Characterization Methods for the Determination of Inhalation Exposure to Airborne Nanomaterials - Safety of Nanomaterials along Their Lifecycle - page 24

time-integrated over the sampling time. In this chapter, the available measurement
techniques are classified according to a scheme presented by Kuhlbusch et al. (2011),
that is,

• Size integrated, time resolved

• Size resolved, time resolved

• Size resolved, time integrated

• Size integrated, time integrated

The aim of this chapter is to give an overview of the commercially available tech-
niques for assessing exposure to airborne nanomaterials. Although nanomaterials are
defined to be in dimensions at least smaller than 100 nm, measurement techniques
for the micron particle size range are also described here, because nanoparticles tend
to agglomerate quickly and can therefore form rather large agglomerates.
These instruments use a variety of measurement principles, and in order to be able
to interpret the data obtained from particle sizing instruments, some knowledge on the
measurement technique is required. The reason for this is that the measured particles
are assumed to be spheres so that the particle size can be expressed as their diameters.
If the particles are nonspherical, the particle sizes are given as equivalent diameters,
where the equivalency describes that the particle considered behaves in the measuring
instrument like a spherical particle of this particular size. The most common equiva-
lent particle diameters delivered by aerosol measurement techniques are the electrical
mobility diameter d m (also referred to as Stokes diameter), the aerodynamic diameter
d ae , and the optical or polystyrene latex (PSL) equivalent diameter d PSL . The electri-
cal mobility diameter describes that the particle under consideration behaves in the
electric field of a differential mobility analyzer (DMA) like a singly charged sphere
of this particular diameter. For a spherical particle, the electrical mobility diameter is
equal to the geometric diameter. The mobility equivalent diameter also describes the
diffusional particle motion well. The aerodynamic diameter is used to describe the
particle size in an inertial separators. The inertial particle motion not only depends on
the particle geometry but also on the (effective) particle density r p,eff :

ρ
ρ

peff ,

dd
ae

=⋅

(2.1)

m

0

where r 0 is the unit density, that is, 1 g/cm³ or 1000 kg/m³. The effective density
equals the bulk density in the case of compact particles, but can be significantly
lower in the case of (loose) agglomerates (McMurry et al. 2002; Ristimäki et al.
2002).
Optical particle sizing instruments measure the light scattered by the particles.
The light scattering of particles depends not only on particle size but also on their
geometry and refractive index. In some cases optical spectrometers are calibrated for
a particular aerosol, taking into account the particles' morphologies and refractive
indices. It is, however, more common to calibrate optical particle counters with PSL
spheres of known properties. The determined particle size is then expressed as PSL