The spatial variability of polarization switching in ferroelectric lead zirconate-titanate nanoparticle arrays and within a single sub-100 nanometer nanoparticle is investigated by using switching spectroscopy piezoresponse force microscopy. Strong variations of switching properties within a single nanoparticle are observed and attributed to polarization pinning by geometric effects and interfacial dislocations. The spatial distributions of imprint bias and the thickness of the frozen polarization layer within the nanoparticle are reconstructed.