*
& UV Radiation Exposure
Danielle LeBrun & Bertrand Tematio
* Ozone exist as a
triatomic molecules,
containing three oxygen
(O3) atoms covalently
bonds.
* Ozone is unstable and
will readily combine with
other atoms.
*
* Ozone is found in the
stratosphere between 10 –
50km, where it blocks the
sun's ultraviolet (UV) waves
and prevents them from
reaching the earth's surface.
* Ozone is also found in the
troposphere between 0 –
10km, where it can damage
living tissue and humanproduced objects. It helps
keep the air fit to breathe.
*
* UV radiation is most emitted by
the sun and has a shorter
wavelength than visible light. It
absorbs in the 200 to 400 nm
region in the electromagnetic
spectrum.
* UV radiation is subdivided into
three distinct spectral ranges:
UVA ( 320 – 400 nm) relatively
harmless.
UVB (290 – 320 nm) causes
sunburn, skin cancer, and other
disorders.
UVC ( 200 – 290 nm) extremely
harmful
*
* Short wavelengths have more energy per
photon than long wavelengths.
* Ozone is formed by the action of the sun's
ultraviolet radiation shorter than 240nm on
oxygen molecules.
* Ultraviolet light splits the molecules apart by
breaking the bonds between the atoms. A
highly reactive free oxygen atom then
collides with another oxygen molecule to
form an ozone molecule
O2 + UV (<240nm)  O + O Slow (1)
O + O2 + M  O 3 + M
Fast (2)
M denotes particles in the atmosphere that
carries off the excess energy of the reaction
*
* Because ozone is unstable, ultraviolet light
quickly breaks it up, and the process begins
again.
O3 + h  O + O2
O3 + O  2O2
Fast (3)
Slow (4)
NO + O2  NO2 + O2 fast
NO2 + O  NO + O2 fast
Net: O3 + O  2O2
fast
Nitrogen dioxide (NO2) is a
brownish gas that is observed in
major cities
In the lower stratosphere, one NO
molecule can destroy hundreds
or thousands of O3 molecules.
*
Cl + O3  ClO + O2 very fast
ClO + O  Cl + O2 very fast
Net: O3 + O  2O2
very fast
This cycle is faster than the nitrogen
cycle, therefore, destroys more
effectively ozone.
NB: Bromine (Br) radicals and
hydroxyl (OH) radicals catalytic
cycle can also destroy ozone.
* Good Ozone
* Ozone and oxygen absorb 95 to 99.9% of the
sun's ultraviolet radiation
* only ozone effectively absorbs the most
energetic ultraviolet light (UV-C and UV-B)
* Life can not exist without the protection
provided by the Ozone
*
* 10% of the ozone in the earth's atmosphere is
found in the troposphere.
* Ozone in high concentration is toxic to plants
and irritates both humans eyes and lungs.
* Ozone is a key component of the photochemical
smog observe in some cities.
* Ozone in the troposphere cleans the lower
atmosphere of pollutants such carbon monoxide
(CO) and sulfur dioxide (SO2) that keep the air
fit to breath.
*
* The hole in the earth's ozone layer, over Antarctica, was first detected in 1976
* They are caused by the formation of ice particles known as polar stratospheric
clouds (PSCs) which alter the chemistry of lower stratosphere by:
(1) coupling between the odd nitrogen and chlorine cycles
ClO + NO2  ClONO2 + M
(2) by providing surfaces on which heterogeneous reactions can occur.
HCl + ClONO2  Cl2 + HNO3
Cl2 + h  2Cl
Atomic chlorine attacks the ozone immediately that is formed
* More research is still needed
*
* CFCs (chlorofluorocarbons) have been extensively used since the
1960s as refrigerants, industrial cleaning solvents, propellants in
aerosol spray cans, and to make Styrofoam. They had interesting
thermodynamic properties, they were inert and nontoxic to humans.
* CFCs drift slowly upward to the stratosphere, where they are broken
up by ultraviolet radiation, releasing the chlorine that catalytically
destroys ozone. The most common compounds are freon-11 (CCl3F)
and freon-12 (CCl2F2)
CCl3F + UV (<220nm)  CFCl2 + Cl
CCl2F2 + UV (<220nm)  CF2Cl + Cl
*
* 1. UV radiation breaks off a chlorine atom from a CFC molecule.
* 2. The chlorine atom attacks an ozone molecule, breaking it
apart and destroying the ozone.
* 3. The result is an ordinary oxygen molecule and a chlorine
monoxide molecule.
