THE OZONE LAYER
Ozone is created and destroyed via the Chapman mechanism, postulated in 1930.
The oxygen molecule, O2 , is slowly dissociated by solar ultraviolet radiation in the stratosphere. This leads to the formation of the individual atoms, O and O. These molecules are unstable, highly reactive, and hence readily attach themselves to other oxygen molecules, forming ozone. Due to its reactive nature, ozone can break down and recombine with other free oxygen atoms to reform oxygen molecules.
The Chapman Mechanism:
O2 + hu ® O + O
O + O2 + M ® O3 + M
O3 + hn ® O2 + O
O3 + O ® O2 + O2
where M is some arbitrary molecule
In 1964, it was discovered that the Chapman mechanism overpredicted the amount of ozone in the atmosphere. Catalytic cycles are required to destroy this excess of ozone.
X + O3 ® XO + O2
XO + O ® X + O2
Net:
O3 + O ® O2 + O2
Several species have been suggested for the species of X. The most important of these are shown in the table below.
| Species, X | Source | Cycle |
H |
OH + CO ® H + CO2 | H + O3 ® HO + O2 HO + O ® H + O2 Net: O3 + O ® O2 + O2 |
OH |
CH4 + O (1D) ® OH + CH3 H2O + O (1D) ® OH + OH |
OH + O3 ® HO2 + O2 HO2 + O ® OH + O2 Net: O3 + O ® O2 + O2 |
NO |
N2O + O (1D) ® NO + NO | NO + O3 ® NO2 + O2 NO2 + O ® NO + O2 Net: O3 + O ® O2 + O2 |
Cl |
CH3Cl + hu ® CH3 + Cl | Cl + O3 ® ClO + O2 ClO + O ® Cl + O2 Net: O3 + O ® O2 + O2 |
O (1D) is the first excited state of oxygen.
These catalytic cycles would in theory remove all ozone, but are prevented when chain terminating reactions occur. For example, the chlorine cycle:
Cycle
Cl + O3 ® ClO + O2
ClO + O ® Cl + O2
Net:
O3 + O ® O2 + O2
Termination
Cl + CH4 ® HCl + CH3
ClO + NO2 + M ® ClONO2 + M
Chlorine spends most of its time in the forms of HCl and ClONO2, which are known as reservoir compounds. In this form chlorine is not available to deplete ozone.