Lecture 15 Antarctic Ozone Hole ATOC/CHEM 5151

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Elaine Gardner
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1 Lecture 15 Antarctic Ozone Hole ATOC/CHEM

2 Ozone Hole Theories 1. Solar activity: During periods of high solar activity, energetic particles are deposited high in the atmosphere, creating NOx. Perhaps this NOx was transported down into the stratosphere, where it destroyed ozone. 2. Dynamics: When sun returns to the polar regions during springtime, slightly positive net heating occurs. This could lead to upward vertical motion, carrying low ozone mixing ratios upward into the lower stratosphere. 3. Chemistry: Many possible catalytic cycles proposed and tested with laboratory measurements and models. All involved chlorine in some form. 2

3 Theories Tested NOZE showed: Low N 2 O in the stratosphere dynamics Low column NO 2 solar Some ClO (but uncalibrated, so hard to determine how much) Enhanced OClO 3

4 Chemical Theories McElroy & Wofsy: ClO + BrO Br + Cl + O 2 Br + O 3 BrO + O 2 Cl + O 3 ClO + O 2 Problem: not much bromine Solomon et al. OH + O 3 HO 2 + O 2 Cl + O 3 ClO + O 2 HO 2 + ClO HOCl + O 2 Molina and Molina; Hayman et al. ClO + ClO + M Cl 2 O 2 + M HOCl + hν OH + Cl Problem: photolysis of HOCl too slow Cl 2 O 2 + hν 2 Cl + O 2 2[Cl + O 3 ClO + O 2 ] Problem: competition between photolysis and thermal decomposition of Cl 2 O 2 4

5 AAOE 1987 Airborne Antarctic Ozone Expedition Unprecedented cooperation and scale of field study 5

6 The Smoking Gun 100 ppt 1200 ppt 6

7 Low abundance of nitrogen oxides Signs of air descent: low N 2 O relative to midlatitudes; high HF relative to midlatitudes Apparent removal of HCl and ClONO 2 Large abundances of ClO Apparent depletion of H 2 O and total reactive nitrogen (NOy) Other Findings 7

8 Formation of Polar Stratospheric Clouds At threshold temperature (T = 196K), PSCs form. This can be as early as May/June! HNO 3 and H2O cocondense in a 1:3 ratio, called nitric acid trihydrate (or NAT). At lower temperatures (T = 188 K), water condenses by itself. 8

Polar Vortex Setup Increasing darkness leads to radiational cooling. Large temperature gradients form, leading to the formation of the polar night jet.

9 Polar Vortex Setup Increasing darkness leads to radiational cooling. Large temperature gradients form, leading to the formation of the polar night jet. Air within the jet boundaries is isolated. Cooling continues and air descends from above, bringing large abundances of O 3, Cly, HF, NOy (and low N 2 O, CH 4 ). 9

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11 Significance of PSCs Provide a reactive surface Essentially catalyze reactions that would not occur in the gas-phase. 11

12 Important PSC reactions 12

13 Effects of PSC Reactions Two primary chemical effects 1. Conversion of chlorine reservoirs (ClONO 2 and HCl) to more photolabile forms such as Cl 2 and HOCl. 2. Sequestration of nitrogen oxides as frozen HNO 3. Helps to explain the observations Low ClONO 2 and HCl Low NOy 13

14 Comparison of Rates Compare the rates of heterogeneous and gasphase loss of ClONO 2. Surface area in NAT PSC ~ 1 x 10-8 cm 2 cm -3 For ClONO 2 + HCl, γ = 0.1 The rate of a heterogeneous reaction is Rate = 0.25<v> SfcA γ [ClONO 2 ] For gas-phase reaction, k ~ 1 x cm 3 s -1 and [HCl] = 3 x 10 9 cm -3 14

15 Wintertime Heterogeneous processing converts ClONO 2 and HCl into Cl 2 (and a bit of HOCl). When the sun returns in August, Cl 2 and HOCl photolyse very quickly. There are now ~ 3 ppb of Cl atoms!!! Cl + O 3 ClO +O 2 But, need to deplete several ppm of O 3!! 15

16 Destroying ozone Only way to destroy more ozone is to have catalytic chemistry. ClO + ClO + M Cl 2 O 2 Cl 2 O 2 + hν 2 Cl + O 2 2 [Cl + O 3 ClO + O 2 ] Net: 2 O 3 3 O 2 Why can t this chemistry happen at midlatitudes? 16

17 Destroying ozone part II Dimer chemistry isn t enough BrO + ClO BrCl + O 2 BrCl + hν Br + Cl Br + O 3 BrO + O 2 Cl + O 3 ClO + O 2 Net: 2 O 3 3 O 2 Chlorine-only cycle, ~70%; BrO/ClO is rest 17

18 De-noxification If this halogen chemistry happened at midlatitudes, it would be mediated by NOx: ClO + NO 2 + M ClONO 2 + M ClO + NO Cl + NO 2 Cl + CH 4 CH 3 + HCl But, there is only ~ 100 ppt NOx compared to 3 ppb ClOx, so chlorine chemistry runs unimpeded. 18

19 Putting it all together 19

20 Recovery Once all ozone is destroyed, Cl atoms react with CH 4 to make HCl. PSCs eventually evaporate, so chlorine activation ceases. HNO 3 returns to gasphase and NOx can be formed (OH + HNO 3 ) Vortex eventually breaks down (Nov/Dec) and mid-latitude air mixes with ozonedepleted air; ClONO 2 reforms 20

21 Time Line of ozone depletion 21

22 Video Time line of ozone hole 22

23 Ozone Hole Size 23

24 Ozone Hole Depth 24

25 Variations in Ozone Hole 25

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2. Sketch a plot of R vs. z. Comment on the shape. Explain physically why R(z) has a maximum in the atmospheric column.

2. Sketch a plot of R vs. z. Comment on the shape. Explain physically why R(z) has a maximum in the atmospheric column. 190 PROBLEMS 10. 1 Shape of the ozone layer Consider a beam of solar radiation of wavelength λ propagating downward in the vertical direction with an actinic flux I at the top of the atmosphere. Assume