atmospheric chemistry of the ozone layer
DESCRIPTION
Atmospheric Chemistry of the Ozone Layer. Levels of Atmospheric Ozone have been Dropping. NASA - http://toms.gsfc.nasa.gov. Decreasing Level of atmospheric ozone is harmful There has been an increase in the number of cases of skin cancer and cataracts - PowerPoint PPT PresentationTRANSCRIPT
![Page 1: Atmospheric Chemistry of the Ozone Layer](https://reader036.vdocument.in/reader036/viewer/2022081508/56815577550346895dc33ec7/html5/thumbnails/1.jpg)
Atmospheric Chemistry of
the Ozone Layer
![Page 2: Atmospheric Chemistry of the Ozone Layer](https://reader036.vdocument.in/reader036/viewer/2022081508/56815577550346895dc33ec7/html5/thumbnails/2.jpg)
Levels of Atmospheric Ozone have been Dropping
NASA - http://toms.gsfc.nasa.gov
![Page 3: Atmospheric Chemistry of the Ozone Layer](https://reader036.vdocument.in/reader036/viewer/2022081508/56815577550346895dc33ec7/html5/thumbnails/3.jpg)
Decreasing Level of atmospheric ozone is harmful
There has been an increase in the number of cases of skin cancer and cataracts
Evidence of damage to plant and marine life
![Page 4: Atmospheric Chemistry of the Ozone Layer](https://reader036.vdocument.in/reader036/viewer/2022081508/56815577550346895dc33ec7/html5/thumbnails/4.jpg)
What is ozone?
Where in the atmosphere is it found?
What is its purpose in the atmosphere?
What is its chemistry?
Why are levels of atmospheric ozone dropping?
Finally, what is the Ozone Hole?
![Page 5: Atmospheric Chemistry of the Ozone Layer](https://reader036.vdocument.in/reader036/viewer/2022081508/56815577550346895dc33ec7/html5/thumbnails/5.jpg)
Structure of Ozone
OO33
O Atoms
![Page 6: Atmospheric Chemistry of the Ozone Layer](https://reader036.vdocument.in/reader036/viewer/2022081508/56815577550346895dc33ec7/html5/thumbnails/6.jpg)
Where is ozone found in the atmosphere ?
NASA Goddard Space Flight Center
Note, higher concentration in stratosphere, compared with troposphere
![Page 7: Atmospheric Chemistry of the Ozone Layer](https://reader036.vdocument.in/reader036/viewer/2022081508/56815577550346895dc33ec7/html5/thumbnails/7.jpg)
Chemical Kinetics and Photochemical Data for Use in Stratospheric Modeling - JPL Publication97-4
Solar Flux
Role of Ozone
![Page 8: Atmospheric Chemistry of the Ozone Layer](https://reader036.vdocument.in/reader036/viewer/2022081508/56815577550346895dc33ec7/html5/thumbnails/8.jpg)
Solar Flux
Chemical Kinetics and Photochemical Data for Use in Stratospheric Modeling - JPL Publication97-4
Role of Ozone
![Page 9: Atmospheric Chemistry of the Ozone Layer](https://reader036.vdocument.in/reader036/viewer/2022081508/56815577550346895dc33ec7/html5/thumbnails/9.jpg)
Role of Ozone
Absorption Spectrum of Ozone
![Page 10: Atmospheric Chemistry of the Ozone Layer](https://reader036.vdocument.in/reader036/viewer/2022081508/56815577550346895dc33ec7/html5/thumbnails/10.jpg)
Role of Ozone
“The Ozone Depletion Phenomenon”, Beyond Discovery, National Academy of Sciences
![Page 11: Atmospheric Chemistry of the Ozone Layer](https://reader036.vdocument.in/reader036/viewer/2022081508/56815577550346895dc33ec7/html5/thumbnails/11.jpg)
UV A (~400 to 350 nm) not absorbed by earth’s atmosphere
Role of Ozone
UV B (~ 350 to 270 nm) partially absorbed by earth’s atmosphere
UV C (~270 to 150 nm) completely absorbed by earth’s atmosphere
UV B is harmful to life on earth
![Page 12: Atmospheric Chemistry of the Ozone Layer](https://reader036.vdocument.in/reader036/viewer/2022081508/56815577550346895dc33ec7/html5/thumbnails/12.jpg)
Chapman mechanism
How is ozone formed?
