Absorption of Light by Gases in the Atmosphere

Fossil fuels are a concern because they increase the absorption of incoming solar radiation above the amount absorbed naturally. This is referred to as the enhanced greenhouse effect.

When a gas molecule absorbs energy, it has no place to store that energy other than in some form of movement of the molecule itself. According to the kinetic theory of matter, molecules are constantly in motion. The molecules move faster when their temperature goes up. Gas molecules in the atmosphere can absorb energy passing through the atmosphere in the form of visible light (from the sun) or invisible electromagnetic waves (from the earth). Some molecules are able to move in more ways than others, enabling them to be better absorbers of certain kinds of light (or electromagnetic) energy.

If we look at the chemical structure of some of the gases in the atmosphere, we can understand better how they absorb light energy passing through the atmosphere. Let's start with nitrogen (Figure 5-1). Nitrogen gas molecules each have two atoms of the element nitrogen bonded together to form a molecule of nitrogen gas that has the formula N2 (which means two nitrogen atoms).

Because the nitrogen gas molecule is so simple, it cannot do very much with the light energy that it absorbs. It can spin or vibrate only a little bit by stretching and pulling. Oxygen acts pretty much the same way (Figure 5-2).

Figure 5-1 Nitrogen molecule.

Figure 5-1 Nitrogen molecule.

Because of their structural simplicity, nitrogen and oxygen, which together make up 99 percent of the earth's atmosphere, absorb relatively small amounts of the visible light energy coming from the sun that passes through the air. This leaves the "glass" of the earth's greenhouse pretty clear.

Water vapor, however, is a different story. Water (H2O) has two hydrogen atoms bonded to a single oxygen atom. The oxygen atoms are bent to form a 105-degree angle (Figure 5-3).

The water molecule can twist, turn, gyrate, bend, flex, and do its own little chemical dance. Because water has more ways to move, it is better able to absorb light energy. Water vapor in the atmosphere absorbs certain colors of light (visible wavelengths) on the way down to the earth's surface as if there were a film on the glass of earth's greenhouse causing it to be less than perfectly transparent. Water also absorbs some of the heat energy in the form of infrared light (long invisible wavelengths) emitted by the earth. For this reason, water is a natural greenhouse gas and contributes to the absorption of heat in the atmosphere. However, as a consequence of global warming, there is more water vapor in the atmosphere, and this, in turn, absorbs even more heat.

The carbon dioxide molecule, like the water molecule, has three atoms. These atoms, rather than being bent, are arranged in a straight line (Figure 5-4). Because of its molecular structure, carbon dioxide happens to be especially well suited to absorb certain wavelengths (mostly the invisible infrared radiation emitted by the earth). It does not strongly absorb the light energy coming from the sun.

Carbon dioxide lets sunlight through the atmosphere but does not let the energy emitted by the earth pass back into space. In this way, carbon dioxide acts a greenhouse gas.

Methane has four hydrogen atoms bonded to a central carbon atom. Looking at the structure of the methane molecule (Figure 5-5), one might (correctly) guess that it has a number of ways to vibrate as it absorbs light (and other invisible electromagnetic)

Figure 5-2 Oxygen molecule.

Figure 5-2 Oxygen molecule.

Figure 5-3 Water molecule.

Figure 5-4 Carbon dioxide molecule with the two main vibrational modes that absorb infrared energy.

Figure 5-4 Carbon dioxide molecule with the two main vibrational modes that absorb infrared energy.

Figure 5-5 Methane molecule.

waves. Like a good dancer, methane has "lots of moves." For this reason, methane is an "energy sponge" that absorbs sunlight 20 times more strongly than carbon dioxide.

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