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slide 3 Executive Summary: Arctic observations Arctic spring total ozone over the last decade were lower than in the eighties
Citation preview
19.7.2007 slide 1
Polar Ozone: Past and presentChapter 4 of WMO 2006 Ozone Assessment
Summary
Part 1 Polar stratospheric observations update
Part 2 Progress in our understanding of physical and
chemical processes
Mark Weber2007/07/19
19.7.2007 slide 2
Part 1Polar stratospheric observations update
19.7.2007 slide 3
Executive Summary: Arctic observations
Arctic spring total ozone over the last decade were lower than in the eighties
19.7.2007 slide 4
Executive Summary: Arctic observations
Antthropogenic chemical loss and variability and transport are equally important for year-to-year Arctic ozone variability
transport
chemicalloss
19.7.2007 slide 5
Executive Summary: Arctic observations
Spring Arctic ozone is highly variable depending on dynamical conditions, variability in polar temperatures and ozone transport are dynamically driven
Eddy heat flux is a measure of planetary wave activity
19.7.2007 slide 6
Executive Summary: Arctic observations
Spring Arctic ozone is highly variable depending on dynamical conditions, variability in polar temperatures and ozone transport are dynamically driven
Fig. 4-5, 4-1, 4-4
19.7.2007 slide 7
Executive Summary: Arctic observations
For coldest Arctic winter, the volume of air with temperatures low enough for PSC formation has increased significantly since the 1960s
19.7.2007 slide 8
Executive Summary: Arctic observations
Column ozone loss in Arctic winter 2004/05 was among the largest ever observed, with particular high losses below 18 km
Observed chemical ozone loss from satellites (HALOE, ACE), ozone sondes, and groundbased DOAS (SAOZ)
19.7.2007 slide 9
Executive Summary: Arctic observations Column ozone loss in Arctic winter
2004/05 was among the largest ever observed, with particular high losses below 18 km
Vortex averaged ozone loss from satellites (MLS, SAGE II, POAM III) and ozonesondes
Note difference in altitudes of maximum loss from year-to-year (2000 vs. 2005)
19.7.2007 slide 10
Executive Summary: Antarctic observations
Antarctic ozone depletion has stabilized during the last decade (1995-2005)
Explanations:Saturation of ozone
loss (complete loss between 15 and 20 km altitudes)
Higher levels of dynamical forcing (see 2002 ozone hole anomaly)
<220 DU
19.7.2007 slide 11
Executive Summary: General
Large inter-annual variability in polar stratospheric temperatures complicate the interpretation of trends
19.7.2007 slide 12
Part 2 Progress in our understanding of physical and chemical
processes
19.7.2007 slide 13
Current understanding of physical and chemical processes
Chlorine cycle: Chlorine activation on PSCs (<195
K) Photolysis rates of ClOOCl is highly
uncertain (absorption cross-section) Uncertainty of the relative role of
ClOOCl+M2ClO+M (~10%, null cycle) vs. Reaction (1b)
Bromine cycle BrO abundance is not dependent on
PSC existence About half of the polar ozone loss
may be due to the bromine cycle total inorganic bromine (Bry, source
of BrO) is highly uncertain due to uncertainties in the very-short-lived (VSL) bromo-carbons entering the stratosphere
1
2 1
3 2 1
3 2
( )2 ( )
2 ( ): 2 3
a
b
c
ClO ClO M ClOOCl M kClOOCl h Cl O JCl O ClO O k
Net O O
2 2
3 2 2
3 2 2
3 2
( )( )
( ): 2 3
a
b
c
BrO ClO h Br Cl O JBr O BrO O kCl O ClO O kNet O O
loss cycle 1:
loss cycle 2:
19.7.2007 slide 14
Chlorine cycle
Best agreement with ClOx observations using photolysis rates (absorption cross-sections) from Burkholder et al. 1990
[ ] / [ ][ ] /[ ]
OBS
MODEL
ClOOCl ClOClOOCl ClO
J1b
k1a
1
2 1
3 2 1
3 2
( )2 ( )
2 ( ): 2 3
a
b
c
ClO ClO M ClOOCl M kClOOCl h Cl O JCl O ClO O k
Net O O
19.7.2007 slide 15
Bromine cycle
Measured BrO ( about 20 ppt) is higher than modelled assuming methyle bromide (CH3Br) and halons as primary source
Contribution from VSLS?
modelled BrO from CH3Br and halons
BrO from ballon measurements
19.7.2007 slide 16
Bromine cycle
Modelled ClOx (blue solid line)(blue solid line) in best agreement with observations (grey symbols) using Burkholder J1a rates (ClOOCl photolysis) and measured BrO
Contribution from bromine cycle to chemical loss is on the order of 30-50%
Arctic winter 1999/2000
19.7.2007 slide 17
Arctic O3 column loss and PSC volume
Color: Observation from HALOE (circles) and ozone sondes (squares) Black: CTM model results using lower grid resolution (light) and higher grid resolution, improved denitrification/ sedimentation, and updated reaction rates (solid)
Modelled polar ozone loss is in better agreement with observation/ measurements
This linear relationship is an useful diagnostics for climate models
19.7.2007 slide 18
Summary
Although the forcing of polar temperatures and vortex strength from planetary and gravity waves are well established, the causes of forcing variability (on dcadal scale) are still unknown
Cold Arctic winters have gotten colder over four decades, the temperature change exceeds that expected from changes in greenhouse gases, the reason for this is still unknown
BrO may play a larger role in polar ozone loss (up to 50%), but there is still large uncertainty in the polar bromine budget.
The exact NAT nucleation mechanism in PSCs is still not completely understood, although progress has been made in parameterising denitrification in models.