1. Atmospheric Thermodynamics Cloud Condensation Nuclei
Matt Andersen, Greg Banks, Logan Kluth, Alex Taylor

2. Overview Introduction to Cloud Formation
Kelvin Equation and Cloud Condensation Nuclei (CCN)
Impact of CCN on the Environment

3. Important Terms Cloud Condensation Nuclei (CCN)
Particles in the air that allow the formation of water
droplets
Cloud Albedo
the extent to which it diffusely reflects light from the sun
Relative Humidity
the amount of water vapor that exists in a gaseous mixture of air and water
Radiative Forcing
the change in net irradiance
Irradiance
the difference between the incoming radiation energy and the outgoing
radiation energy in a given climate system
Aerosol
a suspension of fine solid particles or liquid droplets in a gas

4. Basics of Cloud Formation
Relies on warmer temperatures at the Earth’s surface
Promotes evaporation of liquid water into water vapor
Water vapor rises
Higher Altitudes
Temperature decreases by 1°C per 100 meters
Pressure decreases
As water vapor rises, cooler air and lower pressure cause condensation
in an adiabatic process

5. Basics of Cloud Formation
Start with chemical potentials:
In terms of base and correction factor:
Alpha is water’s activity in solution
Using same form for pure substances (alpha=1):
Combining these equations and simplifying yields the equation for relative humidity:

6. Derivation of Kelvin Equation
Importance of CCN
Derivation of Kelvin Equation
Start with expression for ΔG:
For transfer of particles dn from bulk fluid (Po) to droplet (P):
This change has a corresponding change in surface area, which affects free energy:
Where gamma is the surface tension of the droplet

7. Derivation of Kelvin Equation
Importance of CCN
Derivation of Kelvin Equation
Concurrently, a change dn causes a volume change:
Independently, given density = mass/volume = d(mass)/d(volume):
Substituting in for dr:
Finally, combining our expressions for dG, we arrive at the Kelvin Equation: or

8. Derivation of Kelvin Equation
Importance of CCN
Derivation of Kelvin Equation
The key relation of the Kelvin equation for our purposes is the relationship between vapor pressure and droplet radius (r)
Exponential inverse relationship
Smaller radius results in drastic increase of vapor pressure
Initial condensation begins with small number of water molecules forming a “nucleus”
Other molecules condense around this cluster
Problem! The radius of a cluster of water molecules is very small (10^-9 m)
Solution: Condensation requires pre-existing particles which establish a minimum radius, allowing water molecules to condense around
These particles are called Cloud Condensation Nuclei (CCN)

9. Environmental Impact of CCN
Where do CCN Come From?
Natural
Volcanic Ash
Sea Spray
Forest Fires
Anthropogenic (man-made or stemming from human activity)
Combustion reactions
Contribute massive amounts of CCN into the atmosphere
Chemical Byproducts that diffuse into the air
How prevalent are CCN? How much is out there?
Estimates in 2006 placed the mass of aerosols released into the atmosphere at about 2x10^12 kg. This number is increasing.

10. Environmental Impact of CCN Effect of CCN on Droplets in Clouds
Table 1: Cloud and Subcloud Aerosol Properties
Property
Clean Air
Polluted Air
Aerosol Particle Concentration
176 cm-3
806 cm-3
CCN Concentration
At S = 0.7%
116 cm-3
668 cm-3
At S = 0.04%
2 cm-2
119 cm-2
Cloud Drop Concentration
cm-3
cm-3
Mean Cloud Drop Diameter
18 μm
7-9 μm
Drizzle Drop Concentration
800 L-1
80 L-1
Increasing number of CCN results in:
Smaller droplets
More droplets
Increased concentration of droplets

11. Environmental Impact of CCN Effect on the Cloud as a Whole
Increase the number of CCN  Increase in Albedo
Remember: Albedo is the extent to which the cloud diffusely reflects light from the sun
So more CCN results in more diffusely reflected light
Thus the Earth receives less sunlight, which cools it
But it’s not that simple…

12. Environmental Impact of CCN Effect on Temperature of the Surface
Increasing the albedo can actually result in an increase or decrease in surface temperature depending on the reflectivity of the surface of the earth
If the surface is dark (not reflective) such as ocean water, then the clouds cool the surface
If the surface is light (reflective) such as snow, then the clouds warm the surface

13. Environmental Impact of CCN Effect on Temperature of the Surface
OCEAN (Not Reflective)
SNOW (Reflective)
Incoming Light
0.5 2 10 10 3 10 3 10 9
0.3
1.2 5 1 5 4 1 +9 +1
+6.5 +5
Radiative Forcing
Cloud Causes Cooling
Cloud Causes Heating

14. Environmental Impact of CCN
Annual Average Surface Temperature and Controversy
Averaged over a year, including seasonal snow, increasing the number of CCN is expected to result in a net decrease in temperature
The magnitude of the cooling effect is under some debate since increasing the CCN count of a cloud will cause it to shrink and will cause it to draw closer to the surface of the Earth, thus intercepting less light. However, trends show that increasing CCN does result in cooling.

15. Environmental Impact of CCN Significant Effect of CCN
Increasing the number of CCN in a cloud
Increase in overall number of droplets
Decrease in size of each droplet
Also, the overall amount of precipitation decreased while the average size of raindrops increased
Possibly due to collision-coalescence theory
Raindrops acquire other drops during fall
Number of raindrops hit increases when overall number of droplets increases
Easier to collect a little drop than it is to collect a big drop
Property
Clean Air
Polluted Air
Aerosol Particle Concentration
176 cm-3
806 cm-3
CCN Concentration
At S = 0.7%
116 cm-3
668 cm-3
At S = 0.04%
2 cm-2
119 cm-2
Cloud Drop Concentration
cm-3
cm-3
Mean Cloud Drop Diameter
18 μm
7-9 μm
Drizzle Drop Concentration
800 L-1
80 L-1

16. Environmental Impact of CCN Effect of Human Activity
In addition to increasing the number of CCN in the atmosphere we’re also increasing the diversity of CCN, which produces some unexpected results
For example, when sulfate is released into the atmosphere it reacts with water when water condenses on it
The result is acid rain

17. Environmental Impact of CCN Why not just increase safe-CCN emissions?
Increasing CCN would result in also increasing greenhouse gasses
CCN
Last for about 10 days
Potentially unhealthy
They cool the earth during the day
Act as a thermal blanket at night (since clouds trap heat)
Net Cooling effect
Other greenhouse gasses (such as CO2)
Last for up to 100 years
Constantly warming the earth
If there was a way to inject safe CCN into the atmosphere without creating
greenhouse gasses then it could be a viable counter to global warming

18. What Does All This Mean CCN Impact on Climate Change
They affect clouds and clouds are an integral component in climate change
Understanding how we affect clouds helps us in understanding our impact on climate change and the environment
Keep it in Perspective
The global climate is a very complex system
We don’t understand all of the components of global climate and since they’re all interrelated, it is hard to isolate a single system or our impact on that system