Chapter 2:
Protecting the Ozone Layer
1. Why do we need to do to protect the ozone layer? 2. Isn’t ozone hazardous to
human health?
3. Why is the ozone layer getting smaller?
4. What can we do (if anything) to
help stop the depletion of our ozone layer?
Ozone Formation
Ozone Formation
Energy + 3 O2 2 O3
Ozone is an allotropic form of oxygen. The name ozone comes from a Greek word meaning “to smell”
Energy must be absorbed
(endothermic) for this reaction to occur.
Element Allotropes oxygen O2, O3
carbon graphite, diamond, buckminister fullerenes 2.1 An allotrope is two or more forms of the same element that differ in
O
8
16.00
Atomic number (Z)
Mass number (A)
-The number of protons.
(nuclear charge)
-The sum of the protons and neutrons.
The electrons in the outermost
energy levels are called
valence electrons.
2.2 The group number (of the representative elements) on the
periodic table tells you the number of valence electrons.
1A
2A 3A 4A 5A 6A 7A
8A
Group 1A: 1 valence electron
2.2
Isotopes
are two or more forms of the same element
(same number of protons) whose atoms differ in
number of neutrons, and hence in mass.
Isotopes of carbon:
C-12, C-13, C-14
also written as:
12C
13
C
14C
• Carbon 14 ( 6C)
– Used in dating old materials – Atomic number: 6
– Atomic mass: 14
– # of neutrons = Atomic mass-Atomic #
– 6 protons, 6 electrons, 8 neutrons
Representing molecules with
Lewis structures
:
2.3
Consider water, H
2O:
1. Find sum of valence electrons:
1 O atom x 6 valence electrons per atom = 6 + 2 H atoms x 1 valence electron per atom = +2
8 valence electrons 2. Arrange the electrons in pairs; use whatever electron pairs
needed to connect the atoms, then distribute the remaining electron pairs so that the octet rule is satisfied:
O
H
H
lone pair
bonding pair
O
2.3
Representing molecules with
Lewis
structures
:
Typical valence for selected atoms = the # of bonds an atom typically forms
Element Typical
valence Classification
H,
X (X= F, Cl, Br, I)
1 monovalent
O 2 divalent
N 3 trivalent
2.3
Representing molecules with
Lewis
structures
:
Multiple bonds
O
O
H
C
C
H
Triple bond Double bond
Occasionally a single Lewis structure does not adequately represent the true structure of a molecule, so we use resonance forms:
N O
O O
N O
O O
N O
2.4
The Nature of Light
Low E
High E
Wavelength () = distance traveled between successive peaks (nm). Frequency () = number of waves passing a fixed point in one second
The Electromagnetic Spectrum
Visible: = 700 - 400 nm
M
M
UD
A
S
I
R
Infrared (IR) : longest of the visible spectrum, heat ray absorptions cause molecules to bend and stretch.
Microwaves: cause molecules to rotate.
Short range: includes UV
(ultraviolet), X-rays, and gamma rays. Decreasing wavelength
2.5
The energy of a photon of electromagnetic radiation
is calculated by:
E = h
where
h
= 6.63 x 10
-34J
.s (Plank’s constant)
The wavelength and frequency of electromagnetic
radiation are related by
:
c =
where
c
= 3 x 10
8m/s (the speed of light)
UV radiation has sufficient energy to cause molecular bonds to break
2.5 What is the energy associated with a photon of light with a wavelength of 240 nm?
C =
= C
x 108 m/s
E = h
240 nm x
nm
10-9 m = 1.3 x 1015 s-1
E = (6.63 x 10-34 J.s) (1.3 x 1015 s-1)
The Chapman Cycle
2.6
A
steady state
condition
≤
Factors influencing steady-state
Factors influencing steady-state
concentration of O
concentration of O
33• Intensity of UV radiation
– Expect O3 concentration will depend on season and
sun cycle
• Concentration of O
2and other reacting species
• Temperature
• Reaction rates (some of the steps are faster
than others)
2.7
Biological Effects of Ultraviolet Radiation
The consequences depend primarily on:
1. The energy associated with the radiation. 2. The length of time of the exposure.
3. The sensitivity of the organism to that radiation.
An Australian product uses “smart bottle” technology; bottle color changes from white to blue when exposed to UV light.
