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Objective

Objective. Finish with PM measurements Discuss Friday’s filed measurements. 1. Properties. ASHRAE Transaction 2004. Properties and sources. ASHRAE Transaction 2004. Optical instruments Mie Theory for Scattering. Forward-scattering and back-scattering

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Objective

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Presentation Transcript

  1. Objective • Finish with PM measurements • Discuss Friday’s filed measurements 1

  2. Properties ASHRAE Transaction 2004

  3. Properties and sources ASHRAE Transaction 2004

  4. Optical instruments Mie Theory for Scattering • Forward-scattering and back-scattering • Functions of (λ, θ, dp, Vp) • Often see size parameter, α = πd/λ

  5. Light scattering methods (Detection Sensor) MEASURE MASS CONCENTRATION • Photometers • Typically relative instruments • Sensitive to particle speed • Nephelometer • Measure scattering for aerosol sample (~ 1L) over wide range of angles (q) • 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

  6. 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)

  7. Cascade Impactor

  8. 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

  9. 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

  10. 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

  11. 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)

  12. APS air and particle flow diagram

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  14. 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

  15. The SMPS Consists of • Electrostatic classifier (EC) • Differential mobility analyzer (DMA) • Condensation particle counter (CPC)

  16. SMPS Best for 2.5nm - 0.5mm • 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

  17. 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

  18. 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?

  19. 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

  20. Other Particle Measurement Issues • Sampling line losses • Sampling particles in moving air stream • Particle composition • Bioaerosol sampling

  21. 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 24

  22. Isokinetic Sampling • http://www.knowledgepublications.com/hydrogen/images/Hydrogen_Gen_Gas_Gas_Stream_Lines.gifΩ√ 25

  23. 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 26

  24. Bioaerosol Sampling • Many issues • Fungi, bacteria, other stuff, metabolic byproducts • Quantitative or presence/absence • Culturable, viable, DNA-based • Inhibitors 27

  25. 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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