Environmental Engineering Reference
In-Depth Information
advantage.of.the.differences.in.settling.rates.between.particles.of.different.aerody-
namic.diameters..A.cascade.impactor.has.co-linear.plates.in.series.of.pairs.through.
which.air.is.drawn..The.irst.plate.of.each.pair.has.a.small.nozzle.or.nozzles.in.it.
to.control.low.velocity..After.the.sample.passes.through.the.nozzle(s),.it.is.turned.
sharply.before.the.solid.plate,.which.acts.as.a.collection.plate..Particles.larger.than.
the. stage. cut. diameter. (which. is. a. function. of. the. low. velocity. and. the. distance.
between.the.plates).cannot.follow.the.low.stream.lines.but.fall.onto.the.collection.
plate..Particles.smaller.than.the.stage.cut.diameter.continue.to.the.following.impac-
tor.stages..Ambient.air.cascades.through.succeeding.stages,.which.have.successively.
smaller.oriices.and.consequently.higher.oriice.velocities..Collection.plates.at.each.
successive. stage. will. collect. successively. smaller. particles.. While. most. available.
units.were.designed.to.meet.the.regulatory.requirements.to.monitor.for.particulate.
matter.with.aerodynamic.diameters.of.2.5.µm.(PM2.5).or.less.as.a.category,.newer.
units. designed. with. up. to. 13. stages. can. separate. particulate. down. to. 30. nm. [34]..
Samples.are.time-integrated.and.may.be.collected.from.the.plates.for.further.charac-
terization.analyses.by.electron.microscopy.or.other.techniques..A.micro-oriice.uni-
form.deposit.impactor.(MOUDI).allows.collection.of.nanoparticles.in.three.stages:.
<32.nm,.<18.nm,.and.<10.nm..These.units.have.been.used.to.characterize.nanopar-
ticles.from.vehicular.emissions.[35].
...
electrical low Pressure impactor (elPi)
The. electrical. low. pressure. impactor. (ELPI). is. an. extension. of. cascade. impactor.
technology.that.includes.the.multi-stage.cascade.impactor.with.detector.technology.
to.provide.real-time.data.for.both.particle.size.and.concentration..This.makes.it.pos-
sible.to.measure.rapidly.changing.conditions.in.ambient.air..The.design.of.the.ELPI.
is.based.on.combining.electrical.detection.principles.with.low-pressure.impactor.size.
classiication..The.gas.sample.containing.the.particles.passes.through.an.electrical.
discharge.that.ionizes.aerosol.particles..The.charged.particles.then.pass.into.a.low-
pressure. impactor. with. electrically. isolated. collection. stages.. The. electric. current.
carried.by.the.charged.particles.into.each.impactor.stage.is.measured.in.real-time.
by.a.sensitive.multichannel.electrometer..A.version.designed.for.ambient.or.indoor.
monitoring.can.detect.down.to.90.nanoparticles.in.the.30-nm.or.smaller.range.per.
cubic.centimeter.(nm/cm
3
),.and.can.measure.a.mass.as.small.as.0.005.micrograms.
per. cubic. meter. (µg/m
3
). [36].. The. ELPI. has. been. used. for. indoor. air. monitoring,.
vehicular.emission.studies,.and.ambient.air.monitoring.[37].
...
dynamic light scattering (dls)
Where. the. electrostatic. classiiers. measure. the. size. distributions. of. particles. sus-
pended. in. air,. dynamic. light. scattering. instrumentation. determines. size. distribu-
tions.for.particles.suspended.in.the.liquid.phase..Light.passing.through.a.liquid.or.
suspension. of. nanoparticles. will. be. scattered,. and. for. nanoparticles,. the. intensity.
of.the.scattered.light.will.luctuate..This.luctuation.results.from.the.random.move-
ment.of.the.nanoparticles.as.a.result.of.their.random.bombardment.by.the.molecules.
of.the.luid..The.velocity.and.distance.of.this.movement.(called.Brownian.motion),.
and.the.subsequent.luctuation.of.scattered.light.intensity,.depend.on.the.size.of.the.
