The paper points out that the growth rate of aerosol particles is little affected by a recently proposed uncertainty (Tsalikis et al (2023), (2024)) in the mean free path of air molecules. This is because the relevant molecular path length affecting the rate of growth is the mean free path of vapor molecules prior to collisions with an aerosol particle rather than the mean free path of air. Numerous equations for the aerosol growth rate for different regimes of particle size are reviewed to demonstrate this. 

The more interesting discussion in this paper lies in the Supplement, where an uncertainty in the vapor mean free path of a similar degree to that proposed for air is shown to make a substantial difference to the particle growth rate. I do wonder why the authors claim that the vapor mean free path is not uncertain when recent studies suggest there is uncertainty in the mean free path for air molecules. I recommend that the authors bring the material in the Supplement into the main body of the paper and comment further on this matter.

This opinion piece of Cai & Kulmala tests whether recent revisions to the mean free path of air alter the growth rates of aerosol particles. They present a thorough and very well written theoretical overview, before a handful of calculations showing that there is no notable difference. I echo the other reviewer that the discussion in the supplement is interesting, but I am ambivalent about whether it belongs in the main text. I have no specific comments. This work is highly useful and I enjoyed reading it, and therefore recommend it for publication.