Explanation of the ozone hole and verification of the connections between ozone depleting substances...
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Transcript of Explanation of the ozone hole and verification of the connections between ozone depleting substances...
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, USA
Co-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.