slide 1 Polar Ozone: Past and present Chapter 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 Weber 2007/07/19

slide 2 Part 1 Polar stratospheric observations update

slide 3 Executive Summary: Arctic observations  Arctic spring total ozone over the last decade were lower than in the eighties

slide 4 Executive Summary: Arctic observations  Antthropogenic chemical loss and variability and transport are equally important for year-to-year Arctic ozone variability transport chemical loss

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

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

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

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)

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)

slide 10 Executive Summary: Antarctic observations  Antarctic ozone depletion has stabilized during the last decade ( )  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

slide 11 Executive Summary: General  Large inter-annual variability in polar stratospheric temperatures complicate the interpretation of trends

slide 12 Part 2 Progress in our understanding of physical and chemical processes

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+M  2ClO+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 (Br y, source of BrO) is highly uncertain due to uncertainties in the very-short-lived (VSL) bromo-carbons entering the stratosphere loss cycle 1: loss cycle 2:

slide 14 Chlorine cycle  Best agreement with ClOx observations using photolysis rates (absorption cross-sections) from Burkholder et al J 1b k 1a

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

slide 16 Bromine cycle (blue solid line)  Modelled ClO x (blue solid line) in best agreement with observations (grey symbols) using Burkholder J 1a rates (ClOOCl photolysis) and measured BrO  Contribution from bromine cycle to chemical loss is on the order of % Arctic winter 1999/2000

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

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.