Section 7 Stratospheric (Ozone) chemistry - ?· 1 Section 7 Stratospheric (Ozone) chemistry 1840: Discovery…
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Section 7Stratospheric (Ozone) chemistry
1840: Discovery in Munich by German chemist C.F. Schnbein
ozone had a distinct smell and was almost colourless (ozone gas in high concentrations is light blue)
name taken from Greek ozein meaning to smell
late 19th century: studies of solar radiation spectrum revealed a layer of ozone above the tropopause
1930: Sydney Chapman attempted to explain the existence of the stratospheric ozone layer by the so-called Chapman cycle
1970s: - budgeting work on stratospheric ozone including catalytic NOx reaction cycles by Paul J. Crutzen
- laboratory discovery of the role of CFCs concerning ozonedepletion by Mario Molina and Sherwood Rowland
1984: Detection of ozone hole at the British Antarctic Survey station Halley Bay.
1995: Nobel price in chemistry for Crutzen, Molina and Rowland
Tropospheric ozone levels are increasing Contributes to global warming Air quality problems
Stratospheric ozone levels are decreasing More UV reaches the troposphere, affecting air chemistry
and human & ecosystem health Both processes are primarily due to human activities.
Solar radiation spectrum: blackbody at 5900 K
Tropospheric ozone (northern mid-latitudes)
1870 1990 0
In the stratosphere O2 and O3 absorb UV radiation
temperature increases with altitude
protects troposphere from UV
The ozone layer thus creates the stratosphere, and traps air in the troposphere!
Sydney Chapman proposed a chemical mechanism:
O2 + h O + O ( < 240 nm)
O + O2 + M O3 + M
O3 + h O2 + O(1D) ( < 320 nm)
O(1D) + M O + M
Net: O3 + h O2 + O
O3 + O 2O2
Ground-state (but still reactive with O2) O(3P)
Lots of energy needed
O3 bonds weak
Loss of O3:
Why is there ozone in the stratosphere?
1) O2 + h (< 240 nm) 2O
2) O + O2 + M O3 + M (*)
3) O3 + h (240 nm
Steady state solution
Odd oxygen family Ox = O + O3
Chemical steady-state assumed for species if production and loss rate constant over lifetimeShortest-lived species (O):
similar for [O] between R3 and R2(& neglecting slow R1 and R4)
Observations agree closely with Chapman
[Ox] = [O] + [O3] [O3]
Steady state approximation: k2[O][O2] [M]=k3 [O3]
[Ox] = [O] + [O3] [O3]
[O3] controlled by slow production and loss by R1 and R4 (NOT fast production and loss of O3 from R2 and R3)
Effective O3 lifetime Ox:
Ox = [Ox]/2k4[O][O3] 1/ 2k4[O]
(factor of 2 can be derived formally from mass balance)
What have we learned?
upper stratosphere: Ox short enough that steady state can be assumed:
(applying the P.S.S. approximation for [O])
Values of Ox < 1 day in the upper stratosphere
several years in lower stratosphere
k1(z) and k3(z) are photolysis rate constants (J, not reaction rate constants)
k = qX()X ()Id
I = I, e-/cos
= (O2 [O2] + O3 [O3])dz
*: numerical solution obtained by starting from top of atmosphere and going downward incrementally
Actinic flux [O] & [O3]
Solar zenith angle Optical depth
CHAPMAN CYCLE provides qualitative agreement with observations
Maximum Ox production: 2k1[O2]
Lower stratosphere: s.s. not expected because of long Ox ALSO DYNAMICS
Upper stratosphere: flaw in theory
At high altitudes, lots of UV light reaction 1 efficient but total gas molecules concentrations are low not much O2 around to produce O atoms.
At low altitudes, lots of O2, but little UV light not enough photolysis events, low O production.
Optimal O3 production thus occurs at intermediate altitudes. Chapman functions (plotted on the previous page, see e.g.
Wayne pg 161 -> for some derivations) predict the altitude of the maximum reasonably well, but overestimate the concentration of ozone severely. Also, horizontal O3 patterns are not correctly predicted.
The natural ozone layerFigure is compilation of available measurements from 1960s
Theory predict maximum O3 production in the tropics
But [O3] is not largest in the tropics
To explain this (and low strat. H2O) Brewer and Dobson suggested a circulation pattern
Region of largest production
O3 maxima occur toward high latitudes in late winter/early spring - the result of the descending branch of the B-D circulation
Virtually no seasonal change in the tropics
More accurate data has led to improvements in our understanding of this simple circulation pattern.
