CALCULATION OF PHOTOLYSIS RATES he photolysis rate constant (also called photolysis frequency or J- quantum yield absorption x-section actinic flux (omnidirectional) photon is absorbed Molecular cross- section A Absorption cross- section s photon is not absorbed Probability of absorption for incoming photons = σ/A [] ... [] dX X h kX dt 0 () ()() X X k q I d s
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CALCULATION OF PHOTOLYSIS RATES k is the photolysis rate constant (also called photolysis frequency or J-value) quantum yield absorption x-section actinic.
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CALCULATION OF PHOTOLYSIS RATES
[ ]... [ ]
d XX h k X
dt
k is the photolysis rate constant (also called photolysis frequency or J-value)
0
( ) ( ) ( )X Xk q I d
quantumyield absorption
x-section
actinic flux (omnidirectional)
photonis absorbed
Molecularcross-section A
Absorptioncross-section s
photonis notabsorbed
Probability of absorption for incomingphotons = σ/A
CALCULATION OF 3-BODY REACTION RATES
* (1)
* (2)
* * (3)
* heat (4
)
Net:
A B AB
AB A
A B
B
AB M AB M
M
M B
M
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 AB
dt k k M
General solution:
1
[ ][ ][ ]
d ABk A B
dt
1 3
2
[ ][ ][ ][ ]
k kd ABA B M
dt k
High-pressure limit (Rate(2) << Rate (3)):
1 Dobson Unit (DU) is defined to be 0.01 mm thickness at STP
1, A sink for HOx radicals in the stratosphere is formation of hydrogen peroxide (H2O2):
H2O can then go on to either photolyze or react with OH:
Is this an effective termination pathway for HOx-catalyzed ozone loss?
2. Write a catalytic cycle of propagation reactions starting with the reaction
and based on the reactions we have seen so far. Does your cycle destroy ozone or is it a null cycle?
2 2 2 2 2HO HO H O O
2 2
2 2 2 2
2H O h OH
H O OH H O HO
2 2HO NO OH NO
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!
STRATOSPHERIC OZONE BUDGET FOR MIDLATITUDES CONSTRAINED FROM 1980s SPACE SHUTTLE OBSERVATIONS
STRATOSPHERIC DISTRIBUTION OF CF2Cl2 (CFC-12)
ATMOSPHERIC CYCLING OF ClOx AND Cly
SOURCE GAS CONTRIBUTIONS TOSTRATOSPHERIC CHLORINE (2004)
CHLORINE PARTITIONING IN STRATOSPHERE
Decrease of Cl-containing gases following Montreal protocol
= 45 years
= 100 years
= 26 years = 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
Questions
1. It has been argued that a fleet of supersonic aircraft releasing NOx in the lower stratosphere would decrease chlorine-catalyzed ozone loss. Why? [Hint: think of the chlorine reservoirs]
2. Peroxynitric acid (HNO4) is produced and removed in the stratosphere by
What is the effect on stratospheric ozone? Think of the effects on both the NOx and HOx budgets.
3. Photochemical model calculations for the stratosphere including only the Chapman mechanism overestimate observed ozone levels by a factor of 3. However, in a budget calculation constrained by ozone observations we find that the O3 + O reaction accounts for only 10% of the Ox sink. Can you reconcile these two results?
OZONE TREND AT HALLEY BAY, ANTARCTICA (OCTOBER)
Farman et al. paper published in Nature
1 Dobson Unit (DU) = 0.01 mm O3 STP = 2.69x1016 molecules cm-2
SPATIAL EXTENT OF THE OZONE HOLE
Isolated concentric region around Antarctic continent is called the polar vortex.Strong westerly winds, little meridional transport
VERTICAL STRUCTURE OF THE OZONE HOLE:near-total depletion in lower stratosphere
Argentine Antarctic station southern tip of S. America
Sep. 2, 1987
Sep. 16
20 km altitude
High ClO in polar vortexSept. 1987 ER-2 aircraft measurements at 20 km altitude south of Punta Arenas
ClO
ClO
O3
O3
Edge ofPolar vortex
Measurements by Jim Anderson’s group (Harvard)
SATELLITE OBSERVATIONS OF ClO IN THE SOUTHERN HEMISPHERE STRATOSPHERE
WHY THE HIGH ClO IN ANTARCTIC VORTEX?Release of chlorine radicals from reactions of reservoir species in
polar stratospheric clouds (PSCs)
PSC FORMATION AT COLD TEMPERATURES
PSC formation
Frost point of water
Seasonal PSCs in the Antarctic stratosphere
HOW DO PSCs START FORMING AT 195K?HNO3-H2O PHASE DIAGRAM
Antarcticvortexconditions
PSCs are not water but nitric acid trihydrate (NAT) clouds
DENITRIFICATION IN THE POLAR VORTEX:SEDIMENTATION OF PSCs
CHRONOLOGY OF ANTARCTIC OZONE HOLE
Chronology of 2013 ozone hole
http://ozonewatch.gsfc.nasa.gov/
Increasing CO2 cools the stratosphere
15 m(220 K)
Add CO2 to stratosphere (T2 ). At 15 m:
fT14
T1 = 220 K
fT24
fT24
Net heating = f(T14 - 2T2
4 ) < 0
Greenhouse gases warm the surface but cool the stratosphere
Questions
1. What ratio of HCl to ClNO3 concentrations in Antarctic fall will lead to the largest ozone depletion the following spring?
2. Satellite observations of ClO in the Antarctic stratosphere in the middle of winter show a "collar" of maximum values around 60 degrees S. Why isn't ClO highest over the South Pole, where temperatures are lowest?