Environmental Physics
Lecture 3
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web: http://people.physics.anu.edu.au/~jps107/
ChloroFlouroCarbons (CFCs)
Man-made chemicals
CFCl3 (Freon-11), CF2Cl2 (Freon-12)
Used in aircon, refrigerators, foam, solvents, propellants
Long lifetime in atmosphere
CFCL3 – 45 yrs, CF2Cl2 – 100 years
Can travel up to the ozone layer
Takes around 15 years
Chemical reactions of CFCs
CFCl3 + hν (>3.5 eV, <350nm)--> CFCl2 + Cl
O3 + Cl --> O2 + ClO
ClO + O --> Cl + O2
ClO + O3 --> Cl + 2O2
Chlorine atom is a catalyst
How will this affect ozone concentration?
Ozone hole
Ozone depletion
Reduction in ozone corresponds to increase in
stratospheric CFCs
Why has it stopped?
Ozone Hole
Montreal Protocol instituted in 1989
Phase out production and use of ozone
depleting substances contain either chlorine
or bromine.
If adhered to, the ozone layer is expected to
recover by 2050
The greenhouse effect
What is a greenhouse?
What does it do?
How?
Absorption of light – what is the type of light we get
from the sun?
Greenhouses
The glass or plastic in a greenhouse roof and walls is
transparent to visible and infrared light.
Light is absorbed by the material inside a greenhouse
What materials?
Light is converted to heat energy
How? What happens when a photon is absorbed by a
solid? Do all photons get absorbed?
Absorb energy by phonon excitation – the material
vibrates
It then heats up.
Greenhouses
Heat is transferred to the gas (air) inside the
greenhouse
What form does this heat take?
What happens to the air?
If there is no roof or walls, the hot air can mix with
colder air and it cools down
The greenhouse traps the hot air and it stays inside
The greenhouse is warmer than the surrounding
environment
Greenhouse effect
What are we talking about?
Earth’s atmosphere
This is not exactly the same as how a greenhouse
works!
Let’s talk about blackbodies and blackbody radiation
What is a blackbody?
Hypothetical object, absorbs all radiation
Will also radiate energy – wavelength of radiation emitted
will depend on temperature
Greenhouse effect
Consider the Earth – it absorbs light from the Sun and
warms up
This is similar to a blackbody
Energy from the sun is around 3.5 to 7 kWh/m2 each day
However, the Earth also reflects light
Reflects around 30% of
sunlight.
What does this mean?
Greenhouse effect
From consideration as a blackbody, the Earth would
have an average surface temperature of around
-18oC
Real average surface temperature is actually 14oC – why?
Atmosphere!
The atmosphere traps heat that would otherwise radiate
away into space
What is the atmosphere made of?
Nitrogen (N2) - 78 %, Oxygen (O2) - 21% , Argon (Ar) - 0.9%,
Carbon dioxide (CO2) - 0.04% + others (not including water)
Greenhouse effect
How does the atmosphere trap heat?
Does it work like the greenhouse in our previous
example?
No! There is no confinement of hot gas, it can freely
circulate throughout the atmosphere
Earth radiates photons (energy) in the wavelength
range 4 – 100 µm
N2, O2 and Ar DO NOT absorb radiation in these
wavelengths
CO2 (and H2O) DO absorb these wavelengths
Greenhouse effect
What happens when the infrared (IR) radiation is
absorbed?
Greenhouse effect
So what happens to the radiation from the Earth’s
surface?
It is absorbed by CO2 (and other gases, H2O, methane
(CH4), ozone (O3) etc…)
When reemitted, some can travel back towards Earth
This radiation is trapped and can then heat up the Earth further
This has the effect of warming the Earth more than
would be expected from just the “blackbody effect”
Makes life possible…
Greenhouse effect
In 1850, Carbon dioxide (CO2) concentration was
around 0.028 %
In 1950, around 0.031 %
In 2010, around 0.038 %
What does an increase
in CO2 mean?