ATMOSPHERIC HALOGEN CHEMISTRY

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ATMOSPHERIC HALOGEN CHEMISTRY
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Stratospheric ozone loss catalyzed by chlorine
(ClOx Cl ClO) radicals

• Initiation Cl radical generation from
  non-radical precursors (e.g., CFC-12)
• CF2Cl2 hn g CF2Cl Cl
• Propagation
• Cl O3 g ClO O2
• ClO O g Cl O2
• Net O3 O g 2O2
• Termination Recycling
• Cl CH4 g HCl CH3 HCl OH gCl H2O
• ClO NO2 M g ClNO3 M ClNO3 hn gCl NO3

O3 loss rate
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ATMOSPHERIC CYCLING OF ClOx AND Cly
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SOURCE GAS CONTRIBUTIONS TOSTRATOSPHERIC
CHLORINE (2004)
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CHLORINE PARTITIONING IN STRATOSPHERE
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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
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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!
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VERTICAL STRUCTURE OF THE OZONE HOLEnear-total
depletion in lower stratosphere
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ASSOCIATION OF ANTARCTIC OZONE HOLEWITH 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 vortex
ClO
Sep. 2, 1987
20 km altitude
Measurements by Jim Andersons group (Harvard)
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SATELLITE OBSERVATIONS OF ClO IN THE SOUTHERN
HEMISPHERE STRATOSPHERE
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WHY THE HIGH ClO IN ANTARCTIC VORTEX?Release of
chlorine radicals from reactions of reservoir
species in polar stratospheric clouds (PSCs)
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PSC FORMATION AT COLD TEMPERATURES
PSC formation
Frost point of water
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HOW DO PSCs START FORMING AT 195K?HNO3-H2O PHASE
DIAGRAM
Antarctic vortex conditions
PSCs are not water but nitric acid trihydrate
(NAT) clouds
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TRENDS IN GLOBAL OZONE
Mt. Pinatubo
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(No Transcript)
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LONG-TERM COOLING OF THE STRATOSPHERE
Sep 21-30, 25 km, 65-75S
Increasing CO2 is expected to cool the
stratosphere
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BROMINE-CATALYZED OZONE DESTRUCTION IN ARCTIC
SPRING BOUNDARY LAYER
Depleted Hg(0) also found in these events
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BROMINE EXPLOSION MECHANISM
BrNO3 chemistry still very uncertain
Bromide in sea salt aerosol or at sea ice surface
(frost flowers) is converted to BrO by a
self-amplifying mechanism
Br-
Fan and Jacob 1992
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FROST FLOWERS IN ANTARCTICA
cm-high crystals with very high salt
concentrations formed from new sea ice provide
large surface area and Br- concentration for
bromine explosion.
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TROPOSPHERIC BrO SEEN FROM SPACE
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BROMINE SOURCES TO TROPOSPHERE
Lifetime 1 year sources biogenic anthropogenic
Lifetimes of days, oceanic sources
There is also a significant source from volcanoes
There is indirect evidence that these sources
maintain 1 ppt BrO background in troposphere
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EVIDENCE FOR HIGH Cl ATOM CONCENTRATIONSDURING
SPRING ARCTIC O3 DEPLETION EPISODES
Jobson et al. 1994
The Arctic data imply Cl 104 atoms cm-3 also
some speculative evidence for concentrations of
that order in the marine boundary layer
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COASTAL IODINE EMISSION FROM MARINE ALGAE
Unlike Cl and Br, I cannot originate
significantly from sea salt
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GENERAL MECHANISM FOR HALOGEN TROPOSPHERIC
CHEMISTRY