Air-blasted atomization is the result of shearing instabilities triggered by the interactions between the liquid and gas flows. Many recent works have performed accurate Direct Numerical Simulation of liquid sheet disintegration. Indeed, high resolution DNSs are able to reproduce the smallest scales of atomization. Unfortunately this numerical accuracy is too expensive for industrial or parametric studies. LES is becoming on the other hand an efficient tool for simulating complex unsteady flows. However, in the case of assisted atomization no modelling is yet available for taking into account the sub-grid interfacial topological changes. A promising approach consists in building a transport equation for the sub-grid surface density, similar to the classical turbulence sub-grid quantities. In this approach, a source term takes in account the increase of the total liquid surface induced by the assisted atomization process. A definitive closure of this term is however not universally acknowledged. The objective of the present work is to contribute to the understanding and the modelling of the sub-grid surface source term. Several DNSs of a periodical planar liquid sheet atomization are presented, in which the total liquid surface evolution is measured in time. The influence of several inflow parameters, as density and velocity ratio, as well as initial boundary layer thickness, is evaluated on this quantity. Results show how surface growth rates as well as caracteristic times can be effectively measured in this kind of simulation.