If you are talking about filtration, then you do not have a solution, but a dispersed system of suspension of polystyrene particles in nitric acid. In order to select a filter with certain holes, you must first determine the size of your particles.
First estimate the particles size by using dynamic light scattering (DLS) / transmission electron microscopy (TEM)/ nanoparticle tracking analysis (NTA). Based on the PSD of D10, choose appropriate size filter medium to filtrate the NP's.