CREATE Summer School Tim Canty Understanding Halogens in the Arctic Today: Will focus on chlorine and bromine How lab studies affect our view of ozone loss Using observations and models to probe the atmosphere Lecture C 24 July 2012 1
Polar Ozone Depletion Figure 2 8, WMO/UNEP (2011) 2
Polar Ozone Depletion Ozone loss primarily due to chemical reactions involving chlorine and bromine Figure 2 8, WMO/UNEP (2011) 3
Chlorine Source Gases CFCs ChloroFluoroCarbons CFC usage, ~1986 percent of global release: Propellants : 28% Foam Blowing : 26% Refrigerants : 23% Cleaning Solvents : 21% 4
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 Picture courtesy of R. Salawitch 5
Polar Ozone Loss Cycles Cycle (1): ClO + ClO + M ClOOCl + M ClOOCl + h ClOO + Cl ClOO + heat Cl + O 2 Cl + O 3 ClO + O 2 Cl + O 3 ClO + O 2 Net: O 3 + O 3 3O 2 Cycle (1) accounts for ~60% of polar ozone loss 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, 2001 6
Polar Ozone Loss Cycles Chlorine dimer Cycle (1): ClO + ClO + M ClOOCl + M ClOOCl + h ClOO + Cl ClOO + heat Cl + O 2 Cl + O 3 ClO + O 2 Cl + O 3 ClO + O 2 Net: O 3 + O 3 3O 2 Cycle (1) accounts for ~60% of polar ozone loss 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, 2001 7
Polar Ozone Loss Cycles Chlorine dimer Cycle (1): ClO + ClO + M ClOOCl + M ClOOCl + h ClOO + Cl ClOO + heat Cl + O 2 Rate limiting step Cl + O 3 ClO + O 2 Cl + O 3 ClO + O 2 Net: O 3 + O 3 3O 2 Cycle (1) accounts for ~60% of polar ozone loss 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, 2001 8
Photolysis Frequency For a specific spectral interval, the photolysis frequency (partial J value) of a gas is given by the product of its absorption cross section and the solar irradiance: J gas (z, ) = Quantum_Yield( ) gas (,T) F(z, ) Units: s 1 nm 1 The total photolysis frequency (J value) is found by integrating J gas (z, ) over all wavelengths for which the gas photodissociates: J gas (z) max min do J gas (z,λ) d Units: s 1 3 1 Rate of Reaction = J [O 3] Units of J are s dt More precisely, calculations of photolysis frequencies consider the spectral actinic flux, which represents the amount of available photons integrated over all angles, rather than solar irradiance. These two quantities differ because of scattering of solar radiation by gases and aerosols, and reflection of radiation by clouds and the surface. 9
Chemical loss of polar ozone is controlled by: 1. Amount of activated chlorine (ClO x = ClO + 2 ClOOCl) 2. Photolysis frequency (J value) of ClOOCl Controlled by temperature, PSCs, etc Rate of this process = J [ ClOOCl] ClO + ClO + M ClOOCl + M Cl + O 3 ClO + O 2 Cl + O 3 ClO + O 2 ClOOCl + h ClOO + Cl ClOO + heat Cl + O 2 Net: O 3 + O 3 3 O 2 Molina and Molina 1987 10
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 + h 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 Can use observations of ClO and ClO x to test understanding of J[ClOOCl]. 11
ClOOCl Cross Section Pope et al., 2007 assumed some contamination due to Cl 2. Subtracted Cl 2 from signal to find, what they believed, was actual ClOOCl signal. 12
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von Hobe et al., ACP, 2007 14
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): Daytime: ClO + ClO + M k F Stimpfle et al., JGR, 2004 defined the β ratio : J ClOOCl + M = [ ClO model ] [ ClO model ] / [ ClOOCl model ] [ ClO meas ] [ ClO meas ] / [ ClOOCl meas ] (J / k F ) model (J / k F ) actual A value of β=1 indicates good agreement between observations and models 15
Cross Section extend to 550nm Thick error bars: standard deviation of points Thin error bars: uncertainty in & 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
ClOOCl Cross Section 17
