Jan 21 Atmospheric structure & Composition (Chapter 3)

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The Atmosphere
It appears that
approximately an inch (2.5
cm) of ice is coating this
hapless tree. The icy fangs
are an indication that melt is
underway. Typically, a
quarter inch (0.65 cm) of ice
is all that’s needed for tree
branches to begin to snap.
Frequency, Wavelengths & Energy of
Photons
Energy emitted from the sun
(i.e electromagnetic radiation
exhibits both a wave-like
(electromagnetic wave) and
a particle-like (photon)
nature.
Duality of light and matter
In 1690 Christiaan Huygens theorized that light was
composed of waves, while in 17-4 Isaac Newton explained
that light was made of particles. Experiments supported
each of their theories. However neither a completelyparticle theory nor a completely-waver theory could explain
all of the phenomena associated with light.
So how can something be both a particle and a wave at the
same time?
The wave of light is simply the probability of where the
particle will be.
Let’s hear a little quantum physics explained.
Energy in the form of photons is absorbed or
emitted as electrons change energy levels
within the structure of an atom.
Photon = A particle-like unit of electromagnetic
energy (light) emitted or absorbed by an atom
when an electrically charged electron changes
state.
Recall that
Photons are energy packets having a
well-defined wavelength and
frequency.
a) An electron in its ground state about to
absorb a photon
b) The electron leaps to a higher level as the
photon is absorbed.
As an electron emits or “gives off”
electromagnetic energy (in the form of a photon),
it jumps from a Higher to a Lower energy state
(level).
a) An electron in its excited or quantized state.
b) The electron leaps to a lower energy level
and a photon is emitted.
Summary of quantum mechanics & the link to absorption of
electromagnetic energy at the subatomic scale.
If a photon of electromagnetic energy strikes an atom,
And if the Frequency of the electromagnetic radiation is such
that it is equal to the difference in energy of the ground level
& the first excited level,
The electron Absorbs the photon energy and….
The electron is “moved” (quantum leap) to “Level 2”
Quantum behavior of Molecules
Quantum theory also involved the behavior of
molecules: the molecular-scale motion (I.e. rotation,
bending, & vibration) of molecules.
Molecular motions in the gases Water Vapour and
Carbon Dioxide (H2O and CO2) explain why some
gases contribute to the greenhouse effect and others
do not.
Both Sun & Earth are
radiating energy…
…At different
electromagnetic
wavelengths
….and at different
frequencies
Wavelengths
Quantifying Frequency & Wavelengths
First we’ll talk about the WAVE-like behavior
of electromagnetic energy:
Wavelength = the distance between adjacent
crests (or troughs) (symbol = lambda )
Frequency = how fast the crests move up and
down (symbol = nu )
Speed = how fast the crests move forward
(symbol = c) the speed of light
The pattern of wavelengths absorbed by a
particular atom or combination of atoms, (e.g.
a gas molecule of CO2 or H2O)
Is called its Absorption Spectrum or its absorption
curve
Solar
Radiation
greatest
intensity in
SHORT
wavelengths
Earth
(terrestrial)
Radiation
entirely in
LONG
wavelengths
(high energy &
frequency)
(low energy &
frequency)
Quantum behavior of Molecules
Quantum theory also involved the behavior of
molecules: the molecular-scale motion (I.e. rotation,
bending, & vibration) of molecules.
Molecular motions in the gases Water Vapour and
Carbon Dioxide (H2O and CO2) explain why some
gases contribute to the greenhouse effect and others
do not.
Nitrogen Gas
Molecule
N2
Water Molecule
H2O
Carbon Dioxide
Molecule
CO2
Nitrogen Gas
Molecule
N2
Not a Greenhouse Gas
Water Molecule
H2O
Greenhouse Gases
Carbon Dioxide
Molecule
CO2
When the H2O molecule emits a photon its rotation
rate decreases.
When it absorbs a photon, the rotation rate
increases.
Molecules can also absorb and emit IR radiation by
changing the amplitude with which they vibrate.
