A numerical and experimental investigation of slit nozzle virtual impactor with three-partitioned horizontal inlet configuration and typical vertical inlet configuration was performed for improving the collection efficiency and reducing the wall loss. All parameters such as length and width of acceleration nozzle, width of collection nozzle, distance between the two nozzles, and the inlet flow rate were kept constant and only the inlet configuration was changed. The ratio of minor to total flow rate was fixed to 0.1. Parametric analysis was performed to study the flowrate influence of incoming clean air and aerosol on the performance of three-partitioned horizontal inlet slit nozzle virtual impactor. The square root of Stokes number, (Stk)1/2, was used to characterize the collection efficiency and wall loss of both configurations. It was observed that by using the three-partitioned horizontal inlet, the cut-off diameter was reduced from (Stk)1/2 = 0.80 to (Stk)1/2 = 0.41 and the wall loss near the cut-off diameter decreased from 25%–45% to below 4.5%, as compared to vertical inlet configuration. Moreover, with three-partitioned horizontal inlet, contamination by particles smaller than cut-off size in the minor out flow section of virtual impactor was reduced to almost zero percent.