10. stratospheric chemistry - harvard...
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10. Stratospheric chemistry
Daniel J. Jacob, Atmospheric Chemistry, Harvard University, Spring 2017
The ozone layer
Latest satellite ozone data (March 12):http://www.temis.nl/protocols/O3total.html
Dobson unit: physical thickness (0.01 mm) of ozone layer if compressed to 1 atm, 0oC
1 DU = 2.69x1016 molecules cm-2
Structure of the natural ozone layer
Based on ozonesonde observations in the 1970s
Ozone number density, 1012 molecules cm-3
The natural ozone layer: chemical production (P) and loss (L)Ozone number density, 1012 molecules cm-3
P > L P > L
L > P
L > PL > P
Brewer-Dobson stratospheric circulation
Chapman mechanism for stratospheric ozone (1930)
2
2 3
3 2
3 2
(R1) O O + O ( < 240 nm)(R2) O + O M O M(R3) O O O ( 320 nm)(R4) O O 2O
h
h
ν λ
ν λ
+ →+ → +
+ → + <+ →
O O3O2slow
slow
fast
Odd oxygen family[Ox] = [O3] + [O]
R2
R3R4
R1
Calculation of photolysis rates
[ ]... [ ]d XX h k Xdt
ν+ → = −
k is the photolysis rate constant (also called photolysis frequency)
0
( ) ( ) ( )X Xk q I dλ σ λ λ λ∞
= ∫quantumyield absorption
x-section
actinic flux (omnidirectional)
photonis absorbed
Molecularcross-section A
Absorptioncross-section σ
photonis notabsorbed
Probability of absorption for incomingphotons = σ/A
Solar spectrum and absorption cross-sections
O2+hv O3+hv
Calculation of 3-body reaction rates
* (1)* (2)* * (3)
* heat (4
)Net:
A B ABAB A
A B
BAB M AB MM
M BM
A M
+ →→ ++ →
+ + →→ +
+
+
Low-pressure limit (Rate(2) >> Rate (3)):
A and B are reactants;AB* is the activated product;AB is the stable product;M is the “third body” (N2, O2 )
1 3
2 3
[ ][ ][ ][ ][ ]
k k A B Md ABdt k k M
=+
General solution:
1[ ] [ ][ ]d AB k A Bdt
=
1 3
2
[ ] [ ][ ][ ]k kd AB A B Mdt k
=
High-pressure limit (Rate(2) << Rate (3)):
Energy states of the O atom (1s22s22p4)
multiplicity total electronicorbital angularmomentum number
Multiplicity = 2S+1, where S is the spin. The spin of an electron is ±1/2.
Hund’s Rule: lowest-lying energy state is the one of maximum multiplicity
EnergyO(1 S)O(1D)O(3P)
determined by the arrangement of the four electrons in the 2p orbitals
O.
.
: :
.
.O(3P) is a diradical
Steady-state analysis of Chapman mechanismLifetime of O atoms:
O 22 2 4 3 2 O2
[O] 1 ~ 1 s [O][O ][M]+ [O ][O]
= ≈ak k k C n
τ
…is sufficiently short to assume steady state for O:
3 O2 2 3 3 2
3 2 2 3
x 3
[O]2 3 [O][O ][M]= [O ] 1[O ]
[O ] [O ]
= ⇒ ⇒ = = <<
⇒ ≈O a O
kR R k kk C n
ττ
…so the budget of O3 is controlled by the budget of Ox.
Lifetime of Ox:
xOx
4 3 4
[O ] 1 2 [O ][O] 2 [O]k k
τ = ≈
Steady state for Ox:1
321 2 2
3 O23
1 2 44
3 [O2 1 2 4 [O ] [O O] ]][ aR R k k k C nk k
k
=⇒
= = ⇒
τOx
Photolysis rate constants: dependence on altitude
0X+ ... ( ) ( )X Xh k q I dλν λ σ λ λ
∞→ = ∫
quantumyield
absorptionX-section
photonflux
2 2 3 3optical depth ( ( ) ( ))O O O Od n z n z dzδ σ σ= +
( )I z dz+
( )I z
2 2 3 3
( ) ( ) e
( ( ') ( ')) 'O O O Oz
I z I
n z n z dz
δ
δ σ σ
−
∞
= ∞
= +∫
Chapman mechanism vs. observations
-3
shapedeterminedby k1nO2
Chapman mechanism reproduces shape, but is too high by factor 2-3 missing sink!
