Mechanical properties and measurement techniques

Question 1

Mechanisms that continuously change the properties of an aerosol

Answer 1

Coagulation, Evaporation and condensation (particle-gas equilibrium of volatile components), Electrical ageing (Bolzmann distribution) and Deposition mechanisms

Question 2

Deposition mechanisms

Answer 2

Sedimentation/Gravitational settling
• Impaction
• Interception
• Diffusion
• Electrical forces
• Thermophoresis

Question 3

Impaction, Stokes number (impaction paramter)

Answer 3

Particles collide with and stick to an obstacle. If stk«1 the particle will follow the streamlines but if stk»1 the particle will collide with the obstacle. STK depends on the stopping distance of the particle and the characteristic dimensions of the obstacle.

Question 4

Impactor characteristics

Each plate has a cut off diameter where Sqrt(stk)=0.49. Depends on nozzle size and distance between nozzle and plate

Answer 4

Each plate has a cut off diameter where Sqrt(stk)=0.49. Depends on nozzle size and distance between nozzle and plate

Question 5

Impactor limitations

Answer 5

bounce off, over filled plates, inter stage losses

Question 6

Fibrous filters

Answer 6

• A mat of fine fibers, most of them
perpendicular to the direction of flow
• Cellulose, glass, quartz and polymer
fiber filters
• Fibers diameters 0.1 – 100 mm
• Porosities 70 – 90%
• Low air velocity through the filter 0.1
m/s
• Low pressure drop

Question 7

Porous membrane filters

Answer 7

• Complex irregular pore structure
• Polymers, cellulose, sintered metal and
ceramic micropourous filters
• Pore size 0.02 – 10 mm
• Porosities < 85%
• High collection efficiencies
• The highest pressure drop among all
filters

Question 8

Straight-through pore filters

Answer 8

• Polycarbonate films with cylindrical pores
perpendicular to the surface of the filter • Pores 0.1 – 8 mm • Porosities 5 - 10%
• Collection efficiencies and pressure drops
intermediate
• Useful for collection of particles for
scanning electron microscopy

Question 9

What should be considered when choosing a filter?

Answer 9

The purpose for which filter is used for
Collection efficiency for particle size of interest
Pressure drop versus flow rate
Type of after sampling analysis (e.g. characteristics of the filter needed for gravimetric analysis or chemical or thermal analysis)

Question 10

Light scattering from different particles

Answer 10

•For particles <0.05 micro m: Rayleigh scttering
Strong size (d^6) and wavelength (^-4) dependency
•Particles 0.05-10 micro m, complex Mie scattering region
•Largest sizes: Geometrical optics approximation d2
* Different sizes give of different scattering

Question 11

Scattering from small particles

Answer 11

Significant back scattering from small particles. Cools the earth.

Question 12

Optical measurement instruments

Answer 12

• Photometer - concentration measurement only
• Optical Particle Counters (OPC) - number concentration and size
distribution

Question 13

Photometer

Answer 13

Size range 0.1-10 micrometers
Measures number concentration and particle size
Not good at estimating mass concentration
Can be combined with a collection filter
Particle optical properties must be known and constant

Question 14

Optical Particle Counter (OPC)

Answer 14

Size range 0.05-~1000 micrometers
Measures number concentration and particle size distribution (equivalent optical diameter)
Particle optical properties must be known and constant

Question 15

Condensation Particle Counter (CPC)

Answer 15

Size range >3 nm

Enhances the diameter by condensation so number concentration can be found.

Question 16

Aerodynamic Particle Sizer (APS)

Answer 16

Size range 0.5-20 micrometers

Time of flight measurement then measures by light scattering