Atmospheric chemistry
Lecture 4:
Stratospheric Ozone Chemistry
Dr. David Glowacki
University of Bristol,UK
david.r.glowacki@bristol.ac.uk
Yesterday…
• We discussed tropospheric chemistry
• The troposphere is a massive chemical reactor that
depends on pressure, temperature, sunlight, and ground
level chemical emissions
Today…
• We will discuss some of the chemistry in the stratosphere
• Stratospheric chemistry is a little bit simpler than
tropospheric chemistry because there’s less pollutants
• Also, the molecules involved are smaller so there’s fewer
branching reactions
Integrated column - Dobson unit
Atmospheric O3 profiles
• In the 1920s, observations of the solar UV spectrum
suggested a significant atmospheric [O3]
O3 altitude profile measured from satellite
• At the ground: [O3] ~ 10-100 ppb
• In the stratosphere: [O3] ~ 5-10 ppm
The Chapman Cycle
O2 + hv  O + O
O + O2 + M  O3 + M
O3 + hv  O2 + O
O3 + O  O2 + O 2
(4)
(1)
(2)
(3)
Chapman Cycle Step 1: O2 + hv  O + O
O2  O(3P) + O(3P) - Threshold  < 242 nm
O2  O(3P) + O(1D) - Threshold  < 176 nm
Chapman Cycle Step 2: O + O2 + M  O3 + M
M
O
OO
M = O2 or N2
O3
O + O2 reaction coordinate
UV absorption
of O+
298
KO + O
3 at
Chapman
Cycle spectrum
Step 3: O
hv

3
2
λ < 336 nm
Hartley
bands
Very strong
absorption
Small but
significant
absorption out to
350 nm (Huggins
bands)
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)
UV absorption
spectrum
O3 at 298
Chapman
CycleofStep
4 K
O3 + O  O2 + O 2
Occurs via an abstraction mechanism
The Chapman Cycle
O2 + hv  O + O
O + O2 + M  O3 + M
O3 + hv  O2 + O
O3 + O  O2 + O 2
(4)
Rate coefficients for each
reaction have been
measured in the lab
(1)
(2)
(3)
Solving for [O3] using the Chapman Mech
(1)
(2)
(3)
(4)
(B1)
(A1)
(A2)
k1[O2 ]
[O] 
k 4 [O3 ]
Substitute (A2) into (B2)
k1k2 1/ 2
3/2
[O3 ]  
C
n
 O2 a
k
k
 3 4  
[O]
k3

[O3 ] k 2 [M ][O2 ]
[M]  n a
(B2)
(na is the atmospheric number density)
[O2 ]  CO2 n a
(CO2 is the O2 mixing ratio)
How good is the Chapman mechanism?
k1k2 1/ 2
3/2
[O3 ]  
 [O2 ]n a
k3 k4 
• Determining stratospheric [O3] using the above Chapman
equation isn’t entirely straightforward because k1 and k3 are
photolysis 
rates!
k1  j1    A (,T)A (,T)I()d
where
and
k1 & k3 are photolysis rates
Beer Lambert Law
Atmospheric optical depth
How good is the Chapman mechanism?
Altitude
Increasing photolysis
with altitude
Chapman overpredicts
by a factor of 2
The maximum reflects k1,
which is affected by:
(1)Decreasing [O2] with
altitude following the
barometric law
(2)Increasing hv with altitude
k1k2 1/ 2
3/2
[O3 ]  
[O
]n

2
a
k3 k4 
Q: Why does Chapman overpredict?
A: Catalytic Ozone loss cycles
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
X is a catalyst
Catalysed:
X + O3  XO + O2
k5
and is reformed
XO + O  X + O2
k6
Net rxn:
O + O 3  O2 + O 2
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, R5=R6 and hence k5[X][O3] = k6[XO][O]
Rate (catalysed) / Rate (uncatalysed) = R5/R4 = k5[X][O3]/k4[O][O3]= k5[X]/k4[O]
Or Rate (catalysed) / Rate (uncatalysed) = R6/R4 = k6[XO][O]/k4[O][O3]=k6[XO]/k4[O3]
Catalytic ozone loss kinetics
k5 (220K)
k4
k6 (220K)
• X+O3 (k5) and XO+O (k6) are up to a factor of ~104 faster than O + O3 (k4)!
• A little bit of XO makes a big difference!
Catalytic O3 loss via HOx
• OH is an even more efficient
catalyst because the
intermediate HO2 also
destroys O3
• OH in the stratosphere is
generated in the same way it
is generated in the
troposphere
Catalytic O3 loss via NOx
Predominant fate
of stratospheric NO
(null cycle, no net change)
A small fraction of
NO2 reacts with O
Catalytic Loss Cycle
Loss of stratospheric NOx
• Primarily via formation of HNO3, transport to
troposphere, & deposition
daytime
nighttime
• HNO3 & N2O5 are NOx ‘reservoirs’
• Very stable & have a long lifetime
N2O: another source of stratospheric NOx
• Because the N2O lifetime is very
long, it may be transported to
the stratosphere, where it
undergoes the following:
• Consideration of N2O brings the
Chapman model into much
better agreement with
observations
• Ice Core data show increase of
atmospheric [N2O] of ~0.3%
year since 18th century
Some complications to stratospheric O3
chemistry
• Catalytic Loss cycles are coupled to each other
• Aerosols