A.R. (Ravi) Ravishankara NOAA, ESRL/Chemical Sciences Division Boulder CO, USA Co-Chair, SAP

Explanation of the ozone hole and verification of the connections between ozone depleting

substances and the ozone hole

A.R. (Ravi) RavishankaraNOAA, ESRL/Chemical Sciences Division

Boulder CO, USACo-Chair, SAP

Some personal reflections

Many simplifications for the sake brevity and clarity

Ozone destruction here

Ozone Depletion and CFCs: Pre-ozone holeRowland and Molina theory of ozone depletion- mid 1970s.Work done in the laboratory, atmospheric, and modeling to estimated future ozone depletion

CFCs released at Earth’s 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:

€

Cl + O3 → ClO + O2

O + ClO → Cl + O2

net : O + O3 → 2 O2

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

ClONO2

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

20Cl2

Polar cloud surfaces lead to muchmore ozone destruction, in the heart of the layer

Altit

ude

(km

)40

Gases destroy ozone here

Amount of ozone

20

0

Cl

OzoneClO

HCl

ClONO2

Reservoirs

Activated for ozone lossCFCs

chlorine

On the depletion of Antarctic ozone, Nature 321, 755-758 (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

Altit

ude

(km

)

40

Amount of ozone

20

0

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 quantifiedOzone 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

€

ClO + ClO air molecules ⏐ → ⏐ ⏐ ⏐ ⏐ Cl2O2 ; At cold temperatures

Cl2O2light ⏐ → ⏐ ⏐ 2 Cl + O2

2 (Cl + O3 → ClO + O2 )

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.