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Instrument: SMPS : Scanning Mobility Particle Sizer
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The Scanning Mobility Particle Sizer (SMPS) Sizer is based on
the principal of the mobility of a charged particle in an
electric field. Particles entering the system are neutralized
(using a radioactive source) such that they have a Fuchs
equilibrium charge distribution. They then enter a Differential
Mobility Analyzer (DMA) where the aerosol is classified
according to electrical mobility, with only particles of a
narrow range of mobility exiting through the output slit. This
monodisperse distribution then goes to a Condensation Particle
Counter, which determines the particle concentration at that
size. The DMA consists of a cylinder, with a negatively charged
rod at the center; the main flow through the DMA is particle
free 'sheath' air. It is important that this flow is
laminar. The particle flow is injected at the outside edge of
the DMA, particles with a positive charge move across the sheath
flow towards the central rod, at a rate determined by their
electrical mobility.

Particles of a given mobility exit through the sample slit at
the top of the DMA, while all other particles exit with the
exhaust flow. The size of particle exiting through the slit
being determined by the particles size, charge, central rod
voltage, and flow within the DMA. By (in the case of the SMPS)
exponentially scanning the voltage on the central rod, a full
particle size distribution is built up.

Additional information available at

[Summary provided by the Atmospheric Physics Group]