Stratospheric Chemistry: Polar Ozone Depletion AOSC 433/633 & CHEM 433/633. Ross Salawitch

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Stratospheric Chemistry: Polar Ozone Depletion AOSC 433/633 & CHEM 433/633 Ross Salawitch Class Web Site: http://www.atmos.umd.

1 Stratospheric Chemistry: Polar Ozone Depletion AOSC 433/633 & CHEM 433/633 Ross Salawitch Class Web Site: Today: Processes that govern the formation of the Antarctic ozone hole Compare & contrast Antarctic and Arctic ozone depletion Arctic Ozone 2011 Lecture 16 2 April Polar Ozone Depletion Figure 2 8, WMO/UNEP (2011) 2

Southern Hemisphere 3 Polar Ozone Depletion First view from space: Stolarski et al., Nature, 322, 808, 1986.

2 Discovery of the ozone hole: Halley Bay (76 S) October Mean Polar Ozone Depletion HALLEY BAY Farman et al., Large losses of total ozone in Antarctica reveal seasonal ClO x /NO x interaction, Nature, 315, 207, Southern Hemisphere 3 Polar Ozone Depletion First view from space: Stolarski et al., Nature, 322, 808, The paper showed data for Octobers of 1979 through 1985 in black & white contour diagrams. This image, produced soon after, showed color plots of total column ozone during Antarctic spring, including measurements for year

Polar Vortex Circulation During winter: radiative cooling leads to cold air in polar stratosphere large scale low pressure region develops over pole strong polar night jet develops, isolating air at

3 Polar Vortex Circulation During winter: radiative cooling leads to cold air in polar stratosphere large scale low pressure region develops over pole strong polar night jet develops, isolating air at high latitudes from air at low latitudes T continues to fall in the vortex like circulation near the pole 5 Polar Ozone Depletion Theories Soon after the discovery of the ozone hole three theories emerged to explain the rapid springtime loss of ozone over Antarctica: 1. Chemistry due to enhanced levels of ClO, driven by heterogeneous reactions on the surface of polar stratospheric clouds (PSCs) [McElroy et al., Nature, 1986; Solomon et al., Nature, 1986] a) two new catalytic cycles, both involving halogen radicals and requiring ~1 ppb of ClO to be effective (ClO + ClO + M ClOOCl +M; BrO + ClO Br + Cl + O 2 ) [Molina and Molina, JPC, 1987; McElroy et al., Nature, 1986] b) suggestion that PSC particles might be composed of HNO 3 and upon sedimentation could appreciably lower NO x (which would prevent conversion of ClO to ClNO 3 ) [Toon et al., GRL, 1986] c) decreasing ozone column driven by rising ClO, due to buildup of chlorine from CFCs 2. Chemistry due to enhanced levels of NOx, driven by variations in solar UV [Callis and Natarajan, Nature, 1986] 3. Loss by transport due to upwelling of ozone poor air from the troposphere [Tung et al., Nature, 1986] 6

Polar Stratospheric Clouds Studies prior to

, Carl Stormer, Remarkable clouds at high

of polar stratospheric clouds in SH compared

, Polar Stratospheric Cloud Sightings by SAM

4 Polar Stratospheric Clouds Studies prior to the discovery of the ozone hole documented : high altitude (~20 km) mother of pearl clouds over Norway [e.g., Carl Stormer, Remarkable clouds at high altitudes, Nature, 1929] greater prevalence of polar stratospheric clouds in SH compared to NH [e.g., McCormick et al., Polar Stratospheric Cloud Sightings by SAM II, JAS, 1982]. 7 National Ozone Expedition: McMurdo Station, 1986 Photo Geoff Toon 8

5 National Ozone Expedition: McMurdo Station, 1986 Balloon-borne ozonesondes showed: Region of nearly complete removal of ozone between ~12 and 20 km: ALTITUDE (km) OCT 7, DU OCTOBER AVERAGE DU Hofmann et al., Nature, 326, 59, OZONE ABUNDANCE (PARTIAL PRESSURE, mpa) 9 National Ozone Expedition: McMurdo Station, 1986 Ground based measurements revealed: Presence of ~1 ppb of ClO over Antarctica Decreasing column HNO 3 throughout springtime and suppressed column HCl and ClNO 3, consistent with existence of large amounts of ClO [Farmer et al., Nature, 1987] Left: ClO profiles retrieved over McMurdo Station based on ground based microwave spectra acquired 1-22 Sept 1986, for initial mixing ratio guesses of 0.1, 0.5, 1.0, and 2.0 ppb. Because pressure broadening > spectral bandwidth below ~15 km, the initial guess is unaltered by the retrieval algorithm below ~15 km. Right: Time series of ClO over McMurdo, assuming constant ClO mixing ratio vs altitude between km (circles), km (crosses), or km (diamonds). Thin lines connected by dots are stratospheric temperature at 18 km. From DeZafra et al., Nature, 1987 and P. Solomon et al., Nature,

