Curtain Raiser: Montreal Protocol
Trace Gases with a Major Potential Impact
MONTREAL PROTOCOL GASES
Most of the effects of global warming come from the major human-generated greenhouse gases-carbon dioxide, methane, and nitrous oxide. In addition, there are chlorinated hydrocarbons, ozone, and a wide range of trace molecules. Most of these absorb energy very strongly, but they are present in such small concentrations that they currently have very little influence on climate.
In 1989, a conference was held to address the problem of stratospheric ozone depletion. A total of 191 countries approved the treaty, called the Montreal Protocol that resulted from this gathering. The concern of the participants was not directly global warming at the time. Participants addressed the materials used in refrigeration and aerosol can propellants that were found to deplete ozone in the stratosphere.
These gases are relevant to climate change in two ways:
1. These gases absorb infrared energy very strongly. Despite their low concentrations in the troposphere, these gases contribute to global warming.
2. In the stratosphere, ozone strongly absorbs incoming ultraviolet light. To the extent that this occurs, a small amount of solar energy never makes it to the earth’s surface to contribute to global warming. Stratospheric ozone actually contributes a small but measurable cooling effect that may begin to be more significant as the stratospheric ozone layer recovers.
As a result of the participating countries’ actions to reduce halocarbon gases, their emission levels decreased by a factor of 3 in the years since 1980. Since, the Montreal Protocol gases persist for a very long time in the environment, only 1–2 percent is removed each year. The levels of these gases in the atmosphere appear to be stabilizing and in some cases are declining.
OZONE
Ozone can be good or bad depending on where it is in the atmosphere. No matter where it is found in the atmosphere, ozone is made of the same oxygen atoms that make up oxygen gas. Instead of the two oxygen atoms that bond together to form oxygen gas (O2), however, three oxygen atoms bond together to form ozone (O3).
There is a naturally occurring layer of ozone in the stratosphere that absorbs harmful ultraviolet light before it can enter the atmosphere. Chemicals used for spray cans and refrigerants, which contain chlorine and fluorine, tend to decompose ozone in the stratosphere. This effect was most notable over Antarctica, where a hole appeared in the ozone layer. When scientists recognized that this was happening, a ban was placed on the use of these chemicals. Since then, the ozone layer has been recovering.
This resulted in a slight atmospheric cooling as additional ultraviolet light again began to be absorbed in the upper atmosphere, leaving less to be absorbed closer to the earth’s surface. This is the “good” ozone.
The “bad” ozone is the product of combustion and a complex chemical reaction that occurs in the lower layer of the atmosphere, or the troposphere. Ozone in the troposphere is a greenhouse gas that absorbs a small amount of infrared radiation. Compared with the other heavy hitters in the atmosphere, ozone, despite its notoriety of the stratospheric ozone hole, plays only a small role in changing the global temperature.
Creating Ozone
Ozone (O3) is just three oxygen atoms bonded together instead of two, as is found in oxygen (O2) gas. Natural processes such as lightning convert oxygen gas to ozone. Ozone is also found in tailpipe and smokestack emissions resulting from a more complex photochemical reaction.
3O2 + heat → 2O3
Three molecules of oxygen gas (O2) produce two molecules of ozone (O3).
Destroying Ozone
In the reverse of the preceding chemical reaction, the three atoms of the ozone molecule are rearranged to form oxygen molecules. This reaction is catalyzed (or chemically facilitated) by a class of chemicals that contain carbon and chlorine, fluorine, or bromine and a few others..
There are several more complicated steps in this process, but the basic idea is 2O3 → 3O2
Carbon Sinks
DISSOLVING CARBON DIOXIDE
About half the carbon dioxide that humans add to the atmosphere is removed by nature in what is called a sink. Had this not occurred, the carbon dioxide levels and the enhanced greenhouse effect would be even greater than it is currently. Nature removes carbon dioxide from the atmosphere by dissolving it in seawater. Dissolved carbon dioxide forms a mild acid called carbonic acid that slightly decreases the pH level of the water in which it is dissolved. The amount of carbon dioxide that can be absorbed by the oceans reaches a saturation level depending on water temperature. Warm water can hold less carbon dioxide than cold water.
There is a limit as to how much carbon dioxide can dissolve, and since there is not very much vertical mixing in the oceans, it can take centuries for the dissolved carbon dioxide to penetrate below the surface layers. Although the oceans have an enormous capacity to absorb much of the carbon dioxide that is being added to them, only a small part of the oceans near the surface is available to accomplish this. Consequently, injection of carbon dioxide deep into the oceans has been proposed as a possible way to remove carbon dioxide from the atmosphere.
SEQUESTRATION
One idea for getting rid of some of the excess carbon dioxide is to capture and store it somewhere, such as in the oceans. This is called sequestration as an option for reducing emissions from power plants. If carbon dioxide from a power plant is pumped into the ocean and released near the sea floor, the carbon dioxide could react with the solid calcium carbonate (CaCO3) found in sea shells to form soluble calcium bicarbonate [Ca(HCO3)2]. This would provide long-term storage for carbon dioxide that otherwise would have been released to the atmosphere to contribute to global warming.
REMOVING CARBON DIOXIDE FROM THE ATMOSPHERE
Dissolving Carbon Dioxide in the Oceans
Carbon dioxide dissolves in the oceans and forms carbonic acid. This is a natural part of the carbon cycle:
CO2 + H2O → H2CO3
Long-Term Storage of Carbon Dioxide Near the Ocean Floor Carbon dioxide reacts with calcium carbonate (found in sea shells) to form soluble calcium bicarbonate:
CO2 + CaCO3 + H2O → Ca(HCO3)2
Integrated Gasification Combined Cycle (IGCC)
IGCC makes it easier to capture carbon dioxide from the smoke stacks of electrical generating plants. First, the coal (C) is reacted with steam (H2O) to form hydrogen gas (H2) and carbon monoxide (CO).
C + H2O → H2 + CO
Next, the carbon monoxide is converted to carbon dioxide, and the hydrogen is burned to generate additional heat. The carbon dioxide does not get diluted with nitrogen as it is in conventional coal plants and can be removed more easily.
CO + 2H2O → CO2 + 2H2
