CLOUD FORMATION
PARTICLES IN THE ATMOSPHERE
SOURCES OF PARTICLES
About half the particles in the atmosphere are formed from
gaseous emissions
Sulfate is rather ubiquitous
-Half comes from natural biological processes that produce hydrogen sulfide or sulfur dioxide, one quarter from fossil fuels and the other source is sea spray and volcanoes
Gaseous sulfur compounds are both oxidized in a matter of hours or days to form sulfur trioxide which immediately hydrolyzes to form sulfuric acid (H2SO4)
-This acid is very hygroscopic and forms droplets by accreting water vapor from the air
-Any positive ions in the air such as ammonia or free metals will combine in the droplet to form sulfate salts
All particles, whether windblown dust, sea salts, sulfate salts, smoke or whatever, are ultimately removed from the air by one of four mechanisms
1. Dry sedimentation or settling out
2. Impact against an object on or near the ground
3. rainout or snowout, i.e. condensation nuclei
4. Washout, in which falling rain scavenges the particles below the cloud
- 4 is most important, then 3 etc.
Average residence time in the mid latitudes in lower troposphere is less than a week--in the upper troposphere it is a month and in the stratosphere ranges from 1 year at tropopause to 5 years at stratopause
-the smaller particles (the ones that are not removed in most
air pollution devices) tend to have the longer residence times
Classification of Particles by size
l. less than .2 micron---Aitkin Nuclei
2. from .2 to 1 micron---Large Nuclei
3. greater than 1 micron---Giant Nuclei
CONDENSATION NUCLEI
Condensation nuclei give droplet a large enough threshold size so it doesn't evaporate immediately
In clean air condensation of water vapor requires supersaturated conditions. In clean air the degree of supersaturation needed for cloud development increases rapidly as the radius of the droplets decrease. For example, formation of relatively small droplets with radii of .001 micrometer requires a supersaturation of nearly 340 percent. In contrast, relatively large droplets having radii greater than 1 micrometer need only slight supersaturation to form
Why is this?--has to do with the fact that small droplets have large curvature and therefore only weak bonding and it is easier for water droplets to escape
Why won't pure droplets grow without nuclei???
-Surface tension makes it difficult for molecules to stick
together
-When a water molecule enters a droplet it increases the droplets surface area
-Since surface tension opposes an increase in area it
makes it difficult for molecules to enter a
droplet
-But it is easier for molecules to leave a small
droplet as it decreases surface area
-In dirty air condensation can begin at rh of 65 percent (i.e. lots of hygroscopic particles)
-In absolutely clear air free of ions one needs a relative humidity of nearly 800 percent to get condensation.
-Hygroscopic particles are best--esp. salt, sulfuric and nitric acid
-Only about 1% of condensation nuclei on land are hygroscopic as compared to 10% to 20% in maritime air
-But there are many more total nuclei over land
-Usually there are 100 to 1000 satisfactory (i.e. hygroscopic) nuclei per cm3 of air
Notes on droplet size and vapor pressure
-Vapor pressure is inversely proportional to droplet size. Droplets that are small evaporate quicker than those that are large
CAUSES OF CONDENSATION
1. Expansion of gas (adiabatic)
2. Conduction of heat from air to a surface which is colder
3. Direct radiational cooling of air
4. Mixing of warm and cold masses of saturated air
HOW CLOUD DROPLETS GROW
DIFFUSION (good only up to 20 microns)
-size achieved by this method is limited by the rate at which diffusion of water molecules to its surface can occur (i.e., getting to 1 micron takes about a second, getting to 10 microns takes on the order of 1000 seconds, and to get to100 microns would take over a week).
A. Solute Effect
-important for small cloud droplets
-Salt and other soluble particles form a solution
-Vapor pressure is less because some of the spaces on the
exterior of the drop are held by ions which don't have vapor pressure so v.p. of droplet drops
-more difficult for water molecules to escape
B. Curvature Effect (partially counterbalances solute effect.)
-Water molecule is polarized
-Intermolecular forces due to charges are more effective
when curvature is less
-All other things being equal vapor pressure is greater for
small droplets than for large ones
-But surface tension on small droplets is hi and it is
hard to grow and easy to evaporate water
C. Combination
- As drop grows solute effect decreases in importance while
curvature gives droplets a better chance to survive
Diffusion is good only up to about 20 microns, which is about the size of an average cloud droplet
l. drops have to get up to about 40 microns in diameter
before coalescence takes over and they can stick together
when they collide
2. Giant condensation nuclei and ice crystals bridge gap.
CLOUD DEVELOPMENT
1. Air rises toward the top of a well-mixed cloud free layer with temp falling and rh rising because of dry adiabatic expansion
2. Population of condensation nuclei in air is very important both in terms of total numbers and distribution across the various spectra of sizes and materials
When the relative humidity of the air exceeds about 80% many hygroscopic nuclei begin to swell by condensation and their growth as rh exceeds 90% may be sufficient to reduce visibility quite noticeably by a wet haze, especially when the line of slight is nearly horizontal. May be very visible to fliers or people on high mountains. The radii of haze droplets forming on all but giant nuclei are less than 1 micron
While rising air remains unsaturated all haze droplets remain in equilibrium with the ambient rh, swelling as it rises but not yet activated. As soon as the air begins to supersaturate some droplets exceed their critical radius, become activated and grow very rapidly.
At first only the most favored are activated (giant hygroscopic nuclei) but as supersaturation increases increasing numbers of less favored droplets are activated as are some of the largest non hygroscopic nuclei.
In a short time so many droplets are growing so rapidly that they begin to deplete the ambient vapor quite substantially; the growth of supersaturation checks and then reverses.