1. V. Ozone Cycling and CFCs Catalytic Cycles Termination Steps Coupling 3rd Order Kinetics

2. Catalytic Cycles- Ox Chemistry
Chapman Cycle ( © 1930)
O2 + hn → O + O
O + O2 → O3
O3 + hn → O + O2
O + O3 → O2 + O2
Overpredicts [O3] by x2
*geochemical cycles – iron deposition
*acidification -
*Human health – air quality standards based on PM2.5. Small particles can be respired and aren’t caught up in mucus.

3. Cycles aid understanding,
Catalytic Cycles- Famous Set
X + O3 → XO + O2
XO + O → X + O2
O + O3 → O2 + O2
X, XO
H, OH
OH, HOO
Cl, ClO
Br, BrO
NO, NO2
Caveats
Other types of cycles
Cycles aid understanding,
not natural units

4. Fate of Atomic Chlorine
25 km, 218 K
k(Cl + O3) = 8.8 x cm3 molecule-1 s-1
k(Cl + CH4) = 1.8 x 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

5. 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

6. Catalytic Cycles Interact

7. Predicted Affect of CFCs on Ozone
Year of Kinetics Database used for Prediction

8. Association Reactions (XO + NO2)
[M] = total concentration of gas phase molecules
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

9. 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
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

10. Fate of ClO 25 km, 218 K k(ClO + NO2) = 4.2 x 10-13 cm3 molecule-1 s-1
k(ClO + O) = 4.1 x 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 ?

11. Null Cycles ClO + NO2 → ClONO2 ClONO2 + hn → ClO + NO2
Cl + O3 → ClO + O2
ClO + NO → Cl + NO2
NO2 + hn → O + NO
O + O2 → O3

12. Radical Families Compete
(at a given altitude)

13. 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)

14. Decade-Scale Ozone Loss
Causes
CFCs
Global circulation
Polar ozone holes
October

15. 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