Earth's Atmospheric Layers

• Atmospheric composition and Earth's atmospheric layers

Atmosphere

• From the Greek "atmos" (vapor/steam) and "sphaira" (ball/globe).
• In Modern Latin, the atmosphere is also known as "atmosphaera".
• The atmosphere surrounds the Earth as a gaseous envelope.

Layers of the Atmosphere

• The atmosphere consists of multiple layers:
• Troposphere
• Stratosphere
• Mesosphere
• Thermosphere (Ionosphere)
• Exosphere

Troposphere

• It is the closest to Earth's surface, 7-18 km above the equator.
• The most active region where weather events like rain, lightning, and hurricanes occur.
• Contains about 80% of the atmosphere's total air mass and practically all of its water vapor (forms clouds and rain).
• It is the thinnest layer of the atmosphere, about 10 km.
• Temperatures decrease with increasing altitude in the Troposphere.

Stratosphere

• It is composed of nitrogen, oxygen, and ozone.
• It is 19-50 km above the equator.
• Air temperatures increase with altitude because of exothermic reactions triggered by UV radiation from the sun.
• Ozone (O3), is produced in the Stratosphere and serves to prevent harmful UV rays from reaching Earth's surface.

Mesosphere

• It is 30-50 km above the equator.
• Ozone and gas concentration are low.
• Temperatures decrease with increasing altitude again.

Thermosphere (Ionosphere)

• It is 50-400 km above the equator.
• Temperature increases with altitude due to the bombardment of O2, N2, and atomic species by energetic particles, such as electrons and protons, from the sun.
• N₂ → 2N ΔΗ° = 941.4 kJ/mol
• N → N+ + e¯ ΔΗ° = 1400 kJ/mol
• O₂ → O₂ + e¯ ΔΗ° = 1176 kJ/mol
• In reverse, these processes release energy, mainly as heat.
• Ionized particles reflect radio waves back toward Earth.

Exosphere

• It is about 400 km above the equator.
• The outermost layer of the atmosphere.
• Separates the rest of the atmosphere from outer space, being 10,000 km thick.
• It contains a lot of empty space with spread-out hydrogen and helium gases.
• There is no air to breathe, and the Exosphere is very cold.

Atmosphere Composition

• The atmosphere has a total mass of around 5.3 x 10^18 kg.
• Water is excluded from atmospheric composition tables.
• Water concentration in air can vary based on location.

Composition of Dry Air at Sea Level (% by Volume)

• Nitrogen (N2): 78.03%
• Oxygen (O2): 20.99%
• Argon (Ar): 0.94%
• Carbon Dioxide (CO2): 0.033%
• Neon (Ne): 0.0015%
• Helium (He): 0.000524%
• Krypton (Kr): 0.00014%
• Xenon (Xe): 0.000006%

Nitrogen Cycle

• Molecular nitrogen, with its triple bond, is a very stable molecule.

Nitrogen Fixation

• The conversion of molecular nitrogen into nitrogen compounds called nitrates
• Atmospheric nitrogen fixation:
  • N2 (g) + O2 (g) → 2NO (g) (lightning) - requires electrical energy
  • 2NO (g) + O2 (g) → 2NO2 (g)
  • 2NO2 (g) + H2O (l) → HNO2 (aq) + HNO3 (aq)

The Role of Nutrients in the Nitrogen Cycle

• Nitric acid is converted to nitrate salts in the soil.
• Plants uptake nutrients.
• Animals use nutrients to create proteins and biomolecules
• Denitrification reverses nitrogen fixation.
• Anaerobic organisms decay wastes, plants and animals and produce free nitrogen

Industrial Nitrogen Fixation

• N2 (g) + 3H2 (g) → 2NH3 (g) - requires a catalyst
• NH3 + 2O2→ HNO3 + H2O
• NH3 + HNO3 → NH4NO3 (fertilizer)

Oxygen Cycle

• The Oxygen Cycle is complex.
• Is Complicated because oxygen appears in so many chemical forms.
• Atmospheric oxygen is removed by:
  • Respiration
  • Industrial Processes like combustion with produces CO2
• Photosynthesis:
  • The major mechanism
  • Molecular oxygen is regenerated from CO2 and water.

Aurora Phenomenon

• Spectacular celestial light shows:
  • Caused by ejection of electrons and protons during solar flares.
  • Eruptions on the sun's surface. -Aurora borealis (Northern Hemisphere). -Aurora australis (Southern Hemisphere).
• Electrons and protons collide with the molecules and atoms in Earth's upper atmosphere:
  • Molecules and Atoms are ionized and electronically excited
  • hv + N2 → 2N
  • hv + N → N* + e-
  • hv + O2 → O2* + e-

Auroras - Chemical Reactions in the Thermosphere

• Excited molecules and ions return to their ground state, releasing light.
• Excited oxygen atoms emit photons at 558 nm (green) and between 630 and 636 nm (red).
• O* → O + hv
• Blue/violet colors by ionized nitrogen molecule transitions at 391-470 nm:
• N+* → N⁺ + hv 2 2