Basic Aerosol physics

Question 1

Definition: Particle

Answer 1

A discrete bit of condensed matter (solid or liquid)

Question 2

Definition: Aerosol

Answer 2

A metastable suspension of particles in a gas. Gas may, or may not, be air
(but usually is). Aerosol is a two-phase system

Question 3

Usual unit: Number concentration

Answer 3

/cm3

Question 4

Usual unit: Volume concentration/mass concentration

Answer 4

Volume concentrations in µm3
/cm3 are equal to mass concentrations in µg/m3
(for unit density)

Question 5

nanoparticles, PM1, PM2.5, PM10, Visible range

Answer 5

<0.1, <1, <2.5, <10, >10 µm

Question 6

Forces on particle

Answer 6

Drag force: resistance to move by friction from gas. Gravity (or electric field)

Question 7

Terminal settling velocity

Answer 7

Velocity when drag force and gravity is in balance. proportional to density/viscosity, g and d^2.

Question 8

Derivation of drag force formula

Answer 8

The resistance to move is due to the acceleration of the air it pushes aside.

Question 9

Reynolds number

Answer 9

Characterizes the fluid flow around an obstacle. It is the relation between the force that pushes away the
surrounding gas as the particle propagates (force of inertia, Fi) and the friction force (Ff) between the particle and the gas. Proportional to relative velocity between obstacle and gas, particle diameter, and density/viscosity. Assumtions:
- Incompressible gas,
• No particle-particle interaction (no walls
or other particles nearby),
• Constant particle motion (stationary
force balance),
• Solid spherical particle,
• Gas velocity zero at particle surface.

Question 10

Drag force high reynolds number Re>1000

Answer 10

Cd is constant 0.44, inertial forces»friction, Newtons formula applies

Question 11

Drag force Low Reynolds number Re<1

Answer 11

inertial force<

Question 12

Knudsen Number

Answer 12

The extent of contact between particle and gas. Kn=2*lambda(mean free path)/dp
if Kn«1 gas is a continuum, if Kn»1 the particle
perceives the dragforce as a series of pushes (free molecular regime)

Question 13

Cunningham slip correction

Answer 13

correction to stokes law that reduces the drag force because it accounts for slip between gas and particle. Empirical equation

Question 14

Mechanical mobility, B

Answer 14

ratio of velocity to the applied
external force. B is a measure of the relative ease of producing steady
motion for an aerosol particle

Question 15

Relaxation time, tao

Answer 15

Time before 0.63Vts is reached. tao=mass*B

Question 16

Equivalent diameter

Answer 16

The diameter of a spherical particle that would behave as the irregular particle in question

Question 17

Aerodynamic diameter

Answer 17

The diameter of a spherical particle that has the same Vts as the irregular particle in question

Question 18

Stopping distance

Answer 18

Distance before Vts is reached. the distance a
particle will travel in still air if an
external force acting on the particle
was suddenly turned off.

Question 19

Brownian motion

Answer 19

The irregular wiggling motion of an aerosol particle caused by random variations in the relentless bombardment of gas molecules. Acts faster on small particles.

Question 20

Diffusion

Answer 20

The net transport of aerosol particles in a concentration

gradient. Acts faster on small particles.

Question 21

Coagulation

Answer 21

Small particles collide and stick together. Main cause Brownian diffussion. Liquids for larger drops, solids form complex aggregates with fractal properties. Decreases number concentration but mass concentration stays the same. Stronger with a small and a big particle.

Question 22

effects of coagulation

Answer 22

Will make size distribution narrower with time because small particles will coagulate with larger particles. Average diameter will increase.

Question 23

Thermophoresis

Answer 23

Particles are bombarded more strongly from the hot side in a temperature gradient pushing them away from the hot area. This means that warm surfaces are cleaner than cold surfaces.