Atmosphere: Composition and Basic Nutrient Cycles

The Composition & Structure of Earth’s Atmosphere is among the many things unique that allows earth to sustain life forms. Atmosphere is defined as mass of air surrounding a planet, held by planet’s gravity. Earth’s atmosphere extends up to about 1600 km from the earth’s surface. However, 99 % of the total mass of the atmosphere is confined to the height of 32 km from the earth’s surface. The permanent gases whose percentages do not change from day to day are nitrogen, oxygen and argon.  Nitrogen accounts for 78% of the atmosphere, oxygen 21% and argon 0.9%.

Atmosphere:

Composition of Earth’s Atmosphere

The atmosphere is composed of gases, water vapour and dust particles. Nitrogen and Oxygen together account for close to 99% of Atmosphere. However, the composition of the atmosphere is not static and it changes according to the time and place.

Gases:

• Nitrogen:
  • Occupies 78% of Earth Atmosphere (by Volume)
•  Oxygen:
  • Occupies 21% of Earth’s Atmosphere (by Volume)
  • Oxygen is found upto 120Kms height from earth.
• Inactive Gases:
  • The Earth’s atmosphere also includes a variety of inactive (noble) gases like Argon, Neon, Helium and Krypton.
  • Argon occupies the maximum volume with 0.9%
• Carbon-di-oxide
  • Carbon dioxide is meteorologically a very important gas.
  • It is transparent to the incoming solar radiation but opaque to the outgoing terrestrial radiation.
  • It absorbs a part of terrestrial radiation and reflects back some part of it towards the earth’s surface and  is largely responsible for the green house effect.
  • The volume of other gases is constant but the volume of carbon dioxide has been rising in the past few decades mainly because of the burning of fossil fuels. This has also increased the temperature of the air and associated phenomena of Global Warming.
  • Found upto 90 kms from earth.
• Water-Vapour
  • Water vapour is also a variable gas in the atmosphere, which decreases with altitude.
  • In the warm and wet tropics, it may account for even 4% of the air by volume, while in the dry and cold areas of desert and polar regions, it may be less than one per cent of the air.
  • Water vapour also decreases from the equator towards the poles.
  • It also absorbs parts of the insolation from the sun and preserves the earth’s radiated heat and thus, acts like a blanket allowing the earth neither to become too cold nor too hot.
  • Found upto 90 kms from earth.
• Ozone:
  • Ozone is another important component of the atmosphere found between 10 and 50 km above the earth’s surface
  • It acts as a filter and absorbs the ultra-violet rays radiating from the sun and prevents them from reaching the surface of the earth.
• Dust Particles
  • Dust particles include sea salts, fine soil, smoke-soot, ash, pollen, dust and disintegrated particles of meteors.
  • The higher concentration of dust particles is found in subtropical and temperate regions due to dry winds in comparison to equatorial and polar regions.
  • Dust particles are generally concentrated in the lower layers of the atmosphere; yet, convectional air currents may transport them to great heights.
  • Dust and salt particles act as hygroscopic nuclei around which water vapour condenses to produce clouds.

Structure of Earth’s Atmosphere

The density of Earth’s Atmosphere is highest near the surface of the earth and decreases with increasing altitude. The column of atmosphere is divided into five different layers depending upon the temperature condition. They are:

• Troposphere,
• Stratosphere,
• Mesosphere,
• Thermosphere
• Exosphere.

Troposphere:

• The troposphere is the lowermost layer of the atmosphere.
• Its average height is 13 km and extends roughly to a height of 8 km near the poles and about 18 km at the equator.
• Thickness of the troposphere is greatest at the equator because heat is transported to great heights by strong convectional currents.
• This layer contains dust particles and water vapour and all changes in climate and weather take place in this layer.
• The temperature in this layer decreases at the rate of 1°C for every 165m of height.
• This is the most important layer for all biological activity.
• Tropopause:
  • The zone separating the troposphere from stratosphere is known as the tropopause.
  • The air temperature at the tropopause is about minus 800C over the equator and about minus 45oC over the poles.
  • The temperature here is nearly constant, and hence, it is called the Tropopause.

Stratosphere

• The stratosphere is found above the tropopause and extends up to a height of 50 km.
•  Stratosphere is that it contains the ozone layer and hence, this layer absorbs ultra-violet radiation and shields life on the earth from intense, harmful form of energy.

Mesosphere

• The mesosphere lies above the stratosphere, which extends up to a height of 80 km.
• In this layer, once again, temperature starts decreasing with the increase in altitude and reaches up to minus 100°C at the height of 80 km.
• Mesopause: The upper limit of mesosphere is known as the mesopause.

