Atmospheric chemistry Lecture 4: Stratospheric Ozone Chemistry Dr. David Glowacki University of Bristol,UK david.r.glowacki@bristol.ac.uk

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

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 (1) O + O2 + M  O3 + M (2) O3 + hv  O2 + O (3) O3 + O  O2 + O2 (4)

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 = O2 or N2 O3 O + O2 reaction coordinate

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)

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

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

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)

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

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

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 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]

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 HNO3 & N2O5 are NOx ‘reservoirs’ Very stable & have a long lifetime daytime nighttime

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