Transcript (a) Optical particle counter and (b)

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
2
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
• 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
• Particles have very low masses
• Need to collect many particles to have
measurable mass
• Most mass based techniques are integrated
samples
Gravimetric Sampling
Quantitiative
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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
• Extinction
• What are limitations?
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)
• 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)
• 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
Cascade Impactor
Cascade Impactor Curves
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
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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
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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
Table 15.4
The SMPS
Consists of
1. Electrostatic
classifier (EC)
2. Differential mobility analyzer
(DMA)
3. Condensation particle counter
(CPC)
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
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 - 0.5mm
• 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
• 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
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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
• 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
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Sampling line losses
Sampling particles in moving air stream
Particle composition
Bioaerosol sampling
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
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Fungi, bacteria, other stuff, metabolic byproducts
Quantitiative or presence/absence
Culturable, viable, DNA-based
Inhibitors
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