Transcript PChem Team Nano

Matt Andersen, Greg Banks, Logan Kluth, Alex Taylor
•Introduction to Cloud Formation
•Kelvin Equation and Cloud Condensation Nuclei (CCN)
•Impact of CCN on the Environment
•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
•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
• 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:
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
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
• 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)
•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.
Table 1: Cloud and Subcloud Aerosol Properties
Property
Aerosol Particle
Concentration
CCN Concentration
At S = 0.7%
At S = 0.04%
Cloud Drop Concentration
Mean Cloud Drop Diameter
Drizzle Drop Concentration
Clean Air
176 cm-3
Polluted Air
806 cm-3
116 cm-3
2 cm-2
10-100 cm-3
18 μm
800 L-1
668 cm-3
119 cm-2
220-370 cm-3
7-9 μm
80 L-1
•Increasing number of CCN results in:
•Smaller droplets
•More droplets
•Increased concentration of droplets
•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…
•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
OCEAN (Not Reflective)
Incoming Light
SNOW (Reflective)
0.5
2
10
10
10
3
10
2
3
9
+ 0.2
2
+ 0.8
0.3
5
1.2
1
5
4
1
+9
Radiative Forcing
+6.5
Cloud Causes Cooling
+1
+5
Cloud Causes Heating
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.
•
•
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
Aerosol Particle
Concentration
CCN Concentration
At S = 0.7%
At S = 0.04%
Cloud Drop Concentration
Mean Cloud Drop Diameter
Drizzle Drop Concentration
Clean Air
176 cm-3
Polluted Air
806 cm-3
116 cm-3
2 cm-2
10-100 cm-3
18 μm
800 L-1
668 cm-3
119 cm-2
220-370 cm-3
7-9 μm
80 L-1
•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
•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
•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