Notes

Outline

Chapter 1: Composition and Structure of the Atmosphere  Composition  Evolution  Vertical Structure

Thickness of the Atmosphere Most of the atmospheric mass is confined in the lowest 100 km above the sea level. The thickness of the atmosphere is only about 2% of Earth’s thickness (Earth’s radius = ~6500km).

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Water Vapor (H2O) The most abundant variable gas. Water vapor is supplied to the atmosphere by evaporation from the surface and  is removed from the atmosphere by condensation (clouds and rains). 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). 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.

Carbon Dioxide (CO2) Carbon dioxide is supplied into the atmosphere by plant and animal respiration, the decay of organic material, volcanic eruptions, and natural and anthropogenic combustion. Carbon dioxide is removed from the atmosphere by photosynthesis. CO2 is an important greenhouse gas.

Ozone (O3)

"Methane" Methane A variable gas in small but recently increasing concentrations Released to the atmosphere through fossil fuel activities, livestock digestion, and agriculture cultivation (esp. rice) As a very effective absorber of terrestrial radiation it plays an active role in near surface warming

Other Atmospheric Constituents  Aerosols: small solid particles and liquid droplets in the air. They serve as condensation nuclei for cloud formation. Air Pollutant: a gas or aerosol produce by human activity whose concentration threatens living organisms or the environment.

Origins of the Atmosphere 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). Those gases eventually escaped to the space. The release of gases from rock through volcanic eruption (so-called outgassing) was the principal source of atmospheric gases. 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.

What Happened to H2O? 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. è The concentration of  water vapor in the atmosphere was substantially reduced.

What happened to CO2? Chemical weather is the primary process to remove CO2 from the atmosphere. In this process, CO2 dissolves in rainwater producing weak carbonic acid that reacts chemically with bedrock and produces carbonate compounds. This biogeochemical process reduced CO2 in the atmosphere and locked carbon in rocks and mineral.

Carbon Inventory

What Happened to N2? Nitrogen (N2):    (1) is inert chemically,    (2) has molecular speeds too slow to escape to space,    (3) is not very soluble in water. The amount of nitrogen being cycled out of the atmosphere was limited. Nitrogen became the most abundant gas in the atmosphere.

Where Did O2 Come from?  Photosynthesis was the primary process to increase the amount of oxygen in the atmosphere. Primitive forms of life in oceans began to produce oxygen through photosynthesis probably 2.5 billion years ago. With the concurrent decline of CO2, oxygen became the second most abundant atmospheric as after nitrogen.

Formation of Ozone (O3) With oxygen emerging as a major component of the atmosphere, the concentration of ozone increased in the atmosphere through a photodissociation process.

Where Did Argon Come from?   Radioactive decay in the planet’s bedrock added argon (Ar) to the evolving atmosphere. è Argon became the third abundant gas in the atmosphere.

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Vertical Structure of the Atmosphere

Vertical Structure of Composition

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Variations in Tropopause Height

Stratosphere

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Ionosphere  The ionosphere is an electrified region within the upper atmosphere where large concentration of ions and free electrons exist.  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. The ionosphere plays an important role in radio communication.

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