Composition & Structure of Earth's Atmosphere (College Board AP® Environmental Science): Study Guide

Composition of Earth's atmosphere

• The atmosphere forms the boundary between Earth and space

• It is the outer limit of the biosphere

• The atmosphere supports life on Earth

Atmospheric gases

• The atmosphere is mainly composed of nitrogen (about 78%) and oxygen (about 21%)

  • These two gases make up the majority of the atmosphere and play vital roles in supporting life on Earth

• The atmosphere contains smaller amounts of other gases, including:

  • Carbon dioxide

  • Argon

  • Water vapour

  • Various trace gases

• Carbon dioxide is present in relatively low concentrations (around 0.04%)

• It is essential for:

  • Photosynthesis in plants

  • Maintaining the greenhouse effect

• Argon is an inert gas

  • This means it does not participate in chemical reactions but contributes to the overall composition of the atmosphere

• Water vapor plays an important role in:

  • Photosynthesis in plants

  • The Earth's weather patterns

  • The formation of clouds and precipitation

• Trace gases, such as methane, ozone, and nitrous oxide, are present in even smaller quantities

  • However, they still have significant impacts on climate and atmospheric chemistry

Redistribution of atmospheric gases through physical processes

• Gases in the atmosphere are moved around by various physical processes, including:

  • Wind: the main mover of gases, caused by differences in air pressure

  • Convection: warm air rises and cool air sinks, creating vertical movement

  • Diffusion: gases spread from areas of high concentration to areas of low concentration

  • Turbulence: irregular air flow caused by obstacles like mountains and buildings

  • Jet streams: fast-flowing, narrow air currents in the upper atmosphere

Structure of Earth's atmosphere

Atmospheric layers

• Atmospheric stratification:

  • The atmosphere is divided into layers based on temperature gradients

  • The key layers for living systems are the troposphere and the stratosphere

Troposphere

• The lowest layer, extending up to about 10 km from the Earth's surface

  • Weather phenomena, such as clouds, precipitation, and gas mixing, occur here

  • Contains the highest concentration of water vapor, carbon dioxide and other important trace gases

Stratosphere

• Located above the troposphere, extending from about 10 to 50 km above the Earth's surface

  • Contains the ozone layer, which absorbs and blocks most of the Sun's harmful ultraviolet (UV) radiation

• Importance of inner layers:

  • Various reactions in the troposphere and stratosphere are vital for maintaining the balance of gases, regulating climate and supporting life

  • In the troposphere, chemical reactions involving pollutants, greenhouse gases and particles impact air quality and climate

  • In the stratosphere, chemical reactions involving ozone maintain the ozone layer and protect organisms on Earth from harmful UV radiation

Mesosphere

• The mesosphere is the zone in which most meteors burn up as a result of colliding with gas molecules

  • In this layer, temperature decreases as altitude increases

Thermosphere

• In the thermosphere, temperature increases with altitude

  • This is due to solar radiation

  • The auroras occur here

Exosphere

• The outermost layer of Earth’s atmosphere

  • The density of atmospheric gas particles is extremely low, gradually transitioning into space