Abstract: The PIV/PTV two-phase flow measurement is employed to study the "brownout" two-phase flow induced by a near-ground hovering helicopter rotor. The deflection of the rotor tip vortex trajectory and the merging progress of vortex pairs are characterized through Γ2 vortex core identification and modal decomposition. POD results reveal the spatial distribution and frequency characteristics of dominant modes in different wake field regions. The instantaneous particle distributions exhibit wave-liked pattern excited by the wake field. Large quantities of particles are identified in the near region of x/R = 2～3. Correlation analysis with POD mode coefficients indicates a close relationship between particle entrainment in this region and the transport of large-scale flow structures formed in the wake filed.