Electromagnetic Radiation: Interactions in the Atmosphere

Sun’s Radiant Energy Distribution Spectral RegionWavelength Range [  m] Percent of Total Energy Gamma and X-rays< 0.01Negligible Far Ultraviolet Middle Ultraviolet Near Ultraviolet Visible Near Infrared Middle Infrared Thermal Infrared Microwave> 1000Negligible Radio Waves> 1000Negligible

Solar Emittance Curve Radiation leaving the surface of the sun: 6.4 ´ 107 Wm -2 Solar radiation at sea level: 1370 Wm -2 Blackbody radiation curve By comparison, the Earth at 288K emits only 390 Wm -2

Terminology (yuck!) Radiant flux , [W, Js -1 ] Irradiance (flux density), E [Wm -2 ] (its is called Exitance, M when it is away from the surface) Radiance, L, [Wm -2 sr -1 ] Note, all can be functions of wavelength with additional units [  m -1 ]

Electromagnetic Radiation Interactions with Matter Conservation of energy: radiation at a given wavelength is either –reflected (  ) -- property of surface or medium is called the reflectance or albedo (0-1) –absorbed (  ) -- property is absorptance or emittance (0-1) –transmitted (  ) -- property is transmittance (0-1) Radiation Budget Equation:     

Transmission,  Absorption,  (and Emission) Reflection, 

Reflection + Absorption + Transmission = 1 For the Earth’s surface, we ignore Transmission Reflection + Absorption = 1

Reflection –diffuse reflection (rough surface) –specular reflection (smooth surface) Diffuse Specular

Atmospheric Effects EMR is attenuated by its passage through the atmosphere Attenuation = scattering + absorption –Scattering is the redirection of radiation by reflection and refraction –Attenuation is wavelength dependent

–scattering by molecules and particles whose diameters are << –primarily due to oxygen and nitrogen molecules –scattering intensity is proportional to -4 –responsible for blue sky Rayleigh Scattering

A Clear Blue Sky Blue radiation ( = 0.46) Red radiation ( = 0.66) (0.66/0.46) 4 = 4.24 Blue is scattered 4  more than red radiation

Mie Scattering –Spherical particles that have a mean diameter 0.1 to 10 times the incident wavelength –Examples for visible light: water vapor, smoke particles, fine dust –Scattering intensity is proportional to -4 to 0 (depending on particle diameter) Clear atmosphere has both Rayleigh and Mie scattering. Their combined influence is between -0.7 to 

Red Sky at Night At sunset, solar radiation must traverse a longer path through the atmosphere. Viewing a setting sun, the energy reaching the observer is largely depleted of blue radiation, leaving mostly red wavelengths (Rayleigh). Dust/smoke adds additional scattering with a wavelength dependence that increases the red sky effect (Mie)

Non-selective Scattering –aerosol particles much larger than the wavelength (> 10x) –examples: water droplets, ice crystals, volcanic ash, smog –independent of wavelength: 0

Atmospheric Absorption Absorption is the process whereby radiant energy is absorbed by atmospheric constituent and converted to thermal energy Atmospheric absorbers are primarily: H 2 Owater vapor, water droplets CO 2 carbon dioxide O 2 oxygen O 3 ozone Dust and soot

Absorption Energy incident on a molecule causes –small displacements of the atoms from their equilibrium position (vibrations and rotations) –N atoms 3N possible vibrational modes O HH O HH O HH 2 = 6.08  m 1 =  m 3 =  m

Water Vapor Absorption The water molecule has three classical frequencies 1, 2, 3 that correspond to the three wavelengths: mm (symmetric OH stretch) (HOH bend) mm (asymmetric OH stretch) An example of combination: = /  1/   1/    m

Absorption Bands EM spectrum within which radiant energy is absorbed by substances such as water (H 2 O), carbon dioxide (CO 2 ), oxygen (O 2 ), ozone (O 3 ), and nitrous oxide (N 2 O), dust, soot, etc.An absorption band is a portion of the EM spectrum within which radiant energy is absorbed by substances such as water (H 2 O), carbon dioxide (CO 2 ), oxygen (O 2 ), ozone (O 3 ), and nitrous oxide (N 2 O), dust, soot, etc.

Atmospheric “Windows” Regions in the EM spectrum where energy can be fully transmitted  m UV and visible light 3-5  m emitted thermal energy from Earth 8-11  m emitted thermal energy from Earth 1 mm-1 m radar and microwave energy