below 2 nm. Recent studies have shown that by
modifying the operating conditions of, e.g., the
TSI 3025A [133], TSI 3772 [134], TSI 3025
[135], and TSI 3010 [136], detection of sub-2 nm
particles is possible, with high detection efficiency. The studies commonly focus on modifying the temperature difference between the
saturator and the condenser tube [133] and working fluids [137]. These operational conditions
strongly affect the probability of the condensational growth of the smaller nanoparticles.
Particle Electrical Mobility
The method of determining the size distribution of
an aerosol using particle’s electrical mobility was
introduced about 120 years ago. Electrical mobility is perhaps the best and most widely used
technique for measuring ultrafine particles. As
mentioned above, there are significant obstacles
toward using optical techniques for the highresolution characterization of nanoparticles due
to weak light scattering by small particles, smaller
than the wavelength of light. However impaction
methods, e.g., MOUDI, can use pressures below
atmospheric to collect such particles. Over the last
few decades, the electrostatic force has been
exploited for sizing nanoparticles. As mentioned
in section 4, when a particle charged by a corona
or gas phase ions is exposed to an electric field, it
migrates at a velocity that depends on its size and
morphology. A particle’s motion is described by
the balance between the applied electrostatic force
and aerodynamic resistance (drag). The forces on
nanoparticles are made sufficiently high to overcome diffusional effects, yielding high-resolution
sizing. The determination of the size distribution
of an aerosol using these devices requires knowledge of the charge distribution of the particles,
i.e., the charge number on particles with a given
diameter [5].
The differential mobility analyzer (DMA),
introduced by Knutson and Whitby [138], is the
main class of electrical mobility instruments. The
fundamental principle of this technique is shown
in Fig. 26a. The DMA is a cylindrical classifier in
which particles are charged by colliding with a
cloud of ions produced by a unipolar high voltage
central rod (corona electrode). Particle trajectories
then deviate radially from the airstream depending
on their size, toward the outer collection electrode.
The applied voltage of the corona can be varied
between 0 and 5 kV. A particle-free sheath flow
passes through the cylinder. Each ring collection
electrode is connected to an electrometer for measuring the number of particles and their currents
which is related to the particle size.
Airborne Nanoparticles: Control and Detection, Fig. 26 (a) Schematic of a typical DMA. (Reprinted with
permission [139] and (b) SMPS DMA [5])
Airborne Nanoparticles: Control and Detection
123
modifying the operating conditions of, e.g., the
TSI 3025A [133], TSI 3772 [134], TSI 3025
[135], and TSI 3010 [136], detection of sub-2 nm
particles is possible, with high detection efficiency. The studies commonly focus on modifying the temperature difference between the
saturator and the condenser tube [133] and working fluids [137]. These operational conditions
strongly affect the probability of the condensational growth of the smaller nanoparticles.
Particle Electrical Mobility
The method of determining the size distribution of
an aerosol using particle’s electrical mobility was
introduced about 120 years ago. Electrical mobility is perhaps the best and most widely used
technique for measuring ultrafine particles. As
mentioned above, there are significant obstacles
toward using optical techniques for the highresolution characterization of nanoparticles due
to weak light scattering by small particles, smaller
than the wavelength of light. However impaction
methods, e.g., MOUDI, can use pressures below
atmospheric to collect such particles. Over the last
few decades, the electrostatic force has been
exploited for sizing nanoparticles. As mentioned
in section 4, when a particle charged by a corona
or gas phase ions is exposed to an electric field, it
migrates at a velocity that depends on its size and
morphology. A particle’s motion is described by
the balance between the applied electrostatic force
and aerodynamic resistance (drag). The forces on
nanoparticles are made sufficiently high to overcome diffusional effects, yielding high-resolution
sizing. The determination of the size distribution
of an aerosol using these devices requires knowledge of the charge distribution of the particles,
i.e., the charge number on particles with a given
diameter [5].
The differential mobility analyzer (DMA),
introduced by Knutson and Whitby [138], is the
main class of electrical mobility instruments. The
fundamental principle of this technique is shown
in Fig. 26a. The DMA is a cylindrical classifier in
which particles are charged by colliding with a
cloud of ions produced by a unipolar high voltage
central rod (corona electrode). Particle trajectories
then deviate radially from the airstream depending
on their size, toward the outer collection electrode.
The applied voltage of the corona can be varied
between 0 and 5 kV. A particle-free sheath flow
passes through the cylinder. Each ring collection
electrode is connected to an electrometer for measuring the number of particles and their currents
which is related to the particle size.
Airborne Nanoparticles: Control and Detection, Fig. 26 (a) Schematic of a typical DMA. (Reprinted with
permission [139] and (b) SMPS DMA [5])
Airborne Nanoparticles: Control and Detection
123