Cross Section extend to 550nm 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 Thick error bars: standard deviation of points Thin error bars: uncertainty in & k F combined w/ std deviation
Bromine Source Gases Halons: fire extinguishing agents production in developed world halted by Montreal Protocol present emissions primarily from banks CH 3 Br: fumigant; released by biomass burning production halted by Montreal Protocol significant natural & human sources VSL Gases (e.g., CHBr 3, CH 2 Br 2 ): emitted mainly by ocean biology not considered in most ozone calcs chemistry of decomposition products subject of active research 19
Polar Ozone Loss Cycles Cycle (2a): BrO + ClO BrCl +O 2 Br + O 3 BrO + O 2 Cl + O 3 ClO + O 2 BrCl + h 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 + h 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, 2001 20
Arctic Research of the Composition of the Troposphere from Aircraft and Satellites 21
Arctic Research of the Composition of the Troposphere from Aircraft and Satellites 22
Arctic Research of the Composition of the Troposphere from Aircraft and Satellites 23
Aircraft Observations of BrO (or lack of?) Salawitch, et al., Geophys. Res. Lett., 2010GL043798 24
Aircraft Observations of BrO (or lack of?) Salawitch, et al., Geophys. Res. Lett., 2010GL043798 25
Aircraft Observations of BrO (or lack of?) Geographic regions of near-surface Ozone Depletion and elevated BrO generally not co-located with OMI hotspots in total column BrO Salawitch, et al., Geophys. Res. Lett., 2010GL043798 26
Comparing OMI observations to Model Model Input 3D fields of CFC-12 from GEOS-5 assimilation used to specify Br y vs altitude Then, BrO/Br y ratio from WACCM used to estimate BrO at each model grid point Model Output BrO, for SZA of OMI overpass, for three cases: VSL Br y = 0, 5, and 10 ppt VSL: Very Short Lived OMI Total Column BrO Model Stratospheric Column BrO (VSL=0 ppt) 5 April 2010 27
Bromine from VSL (very short lived) Sources Salawitch : PGI= 7 ppt on top of TC 4 Theys: SGI = 5 ppt PGI= 1 ppt TC 4 CH 3 Br, Halons, CH 2 Br 2 WMO CCMVal2 CH 3 Br & Halons TC 4 tropospheric data indicate significant potential for VSL species to supply Br y to stratosphere Salawitch et al., GRL, 2010 28 Salawitch et al., GRL, 2010 & Theys et al., ACP, 2009 Theys et al., ACP, 2011
Model Stratospheric Column BrO OMI Total Column O 3 5 Apr 2008 Tropopause Pressure OMI Total Column BrO VSL=0 ppt VSL=5 ppt VSL=10 ppt 29
Model Stratospheric Column BrO OMI Total Column O 3 6 Apr 2008 Tropopause Pressure OMI Total Column BrO VSL=0 ppt VSL=5 ppt VSL=10 ppt 30
Model Stratospheric Column BrO OMI Total Column O 3 7 Apr 2008 Tropopause Pressure OMI Total Column BrO VSL=0 ppt VSL=5 ppt VSL=10 ppt 31
Model Stratospheric Column BrO OMI Total Column O 3 8 Apr 2008 Tropopause Pressure OMI Total Column BrO VSL=0 ppt VSL=5 ppt VSL=10 ppt 32
Model Stratospheric Column BrO OMI Total Column O 3 9 Apr 2008 Tropopause Pressure OMI Total Column BrO VSL=0 ppt VSL=5 ppt VSL=10 ppt 33
Ground Based BrO, Harestua, Norway (60ºN) Units: 10 13 molecules/cm 2 GOME-2 OMI Ground-based total column Global Trop Extra-Polar Col BrO of 1 to 3 10 13 Theys et al., ACP, 2011 34
Ground Based BrO, Harestua, Norway (60ºN) Units: 10 13 molecules/cm 2 GOME-2 OMI Ground-based total column Sala : PGI= 7 ppt on top of TC 4 TC 4 CH 3 Br, Halons, CH 2 Br 2 WMO Br y WMO CCMVal2 CH 3 Br & Halons WMO Br y implies higher trop burden than meas. Theys, Canty, Hendrick, Van Roozendael, Chance, Kurosu, Suleiman & Salawitch 35
Ground Based BrO, Harestua, Norway (60ºN) Units: 10 13 molecules/cm 2 GOME-2 OMI Ground-based total column TC4 4 CH 3 Br, Halons, CH 2 Br 2 TC 4 CH 3 Br, Halons, & CH 2 Br 2 TC 4 SGI Br y broadly consistent w/ meas trop BrO Theys, Canty, Hendrick, Van Roozendael, Chance, Kurosu, Suleiman & Salawitch 36
