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Environmental Chemistry

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1 Environmental Chemistry
Chapter 2: The Ozone Holes Part 1 - Pollution of the Stratosphere Part 2 - The Ozone Hole Copyright © 2012 by DBS

2 The ozone hole provides a classic case of the workings of science at its best: the unexpected discovery of an important effect, proposals of theories to explain it, quick mounting of a logistically difficult experimental field program to test the theories, and a blending of laboratory data, field observations, and computer models to achieve understanding, in this case within only two years Graedel and Crutzen, 1993

3 2.1 Dobson Units for Overhead Ozone
1 DU is the number of molecules of O3 required to create a layer of O mm (0.001 cm) thick at 0 °C and 1 atm Over the Earth’s surface, the O3 layer’s average thickness is about 300 DU (3 mm layer) if brought to sea level O3 “Hole” [O3] ~100 DU

4 Determination of O3 Concentration
What is the total mass of ozone corresponding to 350 DU? V(O3) = 4/3π [(r+d)3 -r3] [(r+d)3 - r3] = [r3 + d3 + 3rd2 + 3r2d - r3] ~ 3r2d …since r>>d V(O3) = 4/3 π [3r2d] = 4 π r2d = 4 x 3.14 x (6.4 x 106)2 x 3.5 x 10-3 = 1.8 x 1012 m3 or 1.8 x 1015 L 350 DU = 3.5 mm or 3.5x10-3 m r = 6400 km or 6.4x106 m n = PV/RT = 1.0 atm x 1.8 x 1015 L / L atm mol-1K-1 x 273 K = 8 x 1013 moles = 8 x 1013 mols x 48 g mol-1 = 4 x 1015 g

5 1 DU at 1 atm… d = thickness of O3 (1 DU=10-5 m), r = 6.4 x 106 m
From Ideal gas law, PV = nRT = NkBT N = number of molecules kB=R/NA Boltzmann’s const, kB=1.381 x JK-1 NO3 = [(Pstp x V) /(kB.Tstp)] = [(Pstp x 4 πr 2 d) /(kB.Tstp)] Dividing both sides by 4 πr 2 NO3/4πr 2 = [(Pstp x d)/(kB.Tstp)] = 2.69 x 1020 molecules m-2 From before: V = 4 πr2d If all ozone in the atmosphere were spread out around the earth in a homogeneous spherical shell at standard temperature, Tstp ( K) and standard pressure, Pstp ( Pa), one DU is equivalent to a horizontal density (number of ozone molecules per unit area) of 2.69 x1020 molecules per square meter

6 2.2 History of the Annual Hole Above Antarctica
O3 recorded since 1957 by Farman at Halley Bay (British Antarctic Station) Area of USA = 10 x 106 km2 Sept-Oct period is Antarctic Spring

7 The Big Surprise of 1985 Isolated local concern or global problem?
Farman et al. revealed a dramatic and unpredicted decline in stratospheric O3 in a surprising location Antarctica Shocked the world Showed dramatic decline in springtime O3 starting in 1970’s 30% by 1985 70% by 2000 Farman, J. C.;Gardiner, B. G.;Shanklin, J. D., Large Losses of Total Ozone in Antarctica Reveal Seasonal Clox/Nox Interaction. Nature, 1985, 315, Isolated local concern or global problem? Chemical explanation? Physical explanation? Min O3 at Antarctic in Spring (Sep-Nov)

8 TOMS Indirect measurements
Measures O3 by mapping UV light emitted by the Sun to that scattered from the Earth's atmosphere back to the satellite O3 is inferred from Earth’s albedo Shows strong spatial variability Low around equator, high in mid-latitude (why) Very low at Antarctic (especially in September/October)

9 What is the Ozone Hole? Occurs at the beginning of Southern Hemisphere spring (August-October) The average concentration of O3 in the atmosphere is about 300 Dobson Units Not a “hole” but a region of depleted O3 over the Antarctic Any area where O3 < 220 DU is part of the O3 hole Image from: Ozone is ‘thinning’ out

