A penetration and removal efficiency calibration unit for a volatile particle remover in a solid particle counting system provides an accurate and efficient approach to calibration. The calibration unit includes an aerosol inlet, a flow meter downstream of the aerosol inlet, and a mixer. The flow meter receives the aerosol flow from the aerosol inlet and provides an output flow to the mixer. The mixer receives the output flow from the flow meter and also has a dilution gas inlet. The mixer provides a mixer output flow for reception by the volatile particle remover or particle counter. A first flow controller controls flow into the dilution gas inlet. The calibration unit also includes a bypass inlet. A second flow controller controls flow into the bypass inlet, and a control loop controls the bypass flow such that the aerosol flow tracks a reference value.