Lecture 2 Geog 372 - Remote Sensing

Principles of EM Radiation Interaction with Atmosphere and Land Surface

Lecture 2Summer Session14 July 20111First off, any questions from last week?

Today, we are going to talk about what happens to light as it passes through the atmosphere on the way to the sensor. Interaction of EM Radiation with the Atmosphere2Key components of VIS/NIR remote sensing Constituents of the atmosphere that will interact with EM radiation:

GasesWater Water vaporWater dropletsIce particlesParticulate matter smoke, dust, other particles

VIS/NIR SatelliteEM energyEM energyATMOSPHERE31. The ATMOSPHERE!

energy comes in from the sun, but does not just arrive at the surface unchanged. Today well talk about the many things that happen on the way to the Earths surface, the things that happen at the Earths surface, and the things that happen when travelling back to the sensor.

The atmosphere is made of up many things, and they all interact with radiation.

Note that many of these constituents have a complex 3 dimensional distribution in the atmosphere and they also change over time

90 km4Atmosphere consists of multiple layers.Different gas concentrationsThe atmosphere that can influence remote sensing signatures is about 90 km thicktropospherestratosphereThe Troposphere does not have a uniform depth it is deepest at the equator (18 km) and shallowest at the poles (about 10 km) means that there is more the effects of that atmosphere on RS vary geographicallyAtmospheric GasesNitrogen 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.5These gases primarily absorb and scatter EM radiation

Water is present in a variety of forms in the atmosphereGas/vapor, droplets, ice crystalsIts form determines the manner in which it reacts with EM radiationWater in the atmosphereWater is present in a variety of forms in the atmosphereGas/vapor, droplets, ice crystalsIts form determines the manner in which it reacts with EM radiation6Particulate MatterInorganic and organic particles are suspended in the atmosphere from a variety of sourcesDust storms, pollution, fires, volcanic eruptionsThese particles interact with EM energyFrom 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 amount of solar radiation that reaches Earth's surface... Visibility has increased over Europe, consistent with reported European brightening, but has decreased substantially 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. 7

Dust cloud south of Iceland Observed by MODIS8

Smoke plume overEastern US observed by MODIS in July 2002 from Forest Fires (red dots) in Quebec9

Landsat Image of Mt. Pinatubo Eruption10Why 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.11Just read through them basicallyWhat do gases and particles in the atmosphere do to EM radiation?FIVE THINGS:RefractReflectAbsorbScatterTransmit

Important!12Basic EM energy/matter interactionsIncident EM RadiationRefractionReflectionScatteringTransmittingAbsorptionEarth surface13NOTE: All of these things can happen sequentially, i.e. light can refracted then hit an atmospheric particle then scatter, then transmit, hit the earth surface, be reflected.

Index of refraction - nn = c / cnwherec is the speed of light in a vacuum, and cn is the speed of light within a substance such as water or airn of water is 1.33n 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.

14Light can never travel as quickly in a vacuum c, nvca , navasunATMOSPHEREv = in a vacuuma = in the atmosphere = angle15When light hits the atmosphere, not only does it slow down, but it also changes its path direction

Direction and speed always become less when EM energy passes from outer space (a vacuum) into the atmosphere- because of density- but... (NEXT SLIDE)

Multiple changes in direction as the light passes through portions of the atmosphere which vary in optical density. 16It can speed up again and become larger relative to whats normal to the surface if it hits something that is optically less dense.

Note wavelength changes when the speed of light changes, but frequency does not.Reflection the process by which incoming EM radiation is reflected of the surface of an objectIncoming RadiationOutgoing Radiation17In the case of the atmosphere, reflectance can occur from the tops of clouds- TOA

It will depend on the density and type of clouds, as well as the wavelength of EM radiationAbsorptionThe process by which EM radiant energy is absorbed by a molecule or particle and converted to another form of energy 18Many molecules will absorb, not reflect EM energy transform EM energy into molecular movement heat

Thus, that energy is not transmitted through the atmosphere

19This figure shows the amount of light energy that is absorbed by different gases found in the atmosphere

We can see that different gases absorb different wavelengths of EM energy

For example, Oxygen and Ozone absorb most of the EM energy lower than 0.3 um, e.g., much of the UV energy

H20 has a number of strong adsorption bands in the IR region, as does CO2

Absorption is an important process in these longer wavelength regions as well

The atmosphere strongly regulates where you can operate a RS systemScatteringThe process whereby EM radiation is absorbed and immediately re-emitted by a particle or molecule energy can be emitted in multiple-directionsIncoming EM energyScattered energyNote: No EM energy is lost during scattering20Emphasize that scattering is a random processit is not possible to predict the direction of the EM wave leaving a particle

