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Motivation

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. 1.

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Motivation

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  1. 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 1

  2. Arrestance describes how well an air filter removes larger particles such as dirt, lint, hair, and dust 2

  3. 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

  4. 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

  5. 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

  6. Gravimetric Sampling

  7. 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

  8. Optical Measurement • Extinction • What are limitations?

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

  10. Measuring Particles Optically(Detection Sensor) • 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

  11. 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)

  12. Cascade Impactor

  13. Cascade Impactor

  14. Cascade Impactor Curves

  15. 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

  16. 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

  17. 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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  19. 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)

  20. APS air and particle flow diagram

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  22. 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

  23. Table 15.4

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

  25. 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

  26. 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

  27. 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

  28. 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

  29. 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?

  30. 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

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

  32. 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 36

  33. Isokinetic Sampling • http://www.knowledgepublications.com/hydrogen/images/Hydrogen_Gen_Gas_Gas_Stream_Lines.gifΩ√ 37

  34. 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 38

  35. Bioaerosol Sampling • Many issues • Fungi, bacteria, other stuff, metabolic byproducts • Quantitiative or presence/absence • Culturable, viable, DNA-based • Inhibitors 39

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