CHAPTER 10: STRATOSPHERIC CHEMISTRY

THE MANY FACES OF ATMOSPHERIC OZONE Troposphere Stratosphere: 90% of total In stratosphere: UV shield In middle/upper troposphere: greenhouse gas In lower/middle troposphere: precursor of OH, main atmospheric oxidant In surface air: toxic to humans and vegetation

STRATOSPHERIC OZONE HAS BEEN MEASURED FROM SPACE SINCE 1979 Method: UV solar backscatter Scattering by Earth surface and atmosphere   Ozone layer Ozone absorption spectrum   Last Saturdays’s ozone layer… Notice the Antarctic ozone hole

CHAPMAN MECHANISM FOR STRATOSPHERIC OZONE (1930) O O 3 O2O2 slow fast Odd oxygen family [O x ] = [O 3 ] + [O] R2 R3 R4 R1

STEADY-STATE ANALYSIS OF CHAPMAN MECHANISM Lifetime of O atoms: …is sufficiently short to assume steady state for O: …so the budget of O 3 is controlled by the budget of O x. Lifetime of O x : Steady state for O x :  Ox

SOLAR SPECTRUM AND ABSORPTION X-SECTIONS O 2 +hv O 3 +hv

PHOTOLYSIS RATE CONSTANTS: VERTICAL DEPENDENCE quantum yield absorption X-section photon flux

CHAPMAN MECHANISM vs. OBSERVATION -3 shape determined by j 1 n O2 Chapman mechanism reproduces shape, but is too high by factor 2-3  missing sink!

CATALYTIC CYCLES FOR OZONE LOSS: Chapman got it almost right… CATALYTIC CYCLES FOR OZONE LOSS: General Idea O 3 + X  XO + O 2 O + XO  X + O 2 Net: O 3 + O  2 O 2 X is a catalyst The catalyst is neither created nor destroyed…but the rate for the catalytic cycle [odd-O removal in this case] depends on catalyst concentrations

WATER VAPOR IN STRATOSPHERE Source: transport from troposphere, oxidation of methane (CH 4 )   

HO x -CATALYZED OZONE LOSS HO x  H + OH + HO 2 hydrogen oxide radical family Initiation: Propagation: Termination: OH HO 2 H2OH2O slow fast HO x radical family

NITROUS OXIDE IN THE STRATOSPHERE   

Initiation N 2 O + O( 1 D)  2NO O 3 loss rate: NO x -CATALYZED OZONE LOSS (NO x  NO + NO 2 ) Day Night NO y  NO x + reservoirs (HNO 3, N 2 O 5,..) Also emitted Termination Recycling NO 2 + OH + M  HNO 3 + M HNO 3 + h  NO 2 + OH NO 2 + O 3  NO 3 + O 2 HNO 3 + OH  NO 3 + H 2 O NO 3 + NO 2 + M  N 2 O 5 + M NO 3 + h  NO 2 + O N 2 O 5 + H 2 O  2HNO 3 N 2 O 5 + h  NO 2 + NO 3 Propagation NO + O 3  NO 2 + O 2 NO + O 3  NO 2 + O 2 NO 2 + h  NO + O NO 2 + O  NO + O 2 O + O 2 + M  O 3 + M Null cycle Net O 3 + O  2O 2

ATMOSPHERIC CYCLING OF NO x AND NO y

Gas-phase chemistry only Source of Ox Approximate closure! STRATOSPHERIC OZONE BUDGET FOR MIDLATITUDES CONSTRAINED FROM 1980s SPACE SHUTTLE OBSERVATIONS Paul Crutzen shared 1995 Nobel Prize for his work on the NOx catalyzed destruction of ozone

STRATOSPHERIC DISTRIBUTION OF CFC-12

Initiation: Cl radical generation from non-radical precursors (e.g., CFC-12) CF 2 Cl 2 + h  CF 2 Cl + Cl Propagation: Cl + O 3  ClO + O 2 ClO + O   Cl + O 2 Net: O 3 + O  2O 2 O 3 loss rate: ClO x -CATALYZED OZONE LOSS (ClO x  Cl + ClO) Cl y  ClO x + reservoirs (HCl, ClNO 3 ) Termination:Recycling: Cl + CH 4  HCl + CH 3 HCl + OH  Cl + H 2 O ClO + NO 2 + M  ClNO 3 + MClNO 3 + hv  Cl + NO 3

ATMOSPHERIC CYCLING OF ClO x AND Cl y Molina and Rowland shared 1995 Nobel Prize for their work on the ClOx catalyzed destruction of ozone