University of Florida

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Particle Characteristics
Reading Chap 3.1-3.2

• Aerosol size
• Aerosol size distribution
• Representative size
• Weighted distribution
• Log-normal distribution

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Characterizing an Aerosol Particle
Q How do we characterize a particle?
Coal fly ash particles
Iron oxide particles from arc welding
Size, Shape, Density, Composition (toxicity,
corrosivity, reactivity), Phase (liquid, solid)
Q How do we determine particle size?
Q Why should we care the particle size?
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Health Concern deposition in our respiratory
system
dp gt 10 ?m
dp lt 1 ?m
1 lt dp lt10 ?m
Control Devices Collection Efficiency depends
on particle size Lifetime of Aerosols in the
Atmosphere particle size Environmental Quality
(e.g. visibility) particle size
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Size Range of Aerosol Particles
(see also Figure 3.1)
Hinds, Aerosol Technology, 1999
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Aerosol Size Distribution
Q How do we characterize particleS?

• Concentration
• Number concentration by counting
• Mass concentration by weight measurement
• Size
• Spread
• Particle size distribution
• Ex Particles in room air
• N 104 /cm3 M 5.236?10-6 g/cc dp
  10-3 cm 10 ?m

Q Does this mean all the 104 particles in the 1
cc air are 10 mm? What is the effect if we use
this size to represent the system (e.g. in
inhalation system)? How can we better describe
this aerosol system?
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Particle Size Distribution

• Monodisperse - All the particles are of the same
  size
• Polydisperse - Particles are of more than one
  sizes (more realistic)
• Typical data from measurement

Can also be mass?
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Frequency (or count) versus particle size
Frequency or Count
Q Which size has the highest concentration?
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Normalized Count, Frequency/?dp versus particle
size
Q What is the value of the total area?
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Fraction/?dp versus particle size
Q What is the value of the total area?
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Representative Size

• MEAN (arithmetic average)
• the sum of all the particles sizes divided by the
  number of particles
• MEDIAN
• The diameter for which 50 of the total are
  smaller and 50 are larger the diameter
  corresponds to a cumulative fraction of 50
• MODE
• Most frequent size setting the derivative of the
  frequency function to 0 and solving for dp.

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• GEOMETRIC MEAN
• the Nth root of the product of N values
• Expressed in terms of ln(dp)
• Very commonly used because an aerosol system
  typically covers a wide size range from 0.001 to
  1000 ?m

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Weighted Distributions

• Why do we need distributions other than number
  distribution?
• What are the other distributions?
• Definition frequency of the property (e.g.
  number, mass) contributed by particles of the
  size interval
• What is the effect?
• Ex. A system containing spherical particles
• Number Concentration Mass
  Concentration
• 100 /cc 1?m ?1.91g/cm3 10-10 g/cc 1?m
• 1 /cc 10?m
  10-9 g/cc 10?m

Q Do we have more 1 mm or 10 mm particles
(i.e. are the majority 1 or 10 mm)? Q How will
it impact the PSD we see?
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Number Distribution
Mass Distribution
Q What is the mode size of the above
distribution?

• Count Mean Diameter
• Mass Mean Diameter

Q Which one should we use? Mass, number or
other?
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Q If you just developed a particle control
device that can collect 100 for dpgt 5 um and 0
for dp lt 5 um, how will you report your devices
collection efficiency? Tests were done using the
particles shown on page 13.
Lognormal PSD

• Normal Distribution widely used elsewhere (e.g.
  student grade), but typically not for aerosol
  science, because
• most aerosols exhibit a skewed distribution
  function
• if a wide size range is covered, a certain
  fraction of the particles may have negative
  values due to symmetry.

standard deviation
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• The application of a lognromal distribution has
  no theoretical basis, but has been found to be
  applicable to most single source aerosols
• Usefuel for particle of a wide range of values
  (largest/smaller size gt 10)
• How to use it? Simply replace dp by ln(dp).

geometric mean diameter
Q Whats the unit of ?g?
geometric standard deviation
Q How much is ?g for monodisperse aerosol?
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• Features of Lognormal PSD

For a given distribution, the geometric standard
deviation remains constant (nondimensional) for
all weighted distributions.
Q If sg 1.5, how much is d84/d16?
Measurement from a cascade impactor
Download log-probability graph
Is this a log-normal distribution? Whats d50?
sg?
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Quick Reflection