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

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

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

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

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:

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

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

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)

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.

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 10-100 cm-3 220-370 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

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…

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

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 2 + 0.2 2 + 0.8 0.3 1.2 5 1 5 4 1 +9 +1 +6.5 +5 Radiative Forcing Cloud Causes Cooling Cloud Causes Heating

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.

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 10-100 cm-3 220-370 cm-3 Mean Cloud Drop Diameter 18 μm 7-9 μm Drizzle Drop Concentration 800 L-1 80 L-1

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

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

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