6 Chlorine Abundance, Mid-Latitude Stratosphere Northern Hemisphere Mid-Latitudes November 1994 ClO HCl ClNO 3 Cl TOT Books 27.2 Balance 22.3 CCl y APPROX. ALTITUDE (km) ABUNDANCE (parts per billion) Note: Below ~30 km, ClO << ClNO 3 and HCl Zander et al., GRL, 1996 Lecture 15, Slide Heterogeneous Chemistry, Mid-Latitude vs Polar Regions a) What type of aerosol particles are present in the mid-latitude stratosphere? b) What chemical reaction occurs on the aerosol particles present in the mid-latitude stratosphere? ( heterogeneous reaction since it involves a reaction of gases on a particle). c) What is the effect on ClO of the heterogeneous reaction from part b)? d) What type of particles are present in the polar stratosphere during winter? e) What is the effect of these particles on the chemical composition of the polar stratosphere (chemical reactions occurring on the surface of these particles convert species such as and to more reactive species such as and )? f) Why do the heterogeneous reactions of part e) occur only at high-latitudes during winter (you may want to draw upon material presented in Lecture 11 as well as a reading from Lecture 2)? 12

Heterogeneous Chemistry, Mid-Latitude vs Polar Regions In all cases, γ must be measured in the laboratory Reaction probabilities given for various surface types, with formulations of various degrees

7 Heterogeneous Chemistry, Mid-Latitude vs Polar Regions In all cases, γ must be measured in the laboratory Reaction probabilities given for various surface types, with formulations of various degrees of complexity, in Section 5 of the JPL Data Evaluation. Atmospheric Chemistry and Physics by Seinfeld and Pandis provides extensive treatment of aqueous phase chemistry, properties of atmospheric aerosol, organic aerosols, etc. Lecture 11, Slide POLAR OZONE LOSS COLD TEMPERATURES POLAR STRATOSPHERIC CLOUDS (PSCs) REACTIONS ON PSC SURFACES LEAD TO ELEVATED ClO HCl + ClNO 3 Cl 2 (gas) + HNO 3 (solid) ClNO 3 + H 2 O HOCl + HNO 3 Cl 2 + SUNLIGHT + O 3 ClO HOCl + SUNLIGHT + O 3 ClO HNO 3 SEDIMENTS (PSCs fall due to gravity) ELEVATED ClO + SUNLIGHT DESTROYS O 3 BrO : REACTION PARTNER FOR ClO ADDITIONAL O 3 LOSS 14

15 Airborne Antarctic Ozone Expedition: Punta Arenas, Chile,1987 23 August 1987 16

5 RETURN OF SUNLIGHT Ozone ClO 0 62 64 66 68 70 72 3.0 2.0 1.0 0

8 15 Airborne Antarctic Ozone Expedition: Punta Arenas, Chile, August September 1987 ClO (parts per billion) RETURN OF SUNLIGHT Ozone ClO FULLY SUNLIT Ozone ClO OZONE (parts per million) LATITUDE (degrees south) LATITUDE (degrees south) Anderson et al., Science,

