Basic Aerosol physics Flashcards
Definition: Particle
A discrete bit of condensed matter (solid or liquid)
Definition: Aerosol
A metastable suspension of particles in a gas. Gas may, or may not, be air
(but usually is). Aerosol is a two-phase system
Usual unit: Number concentration
/cm3
Usual unit: Volume concentration/mass concentration
Volume concentrations in µm3
/cm3 are equal to mass concentrations in µg/m3
(for unit density)
nanoparticles, PM1, PM2.5, PM10, Visible range
<0.1, <1, <2.5, <10, >10 µm
Forces on particle
Drag force: resistance to move by friction from gas. Gravity (or electric field)
Terminal settling velocity
Velocity when drag force and gravity is in balance. proportional to density/viscosity, g and d^2.
Derivation of drag force formula
The resistance to move is due to the acceleration of the air it pushes aside.
Reynolds number
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.
Drag force high reynolds number Re>1000
Cd is constant 0.44, inertial forces»friction, Newtons formula applies
Drag force Low Reynolds number Re<1
inertial force<
Knudsen Number
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)
Cunningham slip correction
correction to stokes law that reduces the drag force because it accounts for slip between gas and particle. Empirical equation
Mechanical mobility, B
ratio of velocity to the applied
external force. B is a measure of the relative ease of producing steady
motion for an aerosol particle
Relaxation time, tao
Time before 0.63Vts is reached. tao=mass*B
Equivalent diameter
The diameter of a spherical particle that would behave as the irregular particle in question
Aerodynamic diameter
The diameter of a spherical particle that has the same Vts as the irregular particle in question
Stopping distance
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.
Brownian motion
The irregular wiggling motion of an aerosol particle caused by random variations in the relentless bombardment of gas molecules. Acts faster on small particles.
Diffusion
The net transport of aerosol particles in a concentration
gradient. Acts faster on small particles.
Coagulation
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.
effects of coagulation
Will make size distribution narrower with time because small particles will coagulate with larger particles. Average diameter will increase.
Thermophoresis
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.
