The Pumped Counterflow Virtual Impactor (PCVI) is a virtual impactor that allows on-line separation of coarse from fine aerosol. The separation size is variable over the 0.3 to 5 micron diameter range, and the device is small enough to fit in the palm of your hand. With two mass flow controllers, the flows inside the PCVI can be varied to quickly change the particle size that passes through the counterflow and into the sample outlet flow.