9 Polar Ozone Loss Cycles Cycle (1a): ClO + ClO + M ClOOCl + M Cl + O 3 ClO + O 2 Cl + O 3 ClO + O 2 ClOOCl + hν ClOO + Cl ClOO + heat Cl + O 2 Net: O 3 + O 3 3 O 2 Cycle (1b): ClO + ClO + M ClOOCl + M ClOOCl + heat ClO + ClO Net: M + heat M Cycle (2a): BrO + ClO BrCl +O 2 Br + O 3 BrO + O 2 Cl + O 3 ClO + O 2 BrCl + hν Br + Cl Net: O 3 + O 3 3 O 2 Cycle (2b): BrO + ClO ClOO + Br Br + O 3 BrO + O 2 Cl + O 3 ClO + O 2 ClOO + heat Cl + O 2 Net: O 3 + O 3 3 O 2 Cycle (2c): BrO + ClO OClO + Br OClO + hν O + ClO Br + O 3 BrO + O 2 Net: O 3 O + O 2 Cycle (1) accounts for ~60% of polar ozone loss; Cycle (2) accounts for nearly all of the rest Rate constants and products for these reactions worked out by many scientists: Molina and Molina, JPC, 1987 Sander, Friedl, and Yung, Science, 1989 Moore, Okumura et al., Phys. Chem. A,1999 Bloss, Nickolaisen, Sander et al., JPC, Thermal Decomposition 30.5 kcal/mole kcal/mole ClOOCl + M ClO + ClO + M H = 18.1 kcal/mole Lecture 11, Slide 21 k k THERMAL FORMATION e GREACTANTS GPRODUCTS = = Κ ( )/RT EQUILIBRIUM JPL Data Evaluation gives values of K EQUILBRIUM and k FORMATION In equilibrium: K EQ = e (8744/T) cm -3 k THERMAL [ClOOCl] = k FORMATION [ClO] [ClO] where k THERMAL = k FORMATION K EQ Energetically, system favors ClOOCl Entropically, system favors ClO & ClO at low T, ClOOCl stable: energy wins! at high T, ClOOCl unstable: entropy rules! Equilibrium constants given in Section 3 of the JPL Data Evaluation. 18

10 Polar Halogens, Seasonal Evolution From 19 Update After Farman et al., Large losses of total ozone in Antarctica reveal Seasonal ClOx/NOx interaction, Nature, 315, 207, Rising chlorine due to industrial activity Models provide good overall simulation of Antarctic O 3 loss Scientific understanding of polar O 3 depletion led to international ban on CFCs Lecture 2, Slide 17 20

35 OZONE PROFILES, SOUTH POLE: UPDATE Ozone Hole Update, II 30 ALTITUDE (km) 25 20 15 10 5 SEP 29, 1999 90 DU OCTOBER AVERAGE 1967-1971 282 DU 0 0 5 10 15 OZONE ABUNDANCE (PARTIAL PRESSURE, mpa) D.

11 35 OZONE PROFILES, SOUTH POLE: UPDATE Ozone Hole Update, II 30 ALTITUDE (km) SEP 29, DU OCTOBER AVERAGE DU OZONE ABUNDANCE (PARTIAL PRESSURE, mpa) D. Hofmann, NOAA CMDL 21 Arctic Overview Arctic vortex (polar stratosphere): Always warmer than typical Antarctic winter Tremendous year to year variability in temperature Chemical ozone loss occurs only during cold winters Enough HNO 3 usually remains so that ClO recovers to ClNO 3 : faster ClO de-activation (less ozone depletion) compared to Antarctic All of this is due to hemispheric differences in atmospheric dynamics: More vigorous circulation in NH due to much more land-sea contrast, which triggers poleward transport of heat by atmospheric motions (Antarctic ice sheet suppresses poleward transport of heat by atmosphere) Stronger circulation in NH leads to more disturbed vortex (warmer, less PSCs) 22

NH and SH Temperatures ANTARCTIC POLAR VORTEX, MINIMUM TEMPERATURE, 20 km TEMPERATURE (K) 220 200 POLAR STRATOS. CLOUD 180 1978 to 2000 JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC Data Courtesy P.

12 NH and SH Temperatures ANTARCTIC POLAR VORTEX, MINIMUM TEMPERATURE, 20 km TEMPERATURE (K) POLAR STRATOS. CLOUD to 2000 JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC Data Courtesy P. Newman, NASA/GSFC TEMPERATURE (K) ARCTIC POLAR VORTEX, MINIMUM TEMPERATURE, 20 km 1978 to 2000 POLAR STRATOSPHERIC CLOUD SEP OCT NOV DEC JAN FEB MAR APR MAY JUN JUL AUG 23 Arctic Temperature, 1998 &1999 POLAR STRATOS. CLOUD 24

13 Arctic Temperature, 1999 &2000 POLAR STRATOS. CLOUD 25 Arctic Winter of 1999/2000 ARCTIC WINTER OF 1999/2000 COLD (MANY DAYS BELOW PSC THRESHOLD) ELEVATED ClO THROUGHOUT WINTER SIGNIFICANT OZONE DEPLETION Measured Ozone ~88 DU of chemical loss Ozone profile expected in the absence of chemical loss 26

14 Arctic Ozone Loss and Climate Change 100 DU ozone loss 6 7 K temperature change Rex et al., GRL, 2006 URL in Supplemental Reading Surprisingly simple relationship between chemical loss of column ozone and volume of air exposed to PSC temperatures for entire Arctic winter (V PSC ) This relation leads to estimate of ~15 DU additional loss of ozone per degree Kelvin cooling of the Arctic stratosphere 27 The Stratosphere Cools as the Surface Warms! Figure 4 11, WMO/UNEP (2011) 28