O3 columns are smallest in tropics despite this being the main stratospheric O3 production region
Rising tropospheric air with low ozone
B-D circulation transports O3 from tropics to mid- and high latitudes
Recall that Ox is quite long in the lower stratosphere.
Brewer Dobson circulation
From the tropics to the mid- & high latitudes
Highest UV in the tropics highest ozone production
Circulation stronger in the northern hemisphere
Chapman got it almost right, but did not account for
Catalytic cycles for ozone loss: General Idea
O3 + X XO + O2 O + XO X + O2
Net: O3 + O 2 O2 X is a catalyst
The catalyst is neither created nor destroyedbut the rate for the catalytic cycle [odd-oxygen removal in this case] depends on catalyst concentrations
1) Hydrogen oxide (HOx) radicals (HOx = H + OH + HO2)
Initiation: H2O + O(1D) 2OH
Propagation through cycling of HOx radical family (examples):
OH + O3 HO2 + O2 H + O3 HO + O2 HO2 + O OH + O2 OH + O H + O2 Net: O + O3 2O2 Net: O + O3 2O2
Termination (example): OH + HO2 H2O + O2
Source from troposphere
HOx is a catalyst for O3 loss, but not the only one
2) Nitrogen oxide (NOx) radicals (NOx = NO + NO2)
Initiation N2O + O(1D) 2 NO (N2O: laughing gas) Propagation
NO + O3 NO2 + O2 NO + O3 NO2 + O2 NO2 + h NO + O NO2 + O NO + O2 (O + O2 + M O3 + M) Null cycle no OX removed Net O3 + O 2O2
Termination Recycling NO2 + OH + M HNO3 + M HNO3 + h NO2 + OH
NO2 + O3 NO3 + O2 HNO3 + OH NO3 + H2O NO3 + NO2 + M N2O5 + M NO3 + h NO2 + O N2O5 + H2O 2HNO3 N2O5 + h NO2 + NO3
O3 loss rate:
NO3, HNO3, N2O5 are reservoir species.
What have we learned about NOy?
Production Natural NOy by N2O + O(1D) - well understood source
Loss Dominant sink is deposition to troposphere. Residence time for air is 1-2 years. Loss rate well constrained
Cycling O3 loss related to NOy/NOx ratio. Under most conditions s.s. between different NOy species is a good approximation
NOx catalytic cycle reconciled Chapman theory with observations 1995 Nobel Prize
(e.g.) CF2Cl2 + h CF2Cl + Cl
Cl + O3 ClO + O2
ClO + O Cl + O2
Net: O3 + O 2O2
Cl + CH4 HCl + CH3 HCl + OH Cl + H2O
ClO + NO2 + M ClONO2 + M ClONO2 + h ClO + NO2 "
O3 loss rate:
Each chlorine atom destroys on the order of 100 000 ozone molecules before it is removed from the stratosphere.
O + O catalytic cycle (example)OH + O H + O2H + O2 + M HO2 + MHO2 + O OH + O2 Net: O + O + M O2 + M Important at high altitudes
where [O]/[O3] is higher
O3 + O3 catalytic cycle (example)OH + O3 HO2 + O2HO2 + O3 OH + O2 + O2Net: O3 + O3 O2 + O2 + O2 Important at low altitudes where
[O]/[O3] is low
Null cycle (example)NO + O3 NO2 + O2 NO2 + hv NO + O Net: O3 + hv O2 + O No OX loss. Important because
the NOX tied up in null cycle is not removing OX in catalytic cycles.
Holding cycle (example)Cl + CH4 HCl + CH3OH + HCl H2O + Cl Net: CH4 + OH CH3 + H2O Does not involve OX directly,
but Cl atoms tied up as HCl are not participating in catalytic cycles.
Holding cycles involve reservoir species.
Formation of reservoir species
HOClClO + HO2 HOCl + O2ClONO2ClO + NO2 ClONO2HO2NO2HO2 + NO2 + M HO2NO2 + MHNO3OH + NO2 + M HNO3 + M
Creation of odd oxygen!
OH + CO H + CO2H + O2 + M HO2 + M HO2 + NO OH + NO2NO2 + hv NO + ONet: CO + O2 + hv CO2 + O
Mixed null cycles
OH + O3 HO2 + O2HO2 + NO OH + NO2NO2 + hv NO + ONet: O3 + hv O2 + O
Cl + O3 ClO + O2ClO + NO Cl + NO2NO2 + hv NO + ONet: O3 + hv O2 + O
(Note that NO2 photolysis alsocompetes with the reactionNO2 + O NO + O2 which would lead to a net loss o