If the frequency at which a molecule vibrates matches the
frequency of an electromagnetic wave, the molecule
can absorb a photon and begin to vibrate more
vigorously.
As a triatomic molecule,
one way that CO2
vibrates is in a
“bending mode” that
has a frequency that
allows CO2 to absorb IR
radiation at a
wavelength of about 15
micrometers.
Another triatomic molecule: N2O
does the same thing.
Dance your PhD
N2O acts as a greenhouse
gas through the absorption
of radiation in 3 vibrational
modes.
http://www.youtube.com/w
atch?v=L5j6BS3XoLc
With one hand as a
nitrogen atom, torso as
central nitrogen, and the
other hand as an oxygen
atom, the dancers exhibit
the three specific
movements of N2O’s
vibrational modes as it
moves from soil to
atmosphere.
The N2O starts in the soil where it is
produced by microbial activity and “moves
on up” into the atmosphere.
Stepping onto the chairs represents the
progression of N2O to higher levels in the
atmosphere (the stratosphere) where it is
subject to intense Ultraviolet UV radiation
from the sun.
This high energy from the bombarding UV
radiation is shown in the dancers’ high
energy, more spastic dancing.
The high intenesity UV radiation leads to
the destruction of N2O --Jumping from the
chair.
We will learn later that interaction of N2O in the stratosphere
with UV wavelengths is related to Ozone Depletion
…but N2O also vibrates & bends when absorbing Infrared
(IR) wavelengths
…it is the ability to absorb and emit IR radiation that makes
N2O a GREENHOUSE GAS
What defines a GREENHOUSE GAS?
Abbreviation we’ll use = GHG
GHG = a gas that can absorb and emit (re-radiate)
Infrared wavelengths of Electromagnetic Radiation
The Quantum Behavior of certain
molecules with respect to Infrared
radiation is the reason that
Greenhouse Gases are
Greenhouse Gases.
Longwaves
(LW)
Shortwaves
(SW)
Key bands in the spectrum for Global Change:
UV, Visible, IR, NIR
Definition of Greenhouse Gases
Greenhouse gases are gases which both
absorb and emit electromagnetic radiation in the
infrared (IR) part of the spectrum.
Once IR is absorbed by the greenhouse gases
in the atmosphere, it can be emitted back to the
Earth’s surface to heat it all over again.
Or it can be emitted upward to outer space and
be lost from the system altogether.
IR radiation
is absorbed
by GH
gases in the
atmosphere
and emitted
out to space
IR radiation is
emitted fro the
Earth’s surface
right out to
space through
“IR window”
IR radiation is
absorbed by
GH gases in
the atmosphere
and emitted
back to Earth.
Different gases absorb & emit radiation at different
wavelengths
How do we know which wavelengths are
absorbed/emitted by different gases?
The pattern of electromagnetic wavelengths that are
absorbed & emitted by a particular atom (or combination of
atoms)
Is called its Absorption Spectrum or Absorption Curve
Match the gas with its
absorption curve.
Choices: H2O
O2/O3 N2O CH4 CO2
Match the gas with its
absorption curve.
Choices: H2O
O2/O3 N2O CH4 CO2
Choices: H2O
O2/O3 N2O CO2
???
Key concepts to get out of all of this:
1. Solar radiation is mostly in
shortwave (SW) form (visible and
UV).
Most visible and UV wavelengths are
transmitted through the
atmosphere but some (esp.
harmful UV) are absorbed on their
way to Earth’s surface by O2 and
O 3.
2. Most of the incoming solar energy
absorbed by the Earth and the
atmosphere is absorbed at the
Earth’s Surface which then
radiates IR outward to heat up the
atmospehre.
Hence, the Atmosphere is heated
primarily from below
3. Terrestrial radiation is mostly in
longwave (LW) form (IR).
Much of the outgoing terrestrial
radiation is absorbed by H2O and
CO2 (and other GHG’s) before it
escapes to space, and it is reradiated back to the Earth’s
surface.
This is the “Greenhouse Effect”
4. The re-radiation of LW (IR) energy
to the Earth’s surface by GH
gases is what keeps the Earth in
the “just right” temperature range
for water to be present in all 3
phases and just right for US.
