Abstract: A circumferential slot virtual impactor includes a disk-shaped housing with an endless circumferential slot for receiving aerosols. The slot forms an acceleration nozzle, and a receiver nozzle spaced apart radially inwardly from the acceleration nozzle exit. In an annular gap between the two nozzles, negative pressure is selectively applied to draw a major flow of the aerosol axially away from the nozzles, while a minor flow of the aerosol is drawn radially inward and enters the receiver nozzle. A portion of the larger particles leaves the major flow and merges with the minor flow due to particle momentum, thus increasing the large-particle concentration of the minor flow. The acceleration nozzle incorporates convex curvature along its opposed interior surfaces, for a smoother aerosol flow and reduced large-particle deposition.