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

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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

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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t = O

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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

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Impaction

Cross section of fiber(Air filters are often composed of fiber mats, unlike water filters.)

Air Molecules

Interception

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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

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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