Atmos 348 Lecture 7

8/13/2019 Atmos 348 Lecture 7

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ATMOS 348

Atmospheric ChemistryLecture 7: Stratosphere

Don Wuebbles

Department of Atmospheric Sciences

University of Illinois, Urbana-Champaign

February 2004


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A Historical Perspective on Atmospheric

Chemistry

1950s

The atmosphere was viewed as largely a chemical

inertfluid

that moves heat, momentum and moisture

that transports pollutants away from cities

Importance of photochemistry limited to the upper

atmosphere (ionosphere)

Urban Photochemistry (LA smog)


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1970s

The atmosphere started to be seen as a chemically

dynamic system New analytic instrumentation

New measurements of chemical rate constants

Simple atmospheric models


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1970s Stratospheric ozone became a major scientific

issue

Aircraft NOx

Industrially manufactured CFCs

Photochemistry of tropospheric ozone started to beinvestigated at the global scale.


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1980s

Discovery of the stratospheric ozone hole and roleof heterogeneous chemistry

Recognition that air pollution is becoming a global

issue Potential importance of greenhouse gases other

than CO2 in the climate system


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1990s

Role of the biosphere for the chemistry of thetroposphere (e.g., biogenic hydrocarbons)

Role of chemical compounds (including aerosols)

in the climate system Aerosols and cloud microphysics


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1990s

New research infrastructure and approaches fortropospheric studies

Spacecraft

Surface networks Large airborne campaigns

Comprehensive chemical-transport models

International efforts (e.g., IGAC)


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Ozone Density


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Total Ozone (Dobson units)


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Stratospheric Ozone: Physics and Chemistry

Production of Ozone

The Chapman mechanism -- middle/upper stratosphere

O2 + h

O + O (

< 240 nm)O + O2 + M O3 + M (M=N2, O2, Ar, etc.)

O3 + h O2 + O

O + O3 O

2

O + O + M O2 + M (often left out)

Smog chemistry -- troposphere and lower stratosphere

(CH4, CO, HC) + OH HO2

HO2 + NO OH + NO2

NO2 + h NO + O

O + O2 + M O3 + M


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Transport of Stratospheric Constituents


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The Chapman Mechanism

a) O2 + h O + O ja

b) O + O2 + M O3 + M kb

c) O3 + h O2 + O jc

d) O3 + O

O2 + O2 kddnO/dt = 2janO2

kbnOnO2nM + jcnO3

kdnOnO3

dnO3/dt = kbnOnO2nM - jcnO3 - kdnOnO3

Lifetime of O is very short

need very small time steps to integrate equations


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The Concept Of Odd Oxygen

a) O2 + h O + O

b) O + O2 + M O3 + M

c) O3 + h O2 + Od) O3 + O O2 + O2

Interconversion of O and O3 is rapid compared to formationand loss of O3 + O

Define Ox = O3 + O note nO is very small, so nOx nO3

Reaction (a) generates 2 Ox and reaction (d) destroys 2 Ox

Reactions (b) and (c) have no effect on Oxbut havean effect on relative amounts of O and O3 in Ox


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A Steady State for Ozone

a) O2 + h O + O jab) O + O2 + M O3 + M kb

c) O3 + h O2 + O jcd) O3 + O O2 + O2 kd

Control of nO/nO3 by reactions (b) and (c)kbnOnO2nM = jcnO3nO = (jcnO3)/(kbnO2nM)

Rate of formation of odd oxygendnOx/dt dnO3/dt = 2janO2 - 2kdnOnO3

If O3 is at steady-statenO3 = (2janO2)/(2kdnO) = nO2 [(jakbnM)/(kdjc)]

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Observed O and O3 in the Stratosphere

10

15

2025

30

35

40

45

1.E+00 1.E+02 1.E+04 1.E+06 1.E+08 1.E+10

nO (molecules cm-3)a

ltitude

(km)

10

15

20

25

30

35

40

45

0 1E+12

nO3

(molecules cmaltitude

(km)

nO/nO3


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Lifetime for O(3P) in Stratosphere

a) O2 + h O + Ob) O + O2 + M O3 + M

c) O3

+ h O2

+ O

d) O3 + O O2 + O2

Lifetime = (number concentration)/(loss rate)O = nO/(kbnOnO2nM + kdnOnO3) = 1/(kbnO2nM + kdnO3) 1/(kbnO2nM)

For stratospheric conditions

O < 1 s Prod. rate of O and O do not change much on timescales of sPseudo-state approximation for O is appropriate

Rate of prod. of O by reaction (c) >> by reaction (a)nO/nO3 is determined by a balance between rxn. (b) and (c)

Relative Amounts of Stratospheric O and O


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Relative Amounts of Stratospheric O and O3

a) O2 + h O + O

b) O + O2 + M O3 + M

c) O3 + h

O2 + Od) O3 + O O2 + O2

Rate of prod. of O by reaction (c) >> by reaction (a)

nO/nO3is determined by a balance between rxn. (b) and (c)


nO/nO3


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Calculated O3 with Chapman Mechanism

Assuming steady-state for Ox (i.e. for O3)

Steady-state nO3 = (2janO2)/(2kdnO) = nO2 [(jakbnM)/(kdjc)]1/2


Steady-state nO3

has max. at about 25 km agrees with observations

nO3larger than obs. at all alt. does not agree with observations

Questions

Is assumption of steady-state for Ox correct?Is reaction mechanism sufficient to explain nO3

?

a) O2 + h O + Ob) O + O2 + M O3 + Mc) O3 + h O2 + O

d) O3 + O

O2 + O2

LIFETIME OF O IN THE STRATOSPHERE


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LIFETIME OF Ox IN THE STRATOSPHERE

a) O2 + h O + O

b) O + O2 + M O3 + M

c) O3 + h

O2 + O

d) O3 + O O2 + O2

Lifetime = (number concentration)/(loss rate)Ox = nOx/(2kdnOnO3) 1/(2kdnO)

Ox

= several years in the lower stratosphereDo not expect steady-state to hold transport plays a role

Ox < 1 day in the upper strat.

Expect steady-state to hold deficiency in Chapman mech.

Missing Chemistry in Chapman Mechanism


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Missing Chemistry in Chapman Mechanism

calculated

measured

Global Oxproduction rate = 5 times destruction rateImbalance suggests overest. of prodn. or underest. of loss

Oxproduction well constrained by good spectroscopic dataImplies missing chemical sinks for Ox

Reactions of radicals with O and/or O3

But radicals will also be consumed by reaction


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Stratospheric O3: Physics and Chemistry

Destruction of stratospheric ozone

Occurs primarily through catalytic mechanismsExamples:

For X = OH or NO or Cl or Br

X + O3 XO + O2

XO + O X + O2-----------------------------

O + O3 2O2

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Atmos 348 Lecture 7