V. Ozone Cycling and CFCs

Catalytic CyclesTermination StepsCoupling3rd Order Kinetics

Catalytic Cycles- Ox Chemistry

Chapman Cycle ( © 1930)

O2 + h → O + O

O + O2 → O3

O3 + h → O + O2

O + O3 → O2 + O2

Overpredicts [O3] by x2

Catalytic Cycles- Famous Set

X + O3 → XO + O2

XO + O → X + O2

O + O3 → O2 + O2

X, XO H, OHOH, HOOCl, ClOBr, BrONO, NO2

CaveatsOther types of cycles

Cycles aid understanding,not natural units

Fate of Atomic Chlorine

25 km, 218 Kk(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 + h → ClO + NO2

HONO2 + h → OH + NO2

HCl + OH → Cl + HOH

…faster than permanent removal

Catalytic Cycles Interact

Radical Family Column Ozone (1018 molecules cm2)

None (Ox only) 16 NOx 8.3 ClOx 7.5 NOx + ClOx 8.3 HOx 9.8 NOx + ClOx + HOx 9.4 Measured 8-9

Predicted Affect of CFCs on Ozone

Year of Kinetics Database used for Prediction

Association Reactions (XO + NO2)

1}2))](/])[((10[log1{

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])[(

])[,()(

TkMTokF

TkMTk

MTk

MTkaltitudek

o

o

eff

ko(T) and k∞(T) expressed as power lawk(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

[M] = total concentration of gas phase molecules

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 + h → ClO + NO2

Cl + O3 → ClO + O2

ClO + NO → Cl + NO2

NO2 + h → O + NO

O + O2 → 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 troposphereCF2ClH + OH → CF2Cl + HOH

- HFCs have no Cl (Atomic F is ozone-friendly)

Decade-Scale Ozone Loss

CausesCFCsGlobal circulationPolar 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