The most deadly form of skin cancer, melanoma, is
linked with the intensity of UV radiation and the latitude at which you live.
As decreases, damage
to DNA increases.
O
3
concentrations over time
http://earthobservatory.nasa.gov/cgi-bin/texis/webinator/ printall?/Library/Ozone/ozone_2.html
1 Dobson unit (DU) = 1 O3 molecule per 1 billion (109) molecules of air
320 Du is average ozone level over the northern hemisphere 250 DU is typical at the equator
Natural causes of O
Natural causes of O
33destruction
destruction
1) Reactions with water vapor and its breakdown products: H2O + UV photon H• + •OH
These free radical products can
participate in reactions that convert O3 to O2:
O3 + H• O2 + •OH
O3 + •OH 2O2 + H•
Example of a chain reaction. Free radicals are very destructive
2) Reactions with stratospheric nitrogen
2) Reactions with stratospheric nitrogen
monoxide (NO)
monoxide (NO)
• Formation of natural NO: N2O + O 2NO
– N2O produced by microorganisms • Formation of man-made NO: N2 + O2
+ energy NO
– Energy is from lightning or high-temp engines of supersonic transport airplanes (like the
Concorde) that are designed to fly at altitudes of 15-20 km (the region of the ozone layer)
• Lewis dot structure of NO?
• Turns out that NO is also a free radical…
energy
Source of VOC and NO in the Upper Atmosphere leading to Tropospheric Air Pollution Chemistry
CFCs seem to be the culprit
CFCs seem to be the culprit
Properties and uses of CFC • Stable
• Originally developed as refrigerants to replace the toxic ones that had previously been used (SO2 and ammonia—NH3)
• Also used as:
– Solvent for computer chips – Propellants for aerosol cans – Styrofoam
Revolutionized modern life! Throughout the 1960s and 70s, cheap air conditioning using CFCs helped spur the booming growth of Southern cities such as Atlanta, Dallas-Fort Worth and Houston.
Before we knew about the ozone hole…
Before we knew about the ozone hole…
• Two scientists, Roland &
Molina, set out to study fate of atmospheric CFC molecules • The reactions they
hypothesized would occur suggested that CFCs could destroy significant amounts of O3
– The destruction of O3 by CFCs later shown by scientific experiments.
• ~ 5 years after release in troposphere, CFCs make their way into the
stratosphere.
• A high-energy photon corresponding to
wavelengths 220 nm has enough energy to break the chlorine-carbon bond:
Reactions with •Cl
•Cl +O3 O2 + •OCl
•OCl + O •Cl + O2
O3 + O 2O2
You can see that once •Cl is created that it begins a chain reaction—it can destroy many (~ 100,000!) O3
molecules.
•Cl is acting as a catalyst—it is not used up in the reaction
The presence of •Cl will affect the Chapman cycle balance!!!
• Carried to lower
atmosphere by winds—
once in lower atmosphere, there are many other
compounds for •Cl to react with.
• If it encounters another •Cl the following reaction may occur: •Cl + •Cl Cl2
• May react with other species to form stable compounds such as HCl and ClONO2
Proof of relationship b/w ozone
Proof of relationship b/w ozone
depletion and stratospheric Cl
depletion and stratospheric Cl
• 75-85% is from human activity – CFCs
– Rocket fuel from space shuttles (<1%)
• 15-20% is from methyl chloride – Most from natural sources
and burning of biomass
• ~2% - Large, explosive volcanoes – Increase amount of HCl
which can be converted to •Cl – Only a few volcanoes have
Why is the hole over Antarctica?
Why is the hole over Antarctica?
• A special mechanism appears to be in effect here, related to the fact that the lower stratosphere over the South Pole is the coldest spot on Earth.
– During the Antarctic winter (June to September), a strong circumpolar wind develops in the middle to lower atmosphere
• keeps warmer new air (w/ new O3) from entering – Temps can get as low as -90oC!
– At temps < 80oC, clouds of ice crystals containing sulfates and nitric acid can develop in the stratosphere.
http://earthobservatory.nasa.gov/Study/Tango/
• In spring, when the sun comes out, chemical reactions occurring on the surface of these cloud particles convert otherwise safe (non-ozone depleting) molecules to more reactive species.
