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Composition |
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Evolution |
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Vertical
Structure |
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Most of the atmospheric mass is confined in the
lowest 100 km above the sea level. |
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The thickness of the atmosphere is only about 2%
of Earth’s thickness (Earth’s radius = ~6500km). |
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The most abundant variable gas. |
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Water vapor is supplied to the atmosphere by
evaporation from the surface and is
removed from the atmosphere by condensation (clouds and rains). |
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The concentration of water vapor is maximum near
the surface and the tropics (~ 0.25% of the atmosphere by volume) and
decreases rapidly toward higher altitudes and latitude (~ 0% of the
atmosphere). |
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Water vapor is important to climate because it
is a greenhouse gas that can absorb thermal energy emitted by Earth, and
can release “latent heat” to fuel weather phenomena. |
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Carbon dioxide is supplied into the atmosphere
by plant and animal respiration, the decay of organic material, volcanic
eruptions, and natural and anthropogenic combustion. |
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Carbon dioxide is removed from the atmosphere by
photosynthesis. |
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CO2 is an important greenhouse gas. |
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Methane |
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A variable gas in small but recently increasing
concentrations |
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Released to the atmosphere through fossil fuel
activities, livestock digestion, and agriculture cultivation (esp. rice) |
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As a very effective absorber of terrestrial
radiation it plays an active role in near surface warming |
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Aerosols:
small solid particles and liquid droplets in the air. They serve as
condensation nuclei for cloud formation. |
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Air Pollutant: a gas or aerosol produce by human
activity whose concentration threatens living organisms or the environment. |
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When the Earth was formed 4.6 billion years ago,
Earth’s atmosphere was probably mostly hydrogen (H) and helium (He) plus
hydrogen compounds, such as methane (CH4) and ammonia (NH3). |
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Those gases eventually escaped to the space. |
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The release of gases from rock through volcanic
eruption (so-called outgassing) was the principal source of atmospheric
gases. |
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The primeval atmosphere produced by the
outgassing was mostly carbon dioxide (CO2) with some Nitrogen (N2)
and water vapor (H2O), and trace amounts of other gases. |
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The atmosphere can only small fraction of the
mass of water vapor that has been injected into it during volcanic
eruption, most of the water vapor was condensed into clouds and rains and
gave rise to oceans. |
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č The concentration of water vapor in the atmosphere was
substantially reduced. |
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Chemical weather is the primary process to
remove CO2 from the atmosphere. |
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In this process, CO2 dissolves in rainwater
producing weak carbonic acid that reacts chemically with bedrock and
produces carbonate compounds. |
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This biogeochemical process reduced CO2 in the
atmosphere and locked carbon in rocks and mineral. |
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Nitrogen (N2): |
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(1) is
inert chemically, |
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(2)
has molecular speeds too slow to escape to space, |
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(3) is
not very soluble in water. |
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The amount of nitrogen being cycled out of the
atmosphere was limited. |
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Nitrogen became the most abundant gas in the
atmosphere. |
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Photosynthesis was the primary process to increase the amount
of oxygen in the atmosphere. |
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Primitive forms of life in oceans began to
produce oxygen through photosynthesis probably 2.5 billion years ago. |
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With the concurrent decline of CO2, oxygen
became the second most abundant atmospheric as after nitrogen. |
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With oxygen emerging as a major component of the
atmosphere, the concentration of ozone increased in the atmosphere through
a photodissociation process. |
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Radioactive decay in the planet’s bedrock
added argon (Ar) to the evolving atmosphere. |
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č Argon became the third abundant gas in the
atmosphere. |
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The
ionosphere is an electrified region within the upper atmosphere where large
concentration of ions and free electrons exist. |
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The
ionosphere starts from about 60km above Earth’s surface and extends upward
to the top of the atmosphere. Most of the ionosphere is in the
thermosphere. |
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The ionosphere plays an important role in radio
communication. |
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