[University home]

Centre for Atmospheric Science

Aerodynamic Particle Sizer

The Aerodynamic Particle Sizer

The Aerodynamic Particle Sizer model (3321) manufactured by TSI uses the principle of inertia to size particles. In this instrument the particle and sheath flow are constricted through a nozzle, accelerating the airflow. Particles within the airflow are also accelerated, but by different amounts depending on particle surface area and mass, thus particles exiting the jet have a velocity related to their aerodynamic diameter. Aerodynamic diameter is defined assuming spherical particles and unity density.

The APS measures particle velocity by passing the particles through two laser beams separated by about 200microns. An elliptical mirror collects scattered light onto a photodetector. A particle passing through both beams produces two pulses of scattered light, the time delay between the pulses being related to the velocity and hence aerodynamic diameter of the particle. The APS also records the height of the peaks allowing a secondary calculation of particle size based on optical scattering.

Schematic of APS detector block
Schematic of the APS detector block

Particles are only valid if exactly two peaks occur in the scattering signal in the time allotted for the particle to pass through the beams. Rejected particles are separated into three categories: only one peak, scatter signal low when time elapsed – particle too small or missed second beam; only one peak, scatter signal high when time elapsed – particle moving too slowly (very large or recirculating); more than two peaks – coincident particles. The number of particles in each rejected category is counted to allow statistical corrections to be applied if desired.

The size range of the APS is 0.5-20 microns aerodynamic with 52 size bins. Optical sizing is over the same range, with one extra bin covering 0.3-0.5 microns. Size distributions can be produced at a rate of 10Hz.