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420 13 Nanoaerosol

13.6 Nanoaerosol Generation

Nanoaerosol particle instruments are calibrated by suppliers or specialized labora-
tories before they are delivered to end users. In addition to the primary and sec-
ondary calibration methods, calibration shall also include a check on the particle
number counters’s (PNC’s) detection efficiency with particles of a known size,
often being 23 nm (electrical mobility diameter). Proper selection of the test aerosol
particles is essential to the PNC calibration [15]. However, PNCs from different
manufacturers are calibrated using different aerosol materials. For example, emery
oil from TSI and NaCl from GRIMM. When they are used for industrial nano-
aerosol, such as diesel soot, a material with similar behavior with diesel soot should
be used too for the calibration.
An ideal aerosol generator is expected to be able to produce a constant and
reproducible output of stable aerosol particles with adjustable size and concentration
distributions. There are many nanoaerosol generators developed and commercially
available based on different mechanisms. Typical ones are summarized as follows.

13.6.1 Evaporation–Condensation Technique

This method can be used for the generation of solid nanoaerosol particles like NaCl,
C40 (tetracontane), silver or Tungsten. Nanoparticles are produced based on the
principle of evaporation and condensation on nuclei. As shown in Fig. 13.9, the
bulk nanoaerosol material is placed in a ceramic crucible (shown) or a ceramic
heater container (not shown), where the bulk material is heated to its boiling point.
A small flow is introduced into the heater, above the bulk material, to displace the
concentrated vapor. The hot vapor is delivered to an area where it is mixed with the
cool carrier air to enable condensation.
The output is polydisperse nanoaerosol where particles can be as small as 2 nm
[46], which is the starting point of nucleation [29]. The nanoaerosol particle sizes
can be varied by controlling one or both of the following factors.

• the crucible air flow rate (to control the vapor feeding rate)
• the carrier air flow (to control the subsequent cooling rate).

13.6.2 Electrospray Technique

This method is employed mainly for the generation of nanosized liquid droplets. As
shown in Fig. 13.10, the bulk liquid material (e.g., Emery 3004 or PAO 4 cSt) is
feed through a capillary into a container or suspended through a capillary tube from
the container. By applying an electrical field to liquid at the capillary tip, the liquid