Hence, the term scattering

Types of ScatteringRayleigh scatteringMie scattering Non-selective scattering

The type of scattering is controlled by the size of the wavelength relative to the size of the particleScattering is more important for short-wave radiation than long-wave radiation21absorption and re-emission of radiation by atoms or molecules in the atmosphereRayleigh ScatteringOccurs when the wavelength is MUCH LARGER than the particle size

22The gas molecules in the atmosphere, such as N2, O2, O3, CO2, etc., are much, much smaller than the wavelength of the incoming EM radiationthese small gas molecules still result in scattering RAYLEIGH

WOB

Important characteristics of Rayleigh Scattering

Occurs at the molecular levelRayleigh scattering is inversely proportional to the fourth power of the wavelength

Rayleigh scattering ~ 1 / 4

Rayleigh scattering ~ 1 / 4Blue 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 Layer23Wavelengths shorter than 2 um

16 times more scattering of UV radiation (.3 um) than of red light (0.6 um)

Blue light (0.4 um) is scattered 5 times as much as red light (0.6 um)

Reason the sky is blue during the day is that we are seeing the blue light that is scattered within the atmosphere

90 kmMost Rayleigh scattering occurs in the top 10 km of the stratosphere, e.g., at the ozone layer24WOB

Most Rayleigh scattering occurs in the top 4 to 5 km of the Stratosphere, near the OZONE LayerSummary of Rayleigh ScatteringOccurs at the molecular levelThe degree of Rayleigh scattering is inversely proportional to the fourth power of the EM wavelengthMost Rayleigh scattering occurs in the upper 10 km of the stratosphere

25Draw diagram on board and discuss Mie scattering from next pages

Mie ScatteringOccurs when the wavelength particle size

26Mie scattering occurs from larger particles in the atmosphere like dust and soot0.1 to 10 times particle sizeMie ScatteringOccurs with particles that are actually 0.1 to 10 times the size of the wavelengthPrimary Mie scatterers are dust particles, soot from smokeMie scatterers are found lower in the Troposphere27WOBMie scatterers are actually .1 to 10 times the size of the wavelength

They are typically found in the lower parts of the TroposphereWhy because they originate from the earths surface (e.g., dust and smoke particles)Form cloud condensation nuclei get trapped in water particlesAlso can be washed out of the atmosphere by rain

For further discussion of this slide, seehttp://hyperphysics.phy-astr.gsu.edu/hbase/atmos/blusky.html#c528Mie scattering is somewhat more directional than Rayleigh scattering, e.g., there is more forward scattering of EM energyNon-Selective ScatteringOccurs when the wavelength is MUCH SMALLER than the particle size

29Some people non-selective scattering to be just another category of Mie scatteringWOB

All wavelengths are equally affected hence non-selective

Non selective scattering occurs with water droplets and ice crystals

Non-Selective ScatteringIts 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 cloudsParticles are 10 times the size of the wavelength, > 20 um in sizeWhen all wavelengths are scattered or reflected equally, you get pure white light (clouds)!30Particles are typically greater than 10 times the size of the wavelength

Features that contain water droplets and ice crystals appear white because there is equal scattering of all wavelengths

sunReflectedRefractedScatteredAbsorbedTransmitted31WOB!Here are all the things that can happen to EM radiation coming from the sun once it gets to the atmosphere!!

The same things happen to energy originating from the earths surface, e.g., emitted or reflected energy

Some is absorbedSome of it is scatteredSome of it may be reflected from the bottoms of clouds!!And finally, it is refracted as it goes through different layersAtmospheric ExtinctionExtinction 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.32From the next page a simple model of atmospheric attenuation draw on board

We have light passing a distance L through the atmosphere

As it passes through the atmosphere, some of the light energy is scattered or absorbedNOTE: IT IS NOT LOST!

Atmospheric ExtinctionIo - the unattenuated light intensityL - the path length through the atmosphereI - attenuated light intensity33WOBI / Io < 1

Furthermore, the amount of attenuation or extinction is going to be proportional to the distance the light travels through the atmosphere,

I / Io ~ L

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 34So the extinction coefficient is just the combination of two terms that have to do with scatteringone term that has to do with absorption.

