Aerosols and Eriophyid MitesPrincipal Reference: William C. Hinds, Aerosol Technology, 1986. For...
Transcript of Aerosols and Eriophyid MitesPrincipal Reference: William C. Hinds, Aerosol Technology, 1986. For...
Aerosols and Eriophyid Mites
John KarlikUniversity of California
Pine Pollen,Sierra Nevada Mts., California
“Trading Places”
LA, 1948
Beijing, 2013
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.
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.
Physics Affecting Aerosols
How can aerosols float along and be suspended for long periods of time?
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
Reynolds Number (Re)inertial forcesviscous forces
Re expresses whether flow is laminar or turbulent
•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
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)
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
6
FD = 3 π ηVd
•
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
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
•• • •
•••
• ••
•
•
•
• •
•
•
•
••
••
••
•
•
•••
•••
••
• • •
•
••
••••
•
•
•••
•
•
•
•
••
• •
t = O
•h
•P
t > O
h
•P
Time
no
Concentration at P
Stirred Settling in a Chamber(Similar to Outdoor Conditions)
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
• • • • • • • • •
Impaction
Cross section of fiber(Air filters are often composed of fiber mats, unlike water filters.)
Air Molecules
Interception
• • • • • • •
• • • • • • •••
Effectiveness of Removal Mechanisms for Various Particle Sizes and Velocities
Implications for Sampling
Face velocity very important for collection efficiency
Can some sort of filter be used? Ability to recover mites from filter
fibers?
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
•• • •
•••
• ••
•
•
•
• •
•
•
•
••
••
••
•
•
•••
•••
••
• • •
•
••
• •••
••••
•
•••
• ••••• ••
••
• • •••
••
t = O
•h
•P
t > O
•h
•P
Time
no
Concentration at P
Tranquil Settling in a ChamberSimilar to Glasshouse Conditions
Idealized Trimodal Distribution
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
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
VTS at 20 oC (unit density sphere)
Diameter (µm)1
1050
100200300
VTS (cm/s)0.0030.37.6
30120270
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
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
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