General

This manuscript investigates the roles of hygroscopic aerosols in Arctic longwave downward radiation. This is an interesting and potentially important topic as the Arctic is changing rapidly. Most of the methods and analyses are reasonable and clearly presented. My only concern is the impact of potential cloud scenes on the estimated ARE – the numbers are high for RH > 80% and almost comparable with the cloud effect. This finding is significant if the signals are truly from wet aerosols. However, further examination of the measurement data is needed to exclude cloud contamination, especially from nearby regions around the surface site. The manuscript is otherwise well written, and I recommend returning it to the authors for minor revision. A major revision is appropriate if substantial work is needed to examine the measurements.

Minor

L151-153: what is the field of view of FTS? Do you restrict the area coverage that needs to be cloud-free from Cloudnet? Multiple scattering could contribute to the FTS measurement if cloud is present in nearby regions.

L160: please define LBLDIS

Figure 1: The ARE from measurements for sea salt-dominated scenarios (b) can be comparable to 5000/cm3 of sea salt in simulations. Is this a reasonable sea salt number concentration in the Arctic winter? This raises a concern about whether the sky is truly cloud-free in observations.

Figure 2: there is a white line at 180. This might be because the lat/lon do not overlap or connect in the array for 180E and 180W.         To get rid of this, you can manually add an extra column in your data array. Suppose you have a [90, 360] shaped data, you add the 361^st column that is identical to the 1^st column and, correspondingly, your coordinate arrays. Then, you plot the [90, 361] shaped array. This should make the contour connected.

L321-325: then do you still consider the ARE values at RH levels above 80% valid for aerosols (e.g., the beginning of Section 6)? And I assume the RH here is with respect to liquid, right? During the Arctic winter, RH of 80% with respect to liquid could potentially be high enough for ice cloud formation. Distinguishing cloud and aerosol conditions is vital for the estimated ARE for hygroscopic aerosols.

The authors' investigation into the amplification of longwave downward radiation by hygroscopic aerosols in an Arctic field site presents notably high radiative effect values. This Referee maintains a degree of skepticism and recommends a revision of the manuscript to address these concerns convincingly. The following points should be considered:

• Comparison with Existing Studies: Given the dramatic results, it's essential to compare and contrast these findings with existing literature. Are there any reported measurements or theoretical calculations in similar or relevant settings?
• Robustness of Results:  In line with Referee 1's comments, additional effort is needed to ensure the robustness of the results. For example, while the authors discuss the distinctions between dry aerosol particles, wet aerosol particles, and cloud condensation nuclei, further elaboration is necessary.
• Clarity of Methods: The Methods section requires additional clarity. For instance, the introduction of "AREaw from FTS" measurements in section 4.1 that uses LBLDIS model calculations before the latter’s formal introduction in the subsequent 4.2 section, creates ambiguity. The source of the evidence should be explicitly stated.
• Precision in Terminology and Notation:  Equation 1 and its description warrant careful attention. The equation defines AREaw as the difference between all-sky and clear-sky values, yet the description refers to all scenes as "clear-sky" (no clouds), with the "clear-sky" term in the equation implying the absence of both clouds and aerosols. A revised notation and convention, possibly using the term "clean" for scenarios without aerosols, could enhance clarity.
• Methodological Clarity and Validation: The relationship between the various radiation methods introduced needs clarification. Are they complementary or intended for cross-checking? Additionally, the manuscript would benefit from a discussion of any validation efforts undertaken to bolster confidence in the results.

More general comments that could be considered in the revision as well:

• Extrapolation and Temperature Impact: The manuscript's impact could be enhanced by extrapolating the local effects to a larger (regional or global) signal regarding longwave radiation effects. Furthermore, can the measurements provide insights into the contribution of these aerosol effects to observed temperature changes nearby?
• Writing Style and Clarity: The manuscript's readability could be improved. The use of numerous acronyms, while potentially common in this subfield, can hinder comprehension. Careful consideration of whether each acronym is necessary would enhance clarity. For example, the authors use “FTS” but maybe “FTIR” is more appropriate here?

By addressing these points, the authors can strengthen the manuscript and ensure its clarity and impact for a broader audience.