single particles with the FIMS allows it to measure
distributions with a higher signal/noise ratio than
for electrometer-based instruments [146].
With the exception of a few portable detectors,
e.g., the NanoScan SMPS (TSI 3910), commercial mobility analyzers are heavy, large, costly,
and complex. Some of the research goals such as
personal exposure monitoring, deployment on
small aircraft, and spatial gradient mapping,
which require an array of sensors in the urban
environment, cannot be approached with current
tools. This need has driven research on newer
approaches for the particle mobility analyzer.
These instruments include the opposed migration
aerosol classifier (OMAC) [147], cross-flow ion
mobility spectrometer (CIMS) [148], and a miniature electrical-mobility aerosol spectrometer
(MEAS) [149]. The OMAC is similar to a DMA
in that the opposing forces of aerodynamic drag
and electrostatic force are used to sort particles
into size bins. Particles of too-high and too-low
mobility are deposited on the porous electrodes;
only those particles with an electrostatic migration
velocity that balances the flow velocity can pass.
The system uses an applied voltage of ~1 V, much
lower than in a conventional DMA that would use
a potential difference of ~30 V at the same flow
rate. Also, the OMAC is much smaller than a
DMA because the distance between electrodes is
only 1 mm for the same operational condition.
The CIMS consists of a number of channels
between parallel plates that are 1 mm apart. The
charging electrodes are located at the edge of the
channel so that particles, after passing through
these electrodes, migrate across the channels
with the sheath flow air. While high concentrations of particles may not be suited for
electrometer-based tools (FMPS, EEPS), the
CIMS showed a good performance for measuring
high-mobility gas ions and/or nanoparticles, at
high concentrations. The MEAS is a bipartite
rectangular chamber containing an electrostatic
precipitator and a classifier. Particles are charged
in the electrostatic section and subsequently
injected into the classifier region with a narrowrange streamline at the desired location. The
charged particles are then separated based on
their electrical mobilities and collected on plates.
The plates are located inside of classifier and are
connected to electrometers. These electrometers
quantify current signals that are proportional to
the number of particles collected on the plates
[149]. The structure allows the detection of single
particles over a wide range of mobilities.
The equivalent aerodynamic diameter is only a
good approximation for spherical particles; this
fact limits the performance of any mobilitybased analysis method.
Particle Diffusion Mobility
The diffusion battery is a simple and wellcharacterized diffusion-based aerosol classification method that is free from some of the limitations of the techniques discussed so far. Diffusion
batteries were developed to describe aerosols
according to their diffusional mobilities
[150]. The diffusion battery is a versatile, compact, and simple system that can be easily cleaned
after use and does not require particle charging.
A diffusion battery comprises a stack of screens
and air is sampled at different stages of the flow.
The aerosol is passed through the screens, and
small particles with a large diffusion constant are
more likely to deposit; the particle diameter can be
derived from the particle size-dependent deposition rate. The technique is one of a few methods
that can measure nanoparticles down to 0.8 nm,
i.e., a cluster of just a few molecules. To achieve
the size-resolved concentration of nanoparticles,
diffusion batteries are often accompanied by a
CPC, but this combination limits its use as a
personal monitor [151]. Diffusion battery systems
have been made in several designs. Wire screen
batteries are most commonly used to determine
size distributions. Diffusion mobility-dependent
size distributions have been used in countless
studies (see, e.g., [113, 151–154]).
Measurements of Nanoparticle
Composition and Morphology
Measurements of the composition and morphology
of airborne nanoparticles central to determinations
of pollution sources, toxicity, and atmospheric
behavior. The most common techniques are
Airborne Nanoparticles: Control and Detection
125
distributions with a higher signal/noise ratio than
for electrometer-based instruments [146].
With the exception of a few portable detectors,
e.g., the NanoScan SMPS (TSI 3910), commercial mobility analyzers are heavy, large, costly,
and complex. Some of the research goals such as
personal exposure monitoring, deployment on
small aircraft, and spatial gradient mapping,
which require an array of sensors in the urban
environment, cannot be approached with current
tools. This need has driven research on newer
approaches for the particle mobility analyzer.
These instruments include the opposed migration
aerosol classifier (OMAC) [147], cross-flow ion
mobility spectrometer (CIMS) [148], and a miniature electrical-mobility aerosol spectrometer
(MEAS) [149]. The OMAC is similar to a DMA
in that the opposing forces of aerodynamic drag
and electrostatic force are used to sort particles
into size bins. Particles of too-high and too-low
mobility are deposited on the porous electrodes;
only those particles with an electrostatic migration
velocity that balances the flow velocity can pass.
The system uses an applied voltage of ~1 V, much
lower than in a conventional DMA that would use
a potential difference of ~30 V at the same flow
rate. Also, the OMAC is much smaller than a
DMA because the distance between electrodes is
only 1 mm for the same operational condition.
The CIMS consists of a number of channels
between parallel plates that are 1 mm apart. The
charging electrodes are located at the edge of the
channel so that particles, after passing through
these electrodes, migrate across the channels
with the sheath flow air. While high concentrations of particles may not be suited for
electrometer-based tools (FMPS, EEPS), the
CIMS showed a good performance for measuring
high-mobility gas ions and/or nanoparticles, at
high concentrations. The MEAS is a bipartite
rectangular chamber containing an electrostatic
precipitator and a classifier. Particles are charged
in the electrostatic section and subsequently
injected into the classifier region with a narrowrange streamline at the desired location. The
charged particles are then separated based on
their electrical mobilities and collected on plates.
The plates are located inside of classifier and are
connected to electrometers. These electrometers
quantify current signals that are proportional to
the number of particles collected on the plates
[149]. The structure allows the detection of single
particles over a wide range of mobilities.
The equivalent aerodynamic diameter is only a
good approximation for spherical particles; this
fact limits the performance of any mobilitybased analysis method.
Particle Diffusion Mobility
The diffusion battery is a simple and wellcharacterized diffusion-based aerosol classification method that is free from some of the limitations of the techniques discussed so far. Diffusion
batteries were developed to describe aerosols
according to their diffusional mobilities
[150]. The diffusion battery is a versatile, compact, and simple system that can be easily cleaned
after use and does not require particle charging.
A diffusion battery comprises a stack of screens
and air is sampled at different stages of the flow.
The aerosol is passed through the screens, and
small particles with a large diffusion constant are
more likely to deposit; the particle diameter can be
derived from the particle size-dependent deposition rate. The technique is one of a few methods
that can measure nanoparticles down to 0.8 nm,
i.e., a cluster of just a few molecules. To achieve
the size-resolved concentration of nanoparticles,
diffusion batteries are often accompanied by a
CPC, but this combination limits its use as a
personal monitor [151]. Diffusion battery systems
have been made in several designs. Wire screen
batteries are most commonly used to determine
size distributions. Diffusion mobility-dependent
size distributions have been used in countless
studies (see, e.g., [113, 151–154]).
Measurements of Nanoparticle
Composition and Morphology
Measurements of the composition and morphology
of airborne nanoparticles central to determinations
of pollution sources, toxicity, and atmospheric
behavior. The most common techniques are
Airborne Nanoparticles: Control and Detection
125