Without the “Greenhouse Effect” the
Earth would be too COLD for life
as we know it!
Objectives:
To understand:
The vertical structure of the atmosphere & its
relationship to temperature
Which gases are in the atmosphere
where they are concentrated
Why gases at different levels are linked to the
greenhouse effect & ozone depletion
http://earthguide.ucsd.edu/earthguide/diagrams/atmosphere/index.html
The Vertical Structure of the
Atmosphere
Key Concept:
The
atmosphere’s
vertical structure
is defined by
changes in the
trend of
temperature with
height
Atmospheric Pressure & Mass
vary with Height.
Atmospheric Pressure = weight
of the air column above
99 % of mass
lies below
~50km (top of
Stratosphere)
50% of mass
lies below ~6
km (middle
Troposphere)
Why the zig-zags in
the temperature
height graph?
The changes in
temperature with height are
the result of:
Differential absorption of
shortwave (SW) &
longwave (LW) radiation
By atmospheric gases
concentrated at various
altitudes.
Incoming solar SW
(mostly visible & near IR
+ UV)
Outgoing terrestrial LW
(Far IR) radiated from
Earth’s surface
Recall that Earth’s
atmosphere is heated
from below.
On its way to the Earth’s surface,
several things can happen to incoming
Solar radiation
Transmitted (to Earth’s surface)
Absorbed (by gases, dust, clouds)
Scattered/ Reflected
reflected back to space
Scattered (and indirectly transmitted to Earth’s
surface)
Why is the sky Blue?
Review: The pattern of electromagnetic wavelengths that
are absorbed & emitted by a particular atom (or
combination of atoms) is called its Absorption spectrum
(curve)
UV UV UV
UVA
(C & B) Visible
How incoming
Solar radiation of
different
wavelengths gets
Transmitted or
Absorbed by
different gases on
its way to Earth’s
surface.
Near IR
Q1 - The atmospheric layer of the troposphere is
important because:
1. It is the layer closest to the sun, which is the source
of the Earth’s energy
2. It is the layer in which temperature increases with
altitude in the atmosphere and where most of the
atmosphere’s ozone occurs
3. It is the layer in which most of our weather, heat
transfer, & greenhouse gases
Q1 - The atmospheric layer of the troposphere is
important because:
1. It is the layer closest to the sun, which is the source
of the Earth’s energy
2. It is the layer in which temperature increases with
altitude in the atmosphere and where most of the
atmosphere’s ozone occurs
3. It is the layer in which most of our weather, heat
transfer, & greenhouse gases
Q2 -Here are 3 graphs showing “something”
varying with altitude in the atmosphere. Which is
which?
1. A = water vapour, B = pressure, C = Temperature
2. A = temperature, B = pressure, C = ozone concentration
3. A = ozone concentration, B = temperature in the
troposphere, C = temperature in the stratosphere
Q2 -Here are 3 graphs showing “something”
varying with altitude in the atmosphere. Which is
which?
1. A = water vapour, B = pressure, C = Temperature
2. A = temperature, B = pressure, C = ozone concentration
3. A = ozone concentration, B = temperature in the
troposphere, C = temperature in the stratosphere
Q3 - Here is the graph of atmospheric
pressure vs. altitude, with “parcels of
air” shown to depict the density of the
atmosphere’s gases at 3 different
altitudes. If the air in Parcel X is forced
to subside (sink) to the altitude of Parcel
Z, what will happen to the air in Parcel
X?
1. It will get more dense and get cooler
2. It will get more dense and warm up
3. It will get more dense, and no
change in temperature will occur
Q3 - Here is the graph of atmospheric
pressure vs. altitude, with “parcels of
air” shown to depict the density of the
atmosphere’s gases at 3 different
altitudes. If the air in Parcel X is forced
to subside (sink) to the altitude of Parcel
Z, what will happen to the air in Parcel
X?
1. It will get more dense and get cooler
2. It will get more dense and warm up
3. It will get more dense, and no
change in temperature will occur
Atmospheric Composition
Which gases?
