ATMOSPEHRIC CHEM

Atmospheric Chemistry

• Definition: Studies the chemical composition of the Earth's atmosphere and interactions between its gases, liquids, solids, and Earth's surface and biota.

Layers of the Atmosphere

• Troposphere:

  • Lowest layer, harbors all living organisms and human activity.

  • Where weather occurs.

  • Temperature decreases as altitude increases.

• Stratosphere:

  • Second layer above the troposphere.

  • Temperature increases with altitude.

• Mesosphere:

  • Third layer, temperature decreases with height similar to troposphere.

• Thermosphere:

  • Above the mesosphere, consists of very thin air.

  • Houses the ionosphere, filled with electrically charged electrons.

• Exosphere:

  • Outermost layer extending up to 10,000 km above Earth's surface.

Composition of the Atmosphere

• Early Atmosphere:

  • Formed 4.7 billion years ago from volcanic activity releasing gases.

  • Mostly composed of CO₂, H₂O, CH₄, and NH₃.

  • Almost no O₂ initially present.

  • Cooling led to liquid water formation and oceans where early organisms thrived.

  • Photosynthesis began, decreasing CO₂ and increasing O₂ levels over time.

• Current Atmosphere:

  • Composed primarily of nitrogen (N₂ - 78%) and oxygen (O₂ - 21%).

  • Also contains water vapor (H₂O), carbon dioxide (CO₂), and trace noble gases like argon, neon, xenon, and krypton.

Nitrogen Cycle

• Importance:

  • Despite high composition of nitrogen (78%), it is not directly usable by organisms due to strong triple bond.

• Processes:

  1. Nitrogen Fixation: Conversion of N₂ to biologically available nitrogen.

  2. Assimilation: Plants absorb nitrogen as nitrate (NO₃⁻) or ammonium (NH₄⁺) to build amino acids, nucleic acids, and chlorophyll.

     • Some plants (e.g., legumes) have relationships with rhizobia bacteria for direct nitrogen access.

  3. Nitrification: Conversion of NH₃ to NO₂⁻ and then to NO₃⁻ by soil bacteria.

     • Two steps:

       • NH₃ → NO₂⁻ (by Nitrosomonas)

       • NO₂⁻ → NO₃⁻ (by Nitrobacter)

  4. Denitrification: Conversion of NO₃⁻ back to nitrogen gas (N₂) to be released into the atmosphere, maintaining nitrogen balance.

  5. Ammonification: Decomposition of organic nitrogen (from dead organisms) back into ammonia (NH₃) by fungi and prokaryotes.

Oxygen Cycle

• Processes Using Oxygen:

  • Respiration: Oxygen consumed by animals and plants during breathing.

  • Decomposition: Organic matter decomposition consumes oxygen and releases CO₂.

  • Rusting: Oxidation of materials uses oxygen leading to rust formation.

  • Combustion: Fire requires oxygen, producing CO₂ when materials burn.

  • Photochemical Reactions: Absorption of light energy leading to chemical changes in the atmosphere.

Important Chemical Changes in the Atmosphere

1. Photodissociation: Molecule breaks apart when hit by light.

   • Example: O₂ splits into atomic oxygen (O) upon absorbing sunlight.

2. Photoionization: Molecule loses an electron upon absorbing solar radiation, forming positive ions (cations).

Ozone in the Atmosphere

• The ozone layer: Concentrated ozone (O₃) in the stratosphere acts as a protective shield against UV radiation from the Sun.

• Protection Mechanism: O₃ and O₂ absorb 95% to 99.9% of harmful UV rays, particularly UV-C and UV-B.

Acid Rain

• Definition: Precipitation containing acidic chemicals (e.g., sulfuric acid H₂SO₄, nitric acid HNO₃) harmful to the environment.

• Formation:

  • Created when SO₂ and NOₓ are released from burning fossil fuels, reacting with atmospheric components to form acids.

• Forms of Acid Deposition:

  • Dry deposition: Acidic gases and particles fall without precipitation.

  • Wet deposition: Common acid rain mixing acids with precipitation like rain, snow, or fog.

Ozone Depletion

• Critical environmental issue where ozone layer is thinned due to pollution, especially from chlorine and bromine compounds.

• Main Cause: Rise of chlorofluorocarbons (CFCs) into the atmosphere, breaking down under UV exposure to release chlorine (Cl) which damages ozone (O₃) molecules.