This paper proposes a method for determining the supersaturation in a fog or cloud. The topic is highly important due to the crucial role played by supersaturation in determining the concentration and shape of the cloud droplet size distribution. The measurement concept seems novel and robust, and is reasonably well explained in the paper. There are a few places that are not clear, which I note below. I’m not sure why this was submitted to ACP instead of AMT, but that’s for the editors to decide regarding appropriateness. I consider the manuscript suitable for publication after the following comments are addressed.

One key question that arises after reading the paper is how the proposed technique can provide information about supersaturation fluctuations versus mean supersaturation. The authors nicely describe the importance of quantifying fluctuations, but then it is not clearly explained later on how this can be accessed. I believe it is related to the sigma value in Equation 2 (see my comment below) but I did not find a discussion of this topic in the paper, except a brief mention in the Discussion (limitation 1).  Please provide more discussion of this topic.

Lines 84-85: the statement “estimated from vertical velocity measurements” would be better supported by citing a paper that uses that approach, such as the paper by Cooper 1989 (J. Atmos. Sci.). Also, it would be more correct to state “estimated from vertical velocity and droplet size distribution measurements.”

Lines 117-119: “The κ-Kohler theory tells that if the critical diameter of aerosol activation (𝐷_𝑎) and corresponding aerosol hygroscopicity parameter κ are known, the surrounding supersaturation can be retrieved based on air temperature measurements and by assuming 𝜎_𝑠\a the surface tension of water.” This statement is correct, but it assumes knowledge that is not stated, such as how kappa and the critical diameter are related to each other. Please explain more thoroughly.

Line 126: In describing Equation 2 it is stated that “σ is associated with the slope of the curve near D_a”. Please provide a physical interpretation of what factors contribute sigma. Would it be true that for a monodisperse aerosol, and uniform supersaturation, sigma would be zero?

Lines 134-136: This part of the sentence is not clear and should be revised: “which brings uncertainty in 𝐷𝑎 derivations due to that the maximum activation fraction of aerosols larger than 𝐷𝑎 does not equal to unit although usually very close to (Tao et al., 2018b).”

Figure 3: Include brief discussion in the figure caption to explain the underlying concept of the instrument, at least for panel a. For example, explain the purpose of the drier versus the cooler (for controlling humidity).

Discussion: some discussion of the expected precision versus accuracy, as well as estimated uncertainties should be included.