15 Arctic Ozone Loss and Climate Change Lots of year to year variability in V PSC Figure 2 16, WMO/UNEP (2011) Updated from Rex et al., GRL, 2006 URL in Supplemental Reading Peak levels appear to be rising... suggesting coldest winters getting colder 29 Arctic Temperature POLAR STRATOS. CLOUD 30

16 22 March NASA Aura MLS ClO 490 K pot l temp ~18 km 22 March ppb ppb March 32

17 Arctic Ozone 2011 Late March 2011 Early Jan 2011 Ozone Mixing Ratio, K Potl Temp Surface 33 Manney et al., Nature, 2011 Arctic Ozone

18 Arctic Ozone 2011 Alfred Wegner Research Institute, 14 March 2011 press release: NASA 30 March 2011 release: Early press coverage: Later press coverage following publication of Manney et al.: Points of contention: a) Was 2011 really an Arctic Ozone hole? some in the community strongly resist use of these words to describe Arctic Ozone 2011, despite what is written in Manney et al. b) are GHGs responsible for the coldest winters getting colder? vitally important active research 35 Arctic Temperature: Mar 2013 POLAR STRATOS. CLOUD Today 36

27 March Images for 2013 from http://ozoneaq.gsfc.nasa.gov/datadis.md?

19 27 March Images for 2013 from 37 Extra Material 38

20 von Hobe et al., ACP, Chemical loss of polar ozone is controlled by: 1. Amount of activated chlorine (ClO + 2 ClOOCl) 2. Photolysis frequency (J value) of ClOOCl Controlled by temperature, PSCs, etc stay tuned for Tobias s talk Rate of this process = J [ ClOOCl] ClO + ClO + M ClOOCl + M Cl + O 3 ClO + O 2 Cl + O 3 ClO + O 2 ClOOCl + hν ClOO + Cl ClOO + heat Cl + O 2 Net: O 3 + O 3 3 O 2 Molina and Molina 1987 von Hobe et al

21 Chemical loss of polar ozone is controlled by: 1. Amount of activated chlorine (ClO + 2 ClOOCl) 2. Photolysis frequency (J value) of ClOOCl Rate of this process = J [ ClOOCl] ClO + ClO + M ClOOCl + M Cl + O 3 ClO + O 2 Cl + O 3 ClO + O 2 ClOOCl + hν ClOO + Cl ClOO + heat Cl + O 2 Net: O 3 + O 3 3 O 2 Molina and Molina 1987 J is found using a radiative transfer model that relies on laboratory measurement of the ClOOCl absorption cross section We are able to test our knowledge of J based on field measurements of ClO and related species Here, I will provide: a) an update of lab studies since the Pope et al. (2007) experiment b) a retrospective based on observations of ClO and ClOOCl obtained in ClOOCl Cross Section 42

ClO/ClOOCl Partitioning During daytime, when it is cold (T < ~210 K) and chlorine is activated, the partitioning of ClO and ClOOCl depends on rate constant of ClOOCl formation (k F ) and photolysis

22 ClO/ClOOCl Partitioning During daytime, when it is cold (T < ~210 K) and chlorine is activated, the partitioning of ClO and ClOOCl depends on rate constant of ClOOCl formation (k F ) and photolysis frequency of ClOOCl (J): k F Daytime: ClO + ClO + M ClOOCl + M Stimpfle et al., JGR, 2004 defined the ratio : J β = [ ClO model ] [ ClO model ] / [ ClOOCl model ] [ ClO meas ] [ ClO meas ] / [ ClOOCl meas ] (J / k F ) model (J / k F ) actual A value of =1 indicates good agreement between observations and models 43 Cross Section extend to 550nm Thick error bars: standard deviation of points Thin error bars: uncertainty in σ & k F combined w/ std deviation Burkholder JPL06 IUPAC Pope Papanastasiou JPL09 von Hobe Error bars now represent uncertainty in cross sections in addition to uncertainty in k F combined and measurement noise

Stratospheric Chemistry: Polar Ozone Depletion AOSC 433/633 & CHEM 433. Ross Salawitch

Stratospheric Chemistry: Polar Ozone Depletion AOSC 433/633 & CHEM 433 Ross Salawitch Class Web Site: http://www.atmos.umd.edu/~rjs/class/spr2017 Today: Processes that govern the formation of the Antarctic