Transcript Document

ATMS 455 – Physical
Meteorology
• Today’s lecture objectives:
– Nucleation of Water Vapor
Condensation (W&H 4.2)
• What besides water vapor do we need to
make a cloud? Aren’t all clouds alike?
http://www.artcyclopedia.com/feature-2001-08.html
ATMS 455 – Physical
Meteorology
• Today’s lecture topics:
– Nucleation of Water Vapor
Condensation (W&H 4.2)
• Theory
• Cloud condensation nuclei
Introduction
• Clouds form when air becomes
supersaturated wrt liquid water (or ice, in
some cases)
• Supersaturation most commonly occurs in
the atmosphere when air parcels ascend,
resulting in expansion and cooling (WH 2.6)
• Water vapor condenses onto aerosols
forming a cloud of small water droplets
Andy Aerosol
Theory
• But do we really need (Andy) aerosol to
make a cloud droplet? What if we made a
cloud via condensation without the aid of
aerosols*?
Hey!
*homogeneous or spontaneous nucleation
Theory
• Homogeneous (spontaneous) nucleation
– First stage of growth; requires chance collisions of a
number of water molecules in the vapor phase to come
together, forming small embryonic water droplets large
enough to remain intact. Will this happen
spontaneously?
 Spontaneous implies an irreversible process which
implies a total increase in entropy which implies an
upper limit on the change in Gibbs Free Energy
Theory
• Homogeneous (spontaneous) nucleation
(cont.)
– Recall: a system (droplet + environment)
approaches an equilibrium state by reducing its
energy (DE<0) in time
Theory
• Subsaturated conditions (e < es)
If droplet grows (R increases), then DE>0, this won’t happen spontaneously.
Theory
• Subsaturated conditions (e < es)
– Formation of droplets is not favored
– Random collisions of water molecules do occur,
forming very small embryonic droplets (that
evaporate)
– These droplets never grow large enough to
become visible
Theory
• Supersaturated conditions (e > es)
If droplet grows (R increases), then DE can be positive or negative
Theory
• Supersaturated conditions (e > es)
- DE initially increases with increasing R
- DE is a maximum where R = r
- DE decreases with increasing R beyond R = r
Theory
• Supersaturated conditions (e > es)
– Embryonic droplets with R < r tend to
evaporate
– Droplets which grow by chance (collisions)
with R > r will continue to grow spontaneously
by condensation
• They will cause a decrease in the energy (total
energy) of the system
Theory
• Kelvin’s formula can be used to
– calculate the radius r of a droplet which will be
in (unstable) equilibrium with air with a given
water vapor pressure e
– determine the saturation vapor pressure e over a
droplet of specified radius r
Theory
• Kelvin’s formula can be used to
– calculate the radius r of a droplet which will be
in (unstable) equilibrium with air with a given
water vapor pressure e
– determine the saturation vapor pressure e over a
droplet of specified radius r
• r = 0.01 micrometers requires a RH of 112.5%
• r = 1.0 micrometer requires a RH of 100.12%
Theory
• Supersaturations that develop in natural
clouds due to the adiabatic ascent of air
rarely exceed 1% (RH=101%)
• Consequently, droplets do not form in
natural clouds by the homogeneous
nucleation of pure water…
Theory
• …droplets do form in natural clouds by the
heterogeneous nucleation process
• Cloud droplets grow on atmospheric
aerosols
Yes!
Theory
• Droplets can form and grow on aerosol at much
lower supersaturations than are required for
homogeneous nucleation
– Water vapor condenses onto an aerosol 0.3 micrometers
in radius, the water film will be in (unstable)
equilibrium with air which has a supersaturation of
0.4%
Aerosols give a “boost”
to the size of a growing
cloud droplet.
