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Lecture 3 Remote Sensing in the Visible and Reflected IR Region of the EM spectrum - The Effects of the Atmosphere on EM RadiationFebruary 9th 2009
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SyllabusLecture/Hourly Exam Schedule and Assigned Readings (Subject to Change)
Week Date Lecture Topic Reading Part I Remote Sensing Basics
1 26-Jan 1 Introduction to Remote Sensing Ch 1 28-Jan University Closed
2 02-Feb 2 Principles of EM radiometry and basic EM Theory Ch 204-Feb Principles of EM radiometry and basic EM Theory II
3 09-Feb 3 Atmospheric Influences on EM Radiation 11-Feb 4 Photographic Systems/Image Interpretation Ch 3,5
4 16-Feb 5 The Digital Image I Ch 4,1018-Feb The Digital Image II
5 23-Feb 6 Applications with areal and space photography 25-Feb Exam 126-FebLab 1 Introduction to ENVI – manipulation of digital imagery
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Our atmosphere
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Key components of VIS/RIR remote sensing 1. Sun is EM
Energy Source
2. Energy emitted from sun based on Stephan/Boltzman Law, Planck’s formula, and
Wein Displacement Law (Lecture 2)
3. EM Energy interacts with the
atmosphere
4. EM energy reflected from Earth’s Surface – Lectures
7/8
VIS/NIR SatelliteEM energy
EM energy
5. EM Energy interacts with the
atmosphere
Lecture 3
1. Key Atmospheric Constituents• Gases, water, particulate matter
2. Effects of the atmosphere on EM energy
• Reflection, Absorption, Scattering, Transmittance
3. Atmospheric extinction and the attenuation coefficient
4. Net effects of the atmosphere on VIS/IR energy reaching the earth’s surface - atmospheric windows
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Lecture 3 Topics/Key Points
1. Refraction2. Reflection3. Absorption4. Scattering5. Transmittance
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What do gases and particles in the atmosphere do to EM radiation?
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90 km
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Key components of VIS/NIR remote sensing
Constituents of the atmosphere that will interact with EM radiation
• Gases – CO2, N2Ox, CH4, O2, O3
• Water – – Water vapor– Water droplets– Ice particles
• Particulate matter – smoke, dust, other particles
VIS/NIR SatelliteEM energy
EM energy
Nitrogen – N2 – 78%Oxygen – O2 – 21%Argon – Ar – 1%H20 – 0 to 7% Major atmospheric trace gases (less than 0.1%
each)Carbon dioxide – CO2
Ozone – O3
Methane – CH4
Carbon Monoxide – CONitrous Oxide – N2Ox Chlorofluorocarbons (CFCs)
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Atmospheric Gases
Water is present in a variety of forms in the atmosphereGas/vapor, droplets (liquid and frozen), ice
crystalsThe physical state (e.g., gas, liquid, solid)
and density of water determines the manner in which it reacts with EM radiation
The amount of water in the atmosphere is highly variable, depending on climatic processes and earth/atmosphere interactions, particularly the hydrologic cycle
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Water in the atmosphere
When water is present in the form of clouds, it totally blocks radiation in the visible/RIR region of the EM spectrum
In other forms, atmospheric water affects the absorption, scattering, and transmission of visible/RIR radiation through the atmosphere
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Impacts of atmospheric water
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Water is continuously being added to and removed from the atmosphere in a variety of forms through the global water cycle
This water strongly influences EM radiation is passing through the atmosphere – it is a very transient characteristic, e.g., it is always changing
Inorganic and organic particles that have been suspended in the atmosphere from a variety of sources
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Particulate Matter
Natural processes Volcanic eruptions – ash and inorganic compounds
(example - sulfur dioxide) Dust storms – small soil particles (sand, silt, and clay) Wildland fires – soot and ash Biological processes – emissions of complex
hydrocarbons Sea mist – water in droplets blowing of the sea surface
evaporates, leaving sea salts
Human activities Burning of fossil fuels – soot and inorganic compounds Biomass burning – soot, ash
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Sources of particulate matter
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Dust cloud south of Iceland Observed by
MODIS
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Smoke plume overEastern US observed by MODIS in July 2002 from Forest Fires (red dots) in Quebec
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Landsat Image of Mt.
Pinatubo Eruption
Particulate matterHighly variable both spatially and temporally,
driven by the hydrologic cycleA regional phenomenon, dependent on sourcesCorrections must be made to account for the
impacts of particulate matterNeed to understand possible sources for
particulate matter in the regions of interest
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Temporal/spatial variability of atmospheric constituents
Atmospheric waterHighly variable both spatially and temporally,
driven by the hydrologic cycleA global phenomenonCorrections must be made to account for the
impacts of atmospheric waterNeed to understand how hydrologic cycle is
influencing atmospheric water in the regions of study
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Temporal/spatial variability of atmospheric constituents
Trace gases CO2 CO N2Ox CH4CFC’s
Generally well mixed throughout the atmosphere
Change in response to physical, biological and chemical processes
Except for CO2, Spatial/temporal variations do not influence radiation in the VIS/RIR region of the EM spectrum
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Temporal/spatial variability of atmospheric constituents
The constituents of the atmosphere are highly variable both spatially and temporally
These constituents interact with EM energyTo perform quantitative analyses of satellite
remote sensing imagery requires an understanding of and accounting for atmospheric effects
Sophisticated computer models have been developed to quantify the effects of the atmosphere and to normalize remote sensing data for its effects
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The bottom line!!!