10 2.3 Ozone in Temperate Areas
Ozone depletion seen world-wide Losses during 80’s and 90’s were greater at higher latitudes (close to poles) Trend was reversed

11 2.4 The Activation of Catalytically Inactive Chlorine
The ozone hole occurs due to special polar winter weather conditions in the lower stratosphere, where ozone concentrations usually are highest, that temporarily convert all the chlorine that is stored in the catalytically inactive forms HCl and ClONO2, into active forms •Cl and •ClO

12 2.4 The Activation of Catalytically Inactive Chlorine
Conversion of inactive Cl to active •Cl forms on particles formed by a solution of water, sulfuric acid and nitric acid Most parts of the world stratosphere is cloudless Temperature in lower stratosphere over South Pole drops to -80 ºC in Antarctic winter, results in ice crystal formation Total darkness prevents Chapman mechanism Also pressure drop (PV=nRT) in combination with Coriolis force produces an insolated vortex with speeds in excess of 180 mph

13 2.4 The Activation of Catalytically Inactive Chlorine
Particles produced by condensation of gases within the vortex form Polar Stratospheric Clouds (PSCs) Chemical reactions that lead to O3 loss occur in an aqueous layer at the surface of PSCs Exposure of sunlight in the early Antarctic spring (our Fall) initiates destruction of O3

14 Activation of Cl On Ice Particles (Polar Stratospheric Clouds)
Cl resides in stable "reservoir" compounds, HCl and chlorine nitrate (ClONO2) ClONO2(g) + H2O(aq) → HOCl(aq) + HNO3(aq) HCl(aq) → H+(aq) + Cl-(aq) Reaction of the Cl- with HOCl produces molecular Cl2 gas Cl-(aq) + HOCl → Cl2(g) + OH- Net: HCl + ClONO2 → Cl2 + HNO3 Cl2 + hν → 2 •Cl

15 Activation of Cl On Ice Particles (Polar Stratospheric Clouds)
Massive destruction of ozone by atomic chlorine then ensues by catalytic reactions Any •Cl converted to HCl by reaction with CH4 is reconverted by PSCs and sunlight to •Cl Inactivation of •ClO by conversion to ClONO2 does not occur since all NO2 is bound as HNO3 in the PSCs Only when PSCs and vortex have vanished does Cl return to inactive forms Air containing NO2 mixes with vortex in spring to form catalytically inactive ClONO2 Ozone levels return to normal

16 2.5 Reactions that Create the Ozone Hole
Lower stratosphere – where PSCs form and •Cl is activated, [O] is small due to low amount of UV-C O3 destruction based on O3 + O pathway not important here (Mech. I) Most ozone loss in the ozone hole is via Mech. II

17 2.5 Reactions that Create the Ozone Hole
Mechanism II With both X and X’ being atomic •Cl

18 2.5 Reactions that Create the Ozone Hole
Mechanism II: Step 1: Cl• + O3 → ClO• + O2 Confirmation that O3 loss occurs by this reaction is shown below Zurer, 1988 (Baird graph) Anderson, J.G., Toohey, D.W., and Brune, W.H. (1991) Free Radicals within the Antarctic Vortex: The Role of CFC’s in Antarctic Ozone Loss. Science, Vol. 251, pp Anticorrelation of • ClO with O3

19 2.5 Reactions that Create the Ozone Hole
Mechanism II: Step 2a: 2ClO• → Cl-O-O-Cl Dichloroperoxide formation rate is high due to inc. •Cl Step 2b: ClOOCl + hv → ClOO + •Cl Step 2c: ClOO → O2 + •Cl Net: 2ClO• → [ClOOCl] → 2Cl• + O2 Conversion of 2 chlorine reservoir species to chlorine radical

