Figure 2 Schematic diagram of the reactions leading to polar ozone depletion.

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P1: FQP/FGP P2: FQP

July 26, 2000 19:3 Annual Reviews AR109-17

476 ZONDLO ET AL

Figure 2 Schematic diagram of the reactions leading to polar ozone depletion.

and

HCl + HOCl −→ ^PSC Cl 2 + H 2 O. 7.

Molecular chlorine is rapidly photolyzed into atomic chlorine radicals in spring- time at polar sunrise. Because concentrations of atomic oxygen are low at the altitudes of PSCs, the catalytic cycle outlined in Reactions 2 and 3 is not efficient at destroying ozone in this region of the stratosphere. Instead, ozone is destroyed by chlorine radicals through the following catalytic cycle (14):

2 × (Cl + O 3 → ClO + O 2 ) 8.

ClO + ClO + M → (ClO) 2 + M 9.

(ClO) ₂ + hν → ClO ₂ + Cl 10.

ClO 2 + M → Cl + O 2 + M 11.

−−−−−−−−−−−−−−−−−−−−−− − net: 2O ₃ → 3O ₂ .

Reactions 8–11 continue unabated until terminated by Reactions 4 and 5, thereby

reforming ClONO ₂ or HCl. However, PSC surfaces rapidly convert these pho-

tochemically inert forms of chlorine back into active chlorine by heterogeneous

Reactions 6 and 7. Therefore, rapid ozone loss continues, and ozone remains

depleted until the dissipation of the polar vortex. Because PSCs are situated in

the middle of the ozone layer, heterogeneous chemistry on PSCs can destroy large

(18)

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Figure 2 Schematic diagram of the reactions leading to polar ozone depletion.

P1: FQP/FGP P2: FQP

July 26, 2000 19:3 Annual Reviews AR109-17

476 ZONDLO ET AL

Figure 2 Schematic diagram of the reactions leading to polar ozone depletion.

and

HCl + HOCl −→ PSC Cl 2 + H 2 O. 7.

2 × (Cl + O 3 → ClO + O 2 ) 8.

ClO + ClO + M → (ClO) 2 + M 9.

(ClO) 2 + hν → ClO 2 + Cl 10.

ClO 2 + M → Cl + O 2 + M 11.

−−−−−−−−−−−−−−−−−−−−−− − net: 2O 3 → 3O 2 .

Reactions 8–11 continue unabated until terminated by Reactions 4 and 5, thereby

reforming ClONO 2 or HCl. However, PSC surfaces rapidly convert these pho-

tochemically inert forms of chlorine back into active chlorine by heterogeneous

Reactions 6 and 7. Therefore, rapid ozone loss continues, and ozone remains

depleted until the dissipation of the polar vortex. Because PSCs are situated in

the middle of the ozone layer, heterogeneous chemistry on PSCs can destroy large

185 190 195 200

~T NAD

~T SAT

~T NAT (Typ 1 PSC)

~T STS

~T Eis (Typ 2 PSC)

T e mperat ur [ K ]

HCl + HOCl −→ ^PSC Cl 2 + H 2 O. 7.

(ClO) ₂ + hν → ClO ₂ + Cl 10.

−−−−−−−−−−−−−−−−−−−−−− − net: 2O ₃ → 3O ₂ .

reforming ClONO ₂ or HCl. However, PSC surfaces rapidly convert these pho-

~T _NAD

~T _SAT

~T _NAT (Typ 1 PSC)

~T _STS

~T _Eis (Typ 2 PSC)