CFCs in the Stratosphere



Photolysis

Chlorofluorocarbons, because they don't have any C-H bonds, don't react with hydroxyl radical in the troposphere. They have a very long lifetime in the troposphere and eventually migrate to the stratosphere.



The stratosphere is different from the troposphere in a number of ways. To understand the chemistry, we need to know more about the environment in this region of the atmosphere.
  1. The temperature increases from the bottom of the stratosphere to the top. It decreases in the troposphere from bottom to top. This is because the troposphere's heat comes from sunlight absorbed by the Earth's surface while the stratosphere's heat comes from the sun's UV radiation photolyzing ozone and oxygen.

  2. The pressure is very much lower in the stratosphere than in the troposphere.

  3. Because the temperature is so low in the region between the troposphere and stratosphere (tropopause), water condenses out and doesn't cross over to the stratosphere. This makes the stratosphere very dry. The only water is a small amount of solid water within the stratospheric clouds. This water is formed by chemical reactions rather than evaportation from the oceans.

  4. Clouds in the stratosphere are made up of nitric acid particles with small amounts of solid water.

  5. High energy UV radiation is not blocked and is available for photochemistry.


The high energy UV radiation in the stratosphere cleaves C-F and C-Cl bonds. It take light with a wavelength less than 290 nm to break the C-Cl bond in a chlorofluorocarbon.



There is some natural chlorine migration into the stratosphere (HCl release from volcanoes, some migration of CH3Cl from marine organisms) but synthetic chlorofluorocarbons are the major chlorine source in this region. Even though the production of CFCs was restricted or eliminated by an international treaty in 1987, the long lifetime of these molecules in the troposphere and their slow migration to the stratosphere means that they will be a problem for many more years.


Reactions of Chlorine Atom

Chlorine atoms can react with ozone to form molecular oxygen and chlorine oxide. Chlorine atom and chlorine oxide are the most reactive of the ozone-destroying molecules in the stratosphere.






Chlorine Oxide

Chlorine oxide reacts with oxygen atom to regenerate chlorine atom and molecular oxygen.






Chlorine Nitrate

Eventually, many of the chlorine atoms combine with nitrogen oxides. The product, chlorine nitrate, builds up on the surface of nitric acid particle in stratospheric clouds. The Cl-O bond in chlorine nitrate is weak and this molecule absorbs visible light to give Cl and NO3.




Professor Patricia Shapley, University of Illinois, 2012