Chapman Mechanism(~1930, Sidney Chapman)

O2 + h O + O (<242.4 nm)

O + O2 + M O3 + M

O3 + h O + O2 (<280 nm)

O + O3 2O2

O + O + M O2 + M

http://www.epa.gov/indicators/roe/html/roeAirInfo.htmUnited States Environmental Protection AgencyEPA Report on the Environment (ROE)

http://www.atm.ch.cam.ac.uk/tour/part2.html“The Ozone Tour”Centre for Atmospheric Science, University of Cambridge

“The Ozone Tour” -- http://www.atm.ch.cam.ac.uk/tour/part3.htmlCentre for Atmospheric Science, University of Cambridge

Paul J. CrutzenThe NetherlandsMax-Planck InstitutMainz, Germany

Mario J. MolinaUSAM.I.T.Cambridge, Mass.

F. Sherwood RowlandUSAUniversity of CaliforniaIrvine, California

NobelPrize1995

(1/3 each)

Catalytic Decompositionof Ozone

X + O3 XO + O2

XO + O X + O2________________________________

O3 + O 2O2X = HOx (H, OH, HOO) NOx (NO, NO2) ClOx (Cl, ClO)

N2O from troposphere:N2O + O* 2NO in middle & upper stratosphere

NO + O3 NO2 + O2

NO2 + O NO + O2________________________________

O3 + O 2O2

HO + O3 HOO + O2

HOO + O HO + O2________________________________

O3 + O 2O2

Above 45 km, OH dominates, from:O* + H2O OH + OH and

O* + CH4 OH + CH3

X + O3 XO + O2

X + O3 XO + O2

XO + XO X + X+ O2________________________________

2O3 3O2

In lower stratosphere (~15-25 km), [O] is relatively low:

UV-C absorbed by ozone.[O2] is high (so most O quickly reacts with it).

Therefore, the dominant ozone loss mechanism is:

Reaction goes by:

XO + XO [XOOX] X + X+ O2

Rate of O3 production depends on [O2], [O3], h (UV-C)

Destruction is more complex, but depends on [X], UV-B.

If something changes, generally [O3] increases or decreasesuntil it reaches a steady state.

Self-healing: [O3], UV-C, more O3 forms below.

Next: Atomic Cl and Br as X.

Atomic Cl and Br as X:

Cl can destroy tens of thousands of O3 molecules each, butis mainly in inactive forms (HCl, ClONO2) in stratosphere.

ClO + NO2 ClONO2

Cl + CH4 HCl + CH3

CH3 does not operate as an X catalyst, since it combineswith O2 to give CO2.

On crystals:

ClONO2g + HCls Cl2g + HNO3aq

Cl2 + h 2Cl

or:

ClONO2g + HsOaq HOClaq + HNO3aq

HClg H+aq + Cl-

aq

Cl -aq + HOClaq Cl2g + OH-

aq

Crystals bind NO2 that would normally deactivateCl, removing it to the troposphere (denitrification).

Conditions in the Arctic are similar to those in theAntarctic, but not as severe, because the temperatureis not as low there as in the Antarctic.