Aggregation is an inter-particle process which involves a multitude of different physicochemical mechanisms. In the present work, particles in the nano-scale are considered, with such concentration that renders their direct simulation as individual particles intractable. A stochastic aggregation model is presented for large particle populations in a Lagrangian framework. The model allows for simultaneous collisions between numerical parcels present in a certain volume of interaction (e.g. computational cell) and can be directly coupled to an unsteady numerical solver of a continuous flow. The model performance is evaluated against analytic solutions for a sum (Golovin) and constant aggregation kernel.