Aerosol particles and water

Question 1

Why are aerosol water-interactions important?

Answer 1

Aerosols grow/shrink by condensation/evaporation of water

Aerosols are important for the formation of clouds.

Question 2

Relative humidity, saturation and saturation ratio

Answer 2

saturation ratio=p/p0 (p0=Saturation vapour pressure). RH=100*s. Saturation when RH=100%

Question 3

Speed of condensation and evaporation

Answer 3

Faster in smaller systems

Question 4

Growth factor

Answer 4

depends on diameter of dry particle and the diameter of the wet particle GF=d_wet/d_dry

Question 5

Hygroscopic particles

Answer 5

grow at all RH (truly hygroscopic)

Question 6

Deliquescent compounds

Answer 6

• Dry up to high RH
• Then sudden growth to form saturated solution
• Hysteresis: Formation of supersaturated solutions when RH reduced

Question 7

Effects that describe water uptake

Answer 7

Kelvin and Raoult

Question 8

Kelvin effect

Answer 8

Curved surfaces have larger spacing between molecules and will therefore have a harder time keeping the molecules. This means that smaller molecules have it harder to keep water.

Question 9

Raoult effect

Answer 9

when a material is dissolved in water the vapour pressure decreases. This decrease is higher with higher concentrations so the raoult effect is larger on small particles if they have the same amount of material. The vapour pressure is lowered in proportion to the number of ions substituting water molecules. Valid when RH is close to 100%

Question 10

Köhler equation

Answer 10

A combination of the Kelvin and Raoult effects that creates a curve for the diameter depending on the RH or saturation ratio.

Question 11

Activation of droplets

Answer 11

When they’ve passed over the peak of the Köhler equation. They then have the critical diameter and saturation ratio for activation. Highly dependent on the amount of available salt.

Question 12

Homogeneous nucleation

Answer 12

Formation of cloud droplets only with water. Clusters of water molecules smaller than 1 nm

Question 13

Hetrogeneous nucleation

Answer 13

Formation of cloud droplets onto an aerosol particle. Acts according to the Köhler curve.

Question 14

Droplet diffusion growth

Answer 14

Important for small droplets. Growth by condensation. Depends on the difference in water pressure by the droplet and far away from the droplet. This becomes smaller as the particle grows larger so droplet diffusion growth is only relevant for small droplets.

Question 15

Size of rain drop vs. Size of droplet from diffusion growth.

Answer 15

1 mm vs 30 micrometers

Question 16

Further growth in warm clouds

Answer 16

Coalescence - Growth of droplets by collisions. Accelerated by the existence of large droplets with high settling velocity.

Question 17

Further growth in cold clouds

Answer 17

Water droplets become supercooled droplets and ice particles. The saturation vapour pressure is lower over ice than over water so the ice particles grow on expense of the water droplets.