The Passive Cavity Aerosol Spectrometer Probe (PCASP) is fabricated by Droplet Measurement Technologies (DMT) and has the SPP200 electronics. The PCASP measures both aerosol number and size in the nominal size range 0.1 to 3 micrometers. It is an optical particle counter (OPC) and uses a laser to illuminate the aerosol-laden ambient air stream from the instrument inlet. Pulses of light scattered from each particle are counted with the intensity of the light scattered into a fixed solid angle giving a measure of particle size. This instrument is routinely located on the FAAM BAe146.

The instrument is fitted to the aircraft in one of the PMS canister under the aircraft wings. It has a very short, straight inlet and so is a useful complement for cabin based aerosol instruments which necessarily have more convoluted sampling inlets.