Atmospheric windows35Atmospheric windows define the wavelength regions where we can operate remote sensing instruments

In areas where high absorption occurs, we do not get enough signal back to the sensor

Note that for IR energy, beyond 20 um all energy is absorbed by the atmosphere

IN thermal IR systems, you only have two atmospheric windows

3 to 5 um and 8 to 14 um

Thus, thermal IR remote sensing systems are limited to these wavelength regions

36Here we have the net effect of all the things that happen to EM energy in the atmosphere

This represents the amount of solar radiation

At the top of the atmosphereAt sea level

In this figure, a number of absorption windows associated with various gases in the atmosphere are marked

Figure 1-18 from Elachi, C., Introduction to the Physics and Techniques of Remote Sensing, 413 pp., John Wiley & Sons, New York, 1987.Transmission = 100% absorption

(in the context of the atmosphere)37Another way in which atmospheric absorption is thought of is in terms of atmospheric transmission

Transmission = 100% absorption

Here we can see that in terms of FAR IR radiation, there is low transmission out to about 1000 um, and a strong absorption band at 0.5 cm in the microwave region

However, note that there is very high transmission at microwave wavelengths > 0.5 cm.

Microwave remote sensors are largely unaffected by atmospheric absorptionInteraction of EM Radiation with the surface38Key components of VIS/NIR remote sensing

1. Sun is EM Energy Source2. Energy emitted from sun described by Stephan/Boltzman Law, Plancks formula, and Wiens Displacement Law3. EM Energy interacts with the atmosphere4. EM energy reflected from Earths SurfaceVIS/NIR SatelliteEM energyEM energy5. EM Energy interacts with the atmosphere39Revisiting this diagram which we examined earlier we have addressed the first 3 components (and to an extent the 5th), lets now talk about the 4th component what happens when EM energy hits the surface. Radiation Budget Equation Earths Surface Three things can happen to incident EM energy [i] when it interacts with a featureReflectedAbsorbedTransmittedi The degree to which EM energy is reflected, transmitted, and absorbed is dependent on the wavelength of the EM energy40Based on these three processes we can develop a budget for incoming radiation

Write Equation from next page on the board

Note that the amount of R,A,T that occurs depends on the media that the EM radiation encounters, and the wavelength of the EM energy

In discussing these three processes, we refer to what happens to the incoming incident EM radiation, which we refer to as i

Now, these processes are also quite dependent on the wavelength of the incoming radiation, therefore, we refer to i

SEE NEXT PAGE

The three processes that occur to i are

Reflection - R

Absorption - A

And

Transmittance - T

The radiation budget for EM energy incident on a surface is defined as

i = R + A + T

Radiant Flux - The fundamental unit to measure electromagnetic radiation is radiant flux - is defined as the amount of energy that passes into, through, or off of a surface per unit timeInto = absorbedThrough = transmittedOff of = reflectedRadiant flux () is measured in Watts (W)41 Is phi

Go over stuff on next pageRadiation 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 42WOB

This is a slightly different representation than in the book- Jensen reuses his symbols confusing.

Radiant Flux DensityRadiant flux density is simply the amount of flux per unit area

Radiant flux density = /area43There are two ways to think of radiant flux density

That which is incident on a specific surface this is called irradiance

That which is coming off of a specific surface, this is called the exitanceIrradiance versus Exitance Irradiance (E) is the amount of incident radiant flux per unit area of a plane surface in Watts per square meter (W m 2 )

Exitance (M) is the amount of radiant flux per unit area leaving 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.44 - Exitance is Irradiance as well as hemispherical reflectance, AND emitted temperature.Hemispherical reflection, absorption, transmissionHemispherical reflection, absorption, and transmission refer to what happens to all energy that comes inIf it is not absorbed, it can be reflected or transmitted in any direction into a hemisphere energy is conserved, not lost! 45Coming along a z-axis

REMEMBER that the hemispherical reflectance, transmittance and absorbtance when summed together = 1

Hemispherical reflectance (r), absorptance (), and transmittance ()r = R / i

= A / i

= T / i A ratio of radiant flux reflected, transmitted, or absorbed from the surface to the radiant flux incident to it.r + + =1

recall that: i = R + A + T46WOB this below the initial radiation budget i = R + A + T

Discuss why it is called hemispherical reflection see next page

Later, we will discuss the concept of directional reflectionReflectanceThere are several types of surfaces, whose texture and composition influence the way in which light is reflected.Specular reflectors/surfacesDiffuse reflectors/surfacesLambertian reflectors/surfacesSpecular ReflectanceOccurs from very smooth surfaces, where the height of features on the surface