1. Atmospheric chemistry
Lecture 4:
Stratospheric Ozone Chemistry
Dr. David Glowacki
University of Bristol,UK

2. Yesterday… Today… 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

3. Integrated column - Dobson unit

4. Atmospheric O3 profiles
In the 1920s, observations of the solar UV spectrum suggested a significant atmospheric [O3]

5. O3 altitude profile measured from satellite
At the ground: [O3] ~ ppb
In the stratosphere: [O3] ~ 5-10 ppm

6. The Chapman Cycle
O2 + hv  O + O (1)
O + O2 + M  O3 + M (2)
O3 + hv  O2 + O (3)
O3 + O  O2 + O2 (4)

7. Chapman Cycle Step 1: O2 + hv  O + O
O2  O(3P) + O(3P) - Threshold  < 242 nm
O2  O(3P) + O(1D) - Threshold  < 176 nm

8. Chapman Cycle Step 2: O + O2 + M  O3 + M
M = O2 or N2 O3
O + O2 reaction coordinate

9. UV absorption spectrum of O3 at 298 K
Chapman Cycle Step 3: O3 + hv  O2 + O
UV absorption spectrum of O3 at 298 K
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)

10. UV absorption spectrum of O3 at 298 K
Chapman Cycle Step 4
UV absorption spectrum of O3 at 298 K
O3 + O  O2 + O2
Occurs via an abstraction mechanism

11. The Chapman Cycle O2 + hv  O + O (1) O + O2 + M  O3 + M (2)
Rate coefficients for each reaction have been measured in the lab

12. Solving for [O3] using the Chapman Mech
(1)
(2)
(3)
(4)
(B1)
(A1)
Substitute (A2) into (B2)
(B2)
(A2)
(na is the atmospheric number density)
(CO2 is the O2 mixing ratio)

13. How good is the Chapman mechanism?
Determining stratospheric [O3] using the above Chapman equation isn’t entirely straightforward because k1 and k3 are photolysis rates!
k1 & k3 are photolysis rates
where
Beer Lambert Law
Atmospheric optical depth
and

14. How good is the Chapman mechanism?
Increasing photolysis with altitude
Chapman overpredicts by a factor of 2
The maximum reflects k1, which is affected by:
Decreasing [O2] with altitude following the barometric law
Increasing hv with altitude
Altitude

15. Q: Why does Chapman overpredict?
A: Catalytic Ozone loss cycles

16. 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, 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]

17. 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!

18. 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

19. 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

20. Loss of stratospheric NOx
Primarily via formation of HNO3, transport to troposphere, & deposition
HNO3 & N2O5 are NOx ‘reservoirs’
Very stable & have a long lifetime
daytime
nighttime

21. 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

22. Some complications to stratospheric O3 chemistry
Catalytic Loss cycles are coupled to each other
Aerosols