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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. 1.

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  • 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



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


particle measurement
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
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
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
  • 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
Optical Measurement
  • Extinction
  • What are limitations?
mie theory for scattering
Mie Theory for Scattering
  • Forward-scattering and back-scattering
    • Functions of (λ, θ, dp, Vp)
    • Often see size parameter, α = πd/λ
measuring particles optically detection sensor
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
condensation nuclei counter cnc
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)
optical particle counter
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
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
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
  • 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)
particle mobility analyzer
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

  • Electrostatic

classifier (EC)

  • Differential mobility analyzer (DMA)
  • Condensation particle counter (CPC)
how the ec and dma work
How the EC and DMA work


  • Kr-85 bipolar charger


  • 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
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 5 m m
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
control methods devices
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

  • 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?
future measurement exercise
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


other particle measurement issues
Other Particle Measurement Issues
  • Sampling line losses
  • Sampling particles in moving air stream
  • Particle composition
  • Bioaerosol sampling
sampling line losses
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


isokinetic sampling
Isokinetic Sampling


particle composition
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


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