Wednesday, September 8, 2010

Infrared Trapping – the “Greenhouse Effect”

Goals – to look at the properties of materials that make them interact

with thermal (i.e., infrared, or IR) radiation (absorbing and reemitting

that radiation). Gases like H2O, CO2, and CH4 trap some of the thermal

radiation from the surface of the earth before it can escape to space,

thereby warming the surface above the effective radiating temperature.

Some frequencies trapped by

these greenhouse gases and

why do other frequencies

pass through the atmosphere

and escape to space in

spectral regions we call

“windows”

Material covered in this lecture

Atmospheric composition (p 44-46)

Atmospheric Structure (p 46-48)

Heat transport in fluids (p 47, Figure 3-10)

Molecular motions and greenhouse gases (p 48-49)

Selective absorption of gases (p 49, Figure 3-13)

Atmospheric „layers‟ and the greenhouse effect (p 45)

Last Friday - Greenhouse effect demo

Selective absorption.

Greenhouse gases transmit visible light, but absorb infrared light. This increases

the flux of light (or energy) that hits the surface, because the atmosphere will

reradiate some of this energy back to Earth‟s surface.

Let‟s look at atmospheric composition

What we know:

Materials aren‟t all „blackbodies‟ – some will selectively absorb light at different

frequencies. Some gases in Earth‟s atmosphere will allow solar (visible) radiation

to pass through, thereby warming the surface, but they will trap heat (infrared)

radiation from the surface.

Let‟s see how this impacts our calculation of earth‟s temperature.

Atmospheric Composition

The Main Constituents in Earth‟s atmosphere

Atmospheric Composition

The Greenhouse Gases

This is “parts of chemical X per 1 million parts of total air” – that

is, if something represented 1% of air, it would be 10,000 parts

per million (or “ppm”). Note: 10,000/1,000,000 = 0.01, or 1%

How molecules interact with infrared light

Fig. 3.12

By absorbing infrared light, molecules can change

their states – meaning that they can shake, rattle and

roll (vibrate, rotate, bend) faster. By emitting IR

light, they slow down. Different molecules absorb and

emit IR radiation at different frequencies depending

on their atoms and bonds.

Fig 3-12

Fig 3-14

Fig. 3.13

Looking in detail at how infrared light is affected by atmospheric molecules.

Note that the most important „absorbers‟ (i.e., greenhouse gases) in the

atmosphere are H2O (naturally occuring gas) and CO2 (natural and man-made

sources). Detailed studies show that H2O provides most of the thermal warming

of the planet, followed by CO2. Note that ozone (O3) is also a greenhouse gas,

but not a major one.

Box Figure 3-2

Why does a „layer‟ of gas above the surface make the surface

warmer? Treat the layer of infrared-active gas as a „blackbody‟, so

that we can use the simple equation that relates the emission (or

„flux‟) to the temperature of the layer

First, note surface balance with no atmosphere:

sTs4 = S/4 (1 – A)

Incoming: S/4(1-A) + sTe4 now, two terms!

Outgoing: sTs4

Box Fig. 3.2

Add an atmosphere

Box Figure 3-2

New surface balance with atmosphere:

sTs4 = S/4 (1 – A) + sTe

4

The extra term comes from insulating properties of the atmosphere -

the “greenhouse effect”

Question

For an atmosphere that is in radiative balance (i.e., incoming

radiation equals outgoing radiation for all layers), how does

temperature change with height?

Hint – assume that the atmosphere is made up of a large

number of layers, stacked like pancakes.

Add more layers (i.e., thicken the atmosphere with more GH

gases), and the surface gets even warmer because the layer above

it is warmed by the layer above it, and so on….!

sT4

sT4

Surface

Thus, the surface is warmest, and temperature decreases with height up to an altitude of about 10-15 km. The decrease in temperature with height is called the “temperature lapse rate” or, often, just the “lapse rate.” Note that temperature increases again in the stratosphere. This is due to absorption of ultraviolet light by ozone

Fig. 3.9

So we now have a fairly detailed picture of how heat is distributed vertically in the atmosphere. Sunlight that hits the surface heats the surface, which then radiates some of that heat back up in the infrared. Greenhouse gases like H2O, CO2, and a few others absorb some of that radiation, heating the atmosphere. There are also vertical motions (e.g., convection) and evaporation and condensation of water that redistribute heat upward. Air cools when it rises (we’ll talk about this Friday), so this leads to a decrease in temperature with altitude in the lower atmosphere.

Fig. 3.9

sun

Note that pressure always decreases in the atmosphere – with every 16 km of altitude, pressure drops by a factor of 10 (that is, pressure is 1000 millibars at the surface, it is 100 millibars at 16 km, it is 10 mbar at 32 km, etc.).

Fig. 3.9

The sun emits a little bit of light in the ultraviolet (only a few percent of its total output of energy). This light is absorbed by ozone in the upper atmosphere, which creates a warm layer that we call the ‘stratosphere’ (see Page

Fig. 3.9

Other than radiation, what forms of heat transfer are important in the atmosphere

Fig. 3.10

A schematic of Earth’s energy budget a bit more complex model!

Fig. 3.19

Friday – We‟ll take a closer look at the details of energy

balance and water vapor and clouds