1. Key Atmospheric Constituents• Gases, water, particulate matter
2. Effects of the atmosphere on EM energy
• Reflection, Absorption, Scattering, Transmittance
3. Atmospheric extinction and the attenuation coefficient
4. Net effects of the atmosphere on VIS/IR energy reaching the earth’s surface - atmospheric windows
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Lecture 3 Topics/Key Points
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Basic EM energy/matter interactionsIncident EM Radiation
Refraction
Reflection
Transmittance
AbsorptionScattering
Reflection Absorption Scattering Transmittance
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What do gases and particles in the atmosphere do to EM radiation?
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Reflectance – the process whereby incoming EM radiation is reflected off the surface of an object
Incoming Radiation
Outgoing Radiation
Reflection of EM energy in the Visible/RIR region of the EM spectrum occurs primarily from the tops of dense clouds
~25% of incoming solar EM energy in this wavelength region is reflected by clouds
When clouds of particulate matter (e.g., smoke, dust, etc.) are particularly thick or dense, the reflection from the tops of these can also occur
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Atmospheric Reflection
The process by which EM radiant energy is absorbed by a molecule or particle and converted to another form of energy
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Absorption
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UV radiation
Some trace atmospheric gases are strong absorbers of EM energy, but this absorption is confined to specific wavelength regions
Water is a very strong absorber of EM energy in specific wavelength regions > 0.7 m
Atmospheric particles will absorb some EM energy – because they are large, they tend to absorb all wavelengths equally
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Summary of atmospheric absorption
The process whereby EM radiation is absorbed and immediately re-emitted by a particle or molecule – energy can be emitted in multiple-directions
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Scattering
Incoming EM energyScattered energy
Note: No EM energy is lost during scattering
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Rayleigh scatteringMie scattering Non-selective scattering
The type of scattering is controlled by the size of the wavelength relative to the size of the particle
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Types of Scattering
Rayleigh Scattering(also called molecular scattering)
Occurs when the wavelength λ >> the particle size
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Rayleigh scattering ~ 1 / 4
Rayleigh scattering occurs at a molecular level
Through Rayleigh scattering, blue light (0.4 um) is scattered 5 times as much as red light (0.6 um)
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90 km
Most Rayleigh scattering occurs in the top 10 km of the stratosphere, e.g., at the ozone layer
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For further discussion of this slide, seehttp://hyperphysics.phy-astr.gsu.edu/hbase/atmos/blusky.html#c5
The clear sky appears blue because Rayleigh scattering high in the atmosphere influence short wavelength (blue) radiation the most
Note UV radiation is not scattered by the upper atmosphere because it is absorbed by the OZONE Layer
Occurs at the molecular levelThe 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
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Summary of Rayleigh Scattering
Mie Scattering
Occurs when the wavelength particle size
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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
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Mie Scattering
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Where does Mie Scattering Occur?
The sources of Mie scatterers are at the earth’s surface, therefore, Mie scatterers are largely confined to the lower troposphere
The exception are volcanoes, whose plumes of particulate matter are lifted well above the tropopause into the lower stratosphere
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Non-Selective Scattering
Occurs when the wavelength << particle size
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, e.g., > 20 um in size
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Non-Selective Scattering
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For further discussion of this slide, seehttp://hyperphysics.phy-astr.gsu.edu/hbase/atmos/blusky.html#c5
1. Refraction2. Reflection3. Absorption4. Scattering5. Transmittance
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What do gases and particles in the atmosphere do to EM radiation?
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sunReflected
Refracted
Scattered
Absorbed
Transmitted
The fraction or percent of a particular frequency or wavelength of electromagnetic radiation that passes through the atmosphere without being reflected, absorbed or scattered.
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Atmospheric Transmittance
1. Key Atmospheric Constituents• Gases, water, particulate matter
2. Effects of the atmosphere on EM energy
• Reflection, Absorption, Scattering, Transmittance
3. Atmospheric extinction and the attenuation coefficient
4. Net effects of the atmosphere on VIS/IR energy reaching the earth’s surface - atmospheric windows
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Lecture 3 Topics/Key Points
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
Extinction quantifies the amount of atmospheric transmittance
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Atmospheric Extinction
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Atmospheric Extinction
Io - the unattenuated light intensity passing into the atmosphere
L - the path length through the atmosphere
I - attenuated light intensity
I / Io= e- L
where I is the attenuated light intensityIo is the unattenuated light intensity
L is the path length through the a uniform medium such as the atmosphere
is the extinction coefficient in the units of inverse distance
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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
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Extinction Coefficient -
1. Key Atmospheric Constituents• Gases, water, particulate matter
2. Effects of the atmosphere on EM energy
• Reflection, Absorption, Scattering, Transmittance
3. Atmospheric extinction and the attenuation coefficient
4. Net effects of the atmosphere on VIS/IR energy reaching the earth’s surface - atmospheric windows
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Lecture 3 Topics/Key Points
Those regions of the EM spectrum which are to some degree unaffected by attenuation by constituents of the atmosphere, and therefore can be used in vis/RIR instruments for remote sensing of the earth’s surface
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Atmospheric Window
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55Visible
1 window Near IR3 windows
Shortwave IR2 windows
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Our atmosphere
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SyllabusLecture/Hourly Exam Schedule and Assigned Readings (Subject to Change)
Week Date Lecture Topic Reading Part I Remote Sensing Basics
1 26-Jan 1 Introduction to Remote Sensing Ch 1 28-Jan University Closed
2 02-Feb 2 Principles of EM radiometry and basic EM Theory Ch 204-Feb Principles of EM radiometry and basic EM Theory II
3 09-Feb 3 Atmospheric Influences on EM Radiation 11-Feb 4 Photographic Systems/Image Interpretation Ch 3,5
4 16-Feb 5 The Digital Image I Ch 4,1018-Feb The Digital Image II
5 23-Feb 6 Applications with areal and space photography 25-Feb Exam 126-FebLab 1 Introduction to ENVI – manipulation of digital imagery
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