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

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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

60

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Pinatubo

In the stratosphere O2 and O3 absorb UV radiation

temperature increases with altitude

slow mixing

protects troposphere from UV

The ozone layer thus creates the stratosphere, and traps air in the troposphere!

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~tropopause

~3,000 ppb

290 nm).

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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:

FAST

SLOW

SLOW

Why is there ozone in the stratosphere?

(R1)

(R2)

(R3)

(R4)

Chapman cycle

1) O2 + h (< 240 nm) 2O

2) O + O2 + M O3 + M (*)

3) O3 + h (240 nm

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Steady state solution

O O3O2slow

slow

fast

Odd oxygen family Ox = O + O3

Chapman mechanism

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)

R2

R3 R4

R1

O3

O

Observations agree closely with Chapman

[Ox] = [O] + [O3] [O3]

Steady state approximation: k2[O][O2] [M]=k3 [O3]

Lets summarize

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[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

Ox

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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

Ox

Upper stratosphere: flaw in theory

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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

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Other comments

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.

Brewer-Dobson circulationObservation

O3 columns are smallest in tropics despite this being the main stratospheric O3 production region

Explanation

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.

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

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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

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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.

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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

Initiation:

(e.g.) CF2Cl2 + h CF2Cl + Cl

Propagation:

Cl + O3 ClO + O2

ClO + O Cl + O2

Net: O3 + O 2O2

Termination: Recycling:

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.

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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.

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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

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