20 2.5 Reactions that Create the Ozone Hole
Mechanism II: Adding step 2 to 2 x step 1 we obtain: Thus a complete catalytic ozone destruction cycle exists in the lower stratosphere under these special (cold/vortex) conditions 2 x step 1 step 2

21 The New Catalyic Cycle Reactions Responsible for the Hole
Step 1: •Cl + O3 → ClO• + O2 Step 2: 2ClO• → Cl-O-O-Cl Step 2b: Cl-O-O-Cl → •Cl + ClOO Step 2c: ClOO → •Cl + O2 Step 2 net: 2ClO• → ClOOCl + hν → 2 •Cl + O2 Step 1 and 2 represent Mech II Occurs when [O] (needed for Mech I) is low controls season Net: 2O3 → 3O2 The key behind discovery of this catalytic cycle was the laboratory observation that photolysis of the ClO dimer (ClOOCl) takes place at the O-Cl bond rather than at the weaker O-O bond. It was previously expected that photolysis would take place at the O-O bond, regenerating ClO and leading to a null cycle. One molecule of chlorine can degrade over 100,000 molecules of ozone before it is removed from the stratosphere or becomes part of an inactive compound These inactive compounds, for example ClONO2, are collectively called 'reservoirs'. They hold chlorine in an inactive form but can release an active chlorine when struck by sunlight Nearly 75% of the ozone depletion in the antartica occurs by this mechanism (Cl. As a catalyst)

22 Why are ClO Concentrations So High?
During Polar winter Special vortex conditions + Low temperature Denitrification of ClONO2 @ice crystal Cl2 + HNO3 sunlight Stratospheric ‘containment vessel’ over S. pole •Cl

23 2.5 Reactions that Create the Ozone Hole
~ 75 % of ozone destruction in the hole occurs by mechanism II with Cl as the only catalyst Slow step is 2a combination of 2 ClO molecules Rate = k[ClO]2 Double ClO concentration, rate x4

24 2.5 Reactions that Create the Ozone Hole
Ozone loss above Antarctica ~ 2 % per day By early October almost all ozone is lost 15 – 20 km

25 2.5 Reactions that Create the Ozone Hole
Seasonal evolution and decline of Antarctic ozone hole

26 2.6 The Size of the Antarctic Ozone Hole
Measured according to: Surface area of low ozone Minimum overhead ozone (see 2002) Length of time O3 depletion occurs Vertical region over which O3 depletion occurs

27 2.6 The Size of the Antarctic Ozone Hole

28 2.7 Stratospheric Ozone Destruction of the Arctic
Did not start to form until mid 1990s Less severe than Antarctica due to higher temperatures and meteorology

29 Summary Turco, 2002

30 2.8 Increases in UV at Ground Level
Increases in UV-B have been measured in spring at mid-latitude regions 6-14 % increase

31 2.9 CFC Decomposition Increases Stratospheric Chlorine
Increase in stratospheric chlorine primarily due to use and release of chlorofluorocarbons (CFCs) Nontoxic, nonflammable, nonreactive (at Earth’s surface!), and have useful condensation properties (used as coolants) CFCs have no tropospheric sink, so all molecules eventually reach the stratosphere CFCs are heavier than air, why do they rise? Photochemically decomposed by UV-C Atmospheric lifetimes are long

32 2.10 Other Chlorine-Containing Ozone-Depleting Substances
Carbon tetrachloride (CCl4) No tropospheric sink Ozone-Depleting Substance (ODS) Used as solvent and in manufacture of CFCs Long atmospheric lifetime (26 yrs) Methyl Chloroform (CH3CCl3) Used in metal cleaning Approx. half removed by reaction with OH Atmospheric Lifetime (5 yrs)

33 2.12 CFC Replacements CFCs and CCl4 have no tropospheric sinks (not soluble in water/rain), not decomposed by UV-A or visible light HCFCs contain H atoms bonded to C atom. Removed in the troposphere by hydroxyl radicals (H-abstraction) CHF2Cl (HCFC-22) the current replacement for refrigerator coolants Long term ozone destroying potential is small Reliance on HCFCs would lead to build up of Cl Products free of Cl are ultimate replacement