Ground Based BrO, Harestua, Norway (60ºN) Units: 10 13 molecules/cm 2 GOME-2 OMI Ground-based total column Theys: SGI = 5 ppt PGI= 1 ppt Theys Br y Global Trop Extra-Polar Col BrO of 1 to 3 10 13 Theys et al., ACP, 2011 37
Ground Based BrO, Harestua, Norway (60ºN) Units: 10 13 molecules/cm 2 GOME-2 OMI Ground-based total column Theys: SGI = 5 ppt PGI= 1 ppt Canty / Salawitch calc using Theys formulation for Br y & OMI Global Trop Extra-Polar Col BrO of 1 to 3 10 13 Theys et al., ACP, 2011 38
Ground Based BrO, Harestua, Norway (60ºN) Units: 10 13 molecules/cm 2 GOME-2 OMI Sala Strat Col Ground-based total column Sala : PGI= 7 ppt on top of TC 4 Sala Br y Salawitch Br y Suggests Near-Zero Global Tropospheric Extra-Polar Col BrO 39 Theys, Canty, Hendrick, Van Roozendael, Chance, Kurosu, Suleiman & Salawitch
Ground Based BrO, Harestua, Norway (60ºN) Units: 10 13 molecules/cm 2 GOME-2 OMI Sala Strat Col Ground-based total column Sala Br y Our tropospheric BrO background column of 1 to 3 10 13 tends to confirm the findings of: Fitzenberger et al., 2000 Wagner et al., 2001 Richter et al., 2002 Van Roozendael et al., 2002 Hendrick et al., 2007 Theys et al., 2009 Page 1807, Theys et al., 2011 Salawitch Br y Suggests Near-Zero Global Tropospheric Extra-Polar Col BrO Theys, Canty, Hendrick, Van Roozendael, Chance, Kurosu, Suleiman & Salawitch 40
Ground Based BrO, Harestua, Norway (60ºN) Units: 10 13 molecules/cm 2 GOME-2 OMI Sala Strat Col Ground-based total column Sala Br y Our tropospheric BrO background column of 1 to 3 10 13 is in contrast to direct-sun spectral observations of: Schofield et al., 2004 (Lauder, NZ) Schofield et al., 2006 (Antarctica) Dorf et al., 2008 (Tropics) all showing tropospheric columns of 0.2 to 0.3 10 13 molec/cm 2 more study is needed Page 1807, Theys et al., 2011 Salawitch Br y Suggests Near-Zero Global Tropospheric Extra-Polar Col BrO Theys, Canty, Hendrick, Van Roozendael, Chance, Kurosu, Suleiman & Salawitch 41
ARCTAS Flight of 17 April 2008 Where is the stratospheric signature? Where is the tropospheric signature? Black lines show DC-8 flight track; symbols show location of ascent/descent Sunny Choi & Joanna Joiner s analysis of clouds, viewing geometry, and surface albedo suggests 17 April 2008 provided IDEAL CONDITIONS for remote sensing of tropospheric BrO from space! Choi et al., ACP, 2012 Liao et al., ACP, 2012 42
ARCTAS Flight of 17 April 2008 Sala : PGI= 7 ppt on top of TC 4 TC 4 CH 3 Br, Halons, CH 2 Br 2 WMO CCMVal2 CH 3 Br & Halons Choi et al., ACP, 2012 Liao et al., ACP, 2012 43
ARCTAS Flight of 17 April 2008 TC 4 CH 3 Br, Halons, CH 2 Br 2 Choi et al., ACP, 2012 Liao et al., ACP, 2012 44
ARCTAS Flight of 17 April 2008 Theys: SGI = 5 ppt PGI= 1 ppt Choi et al., ACP, 2012 Liao et al., ACP, 2012 45
ARCTAS Flight of 17 April 2008 Sala : PGI= 7 ppt on top of TC 4 Choi et al., ACP, 2012 Liao et al., ACP, 2012 46
Choi et al., ACP, 2012 47
Final Remarks Community believes photolysis of ClOOCl is not as small as suggested by Pope et al., 2007. However, uncertainties remain in ClOOCl thermal decomposition, ClO+BrO branching ratios, reaction rate. One school of thought: Not much BrO just above tropop Existence of global, ubiquitous, extra-polar tropospheric BrO of 1 to 3 10 13 cm 2 (2 to 3 ppt) with important consequences for: Tropospheric O 3 Hg DMS Another school of thought: Lots of BrO just above tropop with important consequences for: Polar Strat Ozone Depletion Mid-Lat Column Ozone Trends Global, ubiquitous, extra-polar tropospheric BrO small or perhaps non-existent 48