Number size distribution measurements are conducted by means of an SMPS unit.
SMPS consisting of an electrostatic classifier (TSI 3080) and a condensation
particle counter (CPC). First, the classifier separates particles by size
according to their electrical mobility. Then, particles which are size selected
with the electrostatic classifier pass to the CPC (CPC 3772, TSI) in order the
number concentration to be measured. Aerosol particles pass first through
a neutralizer which contains a Kr-85 radioactive source. The source brings the
polydisperse aerosol to equilibrium (Boltzmann) state. At this equilibrium state,
the distribution of charges on the particles is given by Boltzmann’s law.
Then, the charged particles enter the Electrostatic Classifier (EC, TSI 3080),
which separates them due to their electrical mobility. The electrical mobility
of a particle is defined as the ratio of the constant limiting velocity a
charged particle will reach in a uniform electric field to the magnitude of this
field [Willeke and Baron] and depends mainly on the particle size and electrical
charge. The EC consists of two concentric metal cylinders. The incoming sample
flow (1lt/min) containing the polydisperse aerosol is directed together with
laminar particle-free sheath air (10lt/min) parallel to the HV-Rod. A negative
voltage applied to the inner cylinder (collector rod), causes the particle of
positive polarity to be attracted to the collector rod. The outer cylinder is
grounded. Particles with positive polarity and higher mobility are precipitated
on the upper portion of the rod and those with lower mobility are carried along
with the excess air. Only a few particles within a narrow range of electrical
mobilities exit through the slit at the bottom of the collector rod. The selected
particle diameter can be changed by ramping the voltage. Particles which are
size selected with the EC are then counted with the Condensation Particle
Counter (CPC, TSI 3772). The CPC counts the aerosol particles that pass the EC.
Inside a CPC incoming particles are enlarged due to vapor condensation. This is
done in a heated saturator where alcohol vapor condenses onto the particles,
causing them to grow into droplets. These droplets are then detected with a
laser beam.
Optical Particle Sizer GRIMM
The Environmental Dust Monitor EDM 107 of Grimm's Enviro-product group,
the entire PM values are on hand in real-time. This reliable top of the line
device was developed especially for mobile and instant on-site measurements of
environmental dust. It is portable, tough and displays simultaneously all three
of the PM values (PM10, PM2.5 and PM1) in real-time. The PM masses are gathered
in a size range from 0.25 to 32 µm in >30 size classes and displayed as PM values.
Retrieved parameters : size distribution 0.25-32μm
Hygroscopic Tandem Differential Mobility Analyzer
A Hygroscopic Tandem Differential Mobility Analyzer (HTDMA) is used to
measure the ability of the particles to take up water. The HTDMA characterizes
the mixing state and the hygroscopicity of the particles as a function of their
size by measuring hygroscopic growth factors of aerosols when exposed to a
certain percentage of relative humidity.
Retrieved parameters : hygroscopic growth factor