Atmospheric chemistry

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Atmospheric chemistry

• Day 2
• Stratospheric chemistry

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O2 ? O(3P) O(3P) Threshold ? 242 nm O2 ?
O(3P) O(1D) Threshold ? 176 nm
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UV absorption spectrum of O3 at 298 K
Hartley bands
Small but significant absorption out to 350 nm
(Huggins bands)
Very strong absorption
Photolysis mainly yields O(1D) O2, but as the
stratosphere is very dry (H2O 5 ppm), almost
all of the O(1D) is collisionally relaxed to O(3P)
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O3 altitude profile measured from satellite
At the ground O3 10-100 ppb, in the
stratosphere O3 5-10 ppm
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Integrated column - Dobson unit
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Total column amount of ozone measured by the
Total Ozone Mapping Spectrometer (TOMS)
instrument as a function of latitude and season
Can we account for the distribution of ozone?
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Timescale Slow (J is small) Fast lt 100 secs Fast
1000 s Slow (activation barrier)
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O lt lt O3 Ox O3 O O3
Odd oxygen ( at least 99 of odd oxygen is O3
below 50 km)
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Altitude/km
z
J1

• J1 rate of O2 photolysis (s-1)
• J3 rate of O3 photolysis (s-1)
• Graph shows the altitude dependence of the rate
  of photolysis of O3 and O2. Note how J1 is very
  small until higher altitudes
• The ratio J1/J3 increases rapidly with altitude,
  z
• As pressure ? exp (-z) then O22 M decreases
  rapidly with z

J3
J1
This balance results in a layer of O3
J3
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HOW GOOD IS THE CHAPMAN MECHANSIM?
The Chapman mechanism overpredicts O3 by a factor
of 2. Something else must be removing O3 (Or
the production is too high, but this is very
unlikely)
Altitude / km
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Catalytic ozone destruction

• The loss of odd oxygen can be accelerated through
  catalytic cycles whose net result is the same as
  the (slow) 4th step in the Chapman cycle
• Uncatalysed O O3 ? O2 O2 k4
• Catalysed X O3 ? XO O2 k5
• XO O ? X O2 k6
• Net rxn O O3 ? O2 O2

X is a catalyst and is reformed
X OH, Cl, NO, Br (and H at higher altitudes)
Reaction (4) has a significant barrier and so is
slow at stratospheric temperatures Reactions (5)
and (6) are fast, and hence the conversion of O
and O3 to 2 molecules of O2 is much faster, and
more ozone is destroyed. Using the steady-state
approximation for XO, R5R6 and hence k5XO3
k6XOO Rate (catalysed) / Rate (uncatalysed)
R5/R4 k5XO3/k4OO3 k5X/k4O Or Rate
(catalysed) / Rate (uncatalysed) R6/R4
k6XOO/k4OO3k6XO/k4O3
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k5
k6
k4
Note that rate coefficients for XO3 (k5) and
XOO (k6) are much higher than for O O3 (k4)
So dont need much X present to make a difference
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HOx
Altitude z / km
Maximum in the O3 mixing ratio is about here
Fraction of odd oxygen loss
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What are the sources of X ?
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CFCs are not destroyed in the troposphere. They
are only removed by photolysis once they reach
the stratosphere.
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Data from NOAA CMDL Ozone depleting gases
measured using a gas chromatograph with an
electron capture detector (invented by Jim
Lovelock) These are ground-based measurements.
The maximum in the stratosphere is reached about
5 years later
45 years
100 years
Why are values in the N hemisphere slightly
higher?
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Removal of the catalyst X. Reservoir is
unreactive and relatively stable to photolysis.
X can be regenerated from the reservoir, but only
slowly. X is reduced by these cycles. For Cl
atom, destroys 100,000 molecules of O3 before
being removed to form HCl
Do nothing cycles Ox is not destroyed Reduces
efficiency of O3 destruction
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Interactions between different catalytic
cycles Reservoir species limit the destruction of
ozone ClONO2 stores two catalytic agents ClO
and NO2
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Effects of catalytic cycles are not additive due
to coupling
Mechanism Ozone Column (Dobson
units) Chapman only (C) 644 C NOx 332 C
HOx 392 C ClOx 300 C NO x HOx
ClOx 376 Coupling to NO leads to null cycles
for HOx and ClOx cycles Increase of Cl and NO
concentrations in the atmosphere has less effect
than if Cl or NO concentrations were increased
separately (because ClOx and NOx cycles couple,
hence lowering X)
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Bromine cycle Br O3 ? BrO O2 Cl O3 ?
ClO O2 BrO ClO ? Br ClOO ClOO ? Cl
O2 Net 2O3 ? 3 O2 Br and Cl are regenerated,
and cycle does not require O atoms, so can occur
at lower altitude Source of bromine CH3Br
(natural emissions from soil and used as a soil
fumigant) Halons (fire retardants) Catalyt
ic cycles are more efficient as HBr and BrONO2
(reservoirs for active Br) are more easily
photolysed than HCl or ClONO2 But, there is less
bromine than chlorine
Bromine is very important for O3 destruction in
the Antarctic stratosphere where O is low
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October ozone column, Halley Bay, Antarctica

• TOMS (on Nimbus 7 satellite)
• o Dobson spectrophotometer

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Total Ozone Mapping Spectrometer (TOMS) Monthly
October averages for ozone, 1979, 1982, 1984,
1989, 1997, 2001
Dobson units (total O3 column)
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October 2000 For the Second time in less than a
week dangerous levels of UV rays bombard Chile
and Argentina, The public should avoid going
outside during the peak hours of 1100 a.m. and
300 p.m. to avoid exposure to the UV rays
Ushaia, Argentina The most southerly city in the
world
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At 15 km, all the ozone disappears in less than 2
months This cannot be explained using gas-phase
chemistry alone
US Base in Antarctica
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Steps leading to ozone depletion within the
Antarctic vortex
ClOBrO?ClBrO2
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Simultaneous measurements of ClO and O3 on the
ER-2 Late August 1987 September 16th 1987
Still dark over Antarctica
Daylight returns
The smoking gun experiment proved the theory
was OK
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Simultaneous measurements of ClO and O3 on the
ER-2 Late August 1987 September 16th 1987
Still dark over Antarctica
Daylight returns
The smoking gun experiment proved the theory
was OK
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Ozone loss does appear in the Arctic, but not as
dramatic
Some years see significant depletion, some years
not, and always much less than over Antarctica
Above Spitzbergen