Reading Assignment

Lecture 3 Remote Sensing in the Visible and Reflected IR Region of the EM spectrum - The Effects of the Atmosphereon EM Radiation9 September 2008

Reading Assignment • Campbell – Chapter 2, Section 2.5 Unless otherwise noted, all images in this lecture are from • Jensen, J.R., Remote Sensing of the Environment - An Earth Resource Perspective, 592 pp., Prentice Hall, Upper Saddle River, NJ, 2007.

Key components of VIS/RIR remote sensing 2. Energy emitted from sun based on Stephan/Boltzman Law, Planck’s formula, and Wein Displacement Law (Lecture 2) 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 Lecture 3 4.EM energy reflected from Earth’s Surface – Lectures 7/8

Lecture 3 Topics/Key Points • Key Atmospheric Constituents • Gases, water, particulate matter • Effects of the atmosphere on EM energy • Reflection, Absorption, Scattering, Transmittance • Atmospheric extinction and the attenuation coefficient • Net effects of the atmosphere on VIS/IR energy reaching the earth’s surface - atmospheric windows

90 km

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 – CO2, N2Ox, CH4, O2, O3 • Water – • Water vapor • Water droplets • Ice particles • Particulate matter – smoke, dust, other particles

Atmospheric Gases 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 – CO Nitrous Oxide – N2O, Chlorofluorocarbons (CFCs)

Water in the atmosphere • Water is present in a variety of forms in the atmosphere • Gas/vapor, droplets (liquid and frozen), ice crystals • The 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

Impacts of atmospheric water • 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

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

Particulate Matter • Inorganic and organic particles that have been suspended in the atmosphere from a variety of sources

Sources of 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

Dust cloud south of Iceland Observed by MODIS

Smoke plume over Eastern US observed by MODIS in July 2002 from Forest Fires (red dots) in Quebec

Landsat Image of Mt. Pinatubo Eruption

Temporal/spatial variability of atmospheric constituents • Trace gases • 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

Temporal/spatial variability of atmospheric constituents • Atmospheric water • Highly variable both spatially and temporally, driven by the hydrologic cycle • A global phenomenon • Corrections must be made to account for the impacts of atmospheric water • Need to understand how hydrologic cycle is influencing atmospheric water in the regions of study

Temporal/spatial variability of atmospheric constituents • Particulate matter • Highly variable both spatially and temporally, driven by the hydrologic cycle • A regional phenomenon, dependent on sources • Corrections must be made to account for the impacts of particulate matter • Need to understand possible sources for particulate matter in the regions of interest

The bottom line!!! • The constituents of the atmosphere are highly variable both spatially and temporally • These constituents interact with EM energy • To 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

Basic EM energy/matter interactions Incident EM Radiation Reflection Absorption Scattering Refraction Transmittance

Snell’s law n1 sin 1 = n2 sin 2 Don’t worry about learning Snell’s law

What do gases and particles in the atmosphere do to EM radiation? • Reflection • Absorption • Scattering • Transmittance

Reflectance – the process whereby incoming EM radiation is reflected off the surface of an object Incoming Radiation Outgoing Radiation

Atmospheric Reflection • 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

Absorption • The process by which EM radiant energy is absorbed by a molecule or particle and converted to another form of energy

UV radiation

Summary of atmospheric absorption • 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

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

Fluorescence Flourescence is where an object illuminated with EM radiation of one wavelength region emits radiation in another wavelength region Example: sulfides will absorb UV radiation and emit EM radiation in the visible region of the EM spectrum

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

Rayleigh Scattering(also called molecular scattering) Occurs when the wavelength >> the particle size

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)

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

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 For further discussion of this slide, see http://hyperphysics.phy-astr.gsu.edu/hbase/atmos/blusky.html#c5

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

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 Occasionally, large, high energy forest fires will loft plumes into the lower troposphere

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

Non-Selective Scattering Occurs when the wavelength << 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, e.g., > 20 um in size

For further discussion of this slide, see http://hyperphysics.phy-astr.gsu.edu/hbase/atmos/blusky.html#c5

What do gases and particles in the atmosphere do to EM radiation? • Refraction • Reflection • Absorption • Scattering • Transmittance

sun Reflected Refracted Scattered Absorbed Transmitted

Atmospheric Transmittance • The fraction or percent of a particular frequency or wavelength of electromagnetic radiation that passes through the atmosphere without being reflected, absorbed or scattered.

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 • Extinction quantifies the amount of atmospheric transmittance

Reading Assignment