aerosols and eriophyid mitesprincipal reference: william c. hinds, aerosol technology, 1986. for...
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Aerosols and Eriophyid Mites
John KarlikUniversity of California
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Pine Pollen,Sierra Nevada Mts., California
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“Trading Places”
LA, 1948
Beijing, 2013
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Aerosol: Definitions An aerosol is solid or liquid particles suspended in a gas. Particulate Matter (PM) is a related term Other related terms:
• Dust: solid particle aerosol formed by mechanical means • Fume: solid particle aerosol produced by condensation of gases• Smoke: visible aerosol resulting from incomplete combustion• Cloud: visible aerosol with defined boundaries
Primary: emitted as-is Secondary: formed in the atmosphere via reactions
Principal Reference: William C. Hinds, Aerosol Technology, 1986.
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For aerosols, the most important descriptive term is particle size, described by diameter.
Aerosols are usually described in size in micrometers (μm). Particle sizes range from 0.001 to > 100 μm.
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Physics Affecting Aerosols
How can aerosols float along and be suspended for long periods of time?
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Initial Description of Aerosol Motion Newton developed a resistance (drag) equation,
valid for all subsonic particle motion, from a study of cannon balls moving through air
Stokes (1851) developed a specialized form of the equation for drag where the Reynolds number is very small• Made certain assumptions to solve the Navier-Stokes
nonlinear partial differential equations
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Reynolds Number (Re)inertial forcesviscous forces
Re expresses whether flow is laminar or turbulent
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•Momentum = Mass x Velocity
Velocity is a vector quantity including both speedand direction. ΔV requires acceleration, and therefore force, since F = ma.
Force may be supplied by:contact with gas moleculesgravityelectrostatic charge difference
Inertia will cause particle to resist acceleration (Newton’s first law of motion)
Aerosol Particle in Motion
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Deposition Mechanisms(also basis for air sampling and cleaning)
Gravitational settling (gravity) Impaction (hits object) Interception (hits edge of object) Diffusion (random molecular motion) Electrostatic attraction (+ or - difference)
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FG = mg FG = ρpπd3g•
Gravitational Settling:Forces Affecting Aerosols
Gravitational Force
m = mass, gg = grav accel, 980 cm s-2
F = force, dynes, g cm s-2
d = particle diameter, cmρp = particle density, g cm-3
Rewriting, with particlemass as density x volume
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FD = 3 π ηVd
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Gravitational Settling:Forces Affecting Aerosols
Drag Force: Stokes Law, very low Re, < 1.0
F = force, dynes, g cm s-2
η = viscosity of gas, poiseV = particle velocity, cm s-1
d = particle diameter, cm
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Gravitational SettlingAt Terminal Settling Velocity (VTS):
FG = FD
ρp d2 g18η VTS =
Where ρp is particle densityd = particle diameterg = gravitational accelerationη = viscosity (of air)
Note dominance of d2 term—settling is very important for
large particles
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Stirred Settling in a Chamber(Similar to Outdoor Conditions)
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Adhesive Forces
Van der Waals forces (temporary dipoles)
Permanent dipoles
Film of water (adhesion/cohesion)
Electrostatic force (charge accumulation)
====> When a particle hits something, it tends to stick
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Impaction
Cross section of fiber(Air filters are often composed of fiber mats, unlike water filters.)
Air Molecules
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Interception
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Effectiveness of Removal Mechanisms for Various Particle Sizes and Velocities
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Implications for Sampling
Face velocity very important for collection efficiency
Can some sort of filter be used? Ability to recover mites from filter
fibers?
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Reynolds Number (Re)
Re is dimensionlessRatio of inertial forces to viscous forcesVelocity (V) is relative motion between gas and particleViscosity(η) of a gas increases with oC, unlike viscosity for liquidsViscosity of a gas is independent of pressure
Re = ρVdη
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Tranquil Settling in a ChamberSimilar to Glasshouse Conditions
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Idealized Trimodal Distribution
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Natural Aerosol
Particle Size (µm) Conc. (N m-3)Viruses 0.015-0.45 --Bacteria 0.3-15 0.5-100Fungi 3-100 100-10,000Algae 0.5 10-1000Spores 6-60 0-100,000Pollen 10-100 0-1000
Ref: Hinds, 1986
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Water as an Aerosol
Particle Size Conc. Mass(µm) (N cm-3) (µg m-3 )
Fog Drops 10-20 -- 104-106
Cloud Drops 10-200 0.5-100 104-107
Drizzle 200-1000 100-10,000 105-107
Rain 1000-8000 10-1000 105-107
Ref: Jacobson, 2002
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VTS at 20 oC (unit density sphere)
Diameter (µm)1
1050
100200300
VTS (cm/s)0.0030.37.6
30120270
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Units of Measurement: Length
Aerosols are usually described in size in micrometers (µm).
µm = 10-6 meter = 10-4 centimeterParticle sizes range from 0.001 to > 100 µm.
PM10 = particles of < 10 µm diameterPM2.5 = particles of < 2.5 µm diameter
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1/218 k Tπ ρ p d3
Diffusion α
Diffusion is the primary transport mechanism for particles < 0.1 µm in size. (k is Boltzmann constant)
Diffusion
Note dependence
on d3
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Electrostatic Attraction
Not usually a major deposition mechanism under ambient conditions
May be used in air cleaners May be deliberately employed in
glasshouses to improve coverage of aerosol pesticides