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1 ATM 507 Lecture 9 Text reading Section 5.7 Problem Set # 2 due Sept. 30 Next Class Tuesday, Sept. 30 Today s topics Polar Stratospheric Chemistry and the Ozone Hole, Required reading: 20 Questions and Answers from the 2010 WMO report on Ozone Depletion: 1

2 Ozone hole observations Best web site: 2

3 Antarctic Ozone Minimum Values

4 October Monthly Means 4

5 5

6 Time Series of minimum ozone 6

7 7

8 Area covered by ozone hole 8

9 Daily Ozone Hole Area 9

10 2014 so far 10

11 October Monthly Averages

12 October Monthly Mean Ozone 12

13 13

14 14

15 Northern Hemisphere March Monthly Averages 15

16 Circumstances surrounding the ozone hole phenomenon 1. Sets up during polar night (i.e., hole occurs as region comes out of polar night) 2. Confined to the Polar Vortex (i.e., very cold) 3. Return of the sun in spring triggers ozone loss. 4. Break-up of the polar vortex, and mixing in of non-vortex air restores normal levels of ozone. 16

17 Why were we caught by surprise? The large observed ozone depletions were inconsistent with the O 3 chemistry as it had been formulated until then (i.e., with the O 3 chemistry we have learned until now). The ozone loss rate was very large and dramatic on the order of 1+% per day for about six weeks Satellite measurements confirmed the phenomenon (as we have seen). An extensive ground-based observation campaign was stage in 1986 from McMurdo station in Antarctica. These measurements ruled out some of the proposed explanations. Three were left standing. 17

18 Ozone Hole Theories Chemical Theory I (requires low NO X ) ClO dimer mechanism (Molina & Molina, 1987) ClO + ClO + M ClOOCl + M ClOOCl + hv Cl + ClOO ClOO + M Cl + O 2 + M 2[ Cl + O 3 ClO + O 2 ] NET: 2O 3 + hv 3O 2 Chemical Theory II ( also requires low NO X ) ClO-BrO mechanism (McElroy et al., 1986) Cl + O 3 ClO + O 2 Br + O 3 BrO + O 2 BrO + ClO Br + Cl + O 2 * NET: 2O 3 3O 2 * There are three channels for the BrO + ClO reaction. This is the one that is most efficient at destroying ozone. 18

19 Ozone Hole Theories (cont.) Dynamical Theories In these theories, rising motions import air (from the troposphere) with low mixing ratios of ozone into the lower and middle stratosphere. (This could possibly be accompanied by a diminished supply of ozone transported from the equatorial regions.) Since the loss rates were so much higher than anything conceived of before, many leading atmospheric chemists believe the dynamical theories were more likely to be correct. 19

20 What did the first observations show? Balloon sondes in 1986 and 1987 showed that the depletion was not uniform, nor was it from the bottom up. ( Not dynamics!) Ozone was being destroyed most dramatically in the km range, with little change above and below. At some altitudes in this range, there were periods when nearly all of the ozone was chewed up. Microwave ground-based measurements observed very large levels of ClO in the Antarctic stratosphere. 20

21 Balloon Sondes Hoffmann et al., 1987 Harder et al.,

22 Ozone Depletion over South Pole Balloon borne ozone profiles measured at South Pole: blue is the average of several profiles measured in September and October during before the Antarctic ozone hole; red is on the day of the maximum ozone loss in 2001; green is the lowest total ozone recorded in 1986, the first year of CMDL's sounding program at the South Pole. Total column ozone is given in Dobson Units (DU) for each of the profiles. 22

23 The animation shows the development of stratospheric temperatures and the Antarctic ozone hole at the South Pole in 2001, starting in January and continuing to the end of the year. The measurements were made with balloon-borne ozone instruments launched at regular intervals at the Amundsen-Scott South Pole Station. The animation clearly will show ozone depletion in the September to October period, following sunrise, when ozone in the 6-14 mile altitude region is destroyed by chemical reactions involving chlorine and bromine from humanproduced chlorofluorocarbons (CFCs). The thermometer gives a measure of the total column of ozone, which is related to the area of the colored portion of the ozone profile, nearly twothirds of which will be lost. 23 Source: NOAA

24 Airplane missions late 1980 s ER-2 range up to ~ 20 km. Real-time measurements of O 3, NO, NO 2, NO y, ClO, BrO, N 2 O, H 2 O, CN (particles), and more. DC-8 Flew at lower altitudes but included some additional instruments like FT interferometer, LIDAR, and whole air sampler. ER-2 measurements provided the smoking gun. 24

