Bremsstrahlung X-rays (continuum or background X-rays) also can be produced by the deceleration of the primary beam electrons by the
electric field of the nuclei of the sample atoms.
An FESEM (field emission SEM) is an SEM
optimized for operation at a low energy range
(<5 kV), using an electron gun that produces a
low- and high-energy electron beam. This kind of
operation produces high resolution and also
enables scanning at low potentials [159].
Transmission electron microscopy (TEM) can
be used to characterize nanoparticle size, morphology, chemical composition, crystallinity and
structure, thermal volatility, and mixing state,
because of its resolution down to fractions of a
nanometer. A TEM is very similar to an SEM. In
TEM, the objective lens is placed after the sample
area in order to image electrons that have been
transmitted through the sample. These electrons
may have been scattered (elastically or
inelastically) or transmitted (without any interactions with the sample atoms). If the sample area is
thicker, fewer electrons are transmitted and the
images will be darker; thus, a thinner sample
will make brighter images. Elastically scattered
electrons do not lose their energy when interacting
with the sample atoms, but the angle at which they
scatter, according to Braggs Law, can be used to
characterize crystalline regions. Inelastically
scattered electrons lose energy due to interactions
with the atoms in the sample providing elemental
analysis and bonding information [160]. Many
examples of the use of SEM and TEM to characterize airborne nanoparticles can be found in the
literature [116, 161, 162].
Future Prospects
In this review, the most important and feasible
techniques for removing nanoparticles from an
airstream have been presented. Despite their very
high filtration efficiency, the existing filtration
methods have a high pressure drop, resulting in
high energy consumption. The electrostatic precipitator does not have this problem; however,
there are limitations to its use because the high
voltage generates ozone, in addition to its size,
capacity, and energy use. There is an inevitable
trade-off between purification performance and
economic and technical feasibility. As a consequence, there is a continual desire to develop
more appropriate and advanced methods. Filtration of nanofibers and nanotubes, with the lowest
possible pressure drop, may be possible by
designing the arrangement of nanofibers or by
using the high-performance monolayers.
The most common and commercially successful nanoparticle-measuring techniques have been
reviewed. Based on the available literature, the
field of nanoparticle measurement has important
gaps including development of new techniques
better suited to different environmental conditions,
improving the sensitivity of electrometer-based
instruments and the response time of chargingbased instruments and addressing inability to successfully use light-based and mass-based techniques. As always, it would be desirable to have
instruments that are smaller and more robust.
Bibliography
1. Cooper CD, Alley FC (2010) Air pollution control: a
design approach. Waveland Press, Long Grove Illinois, USA
2. Hinds WC (1999) Aerosol technology: properties,
behavior, and measurement of airborne particles.
Wiley, New York
3. Biswas P, Wu C-Y (2005) Nanoparticles and the
environment. J Air Waste Manage Assoc
55(6):708–746
4. Mao X, Bai Y, Yu J, Ding B (2016) Flexible and
highly temperature resistant polynanocrystalline zirconia nanofibrous membranes designed for air filtration. J Am Ceram Soc 99(8):2760–2768
5. Kulkarni P, Baron PA, Willeke K (2011) Aerosol
measurement: principles, techniques, and applications. Wiley, Hoboken
6. Liu J-Y, Hsiao T-C, Lee K-Y, Chuang H-C, Cheng
T-J, Chuang K-J (2018) Association of ultrafine particles with cardiopulmonary health among adult subjects in the urban areas of northern Taiwan. Sci Total
Environ 627:211–215
7. Chen R, Hu B, Liu Y, Xu J, Yang G, Xu D, Chen
C (2016) Beyond PM2. 5: the role of ultrafine particles on adverse health effects of air pollution.
Biochimica et Biophysica Acta (BBA)-General Subjects 1860(12):2844–2855
8. Stafoggia M, Schneider A, Cyrys J, Samoli E, Andersen ZJ, Bedada GB, Bellander T, Cattani G,
Airborne Nanoparticles: Control and Detection
127
electric field of the nuclei of the sample atoms.
