did they use the same setup as the UV-APS?

4) Brasseur et al. (2022) announced that “forthcoming studies would explore atmospheric vertical profiles of INPs, INP sources and transport modeling, plausible links between INP abundance and NPF, and the ice nucleation activity of boreal biology such as flora and fungi”. If this work is a part of the forthcoming studies, the authors should present some more details for the comparison of INPs with NPF. Since the information on the correlation coefficients only (Figure 6) is obviously insufficient, the authors should show the time series of the INP and NPF data.

5) The possible links between INPs and low RH are potentially interesting. On the other hand, many earlier studies have reported elevated INP concentrations under high RH conditions at a forested site in Colorado (Huffmann et al., 2013; Prenni et al., 2013; Tobo et al., 2013) and other locations (Wright et al., 2014). I would suggest showing figure(s) that could explain the relationship between INP and RH data. Then, please discuss the discrepancy between this study and the above studies if there was indeed a clear negative correlation between the INP and RH data.

Technical Comments:

6) I would suggest changing the title appropriately, because other related literatures (Paramonov et al. 2020; Brasseur et al., 2023) have already reported INP data below -24 degree C during the HyICE-2014.

7) Please indicate the full name and the abbreviated name in the same manner (e.g., see “Portable Ice Nucealtion Experiment (PINE)” in Line 59 and “PINC (Portable Ice Nucleation Chamber)” in Line 104.

8) Please explain some more details for the experimental setup for the PINE. For example, what is the cut-off diameter of the inlet?

9) I assume that the PINE uses an OPC that can detect larger aerosol particles, liquid cloud droplets and ice crystals as single particles (Line 84), but the size range covered by this OPC is not clear. It is also unclear how the PINE used here discriminated ice crystal signals from ambient aerosol particles and liquid droplets?

10) Lines 96-97: Please explain if the INP number concentrations presented here were corrected to the values at standard temperature and humidity conditions (0 degree C and 1 atom) or not.

11) Line 99: What do you mean by “the uncertainty for PINE is given as 20 %”?

12) Lines 111-115: Please specify the RH setup for mixed-phase cloud conditions in the PINE chamber.

13) Figure 2a: I would suggest including INP data derived from SPIN and PINC during the HyICE-2014 campaign.

14) Lines 262-264: Please explain how the PINE chamber captures deposition and condensation nucleation in the Methods section.

15) Lines 271-272: Please revise this description if the temperatures were between -27 and -30 degree C as explained in the Figure 6 caption.

16) Line 284: What do you mean by “FP3 channel”?  Please also check my comment 3.

References:

Brasseur et al. (2023), https://doi.org/10.5194/acp-22-5117-2022

DeMott et al. (2010), https://doi.org/10.1073/pnas.0910818107

Huffman et al. (2013), https://doi.org/10.5194/acp-13-6151-2013

Paramonov et al. (2020), https://doi.org/10.5194/acp-20-6687-2020

Prenni et al. (2013), https://doi.org/10.1029/2012GL053953

Schneider et al. (2021), https://doi.org/10.5194/acp-21-3899-2021

Tobo et al. (2013), https://doi.org/10.1002/jgrd.50801

Wright et al. (2014), https://doi.org/10.1080/02786826.2014.968244