Principles of EM Radiation Interaction with Atmosphere and Land SurfaceLecture 2 Summer Session14 July 2011
Key components of VIS/NIR remote sensing VIS/NIR Satellite EM energy EM energy • Constituents of the atmosphere that will interact with EM radiation: • Gases • Water – • Water vapor • Water droplets • Ice particles • Particulate matter – smoke, dust, other particles ATMOSPHERE
Atmospheric Gases Nitrogen – N2 – 78% Oxygen – O2 – 21% Argon – Ar – 1% H20 – 0 to 7% Atmospheric trace gases (less than 0.1% each) Carbon dioxide - CO2 , Ozone – O3 , Methane – CH4 , Carbon Monoxide – CO, Nitrous Oxide – N2O, Chlorofluorocarbons (CFCs), and many others Primarily Absorb and Scatter EM Radiation. *water can reflect as well.
Water in the atmosphere • Water is present in a variety of forms in the atmosphere • Gas/vapor, droplets, ice crystals • Its form determines the manner in which it reacts with EM radiation
Particulate Matter • Inorganic and organic particles are suspended in the atmosphere from a variety of sources • Dust storms, pollution, fires, volcanic eruptions • These particles interact with EM energy • From a recent Science article (authored by two UMD Geographers, Drs. Kaicun Wang and Shunlin Liang): • “Visibility in the clear sky is reduced by the presence of aerosols,whose types and concentrations have a large impact on the amountof solar radiation that reaches Earth's surface... Visibility has increased over Europe,consistent with reported European ‘brightening,’ but has decreasedsubstantially over south and east Asia, South America, Australia,and Africa, resulting in net global dimming over land (Wang, Dickinson, and Liang 2009, p. 1468).” • Aerosols can interrupt the passage of light energy through the atmosphere to Earth.
Smoke plume over Eastern US observed by MODIS in July 2002 from Forest Fires (red dots) in Quebec
Why is atmosphere important in RS of land and ocean surfaces? • The constituents of the atmosphere are highly variable both spatially and temporally. • These constituents interact with EM energy. • Performing quantitative analyses of satellite remote sensing imagery requires an understanding of atmospheric effects. • Sophisticated computer models have been developed to quantify the effects of the atmosphere and to normalize remote sensing data for its effects.
What do gases and particles in the atmosphere do to EM radiation? FIVE THINGS: • Refract • Reflect • Absorb • Scatter • Transmit Important!
Basic EM energy/matter interactions Incident EM Radiation Reflection Scattering Refraction Absorption Transmitting Earth surface
Index of refraction - n n = c / cn where c is the speed of light in a vacuum, and cn is the speed of light within a substance such as water or air n of water is 1.33 n of air is 1.000296 **n, on Earth, will always be greater than 1, because light never travels as fast as it does in a vacuum.
sun v = in a vacuum a = in the atmosphere = angle c, nv v ATMOSPHERE a ca , na
Multiple changes in direction as the light passes through portions of the atmosphere which vary in optical density.
Reflection – the process by which incoming EM radiation is reflected of the surface of an object Incoming Radiation Outgoing Radiation
Absorption • The process by which EM radiant energy is absorbed by a molecule or particle and converted to another form of energy
Scattering • The process whereby EM radiation is absorbed and immediately re-emitted by a particleor molecule – energy can be emitted in multiple-directions Incoming EM energy Scattered energy Note: No EM energy is lost during scattering
Types of Scattering • Rayleigh scattering • Mie scattering • Non-selective scattering The type of scattering is controlled by the size of the wavelength relative to the size of the particle Scattering is more important for short-wave radiation than long-wave radiation
Rayleigh Scattering Occurs when the wavelength is MUCH LARGER than the particle size
Rayleigh scattering ~ 1 / 4 Blue light is scattered 5 times as much as red light UV radiation is not scattered by the upper atmosphere because it is absorbed by the OZONE Layer
90 km Most Rayleigh scattering occurs in the top 10 km of the stratosphere, e.g., at the ozone layer
Summary of Rayleigh Scattering • Occurs at the molecular level • The degree of Rayleigh scattering is inversely proportional to the fourth power of the EM wavelength • Most Rayleigh scattering occurs in the upper 10 km of the stratosphere
Mie Scattering Occurs when the wavelength particle size
Mie Scattering • Occurs with particles that are actually 0.1 to 10 times the size of the wavelength • Primary Mie scatterers are dust particles, soot from smoke • Mie scatterers are found lower in the Troposphere
For further discussion of this slide, see http://hyperphysics.phy-astr.gsu.edu/hbase/atmos/blusky.html#c5
Non-Selective Scattering Occurs when the wavelength is MUCH SMALLER than the particle size
Non-Selective Scattering • Its name derives from the fact that all wavelengths (visible/near IR) are equally affected • Particles are very large, typically water droplets and ice crystals of fog banks and clouds • Particles are 10 times the size of the wavelength, > 20 um in size • When all wavelengths are scattered or reflected equally, you get pure white light (clouds)!
