The authoritative public forum
  for Metal Finishing since 1989
  

  -----

CFCs at upper atmosphere, where do carbon and fluorine atoms go?

May 19, 2008

Hi, my name is Katarina
and I am a Grade 10 student who just had SWEET SIXTEEN BIRTHDAY PARTY!
anyway, I was in my science class and we were learning about how chlorine atoms work as catalysts at upper atmosphere, creating ozone depletion.
then, I became aware of that CFCs go to upper atmosphere but the only thing we know next is that chlorine atoms are decomposed and act as catalysts.
I was wondering where do carbon and fluorine atoms go..
do they stay at upper atmosphere or do they go back down?
or do they work as some special chemicals just like chlorine atoms?
so, I asked my teacher..
OH, I have to add that my teacher is like a genius.
he is well known among students as a harsh teacher but his class is so fun that no body skips..(actually, no one dares to..-o-)
so, I was hoping my teacher would be able to tell me the answer right away when I asked him.
HOWEVER! my teacher surprised by telling that he did not know.
I was kinda shocked but I guess my teacher was shocked over himself..
so, I never said another word about it.
but I am dying to know~~~~a_a
I must know..(sorry if it sounds rude..)
anyway, I hope my question sounds like a TECHNICAL question..>¤·<
ciao~~!

Katarina L [last name deleted for privacy by Editor]
student - Toronto, Canada

---

June 3, 2008

Katarina,
It looks like you've posed a question no-one wants to answer! Anyway, the fluorine and carbon atoms do not disappear - they remain in almost the same form as the original CFC , but hang around like a car thief in a dimly lit car park, waiting for the opportunity to be a nuisance. Just remember that, in general, atoms cannot be created or destroyed.

What happens is that the chloroflourocarbon (CFC) goes into the stratosphere where the UV light (hv) from the sun breaks it down into radicals. For instance, if you have a CFC, CCl3F, this will break down to:

CCl2F + hv = CCl2F + Cl.

The CCl2F radical is quite content to just waft around in the upper atmosphere waiting to be reacted again with another spare radical.

The chlorine free radical (Cl) then reacts with the ozone to form oxygen and chlorine oxide (ClO):

O3 + Cl = O2 +ClO

The ClO is unstable and then reacts with an oxygen radical that has been formed by the photodissociation of an oxygen molecule by UV radiation (hv) to give to oxygen radicals:

O2 + hv = 2O

The oxygen radical normally recombine with another O2 molecule to form O3 (ozone)

O + O2 = O3

However, if the oxygen radical reacts with the ClO from Chlorine radical/ozone reaction, it forms a chlorine radical and oxygen molecule:

ClO + O = Cl + O2

This then allows the chlorine radical to react with the ozone to form ClO and the whole process repeats itself again "ad infinitum".

The important thing about this process is that, whilst it seems to be pretty harmless in itself (after all it is only creating nice oxygen from toxic ozone and we all need oxygen to breathe!), the loss of the ozone by this route results on "ozone depletion" and the loss of the ozone results in the harmful UV radiation being able to get through the atmosphere and on to the Earth's surface.

Normally ozone absorbs UV light between wavelengths 200-310nm - this is powerful radiation that can cause skin cancers and other horrible illnesses. However, if the ozone is broken down by an easier process, such as the reaction with Cl radicals, it will allow this nasty UV radiation to get through.

Whilst all this is going on the chlorine and CFC radicals that were originally formed at the start of this process can recombine to form a stable CFC molecule until it gets the opportunity to dissociate again to form the two radicals Cl and (eg) CCl2F.

In this process the CFC acts as a catalyst (a chemical that takes part in a chemical reaction and makes it proceed at a lower enthalpy, thereby making the reaction easier to go than without it). I theory catalysts are not consumed in a chemical reaction, but in practise, because no reaction is 100% efficient, they do get consumed.

Hope this explains the process.

Trevor Crichton
R&D practical scientist
Chesham, Bucks, UK