How a greenhouse gas interacts with earthlight

We have seen that gases are terrible blackbodies because they are very choosy about which frequencies they absorb and emit. What we will now see is that some frequency bands are more important to the climate of the Earth than others. There are two factors to consider. One is the concentration of the gas, which we will discuss below. The other is the frequency of the absorption band relative to the blackbody spectrum for the Earth.

Figure 4.3 shows blackbody spectra again for temperatures ranging from 300 K, a hot summer day, down to 220 K, which is about the coldest it gets in the atmosphere, up near the troposphere at about 10 km altitude. There is also a jagged-looking curve. This is the intensity of light that an IR spectrometer would see if it were in orbit over the Earth, looking down. Figure 4.3 is not data, but rather a model simulation from one of our online models. You can point a web browser at http://understandingtheforecast.org/models/infrared_spectrum.html to run this model yourself. We will do so in the projects at the end of this chapter.

The spectrum of light leaving the Earth going into space ranges between two different blackbody spectra, a warmer one of about 270 K and a colder one from about 220 K.

Wave number (cycles/cm)

Fig. 4.3 The solid line is a model-generated spectrum of the IR light escaping to space at the top of the atmosphere. For comparison, the broken lines are blackbody spectra at different temperatures. If the Earth had no atmosphere, the outgoing spectrum would look like a blackbody spectrum for 270 K, between the 260 and 280 K spectra shown. The atmospheric window is between about 900 and 1000 cm-1, where no gases absorb or emit IR light. CO2, water vapor, ozone, and methane absorb IR light emitted from the ground and emit lower-intensity IR from high altitudes where the air is colder than at the surface.

Wave number (cycles/cm)

Fig. 4.3 The solid line is a model-generated spectrum of the IR light escaping to space at the top of the atmosphere. For comparison, the broken lines are blackbody spectra at different temperatures. If the Earth had no atmosphere, the outgoing spectrum would look like a blackbody spectrum for 270 K, between the 260 and 280 K spectra shown. The atmospheric window is between about 900 and 1000 cm-1, where no gases absorb or emit IR light. CO2, water vapor, ozone, and methane absorb IR light emitted from the ground and emit lower-intensity IR from high altitudes where the air is colder than at the surface.

700 cycles/cm

^ Temperature 220 K

Atmospheric window

900 cycles/cm *

CO2 I

Fig. 4.4 A comparison of the fate of IR light in the optically thick CO2 bend frequency (left) versus the optically thin atmospheric window (right).

CO2 bend

Fig. 4.4 A comparison of the fate of IR light in the optically thick CO2 bend frequency (left) versus the optically thin atmospheric window (right).

The parts of the spectra that seem to follow the colder blackbody curve come from greenhouse gases in the upper atmosphere. They follow the colder blackbody curve because it is cold in the upper atmosphere. The most pronounced of these absorption bands, centered on a wave number of about 700 cycles/cm, comes from the bending vibration of CO2. Light of this intensity that shines from the surface of the Earth is absorbed by the CO2 in the atmosphere (Fig. 4.4). The CO2 in the atmosphere then radiates its own light at this frequency. Remember from Chapter 1 that light emission and absorption is a two-way street.

Other parts of the spectrum, most notably the broad smooth part around 1000 cycles/cm, follow a warmer blackbody spectrum. These come directly from the ground. The atmosphere is transparent to IR light in these frequencies. This band is called the atmospheric window.

The situation is analogous to standing on a pier and looking down into a pond of water. If the water were very clear, you could see light coming from the bottom; you would see rocks or old tires or whatever in the reflected light. If the water were murky, the light you would see would be scattered light coming from perhaps just a few inches down into the water. The old tires would be invisible, alas.

Remember we said that the total energy flux from one of these spectra can be "eye-balled" as the total area under the curve. The areas of the pure blackbody curves are going up proportionally to the temperature raised to the fourth power because of the Stefan-Boltzmann equation (Eqn. 2.1 in Chapter 2). The area trick works with our new jagged spectrum as well. The effect of an atmospheric absorption band is to take a bite out of the blackbody spectrum from the Earth's surface, decreasing the area and therefore decreasing the outgoing energy flux.

Compare the CO2 absorption band at 700 cycles/cm with the absorption band of methane at around 1300 cycles/cm. The CO2 band has a lot more room to change the outgoing IR energy flux than does the methane band, simply because the Earth and the atmosphere radiate a lot more energy near 700 cycles/cm than near 1300 cycles/cm. Both blackbody spectra are of pretty low intensity in the methane band.

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Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

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  • steffen durr
    How a greenhouse gas interacts with earthlight?
    11 months ago

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