Lecture 14 Stratospheric Ozone Loss ATOC/CHEM 5151

Transcription

1 Lecture 14 Stratospheric Ozone Loss ATOC/CHEM

2 Setting the Stage: An Historical Perspective To begin, we need to go back to the 1970s Concern about the environment is high and people are starting to understand the human capacity to perturb the atmosphere Two major issues SSTs CFCs 2

3 Supersonic Transports and NOx NO + O 3 NO 2 + O 2 O + NO 2 NO + O 2 Net: O + O 3 2 O 2 3

4 Predicted Effects of SSTs 4

5 Fate of SST Fleet Although there were environmental concerns, planned SST fleets were ultimately killed by economic considerations too expensive to build and maintain and too little usage foreseen 5

6 CFCs and Stratospheric Ozone F.S. Rowland and post-doc M.J. Molina measure absorption cross-sections of CFCs in the lab. M. J. Molina and F. S. Rowland "Stratospheric Sink for Chlorofluoromethanes: Chlorine atomic-atalysed destruction of ozone," Nature (28 June 1974):810

7 Chlorine chemistry and Stratospheric Ozone Source: CFC destruction, rockets, Space Shuttle Classic catalytic cycle: Cl + O 3 ClO + O 2 O + ClO Cl + O 2 Net: O + O 3 2 O 2 R. S. Stolarski and R.J. Cicerone, Stratospheric Chlorine: A Possible Sink for Ozone, Can. J. Chem., 52, ,

8 Ozone loss by family HOx ClOx NOx 8

9 Public Outcry Works! CFCs banned from spray cans in US, Canada, Scandinavia Dotto & Schiff, The Ozone Wars 9

10 CFC Emissions over Time CFC-12 10

11 Still, regulation was difficult Atmospheric chemistry and associated laboratory sciences were improving rapidly over the same time period, so predictions about ozone loss from anthropogenic sources kept changing 11

12 Still, regulation was difficult NO 3 photolysis channels: O + NO 2 channel found 12

13 Still, regulation was difficult NO 3 photolysis channels: O + NO 2 channel found Discovery of ClONO 2 : reduces ClOx/Cly ratio 13

14 Still, regulation was difficult NO 3 photolysis channels: O + NO 2 channel found Discovery of ClONO 2 : reduces ClOx/Cly ratio Faster rate for HO 2 + NO: NOx shifts to NOy 14

15 1985: Major Changes World Meteorological Organization publishes Atmospheric Ozone 1985: Assessment of our Understanding of the Processes Controlling its Present Distribution and Change We know most everything about stratospheric ozone 15

16 1985: Major Changes Joseph Farman, British Antarctic Survey Publishes data from Halley Bay showing ozone decrease every October. Farman et al., Nature, 315: 207,

17 Satellites see this too eventually Stolarski et al., Nature 322: 808,

18 And, so do balloon sondes 18

19 But, loss is not where expected! 19

20 What is going on??? Recall that ozone loss is happening in early spring over the pole. It is very cold and dark Chemistry should be slow. Air should be sinking, so the ozone column should be thicker, not thinner. 20

21 Theories Farman suggested a relationship to CFCs, which seemed to be increasing in anti-correlation to ozone decrease. Realized that known catalytic cycles (O + ClO) were too slow in lower stratosphere, so proposed: Cl + O 3 ClO + O 2 ClO + NO Cl + NO 2 O + NO 2 NO + O 2 Too slow at km, where ozone loss was occuring 21

22 More 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. 22

23 More 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. 23

24 More 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. 24

25 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 25

26 Testing Theories Measurements in Antarctica National Ozone Expedition (NOZE) 1986 Confirmed column ozone loss Measured ozone profiles by balloon Measured column NO 2, OClO, N 2 O and ClO by spectroscopic methods 26

27 Testing Theories NOZE showed: Low N 2 O in the stratosphere Low column NO 2 Some ClO (but uncalibrated, so hard to determine how much) Enhanced OClO What does this mean for the theories? 27