Option E
Environmental Chemistry
http://visibleearth.nasa.gov/view_rec.php?id=1023
E9: Ozone Depletion (AHL)
A Few Points about E4
E9 Assessment Statements (AHL)
E.9.1
Explain the dependence of O2 and O3 dissociation
on the wavelength of light. (covered in the Summer 2012 Assignment)
!
E.9.3
Describe the mechanism in the catalysis of O3
depletion by CFCs and NOx.
!
E.9.4
Outline the reasons for greater ozone depletion in
the polar regions.
Consider the seasonal variation in temperature in the upper atmosphere. Refer to
surface catalysis on ice particles.
Catalysis of
Ozone Depletion
Catalysis of Ozone Depletion
Recall: The Natural Cycle
2 O3(g) D 3 O2(g)
Chlorofluorocarbons (CFCs) and nitrogen oxides (NOx) from human activities act as catalysts in the destruction of ozone.
O3
>
depletion
O3
formation
Catalysis of Ozone Depletion
1. Chlorofluorocarbons (CFCs) - mechanism
The mechanism for the CFC catalyzed depletion of ozone is described by these steps:
Step 1: The photodissociation of the CFC
produces a very reactive free radical chlorine atom.
The carbon-chlorine bond is weaker than the carbon-fluorine
bond, so a chlorine atom breaks off instead of fluorine.
Step 2: The free radical chlorine atom
catalyzes the decomposition of ozone:
Step 3: A free radical oxygen atom in the
stratosphere reacts with the ClO• free radical, forming more free radical chlorine atoms. A single chlorine atom can destroy thousands of ozone
molecules.
The OVERALL equation for the depletion of
ozone by a free radical chlorine catalyst is:
uv light
CCl3F(g) g •CCl2F(g) + Cl•(g)
O3(g) + Cl•(g) g O2(g) + ClO•(g)
O•(g) + ClO•(g) g O2(g) + Cl•(g)
Cl•
O3(g) + O•(g) g 2 O2(g)
Catalysis of Ozone Depletion
2. Nitrogen Oxides - mechanism
Nitrogen oxides form in the by the reaction between nitrogen and oxgyen gas that occurs in the high temperature of jet engine internal combustion engines.
Nitrogen oxides may catalyze the breakdown of ozone by different mechanisms. One example is shown below:
Step 1: Nitrogen monoxide reacts with
ozone:
Step 2: The nitrogen dioxide that forms
reacts with oxygen free radicals in the
stratosphere:
The OVERALL equation for the depletion
of ozone by a nitrogen oxide catalyst is:
O3(g) + NO(g) g O2(g) + NO2(g)
NO2(g) + O•(g) g NO(g) + O2(g)
NO
O3(g) + O•(g) g 2 O2(g)
Ozone Depletion in
Polar Regions
Ozone Depletion in Polar Regions
August 2000
Arctic
Antarctic
darker blue = lower O3 concentration
Why is ozone depletion more severe in polar regions?
Why are ozone levels lower in the early spring months in these areas than at
other times of the year?
Ozone Depletion In
in Polar Regions
http://www.theozonehole.com/ozonedestruction.htm
The reduction in ozone levels is greater over the poles.
!
What factors contribute to ozone loss with changing latitude?
Ozone Depletion in Polar Regions
Why is the depletion of ozone greater at the poles?
1. severe drop in stratospheric temperatures at the poles
strong circular winds form = polar vortex
vortex prevents warmer air from entering
2. total darknesses at the poles
keeps temperatures extremely low
maintains the polar vortex
Ozone Depletion in Polar Regions
3. polar vortex + extremely cold conditions promote the formation of polar stratospheric clouds at the poles
formation of ice crystals
compounds with Cl (HCl and
ClONO2) trapped inside the
crystals
http://www.nasa.gov/vision/earth/lookingatearth/25TOMSAGU.html
surfaces of ice crystals act as
catalysts for the formation of Cl2
Ozone Depletion in Polar Regions
4. Aaaahhhh ... spring arrives!
Hello Sunshine ... more sunlight available in the spring
Cl2 molecule (and others) start to photodissociate
Cl2 g Cl• + Cl•
Chlorine free radicals catalyze
the destruction of ozone
(see the notes on CFC catalyzed ozone depletion)
http://www.atmosphere.mpg.de/enid/1z2.html
The rate of ozone depletion INCREASES in the spring over
the poles due to the presence of polar stratospheric clouds.
Ozone Depletion - Seasonal Variations
4. Summertime!
warmer air over the poles in the summer
polar stratospheric clouds disappear
ozone depletion slows or stops
Ozone Depletion in Polar Regions
Key Points
The large scale depletion of the ozone layer that forms the
“ozone hole” seen over Antarctica is only possible with the
generation of the extremely cold conditions that produce the
polar stratospheric clouds.
The seasonal variation in the ozone layer over the poles is
related to this dependency on extremely cold temperatures.
Ozone depletion over the Arctic is less than the Antarctic
because the temperatures around the North Pole are
higher than over the South Pole.