25 Smoking Gun Measurements Outside vortex Inside vortex Outside vortex Inside vortex 25

26 What the measurements show Huge increases of active chlorine inside the vortex ( times previously observed mid-latitude values at these altitudes). In late August, ozone inside and outside the vortex are essentially the same. Boundary of the vortex moves around some, and can get a little ragged. After 3 weeks of reaction time, ozone is down by a factor of two (or more) inside the vortex only. At the same time active chlorine (ClO) is up another 50% inside the vortex. Note the very strong anti-correlation between O 3 and ClO near the edge of the vortex on the second flight. 26

27 Why so different than the mid-latitudes? This phenomenon was a huge surprise to the scientific community. As it turns out, this was the first example of significant perturbation of atmospheric chemistry due to heterogeneous chemistry, in this case the chemistry occurring on/in ice clouds. For our purposes: Homogeneous chemistry: gas phase reactions only; Heterogeneous chemistry: gas/solid, gas/liquid, and/or gas/liquid/solid phase reactions all considered. Current understanding of polar ozone depletion centers around Polar Stratospheric Clouds or PSCs. Since there is very little H 2 O in the stratosphere (a few ppmv), very low temperatures are needed to form PSCs. 27

28 Recap Reservoir species and holding cycle reactions tend to keep the stratospheric ozone in balance. There is potential for large ozone destruction if the balance is disrupted. The disruption can occur if there are large unchecked emissions of the source gas in one chemical family (i.e., CFCs); or large removal of the radicals in one chemical family (i.e., NO x removal). Antarctic Ozone Hole appears around Polar night, polar votex, cold temperatures Spring time only (return of sun triggers loss) Occurs in km altitude range New chemistry required to explain What is the link with Polar Stratospheric Clouds? 28

29 PSC Formation In the Antarctic polar night the temperatures do routinely go below 190 K (-83 C) - and stay there. This is cold enough to force condensation. Step 1. As temperatures drop below K, water condenses on the existing background aerosol particles, which are predominantly concentrated H 2 SO 4 droplets (~75% H 2 SO 4, ~25% H 2 O). As temperatures drop further, these particles take up additional water, become larger and more dilute. (Recall, mr of water in the stratosphere is ~ 3-5 ppmv, so VERY cold temperatures needed to condense the vapor.) 29

30 Simple Picture of PSC Formation 30

31 PSC Formation (cont.) Step 2. As the temperature approaches 195 K, small crystals of HNO 3 3(H 2 O) {NAT} form. These are called Type I PSCs. (size ~ 1 µm diameter) Step 3. As the temperature decreases below 190 K, crystals get larger (and more dilute), with diameters of 10 µm and larger. These Type II PSCs can settle out of the stratosphere (due to gravity), removing NO x and H 2 O from the surrounding air. The Denitrification and Dehydration of the remaining air provide the strong perturbation required to disrupt the standard chemistry, i.e., the ozone balance. 31

32 Simple Picture of PSC Formation 32

33 33

34 Temperature and PSC Formation Temperature (white bars) and PSC observations (black bars) For the given temperature bin, ratio of number of PSC observations to number of temperature observations (i.e., for low enough temperatures, PSCs are common.) 34

35 Polar Stratospheric Cloud, seen from the NASA DC-8 on 4 February 2003 (photograph by Mark Schoeberl, GSFC). This cloud was formed due to turbulent air motion over Iceland (mountain waves). Lidar measurements revealed the primary cloud was composed of ice (Type 2 PSC). The photograph was taken just at sunrise for the lower atmosphere. Under these conditions, the polar stratospheric cloud is illuminated directly by the sun while the lower atmosphere is still shaded from the 35 direct sunlight, accounting for the brilliant appearance of this cloud.

36 Polar Stratospheric Cloud, seen from the NASA DC-8 on 14 January 2003 (photograph by Paul Newman, GSFC). These are naturally occurring clouds composed of small nitric acid and water particles. The clouds have a colorful appearance because they contain similar sized, small particles each of which refracts sunlight in a similar manner. This image is notable for illustrating the high altitude (approximately 70,000 ft) of the polar stratospheric clouds, well above the cirrus cloud deck that lies below the DC-8 airplane. Reactions that 36 occur on the surface of these clouds convert chlorine, most of which is due to human sources, from relatively inert forms to compounds that lead to rapid ozone destruction.

