EMR : Atmospheric Interactions

Lecture Outline

Sources of EMR Definitions The Atmosphere The Earth-Atmosphere radiation budget Influence of the Earth’s Atmosphere -

attenuation, scattering, absorption, transmittance, atmospheric windows

Interaction with Earth-surface materials - reflection (albedo), refraction, absorption, emission

Outgoing terrestrial radiation

Sources of Electromagnetic Radiation

Q: Where does EMR come from?

Definitions 1

Attenuation differential reduction in transmission, and absorption of

wavelengths of EM radiation through the atmosphere. The mechanisms by which attenuation occurs are Scattering, Absorption, Reflectance, Refraction,Re-emission (re-radiation)

Scattering when particles in the atmosphere alter both the direction

and intensity of radiation. Amount of scattering increases greatly as wavelength becomes shorter - so which parts of the spectrum will be affected most?

Absorption the retention of radiant energy by a substance or body. This

involves the transformation of some of the incident radiation into heat and its subsequent re-emission as heat (i.e. at a longer wavelength)

Definitions 2

Reflectance this occurs when a ray of light is re-directed as it strikes

a non-transparent surface. The nature of the reflection depends upon sizes of surface irregularities (roughness/smoothness) in relation to the wavelength of radiation considered.

Refraction the bending of light rays at the contact between 2 media

that transmit light. Refraction occurs in the atmosphere as light passes through atmospheric layers of different clarity, humidity, density and temperature.

Re-emittance energy absorbed by the earth or particles in the

atmosphere will eventually be released (often after being transformed to a longer wavelength) as emitted energy in the form of heat/longer wavelength radiation.

Refraction

The bending of light rays at the contact between 2 media that transmit light.

Refraction occurs in the atmosphere as light passes through atmospheric layers of different clarity, humidity, density and temperature.

Re-emittance

Energy absorbed by the Earth or by particles in the atmosphere will eventually be released

(often after being transformed to a longer wavelength) as emitted energy in the form of

heat/longer wavelength radiation.

Reflectance This occurs when a ray of light (or other

EMR) is re-directed as it strikes a non-transparent surface

The nature of the reflection depends upon sizes of surface irregularities (roughness/smoothness) in relation to the wavelength of EMR considered

The Atmosphere

The Atmosphere

Between Space and Earth’s surface is a layer that affects EMR = the atmosphere

Atmosphere fades out at c. 100km above the Earth

The Atmosphere

Made up of gases:

Nitrogen 78.08%

Oxygen 20.94%

Argon 0.93%

Carbon Dioxide 0.0314%

Ozone 0.00000004%

Variable components: Methane, water vapour, Dust particles

The Atmosphere

The Earth-Atmosphere Radiation Budget

Of total EMR penetrating top of atmosphere only ~ 45-51 % reaches Earth’s surface

The rest is ATTENUATED by gases, clouds and particles in atmosphere

At the Earth’s surface some EMR is reflected, some absorbed and some re-emitted

ONLY 69% of EMR leaving Earth’s surface reaches space again

Reflected EMR = Albedo (31% of ingoing EMR)

The Earth-Atmosphere Radiation Budget

Energy from the Sun passes through the

atmosphere. Not all of it reaches the surface.

There is interaction with the atmosphere both on the way in and way out

Influence of the Earth’s Atmosphere

Most of solar radiation reaching earth’s surface is at wavelengths < 4.0µm (UV, Visible and near IR)

EMR emitted from Earth’s surface has longer wavelengths >4.0 µm (mid, thermal IR and microwave)

However SOME shortwave EMR is reflected from Earth’s surface directly back to Space and is collected for Earth Observation

The Earth-Atmosphere Radiation Budget

Influence of the Earth’s Atmosphere

The atmosphere affects EMR in several ways: its direction of transmission its intensity its wavelength/frequencythe spectral distribution of this radiant

energy

Different sizes and types of particles affect EMR in different ways

Atmospheric Attenuation

Differential reduction in transmission, and absorption of wavelengths of EMR through the atmosphere

The processes by which attenuation occurs are: Scattering, Absorption, Reflectance,

Refraction, Re-emission/( re-radiation)

Atmospheric Scattering

Important in lowest 3 km of atmosphere Shorter wavelengths more vulnerable to scattering Weather conditions and pollutants are important Directs EMR from outside sensor’s field of view It illuminates shadows Causes fading of landscape colours in distance 3 types of scattering: Rayleigh, Mie, Non-selectivesee http://hyperphysics.phy-astr.gsu.edu/hbase/atmos/bluesky.html

Rayleigh Scattering

Rayleigh scattering is the scattering of light, or other electromagnetic radiation, by particles much smaller than the wavelength of the light. It occurs when light travels in transparent solids and liquids, but is most prominently seen in gases

Rayleigh Scattering

Mainly results from interaction with atmospheric gases. Occurs when particles causing the scattering are smaller in size than the wavelengths of radiation in contact with them (therefore Rayleigh

Scattering is wl dependent).

