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
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
•
•
•
•
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
•
•
•
•
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
22
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
24
25
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
•
•
•
•
•
•
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
34
Other Particle Measurement Issues
•
•
•
•
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
36
Isokinetic Sampling
•
http://www.knowledgepublications.com/hydrogen/images/Hydrogen_Gen_Gas_Gas_Stream_Lines.gifΩ√
37
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
Bioaerosol Sampling
• Many issues
•
•
•
•
Fungi, bacteria, other stuff, metabolic byproducts
Quantitiative or presence/absence
Culturable, viable, DNA-based
Inhibitors
39