2. Formation of Cloud droplets

1. 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. 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%

4. 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 • 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. e

5. 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.

6. 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.

7. 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.

8. 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.

9. Critical radius • High supersaturation is required for very small droplets to be stable. • Unstable drops will evaporate.

10. 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.

11. 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.

12. * 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*.

13. 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.

14. * 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*.

15. 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.

17. 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

18. Categorized according to their affinity for water. • Hydrophobic • Nucleation is difficult and requires even higher super-saturation. • Neutral • Same supersaturation as homogeneous nucleation. • Hygroscopic • Much lower supersaturation required.

19. 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.

20. 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.

21. 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.

22. Cloud Condensation Nuclei (CCN): The nuclei activated at supersaturations less than a few per cent (S < 1.02) are called CCN.

23. * The size distribution

24. Meteorology 342 • Homework (2) • Problem 6.4 • Problem 6.10