Composition of the present atmosphere

• Early volcanic gases and the primitive atmosphere
• Complex mixture of gases and suspended particles called aerosols
  • Major gases: Nitrogen- 78%; Oxygen -21%; Argon - 0.9% (together 99.96%)
  • Minor-all other gases less than 0.04% (CO₂, CH₄, N₂O, O₃, H₂O)
  • inert gases
  • CO₂, CH₄, N₂O, O₃, H₂O - Greenhouse gases
  • Water vapor - varies from 4% to less than 1%
• Why composition is important: essential components for life metabolism, shielding from harmful rays, and the needed natural greenhouse effect

Interaction of atmosphere and the incoming solar radition:

• Absorption
  • Absorption of solar radiation in the earth's atmosphere
  • Very short wavelengths ( gamma rays and x-rays etc.) do not penetrate
  • Atomic Oxygen and (Nitrogen) remove short wavelength UV to ~ 210 nm.
  • Ozone cuts off UV from 210-310 nm.
  • A portion of the infrared is absorbed by H₂O and CO₂
  • The visible and parts of the radio and longer wavelengths penetrate and are absorbed by the surface of the earth
• Scattering of light
  • By air molecules - based on wavelength
  • Blue wavelengths scattered more than longer wavelengths of visisble light
    • ---sky is blue
  • Aerosol and particulate matter - all visible wavelengths
  • Clouds scatter in all directions- energy coming to earth is reduced
• Reflection
  • overall reflectivity of the planet is called its albedo
    • From cloud cover
    • By ground surface

Solar energy budget of the earth - natural greenhouse

• black body temperature of the earth: +5° C
• with 30% albedo: -25° C
• net surface temperature of the earth: -20° C (frozen planet)
• present average surface temp of earth +15° C
• net increase needed to have the present temp +35° C