the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 26 Feb 2025
Opinion: Influence of mean free path of air on atmospheric particle growth
Abstract. Recent studies by Tsalikis et al. (2023, 2024) predicted that the mean free path of air (λair) could be significantly smaller than widely used values by a factor of ~2. Given the fundamental importance of λair, it has been a question that whether an overestimation of λair would have profound influences on a number of aerosol processes. Here we assume that the newly proposed value of λair is accurate and examine its influences on our understanding of atmospheric aerosols. We show that for collisions-induced aerosol dynamics such as the condensation growth of atmospheric particles, the collision rate and hence the growth rate are determined by an effective mean free path for vapor and particle collision rather than λair. Similar to the cause of a smaller λair, the overlooked force field in hard-sphere models may enhance vapor-particle collisions; however, this enhancement has been accounted for in previous studies. As a result, we find that the smaller λair does not directly influence particle collisions, i.e., it does not challenge the previous understandings on particle growth in the lowertroposhpere. Other potential influences on growth involving a high excess latent heat and the uncertainties in the sub-5 nm size range are also addressed.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Aerosol Research.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this preprint. The responsibility to include appropriate place names lies with the authors.- Preprint
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RC1: 'Comment on ar-2025-8', Anonymous Referee #1, 14 Mar 2025
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.
Citation: https://doi.org/10.5194/ar-2025-8-RC1 -
RC2: 'Comment on ar-2025-8', Anonymous Referee #2, 04 Apr 2025
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.
Citation: https://doi.org/10.5194/ar-2025-8-RC2 - AC1: 'Comment on ar-2025-8', Runlong Cai, 10 Apr 2025
Status: closed
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RC1: 'Comment on ar-2025-8', Anonymous Referee #1, 14 Mar 2025
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.
Citation: https://doi.org/10.5194/ar-2025-8-RC1 -
RC2: 'Comment on ar-2025-8', Anonymous Referee #2, 04 Apr 2025
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.
Citation: https://doi.org/10.5194/ar-2025-8-RC2 - AC1: 'Comment on ar-2025-8', Runlong Cai, 10 Apr 2025
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