Theory
• Aerosol types
– wettable; aerosol that allows water to spread out on it as
a horizontal film
– soluble; dissolve when water condenses onto them
Theory
• Soluble aerosols
– solute effect has an important effect on
heterogeneous nucleation
• Equilibrium saturation vapor pressure over a
solution droplet (e.g. sodium chloride or ammonium
sulfate) is less than that over a pure water droplet of
the same size
Theory
•
e
es
expression may be used to
– Calculate the vapor pressure e’ of the air adjacent to a
solution droplet of specified radius r
– Calculate the relative humidity of the air adjacent to a
solution droplet of specified radius r
– Calculate the supersaturation of the air adjacent to a
solution droplet of specified radius r
Theory
• Kohler curve
Variation of the RH of the air adjacent to a solution droplet as a function of its radius
Theory
• Kohler curve
– Below a certain droplet size, the vapor pressure of the
air adjacent to a solution droplet is less than that which
is in equilibrium with a plane sfc of water at the same
temperature
– As the droplets increase in size, the solutions become
weaker, the Kelvin curvature effect becomes the
dominant influence
– At large radii, the RH of the air adjacent to the droplets
becomes essentially the same as that over pure water
droplets
Theory
• Focus on curve #2 (solution of 10-19 kg of
sodium chloride)
Theory
• Curve #2 (solution of 10-19 kg of sodium
chloride)
Radius of 0.05 mm  RH of 90%  If an initially dry sodium chloride particle of mass
10-19 kg were placed in air with RH equal to 90%, water vapor would condense onto the
particle, the salt would dissolve, and a solution droplet of r = 0.05 mm would form.
Theory
• Curve #2 (solution of 10-19 kg of sodium
chloride)
RH of 100.2%  radius of 0.1 mm  If an initially dry sodium chloride particle of mass
10-19 kg were placed in air with RH equal to 100.2%, a solution droplet of r = 0.1 mm
would form on the sodium chloride particle
Theory
• In both examples the droplets that form are in
stable equilibrium with the air since,
– if they grew a little more, the vapor pressures adjacent
to their surfaces would rise above that of the ambient
air and they would evaporate back to their equilibrium
size
– if they evaporated a little, their vapor pressures would
fall below that of the ambient air and they would grow
back to the equilibrium size by condensation
Theory
• Droplets small enough to be in stable equilibrium
with the air are called haze droplets. All droplets
in a state represented by points on the left hand
side of the maxima in the curves shown in Fig.
4.12 are in the haze state.
Theory
• Curve #2 (solution of 10-19 kg of sodium
chloride) RH of 100.36% , radius of 0.2 mm
(1) Slight evaporation  growth by condensation back to its original size
(2) Slight growth  growth by condensation  continued growth  activated droplet (a
droplet has passed over the peak in its Kohler curve)
Theory
• Curve #2 (solution of 10-19 kg of sodium
chloride) RH of 100.4%
ambient air RH
Growth by condensation, supersaturation of the air adjacent to the droplet would rise.
Once droplet reaches peak in Kohler curve, supersaturation of the air adjacent to the
droplet would still be below that of the ambient air  droplet continues to grow by
condensation.
Theory
• Any droplet growing along a curve which has a peak
supersaturation lying below the supersaturation of the
ambient air can form a cloud droplet (EX1)
• Any droplet growing along a Kohler curve which intersects
a horizontal line in Fig. 4.12, corresponding to the
supersaturation of the air, can only form a haze droplet (2)
EX1
EX2
Cloud condensation nuclei
• Aerosol which serve as the nuclei upon
which water vapor condenses in the
atmosphere are called cloud condensation
nuclei (CCN).
Andy (a.k.a. “CCN”)
Cloud condensation nuclei
• CCN types
– soluble; the larger the size of an aerosol and the
larger its water solubility, the lower will be the
supersaturation at which it can serve as a CCN
– insoluble; the larger the size of an aerosol and
the more readily it is wetted by water, the lower
will be the supersaturation at which it can serve
as a CCN
Cloud condensation nuclei
• For a given environment of 1%
supersaturation:
– soluble; CCN can be as small as 0.01 mm in
radius
– insoluble; CCN need to be at least about 0.1 mm
in radius
Cloud condensation nuclei
• Measuring CCN; thermal diffusion chamber
CCN counted using photographs or by measuring the intensity of light scattered by
droplets in the chamber
Cloud condensation nuclei
• Near the earth’s surface, continental air
masses are generally significantly richer in
CCN than are marine air masses
Cloud condensation nuclei
• Concentrations of CCN over land decline
by about a factor of five between the sfc and
5 km
• Concentrations of CCN over the ocean
remain fairly constant with height
Cloud condensation nuclei
• CCN source region is over land
– Soil and dust particles are not dominant
– Forest fires are sources of CCN
– Sea-salt particles are not a primary source of
CCN
• Gas-to-particle conversion mechanisms
might be important sources of CCN
• Many CCN consist of sulfates