What concentration?
Which ones are Greenhouse Gases (GHG)?
Where do the GHG’s come from?
Most abundant gases in the atmosphere
Gas
Symbol
Nitrogen
Oxygen
Argon
N2
O2
Ar
Total
% by
volume
78.08
20.95
0.93
99.96
% in ppm
780,000
209,500
9,300
Next most abundant gases in the atmosphere
Gas
Symbol
Water
Vapour
Carbon
Dioxide
H2O
CO2
Greenhouse Gases!!
% by
volume
0.00001
to 4.0
0.0390
% in ppm
0.1-40,000
360 (in 1960)
390 (in June!)
Other important Gases
Gas
Symbol
% by volume
% in ppm
Methane
CH4
0.00017
1.7
Nitrous
Oxide
N2O
0.00003
0.3
Ozone
O3
0.0000004
0.01
CFC’s
0.00026
CCl3F
(Freon-11)
0.000000026
CFC’s
0.00047
CCl2F2
(Freon-12)
0.000000047
Greenhouse Gases
Absorption by ALL the gases in the atmosphere put
together i.e. curve for the “Whole Atmosphere”
Water Vapour
Arrives in the atmosphere naturally through evaporation
& transpiration
Because of the unique quantum rotation frequency, H2O
molecules are excellent absorbers of IR wavelengths of
12 m and longer;
Virtually 100% of IR longer than 12 m is
absorbed by H2O vapour and CO2
(12 m close to the radiation wavelength
of 10 m, at which most of Earth’s
terrestrial radiation is emitted.)
IR at 12 m
is absorbed
Water Vapour
H2O has variable concentration and residence time in
the atmosphere depending on location and
atmospheric circulation
At higher air temperatures H2O molecules collide &
rebound more frequently, leading to expansion of the
air & the water vapour in the air.
Hence hot climates can hold more water vapour in the
air
At lower air temperatures as air gets more dense, H2O
molecules are more likely to bond so that a phase
change to liquid water or even solid ice can occur.
Hence in cooler climates, more of the available H2O is
likely to be in the liquid or solid state on the Earth’s
surface
Water Vapour (cont):
H2O is NOT globally increasing in direct response to
human-induced factors, but if global temperatures get
warmer, H2O vapour in the atmosphere will increase…
Why???
…because of more evaporation in the warmer climate!
Carbon Dioxide:
Arrives in atmosphere naturally through the natural
carbon cycle
Because of the unique quantum bending mode
vibration behavior of CO2 it absorbs electromagnetic
radiation of about 15 m
CO2 is an excellent absorber of radiation
of about 15 m
(15 m close to the radiation wavelength
of 10 m, at which most of Earth’s
terrestrial radiation is emitted.)
IR at 15 m
is absorbed
Carbon Dioxide (cont)
Has increased dramatically since the 1800s because of:
1. Fossil fuel combustion: oil, coal, gas --especially
coal, and
2. Deforestation -- which has the effect of
increasing the amount of carbon in the atmospheric
“reservoir” by reducing the photosyntesis outflow and
increasing the respiration inflow.
Carbon Dioxide: Trends
Carbon Dioxide: Trends
Carbon Dioxide (cont):
Residence time in the atmosphere of carbon atoms in the
carbon cycle = ~ 12.7 years;
But residence time of CO2 gas molecules is estimated at
about 100 years
Plus it takes 50 to 100 years for atmospheric CO2 to adjust
to changes in sources or sinks.
If we make changes now, it will still be many, many years
before the effect will be felt!
Methane
Produced naturally in anaerobic processes (e.g.,
decomposition of plant material in swamps & bogs)
Has increased becaue of the following activities: raising
cattle / livestock, rice, production, landfill decomposition,
pipeline leaks
Has relatively short atmospheric residence time (10
years) because it reacts with OH to form H2O and CO2.
Nitrous Oxide
Produced naturally in soils
Has increased because of fossil fuel combustion (esp.
diesel), forest burning, use of nitrogen fertilizers
Has a long atmospheric residence time (~150 years)
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