– ClONO2 & HCl converted to HOCl & Cl2
Summer conditions replenish hole
Summer conditions replenish hole
spring
summer
• In summer, sunlight warms
the atmosphere
– Ice crystals evaporate
• No ice crystals means the conversion of ClONO2 & HCl
to more reactive species halts.
– Air from lower latitudes
flows into the polar regions, replenishing depleted ozone levels.
– Thus, “Ozone Hole” is
largely replenished by the end of summer.
The Global Response
The Global Response
• CFCs in spray cans banned in North America in 1978, and use as foaming agents for plastics discontinued in 1990.
• 1987 Montreal Protocol—established a schedule to reduce production & consumption of CFCs.
• The US and 140 other countries agreed to a complete ban on CFC production after Dec. 31, 1995.
• Other ozone depleting compounds to be eliminated b/w 2002-2010:
•CFBr compounds (fire-fighting) •CCl4 (an industrial solvent) •CH3Br (agricultural fumigant)
Black-market CFCs
Black-market CFCs
• While the protocols set dates for the halt
of production, sale of existing stockpiles
will remain legal.
– US govt. is promoting conversion to less
harmful substitute refrigerants by imposing
a tax of $5.35/lb on CFCs
• Price of CFCs has risen $1/lb in 1989 to $15/lb (as of 2003)
• Creates a black-market for CFCs, which are 2nd
only to illegal drugs as the most lucrative illegal import.
Ban on CFCs is making a difference--slowly
• Stratospheric concentration of chlorine peaked at 4.1 ppb in the mid-90s and is now declining.
• However, the stratospheric chlorine is not expected to reach 2 ppb* until 2050 or 2075.
• *Antarctic ozone hole first appeared when chlorine levels reached 2 ppb.
• DON’T want to go back to previous technologies
(NH3, SO2 as refrigerants)! • Need to balance 3
Fluorocarbons (FCs)
Fluorocarbons (FCs)
• Made of only carbon and fluorine
– Non-toxic
– Nonflammable
– Don’t decompose in the stratosphere
• Why? The C-F bond is stronger—the energy requirement to break this bond is higher than that of a UV photon.
• The negative: No destruction pathways
—
the molecules will accumulate in the
atmosphere and contribute to the
greenhouse effect
by absorbing IR
Hydrochlorofluorocarbons
Hydrochlorofluorocarbons
(HCFCs)
(HCFCs)
• Replacing one or more of the halogen atoms
with hydrogen makes the compound more
reactive
– Compounds decompose long before
reaching stratosphere
• However:
Suitable HCFCs
Suitable HCFCs
• HCFC-22 is the most widely-used HCFC
– Used in air-conditioners and in the production of foamed fast food containers
– 5% ozone depleting potential of CFC-12 • HCFC-141b used to form foam insulation
– 250 million lbs produced worldwide in 1996 for this purpose
• Montreal Protocol calls for a production ban of HCFCs by 2030.
Decompose in the troposphere more readily than CFCs
Hydroflurocarbons (HFCs)
Hydroflurocarbons (HFCs)
• Example: HFC-134a
• Has no Cl atoms to
interact with ozone
• The 2 H atoms facilitate
decomposition in the
lower atmosphere,
without making it
flammable under normal
conditions.
H H C C
http://www.eia.doe.gov/oiaf/1605/vr98rpt/chapter6.html
Timeline
Timeline
• 1928 – CFCs invented
• 1950s-1970s – Consumption and use of CFCs rises rapidly • 1971 – CFCs measured in the atmosphere
• 1974 – Rowland & Molina link CFCs with ozone depletion
• 1985 – First scientific assessment of stratospheric ozone levels • 1987 – “Smoking gun” evidence linking decreasing ozone levels with increasing stratospheric chlorine levels
– Montreal Protocol established a schedule to reduce production & consumption of CFCs.
• 1991 – Multinational fund established to provide financial and technical assistance to developing countries to enable them to comply with control measures.
- B/w 1991 & 2004 $1.6 billion given to more than 100 countries • 1995 – Production ban of CFCs in US and 140 other countries
goes into effect