3.3 Radiative transfer and atmospheric absorption

The atmosphere is a complex system of gases and particles that interact with radiation in various ways. Absorption, emission, and scattering processes play crucial roles in determining how energy moves through the air, affecting our weather and climate.

Understanding these radiative processes is key to grasping how the atmosphere works. From the greenhouse effect to the ozone layer's protective role, these interactions shape Earth's temperature profile and influence global climate patterns.

Radiative Processes in the Atmosphere

Radiation processes in atmosphere

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• Absorption
  • Atmospheric constituents absorb incoming or outgoing radiation
  • Converts absorbed energy into internal energy, heating the atmosphere
  • Main absorbers include water vapor, carbon dioxide, ozone, and aerosols (dust, smoke)
• Emission
  • Atmospheric constituents emit radiation based on their temperature
  • Governed by the Stefan-Boltzmann law $E = \sigma T^4$
  • Emitted radiation can be in any direction (upward, downward, or sideways)
• Scattering
  • Atmospheric particles redirect radiation in different directions
  • Types of scattering depend on particle size relative to radiation wavelength
    • Rayleigh scattering occurs when particle size is much smaller than wavelength (air molecules)
    • Mie scattering occurs when particle size is approximately equal to wavelength (aerosols, cloud droplets)
  • Scattering affects the distribution of radiation in the atmosphere (blue sky, red sunsets)

Radiative transfer and temperature profiles

• Radiative transfer studies how radiation propagates through the atmosphere
  • Involves absorption, emission, and scattering processes
  • Described by the radiative transfer equation (RTE) $\frac{dI_\lambda}{ds} = -\kappa_\lambda I_\lambda + \kappa_\lambda B_\lambda(T) + \frac{\sigma_\lambda}{4\pi}\int_{4\pi}I_\lambda(\Omega')p(\Omega',\Omega)d\Omega'$
    • $I_\lambda$ is spectral radiance
    • $\kappa_\lambda$ is absorption coefficient
    • $B_\lambda(T)$ is Planck function
    • $\sigma_\lambda$ is scattering coefficient
    • $p(\Omega',\Omega)$ is phase function
• Vertical temperature profile determined by balance between radiative heating and cooling at each level
  • Radiative heating from absorption of solar radiation and longwave radiation from surface and lower atmosphere
  • Radiative cooling from emission of longwave radiation to space and to lower levels
  • Lapse rate (vertical temperature gradient) influenced by radiative processes (greenhouse effect, convection)

Atmospheric Absorption and Its Implications

Main atmospheric absorbers

• Water vapor (H2O) is the primary absorber in the troposphere
  • Absorbs in the near-infrared and infrared regions (heat trapping)
• Carbon dioxide (CO2) is an important absorber in the infrared region
  • Absorption bands at 4.3 μm and 15 μm (greenhouse gas)
• Ozone (O3) is the main absorber in the stratosphere
  • Absorbs ultraviolet (UV) radiation, especially in the UV-C range 200-280 nm (protection from harmful UV)
  • Also absorbs in the visible and infrared regions (minor greenhouse gas)
• Aerosols absorb and scatter radiation depending on their composition and size
  • Examples include dust, smoke, and anthropogenic pollutants (air quality, climate effects)

Absorption effects on climate

• Greenhouse effect
  • Atmospheric absorbers trap outgoing longwave radiation
  • Warms the lower atmosphere and surface (livable temperatures)
  • Without the greenhouse effect, Earth's average temperature would be about -18℃
• Radiative forcing
  • Changes in atmospheric composition (increased greenhouse gases) alter the energy balance
  • Positive radiative forcing leads to warming, negative forcing leads to cooling (climate change)
• Climate sensitivity
  • Amount of warming expected from a doubling of atmospheric CO2 concentration
  • Depends on feedback mechanisms (water vapor feedback, ice-albedo feedback)
• Atmospheric window
  • The 8-12 μm region where the atmosphere is relatively transparent
  • Allows some longwave radiation to escape directly to space (regulates Earth's temperature)