Explanation of the ozone hole and verification of the connections between ozone depleting substances and the ozone hole A.R. (Ravi) Ravishankara NOAA, ESRL/Chemical Sciences Division Boulder CO, USA Co-Chair, SAP Some personal reflections Many simplifications for the sake brevity and clarity

Ozone destruction here Ozone Depletion and CFCs: Pre-ozone hole Rowland and Molina theory of ozone depletion- mid 1970s. Work done in the laboratory, atmospheric, and modeling to estimated future ozone depletion CFCs released at Earths surface Cl atoms: Catalysts for ozone destruction Most chlorine from CFCs are in the benign (non-ozone- destruction) chemical forms. Chlorine nitrate and Hydrogen chloride Ozone destroying fraction (chlorine atoms) is usually only about 1% of total! Not much ozone loss in the region of most ozone. Main pathway:

The predicted ozone depletion Predicted ozone depletion- in the future and modest Coupling of ozone depletion due to Chlorine (CFCs) and nitrogen oxides (SSTs) NOx emissions could enhance, instead of depleting, ozone CFC induced depletion predicted for ~2050; chlorine levels of ~9.5 ppbv NAS Reports 1976 ClONO 2

The Ozone Hole Thanks to Dr. Solomon for this slide

The Antarctic ozone hole was very large! Depletions of 30% Larger than many large countries!

Many theories of ozone hole Natural causes - Dynamical reasons - Cosmic rays Human caused - CFCs (+ other halogens) Ozone depletion due to: Super human causes

20 Cl 2 Polar cloud surfaces lead to much more ozone destruction, in the heart of the layer Altitude (km) 40 Gases destroy ozone here Amount of ozone 20 0 Cl Ozone ClO HCl ClONO 2 Reservoirs Activated for ozone loss CFCs chlorine On the depletion of Antarctic ozone, Nature 321, (19 June 1986) Susan Solomon *, Rolando R. Garcia, F. Sherwood Rowland & Donald J. Wuebbles § Courtesy of Dr. Solomon

Expedition to McMurdo station in 1986 clearly implicated chlorine from CFCs and indicated that other theories were unlikely to be correct Courtesy: Dr. Solomon

Altitude (km) 4040 Amount of ozone

The smoking gun! 1987 Aircraft Expedition Measurements right inside the ozone hole

Chlorine from CFCs are responsible for ozone depletion! Pre-sunlight data is not real

Laboratory studies elucidated how particles are formed in the stratosphere. Examined reactivities on/in these particles. Atmospheric observations confirmed these particles. Many other reactions that occur on ice and ice-like substances were elucidated and quantified Complex New Chemistry Quantified Key point: Most (nearly 100%) chlorine in active ozone-destroying form! Compare this with only 1% in active form elsewhere Figure courtesy of Dr. Margaret Tolbert

Reactions with high chlorine quantified Ozone destruction chemistry in the ozone hole different from elsewhere in stratosphere- Very unusual chemistry First suggested as a possible reaction in the 1950s by Norrish et al. Molina et al. suggested it for the atmosphere with high chlorine Laboratory studies quantified the rates of these reactions This high-order dependence on ClO abundance accounted for rapid onset of the ozone hole.

Explanation of Antarctic Ozone hole: The perfect Storm Confluence of a large number of factors added to create ozone hole! Unusually cold conditions allow formation of particles The unusually atmospheric motion that contains air over Antarctica Some reactions that take away the moderating nitrogen oxides Light shines in to the pre-conditioned cauldron and destroys a lot of ozone! Chlorine continues to destroy ozone uncontrolled till the vortex falls apart From Ravishankara, 2010

Graphic realization of what can happen elsewhere! The global nature was evident- no CFCs emitted by Antarctica, yet a huge hole from contributions of others! The predictable periodicity of ozone hole made it easier to diagnoze/understand Huge increases in UV over Antarctica, levels equal to those in San Diego in the summer Viewed as a harbinger of things to come The predictable periodicity of ozone hole also was a clear recurring reminder. The ozone hole was a graphic poster child for the ozone layer depletion You all did the rest- the Montreal Protocol!

Ozone hole discovered Ozone hole explained Ozone hole explanation Verified Potential human caused large ozone depletion was clear Harmful effects of enhanced UV light recognized Ozone layer passed from curiosity- driven issue to societal issue!

Thank you for your attention.