37 PSC Processing of Air 1. Most of the HNO 3 (the reservoir for NO x radicals) is tied up in PSC ice crystals and removed from the air mass. 2. HCl (one of the reservoirs for ClO X ) is also taken up by the ice crystals, but unlike HNO 3, it undergoes a chemical reaction before the crystals settle out: I. ClNO 3 + HCl(s) Cl 2 + HNO 3 (s) Note that ClNO 3 and HCl contain most of the Cl that has been liberated from the chlorine sources. 37

38 Unperturbed partitioning of chlorine species between sources, reservoirs, and active forms Note: ClONO 2 and ClNO 3 are the same thing. 38

39 Textbook Figure 5.14 Another picture of the unperturbed distribution of chlorine species in the stratosphere. Note the dominance of HCl and the importance of ClONO 2 (once the Cl is liberated from the CCl y source gases). 39

40 Chlorine liberation and Ozone destruction I. ClNO 3 + HCl(s) Cl 2 + HNO 3 (s) The ClNO 3 + HCl can proceed as long as the PSCs persist (days weeks). When sunlight returns to the region II. Cl 2 + hv Cl + Cl (very fast) III. 2[Cl + O 3 ClO + O 2 ] IV. 2[ClO + NO 2 + M ClNO 3 + M ] NET: HCl + 2 O NO 2 ClNO 3 + 2O 2 + HNO 3 (s) These reactions (I IV) continue until either: HCl(s) is gone (used up) pre-1979 NO 2 is all converted (via ClNO 3 ) to HNO 3, at which point the ClO builds up and the Cl species are dramatically repartitioned. 40

41 Polar Ozone Chemistry (cont.) KEY IDEA: Inactive forms of Cl (HCl, ClNO 3 ) are transformed into active forms of Cl (Cl 2, then Cl and ClO) through PSC processing. In the simple reaction scheme shown above, if complete processing is assumed (i.e., reaction I fast), then large amounts of active chlorine (as ClO) are built up if: [HCl] i > ½ [NO X ] I (initial NO y ~ 8 ppbv) Summarizing the necessary conditions: Denitrification required (to lower [NO x ]) [HCl] must be above some minimum level 41

42 Other heterogeneous reactions ClNO 3 + H 2 O(s) HOCl + HNO 3 (s) HOCl + HCl(s) Cl 2 + H 2 O(s) Ties up NO x Liberates active Cl Liberates active Cl N 2 O 5 + HCl(s) ClNO 2 + HNO 3 (s) N 2 O 5 + H 2 O(s) 2HNO 3 (s) Ties up NO x Liberates active Cl Ties up NO x These reactions also pave the way for chemical loss of O 3. 42

43 DEHYDRATION DENITRIFICATION OZONE LOSS LIBERATE ACTIVE CHLORINE 43

44 Chemical Loss of O 3 (review) ClO dimer mechanism ClO + ClO + M ClOOCl + M ClOOCl + hv Cl + ClOO ClOO + M Cl + O 2 + M 2[ Cl + O 3 ClO + O 2 ] NET: 2O 3 + hv 3O 2 ClO-BrO mechanism Cl + O 3 ClO + O 2 This dimer mechanism in particular is of much greater importance in this polar chemistry scheme because: 1. Cold temperatures are more favorable to association rxns; 2. There are vastly elevated ClO concentrations due to PSC activation Br + O 3 BrO + O 2 BrO + ClO Br + Cl + O 2 * NET: 2O 3 3O 2 44

45 Contributions of ClO dimer and ClO-BrO cycles 45

46 Chlorine ozone hole chemistry (excluding Bromine) 46

47 Observed Ozone loss in the northern polar region 47

48 February 1989 ER-2 measurement campaign. Flight track from Norway to Greenland and back. Highly elevated ClO in PSC processed air concentration levels comparable to Antarctica. (Shorter lifetime and coverage of PSCs explains much lower total overall ozone loss.) 48

49 Recap 1. PSC formation esp. Type II with large particles that remove gaseous HNO Reactions on PSCs that remove NO x and liberate active ClO X. 3. Cl 2, HOCl, and ClNO 2 photolyze rapidly when the sun returns. 4. NO X buffers the release of active ClO X by forming ClNO 3, which in turn reacts with PSCs to form active ClO x and HNO When air is denitrified, active ClO x builds up and large scale O 3 depletion occurs via ClO + ClO, ClO + BrO, etc. 49