Structure & Composition of the Atmosphere (Cambridge (CIE) IGCSE Environmental Management): Revision Note

Composition of the atmosphere

• 50% of the atmosphere is held within 5.6 km of the Earth's surface due to gravitational force and compression

• The composition of clean air in the atmosphere is:

  • Nitrogen (about 78%)

  • Oxygen (about 21%)

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

• The atmosphere also contains smaller amounts of other gases, including carbon dioxide, argon, water vapour, and trace gases

  • Carbon dioxide is present in small amounts (about 0.04%)

  • Argon is an inert gas that makes up 0.96% of the atmosphere

  • Water vapour content varies depending on temperature and pressure and can exceed 4% in the tropical rainforests and be below 0.2% at the poles

    • Water vapour is essential to Earth's weather, cloud formation, rain, and photosynthesis

    • Air that has no water vapour is classified as dry, clean air

  • Trace gases, like methane, ozone, and nitrous oxide, are even less common but have significant impacts on the weather and the chemistry of the atmosphere

• The atmosphere also contains aerosols and fine particles such as dust, salt, sand and volcanic ash

• The nitrogen and carbon cycle maintain the natural balance of gases in the atmosphere

• Human activities add and remove gases in the atmosphere, which upset the natural balance

Structure of the atmosphere

• Air pressure decreases with height in the atmosphere

  • Air pressure measures how much air there is above the Earth's surface

    • Low pressure means less air above

    • High pressures means more air above

• The atmosphere is stratified into four different layers because of temperature changes

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

Atmospheric layers

Troposphere

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

• It is where weather phenomena, such as cloud formation, precipitation, and the mixing of gases, mainly occur

• The troposphere contains the highest concentration of water vapour, carbon dioxide, and other trace gases that are important for the functioning of living systems and the regulation of climate

• The air in the troposphere is heated from the ground by longwave radiation so it is warmest near the Earth’s surface and cools with increasing altitude

  • Temperature falls about 6 °C for every 1000 meters of altitude

• Air pressure begins to drop but wind speeds increase with height

• The upper limit of Earth's weather and climate is at the tropopause, where temperatures remain fairly constant

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

Stratosphere

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

• Pressure continues to fall, but temperature increases with height, called a temperature inversion caused by the presence of ozone

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

• The upper boundary is called the stratopause, where temperatures remain constant

Mesosphere

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

  • Pressure continues to decrease

  • Temperatures rapidly fall to below -80 °C due to a lack of water vapour, dust or ozone to absorb incoming short-wave radiation

  • Winds reach speeds of up to 3,000 km/hr

  • The mesopause is the upper boundary

Thermosphere

• This layer is around 80-1000 km in height

• Temperature increases with altitude (up to 1500°C)

  • This is due to solar radiation

• The auroras occur here

• The thermopause marks the boundary

• The exosphere is the very outermost part of Earth’s atmosphere

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

The ozone layer

• Ozone is a molecule composed of three oxygen atoms (O₃)

• Ozone is made naturally in the stratosphere, because this area has high levels of solar radiation

• Ozone plays a crucial role in protecting life on Earth by absorbing a significant portion of the Sun's harmful UV radiation

Dangers of UV radiation

• Exposure to high levels of UV radiation can be dangerous for life in the following ways:

  • Photosynthetic organisms, like phytoplankton, are damaged by too much UV radiation because it breaks down DNA and slows photosynthesis

    • This can lead to a decrease in primary production, particularly in aquatic ecosystems

  • High levels of UV radiation can lead to various health issues in humans and animals by damaging cells and tissues – these issues can include:

    • Cataracts

    • Skin cancer

    • Sunburn

    • Premature skin ageing

    • Damage to immune systems