As wl decreases, amount of scattering increases. Example: blue sky

Mie ScatteringCaused by pollen, dust, smoke,

water droplets and other particles in the lower portion of the atmosphere. Occurs when the molecules and

particles causing the scattering are larger than the wavelengths of radiation in

contact with them. Effects are wl dependent.

Example: white light of clouds, mist, fog, white glare around

the Sun

Rayleigh and Mie Scattering

Non-selective Scattering

Occurs in the lower part of the atmosphere when the particles are much larger than

the radiation. Primary cause of atmospheric haze. Not wl dependent.

Example: Haze

See also: http://www.ccrs.nrcan.gc.ca/ccrs/learn/tutorials/fu

ndam/chapter1/chapter1_4_e.html

Atmospheric Absorption

Absorption: the retention of radiant energy by a substance or body

In Earth Observation this involves the transformation of some of the incident radiation into heat and its subsequent re-emission as heat (at a longer wavelength)

Atmospheric Absorption EMR is absorbed by water vapour, carbon

dioxide, ozone and other particles

Ozone: wavelengths < 0.3 µm are almost completely absorbed by ozone therefore wavelengths c. 0.3 µm are not used

in Remote Sensing

Carbon dioxide: absorbs mid- and far-IR wavelengths 13-17 µm

Water vapour: variable effect. 80% absorption at 5.5-7.0 µm and >27.0 µm

Transmittance The combined processes of absorption and scattering by

the atmosphere are expressed by the attenuation/extinction coefficient. T = transmitted radiation / incident radiation

This is the amount of EMR which reaches the Earth’s surface as a ratio of the total radiation incident upon the top of the atmosphere

Transmittance decreases as the combined effects of absorption and scattering accumulate

T depends on wavelength, thickness and transmissivity of substance

Transmission occurs through transparent and opaque surfaces, e.g. glass, leaves, water

Transmittance at the surface: the result of the combined effects of absorption and scattering

Atmospheric Windows Electromagnetic wavelength ranges which have

high transmittance through the atmosphere EMR at these wavelengths suffers little or no

atmospheric attenuation Sensors are designed to record EMR at these

wavelength ranges

Ultra-violet/ visible0.3-0.7 µm

Near/ mid infrared 0.7-0.9µm, 1.55-1.75µm, 2.05-2.4µm

Thermal infrared 3.0-5.0µm, 8.0-14.0µm

Microwave/ RADAR 1mm-1m

Therefore, for earth observation from space we need to choose wavelengths carefully

Absorption and Windows

Atmospheric windows on the Earth

Concerned with electromagnetic wavelength ranges which have high transmittance through the atmosphere

EMR at these wavelengths suffers little or no atmospheric attenuation

If we want to detect various wavelengths of EMR that reach the Earth’s surface we have to build our detectors to be sensitive to wavelengths where there are windows, or

detectable wavelength ranges

Reflection at the surfaceReflection at the surface

Reflection from the Earth surface is: Specular -surface height variation is smaller than the

wavelength (smooth) Diffuse - surface height variation is greater than

wavelength (rough) Lambertian - EMR is reflected with even intensity in all

directions

Reflection at the surface

Lillesand and Kiefer (2000)

Reflection at the surfaceReflection at the surface

Real surfaces are rarely fully specular nor fully Lambertian, most surfaces are approximately Lambertian in the visible and NIR except water

Diffuse reflections of earth surfaces give rise to their colours i.e. spectral data

Outgoing terrestrial radiation Terrestrial radiation has wavelengths

longer than 10 µm There is little overlap between solar and

terrestrial radiation Solar radiation absorbed by the earth’s

surface will eventually be emitted again as longer wavelengths in the form of heat added to geothermal/anthropogenic heat

Reflected and emitted outgoing radiation is subject to all the atmospheric processes described for incoming EMR

Albedo

Albedo is a ratio of scattered to incident electromagnetic radiation power, most commonly light.

It is a unitless measure of a surface or body's reflectivity. The word is derived from albus, a Latin word for "white".

Albedo

Albedo Surface or object Albedo (%)

Fresh snow 75-95 Thick cloud 60-90 Thin cloud 30-50 Ice 30-40 Sand 15-45 Earth’s atmosphere 30 Grassy field 10-30 Dry ploughed field 5-20 Water 10 Forest 3-10 Moon 7

Questions?