22_ Chemistry of the Atmosphere

Chemistry of the Atmosphere

Learning Objectives

• Understand the composition of Earth's atmosphere.

Historical Roots

• Greek origin: atmos (vapor/steam), sphaira (ball/globe)

• Modern Latin: atmosphaera

• Definition: Gaseous envelope surrounding the Earth.

Layers of the Atmosphere

• Troposphere: Closest layer; 7-18 km above equator

  • Most active region for weather phenomena (rain, lightning, hurricanes).

  • Holds ~80% of air mass and nearly all water vapor.

  • Temperature decreases with altitude.

• Stratosphere: 19-50 km above equator

  • Contains nitrogen, oxygen, and ozone.

  • Temperature increases with altitude due to UV reactions producing ozone (O3), which protects against harmful UV rays.

• Mesosphere: 30-50 km above equator

  • Temperature decreases with altitude.

  • Lower concentration of ozone and gases.

• Thermosphere: 50-400 km above equator

  • Temperature increases due to energetic particles from the sun.

  • Ionized particles reflect radio waves back to Earth.

• Exosphere: ~400 km above equator

  • Outermost layer, separating atmosphere from outer space;

  • Consists mainly of sparse hydrogen and helium gases.

  • Very cold; no breathable air.

Atmospheric Mass

• Total mass of atmosphere: ~5.3 x 10^18 kg.

• Water concentration varies significantly by location.

Nitrogen Cycle

• Nitrogen fixation processes:

  • Converts molecular nitrogen to nitrogen compounds (like nitrates).

• Biological fixation: From plants and animal waste; ends with denitrification to revert nitrogen back to molecular form.

  • Vital for plant and animal life.

Chemical Reactions in Nitrogen Cycle

• Atmospheric nitrogen fixation reactions:

  • Example: 2NO (g) + O2 (g) → 2NO2 (g) (lightning induced)

• Nitrate salts taken up by plants, forming proteins in animals.

Industrial Nitrogen Fixation

• Process: N2 (g) + 3H2 (g) → 2NH3 (g)

• Ammonium nitrate production: NH3 + HNO3 → NH4NO3 (fertilizer).

Oxygen Cycle

• Oxygen appears in many forms; complex cycle.

• Removed via respiration and combustion, generating CO2.

• Major regeneration through photosynthesis.

Auroras

• Caused by solar flares ejection of electrons/protons; ionized upper atmosphere.

• Electron collides with N2, O2, creating light emissions:

  • Colors: Green (558nm), Red (630-636nm), Blue/Violet (391-470nm).