Radical reaction chains in the atmosphere
non-radical radical + radicalInitiation:photolysisthermolysisoxidation by O(1D)
radical + non-radical non-radical + radicalPropagation: bimolecularredox reactions
non-radical + non-radicalTermination: radical redox reaction
radical + radical
non-radical + M radical + radical + M 3-body recombination
Water vapor in stratosphere
Source: transport from troposphere, oxidation of methane (CH4)
H2O mixing ratio
Initiation:1
2H O + O( ) 2OHD →
Propagation: 3 2 2
2 3
3
2
2
OH + O HO O HO +Net:
O OH + 2O
2O3O→
→ +→
Termination:2 2 2OH + HO H O + O→
OH HO2H2Oslow
slow
fast HOx radical family
Ozone loss catalyzed by hydrogen oxide (HOx ≡ H + OH + HO2) radicals
Questions
1. In the upper stratosphere, OH reacts with O atoms:OH + O → H + O2
followed by addition of O2 to H:H + O2 + M → HO2 + M
What is the effect on ozone?
1. A termination step for the HOx radical chain isHO2 + HO2 → H2O2 (hydrogen peroxide)Hydrogen peroxide can go on to either photolyze or react with OH:
H2O2 + hν → 2OHH2O2 + OH → H2O + HO2
Whether H2O2 photolyzes or reacts with OH has a large effect on HOx-catalyzed ozone loss, explain why.
Supersonic aircraft (Concorde) cruising at 60,000’
WHAT IS A RATE-LIMITING STEP?
• From IUPAC: “A rate-controlling (rate-determining or rate-limiting) step in a reaction occurring by a composite reaction sequence is an elementary reaction the rate constant for which exerts a strong effect — stronger than that of any other rate constant — on the overall rate.”
It is not necessarily the slowest reaction in the sequence!
NITROUS OXIDE IN THE STRATOSPHERE
H2O mixing ratio
N2O: LOW-YIELD PRODUCT OF BACTERIAL NITRIFICATION AND DENITRIFICATION
Important as• source of NOx radicals in stratosphere• greenhouse gas
IPCC[2014]
Main anthropogenic source: agriculture
ATMOSPHERIC CYCLING OF NOx AND NOy
STRATOSPHERIC OZONE BUDGET FOR MIDLATITUDES CONSTRAINED FROM 1980s SPACE SHUTTLE OBSERVATIONS
Salawitch et al. [1989]
Chlorofluorocarbons (CFCs) in the atmosphere
Chlorofluorocarbons(CFCs)
(ppb)
ATMOSPHERIC CYCLING OF ClOx AND Cly
Chlorine partitioning in stratosphere
WMO [2014]
Source gas contributions to stratospheric chlorine
WMO [2014]
Decrease of Cl-containing gases following Montreal protocol
τ = 45 yearsτ = 100 years
τ = 26 yearsτ = 5 years
• Original Montreal protocol (1987): cap production rates at 1980s levels• London (1990), Copenhagen (1992) amendments: phase-out in developed world• Beijing (1999): worldwide ban on production
Loss of HNO3 by PSC sedimentationsuppresses conversion of ClO to ClNO3 in Antarctic spring
WMO [2014]
Loss of HNO3 by PSC sedimentationsuppresses conversion of ClO to ClNO3 in Antarctic spring
WMO [2014]
CHRONOLOGY OF ANTARCTIC OZONE HOLE
What about the Arctic?
PSCs in Kiruna, northern Sweden
PSC FORMATION AT COLD TEMPERATURES
PSC formation
Frost point of water
WMO [2014]
Ozone depletion can take place in Arctic stratosphere in spring
WMO [2014]Movie of 2015-2016 Arctic ozone season
http://ozonewatch.gsfc.nasa.gov/
1971-present trend in Arctic ozone column (March)
WMO [2014]
March 1979-2016 Arctic ozone movie
http://ozonewatch.gsfc.nasa.gov/
Arctic ozone watch, 2017
Embryo of ozone hole is occasionally seen in Arcticfollowing cold winters
Correlation of Arctic ozone loss with temperature
Rising CO2 warms the surface but cools the stratosphere
ground
troposphere
stratosphere
CO2
WARM
COLD
IPCC [2014]
Global ozone trend
WMO [2014]
Montreal Protocol as example of successful global environmental policy
Montreal Protocol and its amendments have reversed the stratospheric chlorine trend
Antarctic ozone hole is on its way to recovery,Arctic ozone hole appears to have been avoided
Model projections for future ozone recovery
WMO [2014]