O2 + h-> O + O
O + O2 + M-> O3 + M
O3 + h -> O + O2
O + O3 -> 2 O2
“Ozone: What is it and why do we care about it?”, NASA Facts, Goddard Space Flight Center
![Page 13: Atmospheric Chemistry of the Ozone Layer](https://reader036.vdocument.in/reader036/viewer/2022081508/56815577550346895dc33ec7/html5/thumbnails/13.jpg)
Kinetics of Chapman Mechanism
Rate of formation of O and O3
d[O]/dt = 2k1[O2]-k2[O][O2][M] + k3[O3] - k4[O][O3]d[O3]/dt = k2[O][O2][M] - k3[O3]-k4[O][O3]
Steady-State Approximation
d[O]/dt = d[O3]/dt = 0
O2 + h-> O + O k1
O + O2 + M-> O3 + M k2
O3 + h -> O + O2 k3
O + O3 -> 2 O2 k4
![Page 14: Atmospheric Chemistry of the Ozone Layer](https://reader036.vdocument.in/reader036/viewer/2022081508/56815577550346895dc33ec7/html5/thumbnails/14.jpg)
Kinetics of Chapman Mechanism
d[O3]/dt = k2[O][O2][M] - k3[O3]-k4[O] [O3]=0
k2[O][O2][M]/{ k3+k4[O] } = [O3]
k2[O][O2][M]={ k3+k4[O] } [O3]
![Page 15: Atmospheric Chemistry of the Ozone Layer](https://reader036.vdocument.in/reader036/viewer/2022081508/56815577550346895dc33ec7/html5/thumbnails/15.jpg)
Kinetics of Chapman Mechanism
Can re-write [O3] as:
[O3]=k2[O2][M]/ k4
k3 /(k4[O])+1
[O3] = k2[O][O2][M]/{ k3+k4[O] }
(Divide by k4[O] )
![Page 16: Atmospheric Chemistry of the Ozone Layer](https://reader036.vdocument.in/reader036/viewer/2022081508/56815577550346895dc33ec7/html5/thumbnails/16.jpg)
Kinetics of Chapman Mechanism
Since the rate constants and concentration of species are known, can show that:
k3
k4[O]>>1
[O3]≈k2[O2 ][M][O]
k3
Hence,
[O3]=k2[O2][M]/ k4
k3 /(k4[O])+1+
![Page 17: Atmospheric Chemistry of the Ozone Layer](https://reader036.vdocument.in/reader036/viewer/2022081508/56815577550346895dc33ec7/html5/thumbnails/17.jpg)
[O3] depends on rate of reaction 2
and the intensity of light (k3)
Kinetics of Chapman Mechanism
[O3]≈k2[O2 ][M][O]
k3
Reaction 2 is slow (termolecular); makes ozone “vulnerable” to ozone-depleting reactions
O2 + h-> O + O k1
O + O2 + M-> O3 + M k2
O3 + h -> O + O2 k3
O + O3 -> 2 O2 k4
![Page 18: Atmospheric Chemistry of the Ozone Layer](https://reader036.vdocument.in/reader036/viewer/2022081508/56815577550346895dc33ec7/html5/thumbnails/18.jpg)
Competing Reactions
HOx cycle
H, OH and HO2 species formed by reaction of excited O atoms with H-containing atmospheric species like H2O and CH2
O3 + h -> O + O2
O + H2O -> OH + OH
O + CH4 -> CH3 + OH
H2O + h -> H + OH
![Page 19: Atmospheric Chemistry of the Ozone Layer](https://reader036.vdocument.in/reader036/viewer/2022081508/56815577550346895dc33ec7/html5/thumbnails/19.jpg)
Reactions of HOx species with O3
OH + O3 -> HO2 + O2
HO2 + O -> OH + O2
Net Reaction
O + O3 -> 2O2
“Ozone Depletion”
