V. Ozone Cycling and CFCs
Catalytic Cycles
Termination Steps
Coupling
3rd Order Kinetics
Catalytic Cycles- Ox Chemistry
Chapman Cycle ( © 1930)
O2 + hn → O + O
O + O2 → O3
O3 + hn → O + O2
O + O3 → O2 + O 2
Overpredicts [O3] by x2
Catalytic Cycles- Famous Set
X + O3 → XO + O2
XO + O → X + O2
O + O 3 → O2 + O 2
X, XO
H, OH
OH, HOO
Cl, ClO
Br, BrO
NO, NO2
Caveats
Other types of cycles
Cycles aid understanding,
not natural units
Fate of Atomic Chlorine
25 km, 218 K
k(Cl + O3) = 8.8 x 10-12 cm3 molecule-1 s-1
k(Cl + CH4) = 1.8 x 10-14 cm3 molecule-1 s-1
[O3], [CH4] from figures in Section D of Course pack
What fraction of Cl reacts with O3 at 25 km ?
(Spreadsheet in section F)
Catalysis vs. Stoichiometry
Radical Families Interact to
Create Reservoir Species
ClO + NO2 → ClONO2
OH + NO2 → HONO2
Cl + CH4 → HCl + CH3
Reservoir species regenerate radicals…
ClONO2 + hn → ClO + NO2
HONO2 + hn → OH + NO2
HCl + OH → Cl + HOH
…faster than permanent removal
Catalytic Cycles Interact
Radical Family
None (Ox only)
NOx
ClOx
NOx + ClOx
HOx
NOx + ClOx + HOx
Measured
Column Ozone
(1018 molecules cm2)
16
8.3
7.5
8.3
9.8
9.4
8-9
Predicted Affect of CFCs on Ozone
Year of Kinetics Database used for Prediction
Association Reactions (XO + NO2)
[M] = total concentration of gas phase molecules
k (altitude)  keff (T , [ M ]) 
2 }1
{
1

[log
(
k
(
T
)[
M
]
/
k
(
T
))]
o

10

F

ko (T )[ M ]


1

(
k
(
T
)[
M
]
/
k
(
T
))
o



ko(T) and k∞(T) expressed as power law
k(T) = k300 (T/300)-x
Or use the Excel Spreadsheet at my web site and
Table 2 of the JPL Data Evaluation with F=0.6
Termolecular.xls
Reaction
ClO + NO2
ko(300)
1.80E-31
n
3.40E+00
kinf(300)
1.50E-11
m
1.9
h (km)
T
[M]
ko(T)*[M]
kinf(T)
k(P,T)
10
222
8.50E+18
4.3E-12
2.7E-11
2.7E-12
20
215
2.00E+18
1.1E-12
2.8E-11
9.0E-13
25
218
9.00E+17
4.8E-13
2.8E-11
4.2E-13
30
223
3.00E+17
1.5E-13
2.6E-11
1.4E-13
40
240
1.00E+17
3.8E-14
2.3E-11
3.6E-14
50
268
1.70E+16
4.5E-15
1.9E-11
4.3E-15
Fate of ClO
25 km, 218 K
k(ClO + NO2) = 4.2 x 10-13 cm3 molecule-1 s-1
k(ClO + NO) = 2.4 x 10-11 cm3 molecule-1 s-1
k(ClO + O) = 4.1 x 10-11 cm3 molecule-1 s-1
[O], [NO], [NO2] from figures in Section D of Course pack
What fraction of ClO reacts with O at 25 km ?
Null Cycles
ClO + NO2 → ClONO2
ClONO2 + hn → ClO + NO2
Cl + O3 → ClO + O2
ClO + NO → Cl + NO2
NO2 + hn → O + NO
O + O 2 → O3
Radical Families Compete
(at a given altitude)
Ozone-Friendly CFC Substitutes
Chlorofluorocarbons (CFCs)
– don’t react with anything
- don’t absorb light in troposphere
Hydro(chloro)fluorocarbons (HFCs and HCFCs)
- have hydrogen: react with OH in troposphere
CF2ClH + OH → CF2Cl + HOH
- HFCs have no Cl (Atomic F is ozone-friendly)
Decade-Scale Ozone Loss
Causes
CFCs
Global circulation
Polar ozone holes
October
Key Points
• Catalytic cycles
- Interact and compete
- XO + O limits slow
- null cycles
• CFC substitutes removed in troposphere
• Kinetics calculations:
- fate of species
- steady state concentrations
• CFCs drive ozone depletion globally