34 2.12 CFC Replacements Hydrofluorocarbons, HFCs, are the compound of choice in USA e.g. CH2F-CF3 (HFC-134a), CH2F/CHF2CF mix No chlorine atoms! Rest of world uses cyclopentane or isobutane

35 2.13 Halons Halons, used in fire extinguishers e.g. CF3Br, CF2BrCl (hydrogen free) No tropospheric sinks Photochemically decomposed to Cl, Br, F atoms Bromine is significant ozone problem

36 2.14 Can Stratospheric Fluorine Destroy Ozone?
F and HF formed by decomposition of CF, HCFCs, HFCs, and halons Reaction with methane and other H-containing gases is rapid and produces stable HF Why no F cycle? OH + HF endothermic Atomic F is ‘deactivated’ before it can destroy ozone

37 2.15 International Agreements that Restrict ODSs
‘Precautionary Principle’ Use of CFCs in most aerosols banned in 1970s in USA Montreal Protocol (1987) signed by most countries to phase out CFCs Based on Rowland and Molina’s work

38 2.15 International Agreements that Restrict ODSs
CFC production in developed countries ended in 1995 Developing countries had until 2010 CFC-12 has longer atmospheric lifetime than CFC-11 CCl4 slight decline due to lack of sinks CFC-12 > lifetime than CFC-11, no sinks for CCl4 or CFC-113 Use of HCFCs on the rise, temporary substitute for CFCs

39 2.15 International Agreements that Restrict ODSs
Observations in 2000 indicated chlorine content of the stratosphere has peaked Slowness in the decline of stratospheric chlorine due to: Long travel time to rise to middle stratosphere Slowness of removal Continued inputs Recent projections predict the Antarctic hole area will decrease around 2023, fully recover by 2070 Without International agreements to protect the atmosphere predictions indicate large increase in skin cancers around the world

40 Antarctic Hole Size and Minimum O3
Size: This image shows the growth of the average area of the ozone hole from 1979 to 2004 using data from Version 8 of the TOMS algorithm. We define the ozone hole as the area for which ozone is less than 220 Dobson Units, a value rarely seen under normal conditions. It shows that the ozone this low hardly occured at all in 1980, but by year 2000 covered an area larger than North America, 26.5 million square kilometers. Minima : This image shows the lowest value of ozone measured by TOMS each year in the ozone hole. Global average ozone is about 300 Dobson Units. Before 1980 ozone less than 200 Dobson Units was rarely seen. In recent years ozone near 100 Dobson Units has become normal in the ozone hole. Ozone in the year 2002 ozone hole was higher than we have come to expect because of unusually high temperatures in the Antarctic stratosphere. NASA FACTS Cf. Fig. 1-2, 1-3

41 Chapter 2 Homework P2-1: A minor route for ozone destruction involves Mechanism II with bromine as X’ and chlorine as X (or vice-versa). The ClO and BrO free radical molecules produced in these processes then collide with each other and rearrange their atoms eventually yield O2 and atomic chlorine and bromine. Write out the mechanism for this process, and add up the steps to determine the overall reaction. Box 2-1 problem 1: Deduce the overall reaction equation for the reaction sequence given in Box 2-1. P2-6: The free radical CF3O is produced during the decomposition of HFC-134a. Show the sequence of reactions by which it could destroy ozone acting as an X catalyst in a manner reminiscent of OH. P49 Activity: Using the information to be found at and other websites, compare the history of the most recent Antarctic hole to the time evolution of the 2010 hole in Fig Did the maximum depletion, maximum area, and minimum temperate exceed 2010 values and did they occur at about the same time as they did in 2010? Photocopy or download Figure 2-1 and manually add data for more recent years to the two bar graphs. Are there signs yet from your data that the hole is becoming smaller in area or depletion is lessening? (4 points)

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