Transcript Meteorology 342 - Iowa State University

2. Formation of Cloud droplets
2.1 General aspects
2.2 The curvature effect
2.3 The solute effect
2.4 Atmospheric aerosols and CCN
2.1 General Aspects
* Phase changes of water
vapor ---- liquid
liquid ---- solid
vapor ---- solid
* Nucleation processes
Homogeneous: droplets form in a pure environment
Heterogeneous: droplets form on nuclei
* Supersaturation: the excess of relative humidity
over the equilibrium value of 100%
2.2 The Curvature Effect
• Surface tension

– Work per unit area necessary to increase the surface
area.
– Process stores potential energy in the surface.
– Units: J/m2 or N/m.
– For water  ~ 7.5x10-2 N/m at meteorological temps.
• Vapor pressure
e
– The pressure on a liquid or solid surface due to the
partial pressure of the molecules of that substance in
the gas phase which surrounds the surface.
Curved Surface
• Surface energy of a curved surface
– equilibrium vapor pressure.
– rate of evaporation from droplets.
• Surface tension
– droplet tends to assume a minimum area to volume
ratio.
– Lowest possible surface potential energy state.
• Curvature
– Increased vapor pressure at equilibrium compared
with a flat surface.
Pure Water
• Nucleation
– Depends on partial pressure of water vapor in the
surroundings.
– Determines the rate which water molecules impinge
upon the drops.
• Evaporation
– Temperature of droplet and surface tension.
– Surface molecules must obtain enough energy to
overcome the binding forces.
Equilibrium
• Condensation and evaporation take place at the
same rate.
• Vapor pressure = saturation vapor pressure.
• Equilibrium vapor pressure over a droplets
surface.
• Kelvin or Curvature effect
– Enhanced equilibrium vapor pressure over curved
surfaces, such as drops.
Droplet Growth
• Net rate of growth depends on vapor deficit
– e - es(r) = vapor deficit where e is ambient vapor
pressure.
– e - es(r) < 0
Decay
– e - es(r) > 0
Growth
– e - es(r) = 0
Critical size.
Critical radius
• High supersaturation is
required for very small
droplets to be stable.
• Unstable drops will
evaporate.
2
rc 
Rv  LT ln S
Homogeneous nucleation
• Droplets of critical size are formed by random
collisions.
• What if they capture another drop?
–
–
–
–
Drop becomes supercritical.
es(r) decreases.
Rate of growth increases.
Drop grows spontaneously!
• Homogenous nucleation does not take place in
the atmosphere.
– Supersaturation rarely exceeds 1 or 2 percent.
2.3 The Solute Effect
• Cloud drops form on aerosols
– condensation nuclei or hygroscopic nuclei
• Rate of formation is determined by the number
of these nuclei present.
• Nuclei keep supersaturation from exceeding a
few percent.
* Radius smaller than r*
• Solution term dominates.
• Very small solution drops are in
equilibrium with vapor at RH <
100%.
• If RH increases, drop will grow
until equilibrium is again reached.
– This continues up the curve
beyond 100% RH.
• Once S* is reached, the droplets
have critical radius r*.
• Up to r* the droplet is in
stable equilibrium with its
environment.
• Any change in S causes
the drop to grow until
equilibrium is once again
reached.
• Haze particle.
* Radius equal to or larger than r*
• When r=r*, condensation nuclei
is said to be “activated”.
• If S goes beyond S*, the droplet
grows beyond r*.
• Vapor begins to diffuse to the
droplet and it will continue to
grow without the further
increase in S.
• Any change in S causes droplet
to grow or evaporate, but r
deviates from r*.
• Droplet will continue to grow to
cloud drop size if S remains
above the curve.
• Actual clouds
– Growth does not continue
indefinitely
– Too many drops present and
competition for water vapor.
– S tends to lower once
condensation becomes more
rapid than the production of
supersaturation.
2.4 Atmospheric Aerosol and CCN
• 75% of total mass from natural or anthropogenic
sources
–
–
–
–
Wind-generated dust (20%)
Sea spray (40%)
Forest fires (10%)
Combustion and other industry (5%)
• 25% of total mass from conversion of gaseous
constituents to small particles by photochemical
and other chemical processes.
– SO2, NO2, Olefins, NH3
• Categorized according to their affinity for water.
• Hydrophobic
– Nucleation is difficult and requires even higher supersaturation.
• Neutral
– Same supersaturation as homogeneous nucleation.
• Hygroscopic
– Much lower supersaturation required.
Hygroscopic nuclei
• A non-volatile dissolved substance tends to
lower the equilibrium pressure of a liquid.
• When solute is added, solute molecules replace
liquid molecules at the surface.
• If vapor pressure of solute is less than that of the
solvent, the vapor pressure is reduced.
• A solution droplet can be in equilibrium at a
much lower supersaturation than a pure water
droplet of the same size.
Nuclei Formation
• Condensation of gases
– Spherical
• Disintegration of liquids or solids.
– Crystals, fibers, agglomerates, irregular fragments.
• Equivalent spherical diameter
– Diameter of sphere having same volume as the
aerosol particle.
Nuclei Size
• Size: 10-3m to 10m in diameter.
– Salt, dust, combustion particles.
• D > 2m Giant aerosols
• 0.2m < D < 2m Large aerosols
• D < 0.2m Aitken particles
– Overwhelming majority.
Cloud Condensation Nuclei (CCN):
The nuclei activated at supersaturations less
than a few per cent (S < 1.02) are called CCN.
* The size distribution
Meteorology 342
Homework (2)
1. Problem 6.4
2. Problem 6.10