* 4. The chlorine monoxide molecule is attacked by a free oxygen
atom releasing the chlorine atom and forming an ordinary oxygen
molecule.
* 5. The chlorine atom is now free to attack and destroy another
ozone molecule. One chlorine atom can repeat this destructive
cycle thousands of times.
*
* Ratification by international
community of treaties. The Montreal
Protocol in 1987 limited the amount
of freons and halons that could be
released into the atmosphere by any
country.
* Substitution for freons by the
chemical industry. One of these is
replace one of the chlorine or fluorine
atoms with hydrogen atom to make
HCFCs
* Engagement of citizens. Consumers
should start buying ozone friendly
products such as CFC substitutes.
*
* UV Beads as an inquiry based lab
* Beads change color when exposed to UV light
* Design experiments
*
1.
2.
3.
4.
5.
6.
7.
8.
*
Do clouds block some or all UV rays?
Is old sunscreen as effective as new sunscreen?
Do T-shirts block some or all UV rays?
Does windshield glass block some or all UV rays?
Are all of the windows in a car equal when it comes to blocking UV rays?
Does SPF 30 sunscreens give twice the protection of SPF 15?
So UV filtering sunglasses really block UV rays?
Is exposure to UV rays at midday as great as exposure to UV rays at other times
during the day?
9. Is sunlight reflected from a white surface as dangerous as direct sunlight?
10. Does water reflect all or some UV rays?
11. Are UV rays able to travel through water?
12. Design your own experiment. Please have your experiment approved by the
instructor before you begin.
0 = Bead did not change color (remained
white)
1 = Very Pale/Light
3 = Medium
5 = Medium/Dark
9 = Very Dark
GROUND LEVEL OZONE TESTING
This is the color scale that shows the
ozone levels.
* Gives measurable
level of ground
level ozone
Use this graph to get your results from the
color above.
Schoenbein Color Scale
0-3 Little or no change
4-6 Lavender Hue
7-10 Blue or Purple
* Easy to make
*
*
* Preparation of the strip test by the teacher
* Safety: Potassium iodide can cause mild skin irritation
* 1. Place 100 ml of distilled water in a 250 ml beaker.
* 2. Add 1 1/4 teaspoon of corn starch.
* 3. Heat and stir mixture until it gels. The mixture is gelled when it thickens and
becomes somewhat translucent.
* 4. Remove the beaker from the heat source and add 1/4 teaspoon of potassium
iodide and stir well. Cool the solution before applying to the filter paper.
* 5. Lay a piece of filter paper on a glass plate, or hold it in the air, and carefully
brush the paste onto the filter paper. Turn the filter paper over and do the same on
the other side. Try to apply the paste as uniformly as possible.
* 6. Wash hands after applying the potassium iodide mixture. (Although potassium
iodide is not toxic, it can cause mild skin irritation.)
* 7. Set the paper at a low temperature drying oven.
* 8. Cut the filter paper into 1-inch wide strips.
* 9. To store the paper, place the strips in a sealable plastic bag out of direct sunlight.
* Students will be grouped by 4 and they will construct an ozone map of their area.
*
* 1.
Each student gets two strips of Schoenbein paper in a sealed bag.
* 2.
Each student moistens the paper with distilled water and hangs the test
strips at two sites in their home.
* 3.
Each student will expose the paper for approximately 3 days, will record
the test result every day (To observe and record test results, spray the paper
with distilled water. Observe the color)
* 4.
Each group will get the map of the city or a map may be posted in the
classroom.
* 5.
Each student should record their data noting the color change they
observe.
* 6.
Each group can then plot the relative ozone concentrations on a local,
United States or globe.
Schoenbein Color Scale
0
1
2
3
0–3
4–6
7 – 10
4
5
6
7
8
9
10
Little or no change
Lavender Hue
Blue or Purple
Ideas: Have students mail their Schoenbein paper to relative in other parts of the
country or internationally to test ozone levels.
References
http://www.stevespanglerscience.com/experiment/uv-reactive-beads
http://www.teachersource.com/category/light-ultraviolet
http://www.ucar.edu/learn/1_7_2_29t.htm#procedure
http://beprepared.com/product.asp?pn=FS%20C150&sid=GOOGLE
http://www.flinnsci.com/store/Scripts/prodView.asp?idproduct=20762
http://www.flinnsci.com/store/Scripts/prodView.asp?idproduct=14655
http://www.ucar.edu/learn/1_5_1.htm
The earth system, third edition, Kump. Kasting. Crane
Chemistry, 10th edition, Raymond Chang