STRATOSPHERIC OZONE the OZONE HOLE

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STRATOSPHERIC OZONE the OZONE HOLE Powered By Docstoc
					STRATOSPHERIC OZONE & the OZONE HOLE
THE MANY FACES OF ATMOSPHERIC OZONE




                    In stratosphere: UV shield

Stratosphere:
90% of total

                    In middle/upper troposphere: greenhouse gas
      Troposphere

                    In lower/middle troposphere: precursor of OH,
                     main atmospheric oxidant



                    In surface air: toxic to humans and vegetation
      STRATOSPHERIC OZONE HAS BEEN MEASURED
              FROM SPACE SINCE 1979
                                    Method: UV solar backscatter
Global Ozone Column Measurements




                                                         λ1   λ2
                                   Ozone layer

                                   Scattering by
                                   Earth surface
                                   and atmosphere

                                   Ozone
                                   absorption
                                   spectrum


                                                 λ1 λ2
CHAPMAN MECHANISM FOR STRATOSPHERIC OZONE
                  (1930)

    (R1)        O 2 + hν → O + O    (λ < 240 nm)
    (R2)        O + O 2 + M → O3 + M
    (R3)        O3 + hν → O 2 + O   (λ < 320 nm)
    (R4)        O3 + O → 2O 2
                                       Odd oxygen family
                                       [Ox] = [O3] + [O]
    slow              R2
           R1
  O2            O     fast     O3
                      R3
                      R4
                        slow
          CHAPMAN MECHANISM vs. OBSERVATION




                                                              shape
                                                              determined
                                                              by k1nO2


                                                                      -3



Chapman mechanism reproduces shape, but is too high by factor 2-3
   missing sink!
WATER VAPOR IN STRATOSPHERE AND MESOSPHERE




           H2O mixing ratio


   Source: transport from troposphere and ???
                  HOx-CATALYZED OZONE LOSS

          HOx = H + OH + HO2 hydrogen oxide radical family


 Initiation:     H 2 O + O( D) → 2OH
                             1


Propagation:           OH + O3 → HO 2 + O 2
                      HO 2 + O3 → OH + 2O 2
               Net:        2O3 → 3O 2
 Termination:     OH + HO 2 → H 2 O + O 2
               slow
        H2O              OH fast HO2                 HOx radical family



                                 slow
           NITROUS OXIDE IN THE STRATOSPHERE




from CLAES satellite instrument   Tropospheric N2O = 310 ppbv
                  Nitrogen oxide (NOx)radicals
                       (NOx = NO + NO2)

• Initiation    N2O + O(1D)    2NO
• Propagation
      NO + O3     NO2 + O2       NO + O3        NO2 + O2
       NO2 + hν   NO + O         NO2 + O        NO + O2
      O + O2 + M    O3 + M                                     O3 loss rate:
     Null cycle                  Net O3 + O       2O2      −
                                                               d [O3 ]
                                                                       = 2k[NO 2 ][O]
                                                                  dt
• Termination                      Recycling
  NO2 + OH + M      HNO3 + M       HNO3 + hν        NO2 + OH

  NO2 + O3   NO3 + O2              HNO3 + OH NO3 + H2O
  NO3 + NO2 + M    N2O5 + M        NO3 + hν  NO2 + O
  N2O5 + H2O   2HNO3               N2O5 + hν NO2 + NO3

    HNO3 is a “reservoir species”
    Define NOy = NOx + NOx oxidation products
ATMOSPHERIC CYCLING OF NOx AND NOy
    Ozone loss catalyzed by chlorine (ClOx = Cl + ClO) radicals


•   Initiation: Cl radical generation from non-radical precursors (e.g., CFC-12)
                     CF2Cl2 + hν       CF2Cl + Cl

•   Propagation:
                   Cl + O3     ClO + O2                        d [O3 ]
                   ClO + O      Cl + O2        O3 loss rate: −         = 2k[ClO][O]
                                                                  dt
                   Net: O3 + O     2O2


•   Reservoirs:                                         Recycling:
        Cl + CH4    HCl + CH3                           HCl + OH Cl + H2O
        ClO + NO2 + M     ClNO3 + M                     ClNO3 + hν Cl + NO3



    HCl and ClNO3 are Cl-reservoir species (Cl atoms accumulate in these forms))
ATMOSPHERIC CYCLING OF ClOx AND Cly
SOURCE GAS CONTRIBUTIONS TO
STRATOSPHERIC CHLORINE (2004)
CHLORINE PARTITIONING IN STRATOSPHERE
OZONE TREND AT HALLEY BAY, ANTARCTICA (OCTOBER)




                                          Farman et al. paper
                                          published in Nature


    1 Dobson Unit (DU) = 0.01 mm O3 STP = 2.69x1016 molecules cm-2
     2006 Antarctic Ozone Hole: most severe observed


Aura OMI – 8 Oct 2006                          Ozone Hole Minimum




                       Minimum of 85 DU on 8 Oct 2006

The ozone hole is an austral spring phenomenon – it is not there year-round!
VERTICAL STRUCTURE OF THE OZONE HOLE:
   near-total depletion in lower stratosphere
         ASSOCIATION OF ANTARCTIC OZONE HOLE
               WITH HIGH LEVELS OF CLO
 Sept. 1987 ER-2 aircraft measurements at 20 km altitude S of Punta Arenas



                                                  O3
                       ClO

                       O3



                                             Sep. 16
                                                                  Edge of
                                                                  Polar
                    Sep. 2, 1987            ClO                   vortex



                            20 km
                            altitude
Measurements by Jim Anderson’s group (Harvard)
  WHY THE HIGH ClO IN ANTARCTIC VORTEX?
Release of chlorine radicals from reactions of reservoir
    species in polar stratospheric clouds (PSCs)
             PSC FORMATION AT COLD TEMPERATURES




PSC formation



Frost point of water
         HOW DO PSCs START FORMING AT 195K?
              HNO3-H2O PHASE DIAGRAM




Antarctic
vortex
conditions



      PSCs are not water but nitric acid trihydrate (NAT) clouds
TRENDS IN GLOBAL OZONE




             Mt. Pinatubo
                       QUESTIONS

1. What would be the effect on chlorine-catalyzed ozone
   loss of a supersonic aircraft fleet releasing NOx to the
   stratosphere?

2. The ozone hole is of limited vertical extent because
   PSCs form only in the lowest part of the polar
   stratosphere. Why don’t they form at higher altitudes
   too?
           TRENDS IN POLAR OZONE
 Could greenhouse-induced cooling of stratosphere
produce an Arctic ozone hole over the next decade?
      Race between chlorine decrease and climate change
 SKIN CANCER
EPIDEMIOLOGY
 PREDICTIONS