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416 13 Nanoaerosol

particles are attached to the outward one. Uncharged particles are removed with
*
the excess flow. Only particles with the optimum electrical mobility Z p exit the
DMA, and

Q sh r 2

Z ¼ ln ð13:41Þ
p
2pVL r 1
where Q sh is sheath air flow rate (equal to excess air flow rate), r 2 is outer radius of
annular space, r 1 is inner radius of annular space, V is average voltage on the inner
collector rod, L is length between the exit slit and polydisperse aerosol inlet.
Combining Eqs. (13.40) and (13.41) leads to the particle diameter measured as a
function of the collector rod voltage, number of charges on the particle, classifier
flow rate, and geometry of the DMA:

2 neðÞVLC c
d p ¼ ð13:42Þ
3lQ sh ln r 2
r 1
By changing the voltage supplied to the inner cylinder of the DMA, scanning
over the whole particle size interval is possible.
Once the particles are classified according to electrical mobility; their number
concentration is measured by CPC. The CPC counts particles with a diameter from
a few nanometers to one micrometer. It is very difficult to optically detect submi-
cron particles because they have a diameter that is comparable with or even less
than the wavelengths of most lights. To address this challenge, CPC works by
passing the aerosol samples though a supersaturated vapor stream; the vapor con-
denses quickly on the nanosized particles. These large droplets with nanoparticles
as seeds inside can be easily detected or counted by optical methods. The pulses of
scattered light are collected by a photo detector and converted into electrical pulses.
The concentration of particles is obtained from calibration of DC voltage against
known concentrations.
The combination of size and number gives us the particle size distribution. To
obtain this particle size distribution SMPS requires about 2 min.

13.5.2 Particle Classification by Aerodynamic Particle
Focusing

Alternatively, particle can be classified by aerodynamic particle focusing. Under
normal condition, nanoaerosol particle follow the air due to its noncontinuum
behavior. As introduced in Sect. 4.2.4 above, the inertia of a particle in curvilinear
motion is characterized by the Stokes number (Stk), which is defined in Eq. (13.43).