An FESEM (field emission SEM) is an SEM
optimized for operation at a low energy range
(<5 kV), using an electron gun that produces a
low- and high-energy electron beam. This kind of
operation produces high resolution and also
enables scanning at low potentials [159].
Transmission electron microscopy (TEM) can
be used to characterize nanoparticle size, morphology, chemical composition, crystallinity and
structure, thermal volatility, and mixing state,
because of its resolution down to fractions of a
nanometer. A TEM is very similar to an SEM. In
TEM, the objective lens is placed after the sample
area in order to image electrons that have been
transmitted through the sample. These electrons
may have been scattered (elastically or
inelastically) or transmitted (without any interactions with the sample atoms). If the sample area is
thicker, fewer electrons are transmitted and the
images will be darker; thus, a thinner sample
will make brighter images. Elastically scattered
electrons do not lose their energy when interacting
with the sample atoms, but the angle at which they
scatter, according to Braggs Law, can be used to
characterize crystalline regions. Inelastically
scattered electrons lose energy due to interactions
with the atoms in the sample providing elemental
analysis and bonding information [160]. Many
examples of the use of SEM and TEM to characterize airborne nanoparticles can be found in the
literature [116, 161, 162].
Future Prospects
In this review, the most important and feasible
techniques for removing nanoparticles from an
airstream have been presented. Despite their very
high filtration efficiency, the existing filtration
methods have a high pressure drop, resulting in
high energy consumption. The electrostatic precipitator does not have this problem; however,
there are limitations to its use because the high
voltage generates ozone, in addition to its size,
capacity, and energy use. There is an inevitable
trade-off between purification performance and
economic and technical feasibility. As a consequence, there is a continual desire to develop
more appropriate and advanced methods. Filtration of nanofibers and nanotubes, with the lowest
possible pressure drop, may be possible by
designing the arrangement of nanofibers or by
using the high-performance monolayers.
The most common and commercially successful nanoparticle-measuring techniques have been
reviewed. Based on the available literature, the
field of nanoparticle measurement has important
gaps including development of new techniques
better suited to different environmental conditions,
improving the sensitivity of electrometer-based
instruments and the response time of chargingbased instruments and addressing inability to successfully use light-based and mass-based techniques. As always, it would be desirable to have
instruments that are smaller and more robust.
Bibliography
1. Cooper CD, Alley FC (2010) Air pollution control: a
design approach. Waveland Press, Long Grove Illinois, USA
2. Hinds WC (1999) Aerosol technology: properties,
behavior, and measurement of airborne particles.
Wiley, New York
3. Biswas P, Wu C-Y (2005) Nanoparticles and the
environment. J Air Waste Manage Assoc
55(6):708–746
4. Mao X, Bai Y, Yu J, Ding B (2016) Flexible and
highly temperature resistant polynanocrystalline zirconia nanofibrous membranes designed for air filtration. J Am Ceram Soc 99(8):2760–2768
5. Kulkarni P, Baron PA, Willeke K (2011) Aerosol
measurement: principles, techniques, and applications. Wiley, Hoboken
6. Liu J-Y, Hsiao T-C, Lee K-Y, Chuang H-C, Cheng
T-J, Chuang K-J (2018) Association of ultrafine particles with cardiopulmonary health among adult subjects in the urban areas of northern Taiwan. Sci Total
Environ 627:211–215
7. Chen R, Hu B, Liu Y, Xu J, Yang G, Xu D, Chen
C (2016) Beyond PM2. 5: the role of ultrafine particles on adverse health effects of air pollution.
Biochimica et Biophysica Acta (BBA)-General Subjects 1860(12):2844–2855
8. Stafoggia M, Schneider A, Cyrys J, Samoli E, Andersen ZJ, Bedada GB, Bellander T, Cattani G,
Airborne Nanoparticles: Control and Detection
127