RadiationMechanisms of Energy Transfer

Mass flux-sm

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Energy flux- 22 m

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fcRadiation: 1. Electromagnetic wave

Radiation: 2. particles (photons)

Photons have no mass, and travel at the speed of light

hfE Energy of a photo h=6.626068 x10-34 m2 kg / s

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2. Wien’s displacement lawT

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300K

6000K

Laws of blackbody radiation

1. Plank’s law

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Gray body: 10;4 TE

Gray body: 10;4 TE

Radiative equilibrium temperature

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Transmissivity

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AlbedoE

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Surface Remarks Albedo = A Emissivity

Soils Dark, wet 0.05-0.40 0.90-0.98

Light, dry

Desert 0.20-0.45 0.84-0.91

Grass Long (1.0 meters) 0.16- 0.90-

Short (0.2 meters) 0.26 0.95

Agricultural crops, tundra 0.18-0.25 0.90-0.99

Orchards 0.15-0.20 0.15-0.20

Forests Deciduous (bare) 0.15- 0.97-

Deciduous (Leaved) 0.20 0.98

Coniferous 0.05-0.15 0.97-0.99

Water Small Zenith angle 0.03-0.10 0.92-0.97

Large zenith angle 0.10-1.00 0.92-0.97

Snow Old 0.40- 0.82-

Fresh 0.95 0.99

Ice Sea 0.30-0.45 0.92-0.97

Glacier 0.20-0.40

Selective absorption and emission of atmospheric gases

1. Energy level of atoms or molecules

.)0,1,2,....(n nE level,Energy

Quantum jump: transition between different energy levels

2E

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2. Different energy form of a molecule or atom

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What Happens to Incoming Solar Radiation

a. Rotational energy

CO

Rotational energy transition can happenas long as a photon’s wavelength is shorter than 1 cm, usually associated with microwave wavelength.

b. Vibrational energy

Polar molecule haspermanent dipole

Non-polar molecule doesnot have permanent dipole.

Vibrational energy level transition requires a photon's wavelength shorter than 20 micrometer, usually in the infrared band.

Vibration and rotation sometimes combine together to form vibration-rotation mode, the transition between vibration-rotation modes alsoinvolves certain frequencies.

c. Photodissociation

Solar ultraviolet photon

For photodissociation to occur, the wavelength of a photon must be in the ultraviolet band. To dissociate Oxygen the wavelength of radiation must be shorter than 200 nm.

Ozone is a loosely bonded molecule. To dissociate a Ozone molecule, the frequency of a photon can be as low as 300 nm.

d. Electronic excitation

1st Shell 2nd Shell

e. Photoionization

eM M

Electrons may be excited from one shell to another shell by a photon with a sufficiently high energy level. The wavelength is usually shorter than 1 micrometer.

Photoelectron

To photoionize a molecule requires the radiation with a wavelength shorter than 100 nm.

Electronic excitationPhotoionization

M Me

overlap

What gases absorb shortwave radiation? Ozone

What gases absorb and emit longwave radiation? Cloudswater vaporCO2OzoneMethane

Formation of Ozone

Ozone

Sustaining Ozone

Depletion of Ozone

3CFCl assuch CFCs

10-50 km (stratosphere)

23 CFClCl UVCFCl

ClO O Cl

23 O2Cl O ClO

Ozone Depletion

Ozone

The Ozone Hole

Ozone concentration drops sharply over Antarctica

The Ozone Hole

Polar vortex

Cold air-80C

2. As the cold temperatures persist over the polar, polar stratospheric clouds form.

3. Chlorine reservoir species HCl and ClONO2 become very active on the surface of polar stratospheric clouds.

232 ClNHO ClONO HCl

22322 ClOHHClHOCl HOCl;NHO OH ClONO

ClCl Cl2 h

1. Polar winter leading to the stronger circumpolar wind belt (polar vortex) to isolate the cold air within it.

Atmospheric gases absorb all energy at wavelengths emitted from surface except for 8-11 micron window known as AtmosphericWindow.

Oxygen, ozone, carbon dioxide, water vapor are great absorbers of IR radiation.

Trace gases, other important greenhouse gases

CFC-11 CFC-12 HCFC-22

Rayleigh Scattering

The scattering from molecules and very tiny particles (< 1 /10 wavelength) is predominantly Rayleigh scattering.

Mie Scattering

The scattering from relatively large particles (> 1 wavelength) is predominantly Mie scattering, which is not strongly wavelength dependent and produces a sharper and more intense forward lobe

Red Sunset

Making Radiation Measurements

There are three ways to make radiation measurements. •Thermal sensitive devise•Photoelectric cell (photodiode)•Photochemical sensor

What is the basic operating principle for the thermal devise?

1. How could we use a plate to measure broadband radiation? Illuminate the surface with a bright light, or sun light…

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2. What besides radiation will affect temperature of the plate?

Convection and conduction

3. how could their effect on the temperature of the plate be removed?

Using a glass dome

Phototube is an electron tube in which electrons are excited andemitted by light. The simplest phototube is composed of a cathode coated with a photosensitive material. Light falling upon the cathode causes the liberation of electrons, which are then attracted to the positively charged anode, resulting in a flow of electrons (i.e., current) proportional to the intensity of the light.

Solid-state photodetector is the photoconductor whose resistance changes when it is exposed to light. The solid-state photodetector is small, inexpensive, and uses little power.

What is the basic operating principle for the photoelectric cell?

A device that converts light into electricity.

Broadband Radiation Instruments:

Shortwave KLongwave LTotal Q = K + L

What is the basic operating principle for the photochemical sensor?

The photochemical sensor utilizes materials that tend to have chemical reaction due to the absorption of light (including visible, ultraviolet, and infrared). The light excites atoms and molecules (shifts some of their electrons to a higher energy level) and thus makes them more reactive. The bleaching of dyes or the yellowing of paper by sunlight is a good example of photochemical reaction. It is harnessed by plants in photosynthesis and by humans in photography.

Upward stream and a downward stream of radiation

LKQ LKQ

K

K

L

L

= -solar incident

= reflected solar

= emission from sun’s face

= emission from atmosphere

Net radiation *Q QQQ*

Spectral Radiation Instruments:

LKQ ;;

What are solar radiation measurements?

Direct (beam), diffused (sky), and global (total)

Pyranometer measures global-solar shortwave radiation

Solar radiation curve outside atmosphereSolar radiation curve at the sea levelCurve for balckbody at 50000K

Pyranometer sensor

Solar radiation irradiance measurements on a clear day

Shaded Pyranometer measures diffuse solar radiation

The pyranometer has a black thermopile sensor protected by two concentric hemispherical optically ground covers. The detector is independent of wavelength of radiation over the solar energy spectrum. Both the pyranometer and the shading disk are mounted on an automated solar tracker to ensure that the pyranometer is continuously shaded.

Pyrometer — measures global longwave radiation F

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Video pyrometer

Pyrheliometer measures direct beam

used with a solar tracking system to keep the instrument aimed at the sun. A pyrheliometer is used in the same setup with a pyranometer.

Pyrradiometer measures net radiation

For exact determination of net radiation in short- and longwave radiation range (0.3 to >30 um) with two separately working receivers.

The CN1-R Net Pyrradiometer measures the net total radiation flux (solar, terrestrial, and atmospheric) downward and upward through a horizontal surface.