Evolution of the Atmosphere: Structure and Composition Composition Structure Summary Evolution...

©2003, Perry Samson, University of Michigan

Evolution of the Atmosphere: Structure and Composition

Composition

Structure

Summary

Evolution

Issues

Yo, who turned up the



Composition

Structure

Summary

Evolution

Issues Driving Questions

1. The composition of the atmosphere has changed quite naturally in the past. How did THAT happen?

2. How is moisture affected by temperature and why is that important?

3. What are our dominant atmospheric gases and what are their sources?

4. The thermal characteristics of our atmosphere vary with altitude. How and Why?

Today’s Issues:

Last Updated: April 18, 2023



Composition

Structure

Summary

Evolution

Issues Comparison with other

Planets

The composition of the earth’s atmosphere is markedly different from that of our neighboring planets. Nonetheless they originated from the same process.

Atmospheric Composition



Composition

Structure

Summary

Evolution

Issues

0

100

200

300

400

500

600

700

800

0 100 200 300

Distance from Sun(millions of km)

Temperature (°K)

Actual Temperatures

Temperatures decreasing with distance



Composition

Structure

Summary

Evolution

Issues Stages of Atmospheric Formation

1.Origin In formative years the atmosphere was composed of H and He.

2.Chemical/ pre-biological eraAtmosphere formed from volcanic outgassing lead dominated by water vapor, CO2, SO2 and others.

3.Microbial eraInitial O2 formed through photolysis allowed ozone layer, early microbes emitted O2 as waste product.

4.Biological eraSimultaneous decrease in atmospheric CO2 and the increase in O2 due to life processes.




Composition

Structure

Summary

Evolution

Issues Initial Atmosphere

Composed Largely of

Water Vapor and Carbon

Dioxide

The atmosphere grew from the outgassing of the cooling planet. Assuming that the gases we presently observe were also released by early volcanoes the atmosphere would be made of water vapor (H2O),

carbon monoxide (CO), carbon dioxide (CO2), hydrochloric acid (HCl),

methane (CH4), ammonia (NH3), nitrogen (N2), & sulfur gases. The

atmosphere was reducing (no free oxygen).

Volcanic Origins

Gas plume as lava enters the Pacific Ocean at Kilauea volcano, Hawaii.

Photo copyrighted by Paul J. Buklarewicz.



Composition

Structure

Summary

Evolution

Issues Slow removal of

Carbon Dioxide and

Water Vapor through

Precipitation and

Chemical Weathering

• As planet cooled through release of long-wave radiation the temperature fell to a point that water vapor could condense to liquid water.

• The ensuing precipitation removed water to the surface and carried away soluble gasses like carbon dioxide and sulfur dioxide.

• The carbon became sequestered in ocean sediment through chemical weathering.

o Water reacts with CO2 to produce carbonic acid.

o Carbonic acid reacts with Calcium and Magnesium silicate rocks to yield Calcium Carbonate (CaCO3 =

limestone) or Magnesium Carbonate (MgCO3 =

Dolomite) and Silicon Oxide (SiO2).

Example: CaSiO3 + CO2 CaCO3 + SiO2

Weathering Removes CO2



Composition

Structure

Summary

Evolution

Issues The amount of water the atmosphere can hold is a function

of its temperature

.

Cooling Removes Water Vapor

Clausius-Clapyron Relationship

-

10

20

30

40

50

60

70

80

0 5 10 15 20 25 30 35 40

Temperature (°C)

Vapor Pressure (mb)



Composition

Structure

Summary

Evolution

Issues

Photolysis as a source for Oxygen



Composition

Structure

Summary

Evolution

Issues The initial oxygen

formation lead to feedbacks that further

enhanced the formation of

oxygen

Consequences of Oxygen Build-Up

• Some atmospheric oxygen formed through photodissociation of water vapor:

• Some formed as waste product of photosynthetic autotrophs.

• Development of an ozone (O3) layer, which absorbs harmful UV radiation and eventually allowed life on land.

• End of banded iron formations which only formed in low O2

• Beginning of deposition of red beds - iron oxides



Composition

Structure

Summary

Evolution

Issues Oxygen formed was at

first sequestered in

oxidation of iron but

transition to aerobic life

and photosynthesi

s lead to build-up of

atmospheric oxygen.

Arrival of Oxygen



Composition

Structure

Summary

Evolution

Issues Oxygen concentrations

are balanced between

photosynthetic formation by

autotrophs and respiratory

destruction by heterotrophs and

decay.

Arrival of Oxygen



Composition

Structure

Summary

Evolution

Issues Oxygen concentrations

are balanced between

photosynthetic formation by

autotrophs and respiratory

destruction by heterotrophs and

decay.

Arrival of Oxygen



Composition

Structure

Summary

Evolution

Issues Changes in CO2

We know the concentration of some gases have changed significantly over time.

Changes in Atmospheric Composition

1500

3000

4500

6000

7500

Age (Myr BP)

(f)

0

500 400 300 200 100 0

Geochemically inferred.

NOW



Composition

Structure

Summary

Evolution

Issues Units One way to express relative concentrations of gases in the atmosphere is to compare their percentages by volume. A second way is to express their parts per million by volume.


