Chapter One
Composition and
Structure of the Atmosphere
Chapter 1 Outline
1.
2.
3.
4.
Composition of the Atmosphere
Vertical Structure of the Atmosphere
Pressure and Density
Other Planetary Atmospheres
Chapter 1 Outline
1.
Composition of the Atmosphere
•
•
•
•
Physical Properties
Composition
Evolution of the Atmosphere
Permanent (primary) Gases
–
–
•
Variable (trace) Gases
–
–
–
–
2.
3.
4.
Nitrogen (N2)
Oxygen (O2)
Water vapor (H2O)
Carbon Dioxide (CO2)
Ozone (O3)
Others: Aerosols, Methane, …
Vertical Structure of the Atmosphere
Pressure and Density
Other Planetary Atmospheres
Why do we have an atmosphere?
Earth viewed from the Moon
Moon viewed from the Earth
Physical Properties
The Atmosphere
A mixture of gas molecules and very small particles of solid and liquid.
Behaves as a fluid!
The majority of atmospheric mass is contained in a rather thin layer near
the surface (99.9% is below 50 km).
Compared to the radius of the Earth (6500 km) the atmosphere is very thin
(~50 km or ~1%).
• Atmospheric motions (winds) are largely horizontal rather than vertical.
Permanent vs. Variable Gases
Ar
+
H2O
CO2
O3
CH4
Methane
CH4
Evolution of the Atmosphere
Solar System Forms (6 billion years ago)
Planets form from left over dust/gas.
Earth Forms (4.5 billion years ago)
Crust forms
Original Atmosphere:
• Primarily H2, He
• Lost to space
Secondary Atmosphere from Volcanic Outgassing:
H2O (85%)
CO2 (10%)
Trace amounts of N2
Oceans Form
H2O condenses to form oceans.
CO2 dissolves in oceans and precipitates to form limestone.
N2 slowly builds up over time.
Evolution of the Atmosphere (cont.)
Life Forms ~4 Billion Years Ago (BYA):
Cyanobacteria or “blue green algae”
Use chemical reactions to obtain energy, not photosynthesis.
Life Evolves ~2 BYA:
Bacteria evolve into more complex organisms.
• Photosynthesis begins.
Photosynthesis removes carbon dioxide (CO2) and creates oxygen (O2).
• Chemistry shifts dramatically, from reducing to oxidizing conditions
Ozone (O3) formed naturally from O2.
Evolution of the Atmosphere
3 BY old fossil
Modern Bacteria
2 BY old fossilized stromatilites,
NW Canada
Living stromatilites,
Shark’s Bay Australia
Evolution of Atmosphere
Summary
The early atmosphere was likely composed of hydrogen and
helium. These light gases “escaped” the Earth’s gravity to space.
A secondary atmosphere formed through volcanic outgassing;
primarily H2O, CO2.
Precipitation removed excess water vapor -> oceans.
Photosynthesis algae removed high concentrations of CO2 (->
ocean sediments) and produce oxygen O2.
Ozone layer formed from oxygen O2 + O = O3; allowed life to
evolve onto land.
Nitrogen concentrations slowly grew to current levels.
Current Atmosphere
• Permanent gases: N2 (78%), O2 (21%).
• Variable (trace) gases: H2O, CO2, CH4, O3 < 1%.
Important Trace Constituents
• Carbon Dioxide (CO2)
• Water Vapor (H2O)
• Ozone (O3)
• Methane (CH4)
• Aerosols & clouds (solid particles, not a gas)
Carbon Dioxide: CO2
Carbon Dioxide
A trace gas accounting for only 0.036% of total atmospheric mass
Important to Earth’s greenhouse effect
Naturally occurring.
• Added through respiration (breathing), volcanic activity, organic decay,
diffusion from ocean and natural and human-related combustion
• Removed through photosynthesis, diffusion into ocean, weathering of rocks.
Human contribution
• Atmospheric CO2 has increased over the
past 200 years due to burning fossil fuels.
• Spring maximum/Fall minimum reflects
breathing of NH land plants.
