Size and refractive index measurements of spherical particles with a liquid core and a solid crust are performed by dual-mode phase Doppler anemometry. A single particle containing urea-water-solution is suspended in an acoustic levitator and subjected to evaporation. The substance investigated is typically used for NOx reduction of HC engines. Evaporation of the higher volatile water leads to a formation of a solid shell due to crystallisation of urea. The temporal evolution of size and refractive index are measured and compared to numerical simulations.The capability by phase Doppler anemometry for measuring particles with a solid crystalline shell is demonstrated.