EXAMPLES:

N2 78.08% 780,800 ppm

O2 20.95% 209,500 ppm

Ar 0.93% 9,300 ppm

CO2 0.036% 360 ppm



Composition

Structure

Summary

Evolution

Issues Who Let the CO2 Out?

Today’s atmospheric composition is dominated by Nitrogen and Oxygen, neither of which were dominant in outgassing and carbon dioxide has fallen to trace levels.




Composition

Structure

Summary

Evolution

Issues Blackbody Radiation

Selective Absorbers

An object that absorbs all radiation incident upon it is called a “blackbody.”

Radiative Properties

Some gases only absorb in selected wavelengths dictated by their atomic structure. In the atmosphere so-called “greenhouse gases” are “selective absorbers” that absorb in the wavelengths of Earth emissions.

E = T4

max = C/T



Composition

Structure

Summary

Evolution

IssuesSolar

shortwave radiation

is centered at 0.5 µm wavelengt

h

Radiation

0

0.5

1

1.5

2

2.5

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2Wavelength (µm)

Radiation

1 µm = 10-6 m


Composition

Structure

Summary

Evolution

Issues


Selective Absorbers

Radiative Properties

Some gases only absorb in selected wavelengths dictated by their atomic structure. In the atmosphere so-called “greenhouse gases” are “selective absorbers” that absorb in the wavelengths of Earth emissions.



Composition

Structure

Summary

Evolution

Issues Pressure

Temperature

Force exerted per unit area. In most sciences the standard unit of measure is the pascal (Pa), but in meteorology either millibar (1 mb = 100 Pa) or inches of Mercury (in Hg = 3386 Pa) are used.

A measure of the average kinetic energy of the molecules comprising a substance.

The Structure of the Atmosphere



Composition

Structure

Summary

Evolution

Issues Pressure

Temperature

Force exerted per unit area. In most sciences the standard unit of measure is the pascal (Pa), but in meteorology either millibar (1 mb = 100 Pa) or inches of Mercury (in Hg = 3386 Pa) are used.

A measure of the average kinetic energy of the molecules comprising a substance.

The Structure of the Atmosphere



Composition

Structure

Summary

Evolution

IssuesSolar

shortwave radiation

is centered at 0.5 µm wavelengt

h

Radiation

QuickTime™ and aSorenson Video decompressorare needed to see this picture.



Composition

Structure

Summary

Evolution

Issues The thermosphere is the fourth layer of the Earth's atmosphere and is located above the mesosphere. The air is really thin in the thermosphere. A small change in energy can cause a large change in temperature. For this reason the temperature of the thermosphere is very sensitive to solar activity. When the sun is active, the thermosphere can heat up to 1,500°C or higher! The Earth's thermosphere also includes the region of the atmosphere called the ionosphere. The ionosphere is a region of the atmosphere that is filled with charged particles. The high temperatures in the thermosphere can cause molecules to ionize. This is why an ionosphere and thermosphere can overlap.

Thermosphere



Composition

Structure

Summary

Evolution

Issues

Thermosphere



Composition

Structure

Summary

Evolution

Issues In the Earth's mesosphere, the air is relatively mixed together and the temperature decreases with altitude.

The atmosphere reaches its coldest temperature of around -90°C in the mesosphere. This is also the layer in which a lot of meteors burn up while entering the Earth's atmosphere.

Mesosphere



Composition

Structure

Summary

Evolution

Issues In the Earth's stratosphere, the temperature increases with altitude. On Earth, ozone causes the increasing temperature in the stratosphere.

Stratosphere

Ozone is concentrated around an altitude of 25 kilometers in the “ozone layer.” The ozone molecules absorb dangerous kinds of sunlight, which heats the air around them.



Composition

Structure

Summary

Evolution

Issues The troposphere is the layer in contact with the Earth’s surface and hence its temperature structure is predicated by energy transmitted to and from the surface.

Troposphere



Composition

Structure

Summary

Evolution

Issues

SummaryDriving

Questions1. The composition of the atmosphere

has changed quite naturally in the past. How did that happen?

Keywords:

•Outgassing

•Chemical weathering

•Photosynthesis

•Photolysis

•Oxidation



Composition

Structure

Summary

Evolution

Issues

SummaryDriving

Questions2. How does our present atmosphere’s

composition influence the flow of energy to and from the Earth’s surface?

Keywords:

•Selective absorbers

•Longwave radiation

•Shortwave radiation



Composition

Structure

Summary

Evolution

Issues

SummaryDriving

Questions3. The thermal characteristics of our

atmosphere vary with altitude. How and Why?

Keywords:

•Troposphere

Weather

•Stratosphere

Ozone Layer

•Mesosphere

•Thermosphere

Ionosphere, Aurora



Composition

Structure

Summary

Evolution

Issues Energy Balance

Ultimately the selective absorbers influence the flow (flux) of energy to and from the earth’s surface.

Energy Balance Controls Temperature



Composition

Structure

Summary

Evolution

Issues

QUIZ!

Last Updated: April 18, 2023

On a scale of 0 - 10:

The ozone hole and global warming are related.

0No way!

10You Betcha!

5Coward

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Evolution of the Atmosphere: Structure and Composition Composition Structure Summary Evolution...

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