A two-partitioned horizontal inlet was developed for improving the collection efficiency and minimizing the wall loss problem in slit virtual impactor. The two-partitions were provided to simultaneously supply both aerosol and clean air to the virtual impactor. Both numerical and experimental investigations were carried out on the developed inlet configuration by considering different flowrate ratios of aerosol to clean air. The horizontal inlet was helpful in reducing the cutoff diameter, whereas the clean air prevented the particle deposition on the virtual impactor walls. The performance of two-partitioned horizontal inlet was compared with the conventional vertical inlet configuration for PM2.5, PM5 and PM10 virtual impactors. All the operating conditions and geometric parameters, such as the inlet flowrate; the width of collection nozzle; the width, length and span of acceleration nozzle; and the distance between collection and acceleration nozzles, were kept the same and only the inlet configuration was changed. The major-to-total flowrate ratio was kept at 0.9 and minor-to-total flowrate ratio at 0.1. It was observed that by using the two-partitioned horizontal inlet configuration, the cutoff diameters for PM2.5, PM5 and PM10 virtual impactors, were reduced by 16%, 10% and 11%, respectively, while the wall loss of particles near the cutoff size in all three cases were reduced from 16% to about 1%. (C) 2019 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.