Page 165 - Basic physical chemistry for the atmospheric sciences

Photochemistry l .'i l

the vortex have decreased dramatically. There are also sharp de­
creases in the oxides of nitrogen (NO ) and in the water-vapor content
Y
of the air when passing from outside to inside the wall of the vortex.
The denitrification and dehydration are due, respectively, to the con­
version of NO to nitric acid and the condensation of water at the very
Y
low temperatures inside the vortex. These two condensates form two
s
types of PS s . One type consists of nitric acid trihydrate particle ,
C
about 1 µ m i n diameter, which condense at about -80°C . The other
type of PSC consists of ice-water particles (with nitric acid dissolved
in them), about 10 µm in diameter, which condense near - 8 5°C. As
the particles in these clouds s l owly sink, they remove both water and
nitrogen compounds from the stratosphere. As we shall see below,
these processes play important roles in depleting ozone concentrations
in the Antarctic vortex.
Let u s now return to the chemistry associated with the depletion of
stratospheric ozone. Most of the chlorine and chlorine oxide released
into the stratosphere by Reactions (7.34) and (7.35) are quickly tied
up in reservoirs as hydrogen chloride and chloride nitrate by the reac­
tions
Cl + CH � HCl + CH 3 (7. 3 8)
4
ClO + N02 + M� ClON0 2 + M (7.39)
Liberation of the active chlorine atoms from these reservoirs is gener­
ally slow. However, on the surface of the ice particles that form PSCs,
the following catalytic (catalyzed by the ice) reaction can occur
ClON02(s) + HCl(s)� Cl2(g) + HN03(s) (7.40)

where the parenthetical "s" has been inserted to emphasize those
compounds that are on (or in) ice particles. The nitric acid remains in
the ice particles , but Cl2 is released as a gas that s photodissociated in
i
the stratosphere

c12 + h v � c 1 + c 1 (7.4 1 )
I n addition o catalyzing Reaction (7.40), the ice particles play another
t
role: they remove nitrogen from the stratosphere (as HN03) , which
.
limits the forward Reaction of (7 3 9), thereby providing more ClO than
e
would otherwise be availabl . Thus , on both counts, during the austral
winter the ice particles that comprise PSCs in the Antarctic vortex set
the stage for the destruction of ozone by enhancing the concentrations