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 Vapor pressure 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

Curved Surface Surface energy of a curved surface Surface tension 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 Evaporation 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.

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 super-saturation. 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) Problem 6.4 Problem 6.10