Environmental Engineering Reference
In-Depth Information
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aerosol diffusion Charger
The.same.measurement.principle.as.used.for.the.epiphaniometer.is.applied.in.aero-
sol.diffusion.chargers.but.without.the.requirement.for.a.radioactive.source..Ions.are.
produced.in.a.carrier.gas.by.electrical.discharge..The.ions.attach.to.the.surface.of.
the.particles,.which.are.then.collected.in.an.electrically.insulated.particle.ilter..The.
electric.charge.is.converted.to.a.direct.current.(DC).voltage.signal.in.an.electrometer.
ampliier.. Studies. have. shown. that. these. devices. provide. a. good. estimate. of. aero-
sol.surface.area.in.ambient.air.when.airborne.particles.are.smaller.than.100.nm.in.
diameter.[31].
5.1.4
s
iZe
d
istributiOn
Individual.particle.sizes.can.be.measured.accurately.with.TEM,.STEM,.and.AFM,.
but.those.techniques.are.not.time.or.cost.eficient.when.a.complete.size.distribution.
is.required..Size.distribution.analyses.generally.are.conducted.with.aerosols.formed.
when.the.particles.are.suspended.in.air,.or.when.particles.are.in.emulsions.or.suspen-
sions.in.a.liquid.matrix.
...
electrostatic Classiiers
Electrostatic.classiiers.operate.on.the.basic.principle.that.the.velocity.of.a.charged.
spherical.particle.in.an.electrical.ield.relates.directly.to.its.diameter..Particles.are.
suspended.in.air.to.form.an.aerosol,.charged,.and.then.introduced.into.a.cylindri-
cal.apparatus..The.classiier.has.an.outer.cylinder.that.is.a.ground.electrode.and.an.
inner.rod.that.can.have.precisely.controlled.negative.voltage.applied..The.charged.
particles.are.introduced.near.the.wall.of.the.outer.cylinder,.with.a.sheath.of.clean.air.
moving.through.the.cylinder.at.a.constant.low.rate..The.positively.charged.particles.
will. move. toward. the. negatively. charged. center. electrode. at. a. rate. determined. by.
their.operative.diameter.and.the.applied.voltage..Only.those.particles.within.a.nar-
row.velocity.range.will.pass.through.a.thin.sampling.slit.near.the.bottom.of.the.cen-
ter.electrode..Particles.exit.through.this.slit.into.a.particle-counting.instrument..By.
scanning.the.voltage.on.the.central.rod,.analysts.can.obtain.a.full.particle-size.dis-
tribution.for.the.aerosol..It.should.be.noted,.however,.that.the.particle.size.measured.
is.based.on.the.assumption.of.a.spherical.shape,.and.the.dimensions.of.nonspherical.
particles.determined.by.this.technique.will.correlate.with.but.not.necessarily.equal.
those.determined.by.an.imaging.technique.
Various. types. of. (and. names. for). electrostatic. classiiers. are. in. common. use..
These.include.the.differential.mobility.analyzer.(DMA),.nanodifferential.mobility.
analyzer.(NDMA),.the.differential.mobility.particle.sizer.(DMPS),.and.the.scanning.
mobility.particle.sizer.(SMPS)..Electrostatic.classiiers.can.be.used.in.a.variety.of.
ways,.including.real-time.monitoring.of.the.length.of.carbon.nanotubes.during.syn-
thesis.[32].or.to.monitor.emissions.during.use.of.TiO
2
.nanopowder.materials.[33].
...
Real-time inertial impactor: Cascade impactors
Cascade.impactors.have.a.long.history.with.ambient.air.monitoring.programs,.pro-
viding. size. selectivity. to. the. collection. of. suspended. particles.. These. units. take.