sun Reflected Refracted Scattered Absorbed Transmitted
Atmospheric Extinction • Extinction is a term used to account for the loss or attenuation* of radiant energy as light passes through the atmosphere, and includes both scattering and absorption • The amount of atmospheric transmittance depends on the amount of extinction *sunglasses are attenuators they lessen the intensity of visible and UV light.
Atmospheric Extinction Io - the unattenuated light intensity L - the path length through the atmosphere I - attenuated light intensity
Extinction Coefficient - = bm + bp + k where bm is the Rayleigh or molecular scattering coefficient bp is the Mie scattering coefficient (due to the airborne particles) k is the absorption coefficient
Transmission = 100% – absorption (in the context of the atmosphere) • Figure 1-18 from Elachi, C., Introduction to the Physics and Techniques of Remote Sensing, 413 pp., John Wiley & Sons, New York, 1987.
Key components of VIS/NIR remote sensing 2. Energy emitted from sun described by Stephan/BoltzmanLaw, Planck’s formula, and Wien’s Displacement Law 1. Sun is EM Energy Source VIS/NIR Satellite EM energy EM energy 3. EM Energy interacts with the atmosphere 5. EM Energy interacts with the atmosphere 4.EM energy reflected from Earth’s Surface
Radiation Budget Equation – Earth’s Surface Three things can happen to incident EM energy [i] when it interacts with a feature • Reflected • Absorbed • Transmitted i The degree to which EM energy is reflected, transmitted, and absorbed is dependent on the wavelength of the EM energy
Radiant Flux - • The fundamental unit to measure electromagnetic radiation is radiant flux - • is defined as the amount of energy that passes into, through, or offof a surface per unit time • Into = absorbed • Through = transmitted • Off of = reflected • Radiant flux () is measured in Watts (W)
Radiation Budget Equation i = R + A + T R is the amount of energy reflected from the surface A is the amount of energy absorbed by the surface T is the amount of energy transmitted through the surface i is the incident radiation (radiant flux) for a given wavelength
Radiant Flux Density Radiant flux density is simply the amount of flux per unit area Radiant flux density = /area
Irradiance versus Exitance Exitance (M) is the amount of radiant flux per unit area leaving a plane surface in Watts per square meter (W m –2 ) Irradiance (E) is the amount of incident radiant flux per unit area of a plane surface in Watts per square meter (W m –2 ) They both incorporate radiant flux per unit area, but their directionality is what distinguishes them.
Hemispherical reflection, absorption, transmission • Hemispherical reflection, absorption, and transmission refer to what happens to all energy that comes in • If it is not absorbed, it can be reflected or transmitted in any direction into a hemisphere energy is conserved, not lost!
Hemispherical reflectance (r), absorptance (), and transmittance () r = R / i = A / i = T / i A ratioof radiant flux reflected, transmitted, or absorbed from the surface to the radiant flux incident to it. recall that: i = R + A + T r + + =1
Reflectance • There are several types of surfaces, whose texture and composition influence the way in which light is reflected. • Specular reflectors/surfaces • Diffuse reflectors/surfaces • Lambertian reflectors/surfaces
Specular Reflectance • Occurs from very smooth surfaces, where the height of features on the surface << wavelength of the incoming EM radiation • In specular reflection, all energy is reflected in one direction • angle of incidence = angle of exitance
Diffuse Reflectance • Most surfaces are not smooth, and reflect incoming EM radiation in a variety of directions • These are called diffuse reflectors
Lambertian Surface • A perfectly diffuse reflector is called a Lambertian surface • A Lambertian surface reflects equally in all directions