Opinion: Influence of the 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, the question of whether an overestimation of λair would have profound influence on a number of aerosol processes must be answered. 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 collision-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 by λair. Similar to the cause of a smaller λair, the overlooked force field in pure elastic 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 substantially influence particle collisions; i.e., it does not challenge our previous understanding of particle growth in the lower troposphere. Other potential influences on growth involving high excess latent heat and the uncertainties in the sub-5 nm size range are also addressed.