Todays lecture objectives:

1
ATMS 455 Physical Meteorology

• Todays lecture objectives
• Nucleation of Water Vapor Condensation (WH 4.2)
• What besides water vapor do we need to make a
  cloud? Arent all clouds alike?

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ATMS 455 Physical Meteorology

• Todays lecture topics
• Nucleation of Water Vapor Condensation (WH 4.2)
• Theory
• Cloud condensation nuclei

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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
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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
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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

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Theory

• Homogeneous (spontaneous) nucleation (cont.)
• Recall a system (droplet environment)
  approaches an equilibrium state by reducing its
  energy (DElt0) in time

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Theory

• Subsaturated conditions (e lt es)

If droplet grows (R increases), then DEgt0, this
wont happen spontaneously.
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Theory

• Subsaturated conditions (e lt 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

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Theory

• Supersaturated conditions (e gt es)

If droplet grows (R increases), then DE can be
positive or negative
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Theory

• Supersaturated conditions (e gt es)
• - DE initially increases with increasing R
• DE is a maximum where R r
• DE decreases with increasing R beyond R r

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Theory

• Supersaturated conditions (e gt es)
• Embryonic droplets with R lt r tend to evaporate
• Droplets which grow by chance (collisions) with R
  gt r will continue to grow spontaneously by
  condensation
• They will cause a decrease in the energy (total
  energy) of the system

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Theory

• Kelvins 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

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Theory

• Kelvins 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

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Theory

• Supersaturations that develop in natural clouds
  due to the adiabatic ascent of air rarely exceed
  1 (RH101)
• Consequently, droplets do not form in natural
  clouds by the homogeneous nucleation of pure
  water

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Theory

• droplets do form in natural clouds by the
  heterogeneous nucleation process
• Cloud droplets grow on atmospheric aerosols

Yes!
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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.
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Theory

• Aerosol types
• wettable aerosol that allows water to spread out
  on it as a horizontal film
• soluble dissolve when water condenses onto them

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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

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Theory

• 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

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Theory

• Kohler curve

Variation of the RH of the air adjacent to a
solution droplet as a function of its radius
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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

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Theory

• Focus on curve 2 (solution of 10-19 kg of sodium
  chloride)

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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.
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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
25
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

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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.

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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)
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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.
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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
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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)
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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

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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

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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
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Cloud condensation nuclei

• Near the earths surface, continental air masses
  are generally significantly richer in CCN than
  are marine air masses

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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

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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