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Objective

• Finish with PM measurements

• Discuss Friday’s filed measurements

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ASHRAE Transaction 2004

Properties

Properties and sources

ASHRAE Transaction 2004

Optical instruments Mie Theory for Scattering

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

• Often see size parameter, α = πd/λ

MEASURE MASS CONCENTRATION •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

Light scattering methods (Detection Sensor)

Condensation Nuclei Counter (CNC)

PARTICLE COUNTER •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

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

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

The SMPS

Consists of

1. Electrostatic

classifier (EC)

2. Differential mobility analyzer (DMA)

3. Condensation particle counter (CPC)

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 22

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• Quantitative or presence/absence• Culturable, viable, DNA-based• Inhibitors

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Field Measurements (Friday 2:20 p.m.)

• Address • 5404 Odessa Lane, 78731

• Type of measurement • Residential building – characterization

• Envelope air tightness

• Duct testing

• HVAC testing

• Air quality T, RH, PM, CO, O3,…

• …..

• Teams• ….

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