There are two major factors that negatively impact the ozone layer: (1) CFCs and (2) NOx. (1) CFCs released into the atmosphere are destroyed by ultraviolet rays, releasing chlorine which destroys ozone. (2) NOx both provides a catalytic mechanism for ozone destruction and inhibits cycles that destroy ozone, with the net effect depending on altitude. The ozone layer forms as oxygen molecules in the stratosphere are broken down by solar radiation, producing reactive oxygen atoms that combine with oxygen to form ozone molecules.

1. The factors affecting the OThe factors affecting the O33
layerlayer
 There are twoThere are two
major factorsmajor factors
that hasthat has
tremendoustremendous
negativenegative
effects on theeffects on the
OO33 layerlayer
 I. CFCsI. CFCs
 II. NOII. NOxx

2. I. CFC’s and ozoneI. CFC’s and ozone
depletiondepletion
 Chlorofluorocarbons areChlorofluorocarbons are
created and used increated and used in
refrigerators and airrefrigerators and air
conditioners. Theseconditioners. These
chlorofluorocarbons are notchlorofluorocarbons are not
harmful to humans and haveharmful to humans and have
been a benefit to us. Oncebeen a benefit to us. Once
released into thereleased into the
atmosphere,atmosphere,
chlorofluorocarbons arechlorofluorocarbons are
bombarded and destroyedbombarded and destroyed
by ultraviolet rays. In theby ultraviolet rays. In the
process chlorine is releasedprocess chlorine is released
to destroy the ozoneto destroy the ozone

3. Destruction of ozone byDestruction of ozone by
chlorinechlorine
Molecular oxygen is broken down in the stratosphere byMolecular oxygen is broken down in the stratosphere by
solar radiation to yield atomic oxygen, which thensolar radiation to yield atomic oxygen, which then
combines with molecular oxygen to produce ozone. Thecombines with molecular oxygen to produce ozone. The
ozone is then destroyed by chlorine atoms.ozone is then destroyed by chlorine atoms.

4. Ozone destruction by UVOzone destruction by UV
raysrays
 UV radiationUV radiation from the sunfrom the sun
releases the radicals Cl andreleases the radicals Cl and
ClO.ClO.
 Ozone is a highly unstableOzone is a highly unstable
molecule so it readilymolecule so it readily
donates its extra oxygendonates its extra oxygen
molecule to free radicalmolecule to free radical
species such as hydrogen,species such as hydrogen,
bromine, and chlorine.bromine, and chlorine.
 These compound speciesThese compound species
act as catalysts in theact as catalysts in the
breakdown of ozonebreakdown of ozone
molecules.molecules.

5. I.I. Nitric oxideNitric oxide
 NO is producedNO is produced
abundantly in theabundantly in the
troposphere, but all of it istroposphere, but all of it is
converted into NOconverted into NO22 
HNOHNO33 (removed through(removed through
precipitation)precipitation)
 Removal processes:Removal processes:
NONO22 ++ ..
OHOH  HNO3HNO3
ClOClO..
+ NO+ NO22  ClONOClONO22
 NO in the stratosphereNO in the stratosphere
produced from nitrousproduced from nitrous
oxide (Noxide (N22O), which isO), which is
much less reactive thanmuch less reactive than
NO.NO.
NN22O + hvO + hv  NN22 + O+ O
(90%)(90%)
NN22O + OO + O  2 NO2 NO
(~10%)(~10%)

6. The two-sided effect of NOThe two-sided effect of NOxx
 NONOxx provides aprovides a
catalytic chaincatalytic chain
mechanism for Omechanism for O33
destruction.destruction.
 NONOxx inhibit the NOinhibit the NOxx
and ClOand ClOxx cycles forcycles for
OO33 destruction bydestruction by
removing radicalremoving radical
species in the twospecies in the two
cycles.cycles.
 The relative magnitude ofThe relative magnitude of
the two effects is altitudethe two effects is altitude
dependent.dependent.
• >25 km, the net effect>25 km, the net effect
is to destruct Ois to destruct O33..
• (NO(NOxx accounts foraccounts for
>50% of total ozone>50% of total ozone
destruction in thedestruction in the
middle and uppermiddle and upper
troposphere.)troposphere.)
• In the lowerIn the lower
stratosphere, the netstratosphere, the net
effect is to protect Oeffect is to protect O33
from destruction.from destruction.

8. wavelengths below 240 nm

9. The formation of the ozone layerThe formation of the ozone layer
 The formation of the ozone in the stratosphere region begins with the photodissociation of oxygenThe formation of the ozone in the stratosphere region begins with the photodissociation of oxygen
molecules by the solar radiation at wavelengths below 240 nm.molecules by the solar radiation at wavelengths below 240 nm.
 O2 O + OO2 O + O
 The highly reactive O atoms combine with oxygen molecules to form ozone as follows :The highly reactive O atoms combine with oxygen molecules to form ozone as follows :
O3 + MO3 + M O + O2 + MO + O2 + M
M : Some inert substance.M : Some inert substance.
 The role of M in this exothermic reaction is to absorb some of the excess energy released andThe role of M in this exothermic reaction is to absorb some of the excess energy released and
prevent the spontaneous decomposition of the O3prevent the spontaneous decomposition of the O3 ..moleculemolecule
OO33 O + OO + O22
UV < 240nm
UV

10. The formation of the ozone layerThe formation of the ozone layer
 The formation of the ozone in the stratosphere region begins with the photodissociation of oxygenThe formation of the ozone in the stratosphere region begins with the photodissociation of oxygen
molecules by the solar radiation at wavelengths below 240 nm.molecules by the solar radiation at wavelengths below 240 nm.
 O2 O + OO2 O + O
 The highly reactive O atoms combine with oxygen molecules to form ozone as follows :The highly reactive O atoms combine with oxygen molecules to form ozone as follows :
O3 + MO3 + M O + O2 + MO + O2 + M
M : Some inert substance.M : Some inert substance.
 The role of M in this exothermic reaction is to absorb some of the excess energy released andThe role of M in this exothermic reaction is to absorb some of the excess energy released and
prevent the spontaneous decomposition of the O3prevent the spontaneous decomposition of the O3 ..moleculemolecule
OO33 O + OO + O22
UV < 240nm
UV