motivation
DESCRIPTION
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. - PowerPoint PPT PresentationTRANSCRIPT
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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
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
• 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
• Photometers• Typically relative instruments
• Sensitive to particle speed
• Nephelometer• Measure scattering for aerosol sample (~ 1L) over wide
range of angles ()• 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
Measuring Particles Optically(Detection Sensor)
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
• 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)
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 - 0.5m
• 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
• 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 34
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 (>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• Fungi, bacteria, other stuff, metabolic byproducts• Quantitiative or presence/absence• Culturable, viable, DNA-based• Inhibitors
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