Chapter 6 Atmospheric Aerosol and Cloud Processes Spring 2015

Cloud Physics Initiation and development of cloud droplets Special interest: Explain how droplet formation results in rain in approximately 20 minutes (warm clouds). Interval between initial development of a cumulus cloud and first appearance of rain. Laboratory, aircraft, and satellite observations as well as extensive computer modeling.

Atmospheric Aerosol Suspension of solid/liquid material in a gaseous medium. Mostly particles with small settling velocity. D 2um : Aitken nuclei Amounts/Concentrations (typical) Oceans: 10 3 cm 3 Rural : 10 4 cm 3 Polluted Air : 10 5 cm 3

Aitken Nuclei Concentration Scale Height = 7 km Scale Height = 3 km

Size Spectrum Size range: 10-4 um - ~10 um Concentration range: 10 7 /cm 3 10-6 /cm 3 Large/Giant aerosols ( > 2um): On average, concentrations similar in continental, marine, and urban polluted air.

Sources Aitken Nuclei Combustion. Gas -> Particle conversion. Nucleation of aerosol from supersaturated gases. Photochemical reactions. Large/Giant Dust. Pollen and spores from plants. Bursting of air bubbles in the ocean.

Aerosol Production

Removal Scavenging by precipitation 80%-90% Impaction by drops. Improvements in visibility that follow period of precipitation. Affects large particles more ( 2 um). Condensation nuclei ( < 0.1 um) Smaller drops stream around. Gravitational settling Especially 1 um.

Particle Distribution cont.

Nucleation of water vapor - condensation Clouds form when air becomes saturated. Ascent of air parcels (vertical motion) Expansion and adiabatic cooling. How is the condensation of water vapor to liquid water initiated?

Homogeneous nucleation Spontaneous nucleation Formation of a pure water droplet from a supersaturated vapor without aerosol (condensation nuclei) Collisions => water vapor molecules may combine into small water droplets. Condensation involves energy change. Natural systems will evolve to lower energy states. When molecules collide into a drop, does their energy increase or decrease? If it decreases, then drop is in a lower energy, more stable state.

Kelvin's Formula Equilibrium State, but unstable Gives vapor pressure for equilibrium Smaller super-saturation -> larger r Need greater collisional droplet before spontaneous growth. Difficult mechanism

RH and Drop Radius

Kelvin's Formula Equilibrium State, but unstable Gives vapor pressure for equilibrium Smaller super-saturation -> larger r Need greater collisional droplet before spontaneous growth. Difficult mechanism Supersaturations that develop in natural clouds due to ascent rarely exceed 1%! Even drops that form 0.01um in size by collision are well below the critical radius.

Energy and Vapor Pressure

Heterogeneous Nucleation Wettable aerosol: water condenses on, forms thin film. Suppose water dissolves aerosol: Solid particle Solution of water and dissolved aerosol. e s decreases because evaporation from solution is less than evaporation from pure water. e µ concentration of water droplets on the surface of the drop. Solution droplet: some of the surface molecular sites are occupied by molecules of salt (or ions). Vapor pressure is reduced.

E, E, E, what begins with E? e, e', e s Kelvin's formula (e) Can be used in two ways. Calculate the r of a droplet that is in unstable equilibrium with a given vapor pressure, e. Determine the saturation vapor pressure, e, over a droplet of specified radius, r. This will be our most common use of e for Kohler curves. e: saturation vapor pressure adjacent to a pure water drop of radius r and temperature T. e': saturation vapor pressure over a solution with the same size and temperature. e s : saturation vapor pressure over a plane surface of pure water.

Kohler Curve

Cloud Condensation Nuclei Soluble aerosol are favored Can be smaller than insoluable aerosol and still be effective. ~ 1% of continental aerosol. ~ 10% of maritime aerosol. Larger aerosol amounts exist over land. More CCN. Sources? Appear to be land (diurnal cycle and near-surface concentration over land). Burning vegetation. Gas particle conversion.

Growth of Cloud Droplets in Warm Clouds Warm clouds can grow by: Condensation in a super-saturated environment. Colliding and coalescing with other cloud drops. Growth by condensation e > e drop Droplet has passed over peak in Kohler Curve. Consider an isolated droplet with radius r at time t situated in a super-saturated environment.

Drop Growth by Condensation

Observations (red) vs. Theory (blue) (t ~ 5 minutes)

r is in µm, n is the # per liter of air, and v is the terminal fall speed in cm/s.

Growth and Observations Droplet growth by condensation and obs agree well at t=5 min. Note: Drops only extend to 10um. Rate of increase in radius decreases with time. Growth by condensation Growth by condensation alone in warm clouds is much to slow. Rain does form in warm clouds.

r is in µm, n is the # per liter of air, and v is the terminal fall speed in cm/s.

Growth by Collision and Coalescence Growth from relatively small sizes (condensation) to the size of rain drop is achieved by collision and coalescence. Large drops fall faster than small drops Large drops overtake and capture a fraction of these smaller drops. Consider a single drop of radius r 1 (collector drop) which is over taking a smaller drop of radius r 2

Collision y distance between drop and droplet centerlines so that droplet just makes a grazing collision. Center closer than y? Center further than y? Collision efficiency: fraction of those droplets in the path swept out by the collector that actually collide with it.

Efficiencies

Collection

Collection continued Not all drops that collide are collected (coalescence). Droplets bounce off collector drops. Rebound off cushion of air between. E' = coalescence efficiency Fraction of collisions that result in coalescence. Collection efficiency, E c = E * E' How do drops grow by this process?

More Collection Consider a collector drop of radius r 1 which has a terminal fall speed v 1. The drop is falling through a cloud with droplets of size r 2 falling at v 2. Droplets are uniformly distributed in space and will be collected uniformly at the same rate by all collector drops Continuous collision model

Collection

Example problem On a particular day, the orogoraphic cloud on the island of Hawaii is 2 km thick with a uniform liquid water content of 0.5 g/m 3. A drop of 0.1 mm radius at cloud top begins to fall through the cloud. Find the size of the drop as it emerges from cloud base and assume a collection efficiency of 1. Assuming that the terminal velocity of the drop is kr, where k = 8 x 10 +3 s -1, find the time taken by the drop to fall through the cloud.

Exercise 6.3 A drop enters the base of a cloud with radius r o and, after growing with a constant collection efficiency while traveling up and down in the cloud, the drop reaches cloud base again with a radius of R. Show that R is a function of only r o and the updraft velocity w (assumed to be constant).

Continuous collision model - Flaws Predicts raindrops should grow within reasonable time periods (~ 1 hr) We know this process occurs slightly faster (~20 min.) Growth by condensation predicts a relatively uniform size distribution. Similar terminal fall speeds In reality, a few drops end up larger than the rest, become collectors, and go on to produce raindrops. How does this happen?

Bridging the gap from growth by condensation to C-C Existence of giant cloud condensation nuclei (gccn) Only 1 particle in a million is needed. Giant aerosol: wettable with radius > 3 µm. Effect of turbulence on collisions Can cause fluctuating super-saturations. Causes broadening of the drop size distribution. Stochastic collection model Collisions are individual events, distributed in space and time. Small fraction of drops can grow much faster than average.