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
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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

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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

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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

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Gravimetric Sampling
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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

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Optical Measurement

• Extinction
• What are limitations?

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Mie Theory for Scattering

• Forward-scattering and back-scattering
• Functions of (?, ?, dp, Vp)
• Often see size parameter, a pd/?

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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

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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)

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Cascade Impactor
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Cascade Impactor
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Cascade Impactor Curves
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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

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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
• Manufacturers accuracy is not often useful
• Must calculate your own based on knowledge of
  instrument

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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)

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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

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Table 15.4
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The SMPS

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

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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

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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

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SMPS Best for 2.5nm - 0.5mm

• Cant 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 cant control mobility

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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
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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

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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 (gt1 µm) and small (lt
  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/imag
  es/Hydrogen_Gen_Gas_Gas_Stream_Lines.gifOv

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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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