Motivation

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Motivation

• Particles are generally regarded as one of the most serious indoor air quality concerns

• Increasing concern about ultrafine particles• Very high surface area/unit mass• Direct transfer through cell walls• Mechanism for respiratory disease• “Asbestos-like” health effects

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Arrestance describes how well an air filter removes larger particles such as dirt, lint, hair, and dust

Particle Measurement• Distinguish between

• Particle counting• Only counts number of particles, makes no distinction

between sizes

• Particle sizing• Counting and sizing information

• Particle mass• Particle composition• Viable and non-viable bioaerosol assessment

• Sampling issues

Particle Sensors

• Inexpensive (relatively)• Gravimetric for particle mass• Light scattering for large particle mass• Condensation nucleus counter (CNC) for counting small

particles• Cascade impactor for size-resolved mass

• Mid-range• Optical particle counters

• Expensive• Aerodynamic particle sizing for large particles• Differential mobility analyzer for small particles

Gravimetric (Mass-based) Techniques

• Particles have very low masses• Need to collect many particles to have

measurable mass• Most mass based techniques are integrated

samples

Gravimetric Sampling

Quantitiative

• Measure mass of clean filter• Measure mass of filter after exposure• Measure flow rate and exposure time• Calculate concentration

• Corrections for blank filter• Corrections for humidity

Optical Measurement

• Extinction

• What are limitations?

Mie Theory for Scattering

• Forward-scattering and back-scattering• Functions of (λ, θ, dp, Vp)

• Often see size parameter, α = πd/λ

• Photometers• Typically relative instruments

• Sensitive to particle speed

• Nephelometer• Measure scattering for aerosol sample (~ 1L) over wide

range of angles ()• Particle density is function of the light reflected into the detector

– Scatered light depends on properties of the particles such as their shape, color, and reflectivity.

• Determines mass concentration much more accurately than photometer

• Often calibrated to single particle composition

Measuring Particles Optically(Detection Sensor)

Condensation Nuclei Counter (CNC)

• Subject aerosol stream to alcohol (or water) vapor

• Cool air stream to cause condensation• Count particles with an optical particle counter

• Closely related to a condensation particle counter (CPC)

Cascade Impactor

Cascade Impactor

Cascade Impactor Curves

Optical Particle Counter

• Similar to photometer, but particles are isolated• May require dilution

• 0.065 – 20 µm• Practically 0.1 – 5 µm

• Some devices just count

General Discussion of Accuracy

• For what size aerosol?• For what concentration of aerosol?

• Even gravimetric

• For instruments that size• Not counting particle vs. putting particle in wrong

bin

• Manufacturer’s accuracy is not often useful• Must calculate your own based on knowledge of

instrument

Aerodynamic Particle Sizer

• One of many time-of-flight instruments• Two laser beams separated by known distance• Particle is accelerated between beams• Time between beams being broken is

calibrated to test aerosol• 0.5 - 20 um

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APS

• Small particles move at the air velocity• Large particles lag air velocity• Problems

• Small particles not-Stokesian• Bigger density sized as larger particle• Shape also influences drag• Multiple particles in sizing chamber (same as other devices)

APS air and particle flow diagram

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Particle Mobility Analyzer

• Particle is subjected to careful (difusive) electric charging• Charge on particle is proportional to diameter

• Electric mobility is known

• Particles are sorted by charge• Particles are counted by other technique (mostly the

condensation method)• 0.001 – 1 µm

Table 15.4

The SMPS

Consists of

1. Electrostatic

classifier (EC)

2. Differential mobility analyzer (DMA)

3. Condensation particle counter (CPC)

How the EC and DMA work

EC• Kr-85 bipolar charger

DMA• 2 laminar flows

Sheath and aerosol

• 2 concentric cylinders Center negative voltage Electric field

• + particles attracted

through sheath air

• Location depends on electrical mobility, flow rate, and geometry Cycles through different voltages to capture different size particles

How the CPC works

• Interface with EC and DMA to form the SMPS

• Particles are passed through a wick and grown with either water or butanol• Aerosol stream saturated and temperature

equilibrated

• Heterogeneous condensation on condensation nuclei (the particles)

• Grown to 2 to 3 micrometers

• Individual particles passed through light beam and scatter light onto a photodetector

SMPS Best for 2.5nm - 0.5m

• Can’t precisely classify larger particles b/c• Fraction of +1 and +2 charged particles begin to

converge• Changing voltages begins to cause equal fractions of

particles of the same size to fall in different bins

• Smaller particles• Fraction of charged particles gets close to 0, so

different voltages can’t control mobility

Control methods/devices

• No device works (well) for all particle sizes

(a) (b)Efficiency as a function of particle diameter as measured with (a) Optical particle counter and (b) Aerodynamic particle sizer

Summary

• Wide variety of instruments available for particle measurement

• What size of aerosol are you interested in?• Do you need sizing or is counting sufficient?• Do you need real-time data?• What type of aerosol are you trying to

measure?• How much accuracy do you need?• How much money do you have?

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Future Measurement Exercise

• Get manual and record data from:• TSI Aerotrack optical handheld particle counter

(4)• P-Trak (2)• DustTrak• SidePak

• Colocate all instruments in a room and see concentrations that result from different sources

• Main purpose is to understand all instruments 34

Other Particle Measurement Issues

• Sampling line losses• Sampling particles in moving air stream• Particle composition• Bioaerosol sampling

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Sampling Line Losses

• Extensive literature on subject• Generally an issue for large (>1 μm) and small (<

0.05 μm) particles

• What are mechanisms that cause loss and how do we minimize them?

• Calculating line loss• Values from literature, software, or use equivalent

lines• Best approach is measurement

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Isokinetic Sampling

• http://www.knowledgepublications.com/hydrogen/images/Hydrogen_Gen_Gas_Gas_Stream_Lines.gifΩ√

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Particle Composition

• Collect sample of particles on filter• Analyze as you would for liquid or solid

compounds• Challenges?

• SMPS w/ mass spec.• Very expensive and response time issues

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Bioaerosol Sampling

• Many issues• Fungi, bacteria, other stuff, metabolic byproducts• Quantitiative or presence/absence• Culturable, viable, DNA-based• Inhibitors

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