![Page 20: Atmospheric Chemistry of the Ozone Layer](https://reader036.vdocument.in/reader036/viewer/2022081508/56815577550346895dc33ec7/html5/thumbnails/20.jpg)
NOx Cycle
Competing Reactions
NOx species are produced during the reaction of O atoms with N2O (produced in the soil by bacteria)
O + N2O -> 2 NO
![Page 21: Atmospheric Chemistry of the Ozone Layer](https://reader036.vdocument.in/reader036/viewer/2022081508/56815577550346895dc33ec7/html5/thumbnails/21.jpg)
Reactions of NOx species with O3
NO + O3 -> NO2 + O2
NO2 + O -> NO + O2
Net Reaction
O + O3 -> 2O2
“Ozone Depletion”
![Page 22: Atmospheric Chemistry of the Ozone Layer](https://reader036.vdocument.in/reader036/viewer/2022081508/56815577550346895dc33ec7/html5/thumbnails/22.jpg)
Competing Reactions
ClOx cycle
ClOx species are produced from chlorofluorocarbons (CFC’s) and methyl chlorideCFC’s are artificially produced; methyl chloride is a naturally occuring chemical.
Examples of CFC’s : Freons (CFCl3, CF2Cl2)
CCl2F2 + h -> CF2Cl + Cl
CCl2F2 + O -> CF2Cl + ClO
![Page 23: Atmospheric Chemistry of the Ozone Layer](https://reader036.vdocument.in/reader036/viewer/2022081508/56815577550346895dc33ec7/html5/thumbnails/23.jpg)
Reactions of ClOx species with O3
Cl + O3 -> ClO + O2
ClO + O -> Cl + O2
Net Reaction
O + O3 -> 2O2
“Ozone Depletion”
1995 Nobel Prize in Chemistry
![Page 24: Atmospheric Chemistry of the Ozone Layer](https://reader036.vdocument.in/reader036/viewer/2022081508/56815577550346895dc33ec7/html5/thumbnails/24.jpg)
Consequences of Competing Reactions
Catalytic Reactions
Cl + O3 -> ClO + O2
ClO + O -> Cl + O2
- lower activation energy
Ea for Chapman mechanism = 17.1 kJ/mol
Ea for ClOx reaction = 2.1 kJ/mol
catalyst
catalyst
intermediate
intermediate
![Page 25: Atmospheric Chemistry of the Ozone Layer](https://reader036.vdocument.in/reader036/viewer/2022081508/56815577550346895dc33ec7/html5/thumbnails/25.jpg)
Depleting reactions are NOT independent of each other; in fact all occur simultaneously
Effect of competing reaction on rate of ozone formation
Consequences of Competing Reactions
NET LOSS OF OZONE
![Page 26: Atmospheric Chemistry of the Ozone Layer](https://reader036.vdocument.in/reader036/viewer/2022081508/56815577550346895dc33ec7/html5/thumbnails/26.jpg)
Sources of ozone depleting molecules
Naturally occuring species (H2O, N2O, CH4)
Artificial, “man-made” species
CFC’s (CCl3F,CCl2F2, etc.)
CCl4, CHCl3
HBFC (CHFBr2,CHF2Br)
CH3Br
NO from supersonic aircrafts
The artificial compounds have the most severe effect
![Page 27: Atmospheric Chemistry of the Ozone Layer](https://reader036.vdocument.in/reader036/viewer/2022081508/56815577550346895dc33ec7/html5/thumbnails/27.jpg)
What is the “Ozone Hole”?