500,000 Year Record of CO2
Current CO2 360 ppm
Fig. 1.3
Carbon Reservoirs (GTC)
Reservoir: An area of carbon storage
750
Input (sources)
Removal (sinks)
Decomposition
650
36000
4000?
Fig. 1.3
Carbon Fluxes (GTC/yr): Pre-Industrial
Flux: The exchange of carbon between reservoirs
750
Input
Removal
120
120
Decomposition
90
<1
<1
0
90
650
36000
4000?
Atmospheric Residence Time: 750 GTC / (90+120 GTC yr-1) = ~3 yr
Atmospheric Residence Time
Atmospheric Residence Time: The average time a molecule of a particular
gas resides in the atmosphere.
Residence Time = Amount in Reservoir / Rate of Input
Carbon Dioxide (CO2):
750 Gigatons / 210 Gigatons per year = ~3.6 yr
Variable
Oxygen (O2):
750000 Gigatons / 400 Gigatons per year = ~2000 yr
Well mixed
Carbon Fluxes (GTC/yr): Current
750
Input
Removal
122
120
Decomposition
92
<1
2
6
90
650
36000
4000?
Carbon Fluxes (GTC/yr): 2100
1500
Input
Removal
122
120
Decomposition
92
<1
0
0
90
650
36000
4000?
Atmospheric Removal Time: 750 GTC / (4 GTC yr-1) = ~150 yr
Water Vapor
Water Vapor
The most abundant variable gas.
Concentrations vary from nearly 0% over desert and polar regions
to nearly 4% near tropics.
Important in many atmospheric processes.
• Most important greenhouse gas.
• Provides fuel for hurricanes and other severe storms.
Water Fluxes (x1015 kg)
13 in atmosphere
111
385
71
425
1,350,000 in ocean
8,000 in ground
Atmospheric Residence Time = 13 kg / (385+111 kg per yr) =0.026 yr
Ocean Residence Time = 1350000 kg / (385+111 kg per yr) = 2700 yr
Important Trace Constituents
• Carbon Dioxide (CO2)
• Water Vapor (H2O)
• Ozone (O3)
• Methane (CH4)
• Aerosols (dust, sulfate, soot, …)
Ozone (O3) ”Good up high, bad nearby”
Ozone occurs in two places:
• Near the surface it is a pollutant.
• In the stratosphere it is an essential absorber of ultraviolet radiation.
Good Ozone
Naturally occurring
Ozone layer in stratosphere (up high; 20-30 km)
Provides a protective shield from UV radiation.
“Ozone hole”: Anthropogenic depletion of the ozone layer.
Bad Ozone
Bad ozone in troposphere (nearby; at surface)
Produced from photochemical reactions with pollutants (smog)
Aggravates asthma, emphysema, respiratory problems.
Damages vegetation, agriculture, rubber, fabrics, lungs, etc.
Ozone: O3
The Ozone Hole
The Ozone Hole
50 year record of ozone measurements over the south pole from the
British Antarctic Survey.
The Ozone Hole
The purple areas reveal the “ozone hole” over Antarctica
The Ozone Hole
Why does the ozone hole
occur only over the South
Pole?
During SH winter (July) strong winds
create a barrier which isolates the air over
Antarctica.
This barrier allows the air over Antarctica
to get very cold leading to the formation of
Polar Stratospheric Clouds (PSCs).
The Ozone Hole
Once sunlight returns, it breaks
down the Chloroflorocarbons
(CFCs) into free Chlorine (Cl)
atoms which destroy ozone.
The presence of PSCs, greatly
accelerates this process. 1 CFC
molecule can destroy 100,000
O3 molecules.
This causes increases the
amount of harmful UV rays
reaching the surface.
Chloroflourocarbons: CFCs
CFC’s stabilized during 1990s
CFC’s increased during 1980s
The Ozone Hole
Size of ozone hole has stabilized
Rapid increase in ozone hole size
Estimated recovery time: 2065
Methane: CH4
Methane
A very effective greenhouse gas (1 CH4 = 20 CO2).
A variable gas in small, but increasing, concentrations.
Released to the atmosphere through fossil fuel activities,
livestock, agriculture, and decaying organic matter.
