In radiative transfer calculations, the optical properties of atmospheric particles are usually estimated using particle models with spherical symmetry. These spherical symmetric particle models, however, predict zero depolarization properties, which makes them unsuitable for remote sensing applications. More realistic particle models are been developed but the validation of these models needs information of the optical properties of the real atmospheric particles. In this contribution the optical properties of amorphous SOA particles and BC particles coated with SOA were studied in aerosol chamber simulations studies. SOA particles and SOA coatings were produced through the ozonolysis of alfa-pinene precursors. In the first part of this contribution it will be shown that amorphous SOA particles will induce depolarization, unlike SOA particles in the liquid phase. This information can be used to detect the amorphous phase state of SOA with a non-invasive optical method. We will show the results of direct measurements of the SOA phase transition under different atmospheric conditions. In the second part of this contribution the change in the depolarization and in the absorption properties of BC particles as a function of SOA coating thickness are shown. The results are discussed in the light of developing and validating optical particle models.