Climate of the polar realms - The Global Climate System: Patterns, Processes, and Teleconnections - page 150

again. It is estimated that one Cl atom can destroy 100 000 ozone molecules, until
the cycle is broken and Cl is ''stored'' in a reservoir molecule.
Background Box 5.1
The chemistry of stratospheric ozone depletion (after Bridgman 1997 ;
Turco 2002 )
The anthropogenic destruction of stratospheric ozone is caused mainly by chlorine (Cl)
and bromine (Br) atoms. This summary uses chlorine chemistry to illustrate the process:
Chlorofluorocarbons (example CFC-11) enter the stratosphere from the troposphere
where they react with sunlight
CFCl 3 þ hv ! CFCl 2 þ Cl
The Cl atom then reacts with ozone (O 3 )
Cl þ O 3 ! ClO þ O 2
ClO then reacts with single oxygen atoms
ClO þ O ! Cl þ O 2
and the cycle starts again.
Eventually, Cl is stored in reservoir species, when it reacts with methane
Cl þ CH 4 ! CH 3 þ HCl (reservoir, hydrogen chloride)
or with nitrogen dioxide
ClO þ NO 2 ! ClONO 2 (reservoir, chlorine nitrate)
The reaction chain is illustrated in Figure 5.8a.
On the surface of a polar stratospheric cloud, Cl is released from reservoir storage
HCl þ ClONO 2 !!! Cl 2 þ HNO 3 (summary reaction)
and
Cl 2 þ hv ! Cl þ Cl
The strength and consistency of the PNJ and the CPV in the late winter and
early spring create the conditions that lead to major destruction of stratospheric
ozone in the Antarctic atmosphere (Turco 2002 ). The vortex prevents ozone
replenishment from the mid-latitudes. The extremely cold temperatures within
the vortex allow the creation of ice and nitric acid polar stratospheric clouds
(PSCs), especially above the coast. The PSCs act as a reaction surface, allowing
the reservoir species to break apart, freeing Cl atoms once again. In the weak
spring sunlight, Cl destruction of ozone continues unabated.