Methane has stabilized Why???
Aerosols
Aerosols
Any solid or liquid particle in
the atmosphere.
Natural (e.g., dust) and human
(e.g., soot) sources.
Due to small size can remain
suspended long periods of time.
Contribute to cloud formation
precipitation processes by acting
as condensation nuclei.
Aerosols: Saharan Dust Storm
Dust particles are about 10 micro-meters in
size (0.00001 meters). Roughly 1/10 the width
of a human hair. Big ones settle out..
Aerosols: Pollution from China
smaller particles, less than 1 micron.
Will not fall out
Aerosols: Pollution over LA
Chapter 1 Outline
1.
2.
Composition of the Atmosphere
Vertical Structure of the Atmosphere
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•
•
•
•
3.
4.
Temperature Structure
Troposphere
Stratosphere
Mesosphere
Thermosphere
Pressure and Density
Other Planetary Atmospheres
Vertical Structure of the Atmosphere
Thermal Layers of the
Atmosphere
Four distinct layers of the
atmosphere emerge from
identifiable temperature
characteristics with height
Vertical Structure of the Atmosphere
Troposphere
The lowest layer, named as this region
promotes atmospheric overturning
Layer of virtually all weather
processes
Warmed at the surface by solar
radiation
Identified by a steady temperature
decrease with height
Thinnest layer, but contains 80% of
the mass
Due to thermal expansion, the
tropopause is roughly 16 km over the
tropics, but only 8 km at poles
Vertical Structure of the Atmosphere
Updraft has “overshot” the tropopause and
entered the lower stratosphere
Flattened Anvil cloud top reveals
the top of troposphere
Vertical Structure of the Atmosphere
Stratosphere
Area of little weather (“stratified”)
A layer where temperature increases with
height
Inversion caused by the absorption of
ultraviolet radiation by ozone
Although the ozone layer exists through an
altitude between 20-30 km (12-18 mi),
actual concentration of ozone can be as
low as 10 ppm
Vertical Structure of the Atmosphere
Mesosphere and Thermosphere
Combined the two layers account for only
0.1% of total atmospheric mass
Mesosphere, which extends to about 80
km (50 mi) is characterized by decreasing
temperatures with height and is the coldest
atmospheric layer
The upper most layer; slowly merges with
interplanetary space and is characterized
by increasing temperatures with height
Temperatures approach 1500oC, however,
this only measures molecular kinetic
energy as the sparse amount of mass
precludes actual heat content
Electrical Properties
The Ionosphere
Located within the mesoand thermospheres, this
portion of the atmosphere is
replete with ions;
electrically charged
particles
Interactions between the
ionosphere and subatomic
particles emitted from the
Sun excite atmospheric
gases causing the aurora
borealis (northern lights)
and the aurora australis
(southern lights)
Chapter 1 Outline
1.
2.
3.
Composition of the Atmosphere
Vertical Structure of the Atmosphere
Pressure and Density
•
•
•
•
•
4.
Pressure Definition
Ideal Gas Law
Vertical profiles of pressure
Density Definition
Vertical profiles of density
Other Planetary Atmospheres
Atmospheric Pressure
Gas molecules are constantly in motion.
These molecules exert a pressure (force per unit area) when they
strike a surface.
• like ten thousand ping pong balls could knock you over
Molecules move in all directions, so pressure is exerted in all
directions.
• like ping pong balls battering your front and back equally
The Ideal Gas Law:
• The amount of pressure exerted by a gas is the product of how
many molecules, how heavy they are, and how fast they move.
Of course - what else could it depend on?
• Pressure = R x Density (how many, how heavy) x Temperature
(how fast). R is a constant (for a given gas. 287 for air.)
• Consider a balloon … or a dam with a leak...
Pressure
The weight (pressure) of the overlying air
pushes down on the fluid (mercury) and
forces it up in the tube.
.
Air pressure is (almost exactly) equal to
the weight of the air above a particular
level in the atmosphere. It must be, since
it is what holds up the atmosphere (all the
air doesn’t fall down into a puddle).
