Page 435 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

13.5 Nanoaerosol Characterization 415

13.5 Nanoaerosol Characterization

Nanoaerosol can be sampled on a filter or grid for offline analyses of the mor-
phology and composition of individual particles. The most common offline method
is transmission electron microscope (TEM) and energy-dispersive X-ray spectros-
copy (EDX). However, the physical and chemical properties may change due to
agglomeration and/or chemical reactions during the sampling, transport and offline
characterization processes. An online measurement is preferred when it is available.
Online monitoring of nanoaerosol is applied mainly to measure the size distri-
bution. A number of technologies have been developed to measure airborne
nanoparticle size distribution for use in a laboratory setting. And they are briefly
summarized for guidance as follows.

13.5.1 Scanning Mobility Particle Sizer

Scanning mobility particle sizer (SMPS) employs a differential mobility analyzer
(DMA) to classify nanoaerosols based on their electrical mobility after passing
through a bipolar charger; the performance of a DMA is mostly limited by the low
charging efficiency of sub-20 nm nanoaerosol particles. Classified nanoparticles are
sent into a condensation particle counter (CPC), where they grow to 25 μmby
condensation in butanol or water vapor. Then these large particles are counted by
light scattering technique.
The principles of SMPS were introduced by Wang and Flagan [59]. Particles of
different sizes are separated in DMA based on their electrical mobilities that depend
on particle size. The electrical mobility Z p , a measure of the particle’s ability to
move in an electric field, is defined as

neC c
Z p ¼ : ð13:40Þ
3pld p
where n is number of elementary charges on the particle, e is the charge of an ion,
C c is the Cunningham slip correction factor, μ is gas viscosity and d p is particle
diameter.
The polydisperse aerosol enters a bipolar neutralizer in the electrostatic classifier
where aerosol particles reach a state of charge equilibrium due to collisions with
bipolar ions. Then the charged aerosol enters DMA. The DMA consists of two
concentric metal cylinders. The inner cylinder (r 1 ) is maintained at a controlled
negative DC voltage and outer one (r 2 ) is electrically grounded. Thus, an electric
field between the two cylinders is created. The polydisperse aerosol (flow rate Q p )
and sheath air (flow rate Q sh ) from the top of the classifier flow down the annular
space between the cylinders. Due to the action of the electric field, positively
charged particles are attached to the inner electrode while negatively charged