A penetration and removal efficiency calibration unit for a volatile particle remover in a solid particle counting system, the calibration unit comprising: an aerosol inlet; a flow meter downstream of the aerosol inlet, the flow meter being configured to receive an aerosol flow from the aerosol inlet, measure the aerosol flow, and provide an output flow; a mixer configured to receive the output flow from the flow meter and having a dilution gas inlet, the mixer being configured to provide a mixer output flow; a first flow controller for controlling a flow of a particle free dilution gas into the dilution gas inlet of the mixer; a bypass inlet; a second flow controller for controlling a bypass flow into the bypass inlet; and a control loop for controlling the second flow controller such that the measured aerosol flow tracks a reference value, wherein the flow meter is configured to measure the aerosol flow upstream or downstream of a differential mobility analyzer; a portion of the mixer output flow is configured to be provided to a volatile particle remover or a condensation particle counter; the first flow controller is configured to control the flow of the particle free dilution gas to the dilution gas inlet of the mixer; the bypass inlet is configured to receive the bypass flow, the bypass flow being the remainder of the mixer output flow that bypasses the volatile particle remover and the condensation particle counter; and the control loop is configured to control the second flow controller such that the aerosol flow measured upstream or downstream of the differential mobility analyzer tracks the reference value as the flow into the volatile particle remover or the condensation particle counter varies. an aerosol inlet; a flow meter downstream of the aerosol inlet, the flow meter being configured to receive an aerosol flow from the aerosol inlet, measure the aerosol flow, and provide an output flow; a mixer configured to receive the output flow from the flow meter and having a dilution gas inlet, the mixer being configured to provide a mixer output flow; a first flow controller for controlling a flow of a particle free dilution gas into the dilution gas inlet of the mixer; a bypass inlet; a second flow controller for controlling a bypass flow into the bypass inlet; and a control loop for controlling the second flow controller such that the measured aerosol flow tracks a reference value, the flow meter is configured to measure the aerosol flow upstream or downstream of a differential mobility analyzer; a portion of the mixer output flow is configured to be provided to a volatile particle remover or a condensation particle counter; the first flow controller is configured to control the flow of the particle free dilution gas to the dilution gas inlet of the mixer; the bypass inlet is configured to receive the bypass flow, the bypass flow being the remainder of the mixer output flow that bypasses the volatile particle remover and the condensation particle counter; and the control loop is configured to control the second flow controller such that the aerosol flow measured upstream or downstream of the differential mobility analyzer tracks the reference value as the flow into the volatile particle remover or the condensation particle counter varies.