Pressure is sometimes expressed in terms
of “inches of mercury” (diagram)
Typical surface pressure (weight of the
entire column of atmosphere) is ~1000
millibars (1 Bar = “1 atmosphere”), ~15
pounds per square inch, or 30 inches of
mercury.
This is equal to the weight of 10 m (30
feet) of water, so a diver at 90 feet feels 4
atmospheres (4 Bars) of pressure.
vacuum
(no air) in
here
There is more overlying weight (pressure)
near the surface, hence more mercury is
forced up into the tube..
Vertical Pressure Profile
Pressure always
decreases with height.
Pressure at surface = 1000 mb
Pressure at 18 km = 100 mb
100 mb / 1000 mb = 10% above 18 km
or 90 % below 18km
Pressure at surface = 1000 mb
Pressure at 5.5 km = 500 mb
500 mb / 1000 mb = 50% above 5.5 km
Sea Level Pressure Map
Less atmosphere here
Higher pressure means more atmosphere here
Contours are the pressure (in millibars) of the pressure at sea level.
Atmospheric Density
Density measures the concentration of gases.
Density = Mass/Volume (units of kg/m3)
At surface, 1.2 kg of air per cubic metre.
Air is compressible.
Gas molecules are not attached to each other, and resist
being squeezed closer together.
Because of compression from the weight of overlying air,
the atmosphere is denser near the surface than above.
Pressure and Density
Due to compressibility,
atmospheric mass
gradually “thins out”
with height.
less overlying
weight
more overlying
weight
Pressure and Density
People who climb Mt. Everest bring
oxygen tanks because the air is so
“thin”
Easier to hit a home run in Denver
than Miami because there is less
air resistance.
Denver
Miami
Other Measures of Density
Can be viewed in terms of:
Number of molecules per unit volume
(times the mass of each molecule)
• At surface, 1022 molecules of air in each breath
(1 breath = 1 litre = 1000 cm3 = 10-3 m3)
“Mean free path”
• The average distance a molecule travels before
colliding with another molecule
• In a liquid: 10-10 metre
• Air at surface: MFP = 10-9 metre
• At 150 km: MFP = 10 km or more!
Chapter 1 Outline
1.
2.
3.
4.
Composition of the Atmosphere
Vertical Structure of the Atmosphere
Pressure and Density
Other Planetary Atmospheres
Planetary Atmospheres
Surface Pressure
Surface Temp
90,000 mb
500 oC
1000 mb
20oC
1 mb
-20o C
Planetary Atmospheres
Mercury
Virtually no atmosphere due to small size (weak gravity) and high
temperatures. It “boiled off” long ago.
Venus
Very thick atmosphere with a mass 90 times greater than Earth
Primarily CO2 - no life to convert it to O2 like on Earth.
A “runaway” greenhouse effect responsible for very high temperatures
Mars
Much thinner atmosphere (1% of Earths)
Primarily CO2
Thin atmosphere contributes to cold temperatures
Jupiter
Much larger mass (and gravitational pull) has kept the lighter gases
(Hydrogen, Helium). Very thick atmosphere - maybe no solids even!
Planetary Atmospheres
Venus
Earth
Mars
Neither does Mars
Venus has no stratosphere
because there is no ozone.
Top 10 Things You Should Know From Chapter 1
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
Primary (permanent) gases: N2 (78%), O2 (21%), Ar (1%)
Trace (variable) gases: CO2, H2O, O2, small but important!
Evolution: H2/He escaped. Volcanic H20 + CO2 became oceans &
O2 (made from CO2 by life). N2, Ar accumulate, they are inert.
Atmosphere classified into 4 layers based on thermal structure.
Characteristics of Tropo, Strato, Meso and Thermosphere.
99% of atmosphere’s mass lies in the lowest 2 layers.
Atmospheric pressure is the force exerted by molecules hitting
something (like each other). This force holds up the atmosphere
against gravity, so pressure at any level is equal to the weight of
all the air above that level.
Density (mass of air per cubic meter) also decreases with height
Ideal Gas Law: Pressure = R x Density x Temperature
Differences between Earth and other planetary atmospheres