A phenomenological model for the creation and transport of anodic gas bubbles in aluminium reduction cells is presented. The proposed model is a multiscale approach in which molecular species are produced and transported through a supersaturated electrolyte. Sub-grid bubbles are allowed to form through nucleation and the resulting bubble population evolves through mass transfer and coalescence. As sub-grid bubbles reach a certain size, they are transferred to a macroscopic phase, which evolution is governed by a volume of fluid method. The current work describes the underlying theory and motivation for the proposed model and it is used to describe a laboratory-scale electrolysis cell, showing the potential of the suggested approach. The influence of selected properties of the model is identified by means of a factorial analysis.