Investigating Aerosol Activation and Microphysics in Marine Fog using a Lagrangian Cloud Modelling Framework

Cloud microphysics plays a fundamental role in the formation and evolution of marine fog, but it is not fully understood, particularly due to its substantial differences with cloud microphysical behavior. Numerous studies have addressed this by means of direct observations and modelling efforts, however basic questions remain regarding the composition of the fog condensation nuclei and what processes lead to their activation. To achieve the goal of describing this process from a mechanistic point of view, we have performed a set of large-eddy simulations coupled with a Lagrangian cloud modeling framework across varying background aerosol concentrations in a marine fog event based on the recent FATIMA campaign. In this study we perform an analysis of the Lagrangian history of fog droplets and aerosols to determine a deceptively simple question: which aerosols activate, and where do they come from? It is found that most activated aerosols have their initial locations inside the fog layer and are not entrained from above. Particles inside the fog layer exhibit a wide-ranging vertical movement, touring the entire height of the layer several times during the simulations, but are activated at or close to their original locations. This suggests that entrainment does not play an active role in activation, which if true has important implications regarding the origin of aerosols acting as condensation nuclei. In addition, inactivated aerosols account for half of the particle concentration inside the fog layer, with a significant proportion reaching droplet-like sizes and behavior without undergoing activation.