Thermosphere:

• This sphere extends from 80kms to 480kms in altitude.
• The temperature in this zone increase with height.
• High Temperatures are generated in thermosphere because gas molecules absorb shortwave solar radiation.
• Ionosphere: 
  • The ionosphere is located between 80 and 400 km above the mesopause.
  • It forms the bottom-part of thermosphere.
  • It contains electrically charged particles known as ions, and hence, it is known as ionosphere.
  • Radio waves transmitted from the earth are reflected back to the earth by this layer.
  • Temperature here starts increasing with height.

Exosphere:

• The uppermost layer of the atmosphere above the thermosphere is known as the exosphere.

Biogeochemical Cycles or Nutrient Cycles

Energy flow and nutrient circulation are the major functions of the ecosystem. While the flow of energy is unidirectional with loss of energy at each tropic level through respiration, nutrients get circulated between living and non-living components indefinitely. This movement of nutrients and other materials in circular manner is termed as Biogeochemical cycles.

Biogeochemical = Biological Chemical + Geological Process

Types of Biogeochemical cycles:

Biogeochemical cycles are basically divided into two types based on the reserve of principal Nutrients:

• Gaseous cycles  –where the reservoir is the atmosphere or the hydrosphere – Includes Carbon,  Nitrogen, Oxygen and the Water cycle.
• Sedimentary cycles –where the reservoir is the earth’s crust – Includes Sulphur, Phosphorus, Rock cycle, etc.

Carbon Cycle

• Carbon is present in the atmosphere, mainly in the form of carbon dioxide (CO₂).
• Carbon cycle involves a continuous exchange of carbon between the atmosphere and organisms.
• Carbon from the atmosphere moves to green plants by the process of photosynthesis, and the animals get carbon by eating plants.
• Short-term Cycle: By process of respiration and decomposition of dead organic matter it returns back to atmosphere.
• Long-term Cycle: Some carbon accumulates as un-decomposed organic matter in soil or as insoluble carbonates at bottom of aquatic systems. Such carbon turns into fossil fuels like coal and returns to atmosphere when humans burn fossil fuels.
• Carbon Accumulation:  Oceans > Fossils > Living & dead matter > Atmosphere.

Nitrogen Cycle:

• Apart from carbon, hydrogen and oxygen, nitrogen is the most prevalent element in living organisms.
• It is a constituent of amino acids, proteins, hormones, chlorophylls and many of the vitamins.
• Most of Nitrogen in atmosphere is present as N₂(N ≡ N). This nitrogen gets converted to multiple forms.
• Lightning and ultraviolet radiation provide enough energy to convert nitrogen to nitrogen oxides (NO, NO₂, N₂O).
• Humans using industrial process convert atmospheric nitrogen to compounds.
• Microorganisms like bacteria (Rhizobium, Azotobacter) and blue-green algae convert atmospheric nitrogen  into ammonium ions .
• Nitrates synthesized by bacteria in the soil are taken up by plants & are converted into amino acids, which are the building blocks of the proteins.
• With food-chain, nitrogen travels to higher tropic levels.
• Living organisms produce nitrogenous waste products such as urea and uric acid (organic nitrogen).
• These waste products as well as dead remains of organisms are converted back into inorganic ammonia and ammonium ions by the bacteria.
• Some of this ammonia volatilizes and re-enters the atmosphere but most of it is converted into nitrate by soil bacteria.
• Nitrate present in the soil is reduced to nitrogen by the process of denitrification.

Oxygen Cycle:

Water Cycle:

• About 71% of the water on Earth is found in the oceans.  The remaining is held as freshwater in glaciers and ice caps, groundwater sources, lakes, soil moisture, atmosphere, streams and within life.

Distribution of Surface Water on Earth:

• Evaporation is the transfer of water from the surface of the Earth to the atmosphere. By evaporation, water in the liquid state is transferred to the gaseous, or vapour, state.
• Evaporation from snow and ice, the direct conversion from solid to vapour, is known as sublimation.
• Transpiration is the evaporation of water through minute pores, or stomata, in the leaves of plants.
• Water Vapour is the primary form of atmospheric moisture. The transition process from the vapour state to the liquid state is called condensation.
• Precipitation that falls to the Earth is distributed in four main ways: some is returned to the atmosphere by evaporation, some may be intercepted by vegetation and then evaporated from the surface of leaves, some percolates into the soil by infiltration, and the remainder flows directly as surface runoff into the sea.
• By condensation, water vapour in the atmosphere is released to form precipitation.
• Most groundwater is derived from precipitation that has percolated through the soil.