A Passive Cavity Aerosol Spectrometer Probe (PMS model PCASP-100X) was used to
count number concentrations (number per cm3) of particles in 15 channels spaced
pseudo-logarithmically over the diameter range 0.10 micrometres to 3.00
micrometres, to provide a particle size distribution over this range. The
instrument has been used as provided by the manufacturer, except for
modifications to monitor air flows. The instrument operates on the principle
that the light scattered by a particle within an active laser cavity is a
direct function of its size.

The PCASP has a fast response: PMS quote that it can count at least 20,000
particles per second.