Every year, in October, a huge “hole” in atmospheric levels of ozone is observed over the Antarctic.
QuickTime™ and aAnimation decompressor
are needed to see this picture.
August 1 ‘96 - Dec 15
‘96 NASA Goddard Space Flight Center
![Page 28: Atmospheric Chemistry of the Ozone Layer](https://reader036.vdocument.in/reader036/viewer/2022081508/56815577550346895dc33ec7/html5/thumbnails/28.jpg)
QuickTime™ and aAnimation decompressor
are needed to see this picture.
![Page 29: Atmospheric Chemistry of the Ozone Layer](https://reader036.vdocument.in/reader036/viewer/2022081508/56815577550346895dc33ec7/html5/thumbnails/29.jpg)
Why does the Ozone Hole form over the Antarctic and why in spring?
The Antarctic Vortex
Polar Stratospheric Clouds
Concentrations of Active Chlorine
![Page 30: Atmospheric Chemistry of the Ozone Layer](https://reader036.vdocument.in/reader036/viewer/2022081508/56815577550346895dc33ec7/html5/thumbnails/30.jpg)
The Antarctic Vortex
In the winter, the air around the S. Pole cools and circulate west creating a “vortex”
Air is trapped in the vortex along with ozone depleting species
Heat from outside is “shut off”, prolonging the duration of low stratospheric temperatures.
![Page 31: Atmospheric Chemistry of the Ozone Layer](https://reader036.vdocument.in/reader036/viewer/2022081508/56815577550346895dc33ec7/html5/thumbnails/31.jpg)
Polar Stratospheric Clouds
Low stratospheric temperatures result in “ice clouds” called Polar Stratopsheric Clouds
The surface of the ice clouds serve as reaction sites for heterogeneous gas-surface reactions
ClONO2 + HCl -> HNO3 + Cl2
ClONO2 + H2O -> HNO3 + HOCl
![Page 32: Atmospheric Chemistry of the Ozone Layer](https://reader036.vdocument.in/reader036/viewer/2022081508/56815577550346895dc33ec7/html5/thumbnails/32.jpg)
Concentrations of Active Chlorine
The Cl2 and HOCl formed photodissociate to yield reactive Cl atoms
Cl2 + h -> Cl + Cl
HOCl + h -> Cl + OH
Cl + O3 -> ClO + O2
OZONE DEPLETION
![Page 33: Atmospheric Chemistry of the Ozone Layer](https://reader036.vdocument.in/reader036/viewer/2022081508/56815577550346895dc33ec7/html5/thumbnails/33.jpg)
The Antarctic vortex traps CFC’s
The low polar temperatures results in ice particles on which gas-solid reactions can occur efficiently
The same reactions in the gas phase have much higher activation energies. The higher Ea and low temperatures result in very slow rates.
The onset of spring corresponds to higher light intensities and hence photolysis of Cl containing species
“Ingredients” for the formation of the Ozone Hole
![Page 34: Atmospheric Chemistry of the Ozone Layer](https://reader036.vdocument.in/reader036/viewer/2022081508/56815577550346895dc33ec7/html5/thumbnails/34.jpg)
What is being done to reduce ozone depletion?
Montreal Protocol and subsequent treaties ban world-wide usage of ozone depleting substances
Assuming full compliance expect that ozone levels will return to “natural” levels ~2050
http://www.nobel.se/announcement-95/announcement95-chemistry.html
![Page 35: Atmospheric Chemistry of the Ozone Layer](https://reader036.vdocument.in/reader036/viewer/2022081508/56815577550346895dc33ec7/html5/thumbnails/35.jpg)
References
NASA Goddard Space Flight Center (http://www.gsfc.nasa.gov/)
EPA (www.epa.gov)
Center for Atmospheric Science, Cambridge University
www.atm.ch.cam.ac.uk/tour/index.html
Chemical Kinetics and Dynamics,Ch 15, J. Steinfeld, J. Francisco, W. Hase