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

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Presentation on theme: "Atmospheric chemistry Lecture 4: Stratospheric Ozone Chemistry Dr. David Glowacki University of Bristol,UK"— Presentation transcript:

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

2 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 theres less pollutants Also, the molecules involved are smaller so theres fewer branching reactions

3 Integrated column - Dobson unit

4 Atmospheric O 3 profiles In the 1920s, observations of the solar UV spectrum suggested a significant atmospheric [O 3 ]

5 At the ground: [O 3 ] ~ 10-100 ppb In the stratosphere: [O 3 ] ~ 5-10 ppm O 3 altitude profile measured from satellite

6 The Chapman Cycle O 2 + hv O + O(1) O + O 2 + M O 3 + M(2) O 3 + hv O 2 + O(3) O 3 + O O 2 + O 2 (4)

7 O 2 O( 3 P) + O( 1 D) - Threshold < 176 nm Chapman Cycle Step 1: O 2 + hv O + O O 2 O( 3 P) + O( 3 P) - Threshold < 242 nm

8 Chapman Cycle Step 2: O + O 2 + M O 3 + M O + O 2 reaction coordinate O OO M M = O 2 or N 2 O3O3

9 UV absorption spectrum of O 3 at 298 K Hartley bands Very strong absorption Photolysis mainly yields O( 1 D) + O 2, but as the stratosphere is very dry (H 2 O ~ 5 ppm), almost all of the O( 1 D) is collisionally relaxed to O( 3 P) Chapman Cycle Step 3: O 3 + hv O 2 + O Small but significant absorption out to 350 nm (Huggins bands) λ < 336 nm

10 UV absorption spectrum of O 3 at 298 K Chapman Cycle Step 4 O 3 + O O 2 + O 2 Occurs via an abstraction mechanism

11 The Chapman Cycle O 2 + hv O + O(1) O + O 2 + M O 3 + M(2) O 3 + hv O 2 + O(3) O 3 + O O 2 + O 2 (4) Rate coefficients for each reaction have been measured in the lab

12 Solving for [O 3 ] using the Chapman Mech (1) (2) (3) (4) (A1) (A2) (B1) (B2) (n a is the atmospheric number density) (C O2 is the O 2 mixing ratio) Substitute (A2) into (B2)

13 How good is the Chapman mechanism? Beer Lambert Law Atmospheric optical depth k 1 & k 3 are photolysis rates Determining stratospheric [O 3 ] using the above Chapman equation isnt entirely straightforward because k 1 and k 3 are photolysis rates! where and

14 How good is the Chapman mechanism? Increasing photolysis with altitude Chapman overpredicts by a factor of 2 The maximum reflects k 1, which is affected by: (1)Decreasing [O 2 ] with altitude following the barometric law (2)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) 4 th step in the Chapman cycle Uncatalysed:O + O 3 O 2 + O 2 k 4 Catalysed:X + O 3 XO + O 2 k 5 XO + O X + O 2 k 6 Net rxn: O + O 3 O 2 + O 2 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 O 3 to 2 molecules of O 2 is much faster, and more ozone is destroyed. Using the steady-state approximation for XO, R 5 =R 6 and hence k 5 [X][O 3 ] = k 6 [XO][O] Rate (catalysed) / Rate (uncatalysed) = R 5 /R 4 = k 5 [X][O 3 ]/k 4 [O][O 3 ]= k 5 [X]/k 4 [O] Or Rate (catalysed) / Rate (uncatalysed) = R 6 /R 4 = k 6 [XO][O]/k 4 [O][O 3 ]=k 6 [XO]/k 4 [O 3 ]

17 X+O 3 (k 5 ) and XO+O (k 6 ) are up to a factor of ~10 4 faster than O + O 3 (k 4 )! A little bit of XO makes a big difference! k 5 (220K) k4k4 k 6 (220K) Catalytic ozone loss kinetics

18 Catalytic O 3 loss via HO x OH is an even more efficient catalyst because the intermediate HO 2 also destroys O 3 OH in the stratosphere is generated in the same way it is generated in the troposphere

19 Predominant fate of stratospheric NO (null cycle, no net change) A small fraction of NO 2 reacts with O Catalytic O 3 loss via NO x Catalytic Loss Cycle

20 Loss of stratospheric NO x Primarily via formation of HNO 3, transport to troposphere, & deposition HNO 3 & N 2 O 5 are NO x reservoirs Very stable & have a long lifetime daytime nighttime

21 N 2 O: another source of stratospheric NO x Because the N 2 O lifetime is very long, it may be transported to the stratosphere, where it undergoes the following: Consideration of N 2 O brings the Chapman model into much better agreement with observations Ice Core data show increase of atmospheric [N 2 O] of ~0.3% year since 18 th century

22 Some complications to stratospheric O 3 chemistry Catalytic Loss cycles are coupled to each other Aerosols


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