We have already seen how the concept of mass or energy budgets can be used to explain changes in the reservoirs (mass or energy) in a given system. For example, we have examined the Earth's energy budget and have seen how an equilibrium energy level is established and its role in determining the average temperature of the surface. On a homework problem you were asked to consider the water budget for the Great Salt Lake and consider how the lake level, that is the water reservoir, is influenced by seasonal variations in inputs and outputs.

When considering how the atmosphere has changed since the formation of Earth, these same concepts can be used to develop a conceptual understanding of these changes took place. They are also useful when attempting to ascertain what impact anthropogenic emissions will have on the overall composition of the atmosphere.

A dramatic example of how the atmosphere's composition has changed over time can be found when considering the relative concentrations of molecular oxygen, O₂, and hydrogen, H₂. The major components of the atmosphere about 4 billion years ago were:

hydrogen (H₂)
nitrogen (N₂)
carbon dioxide (CO₂)
water (H₂O)

In primeval oceans, early life forms drew their energy by chemosynthetic processes, that is, they broke down existing organic molecules and released chemical potential energy to support cellular processes. No oxygen was needed to support such organisms.

Around 3.5 billion years ago, the first photosynthetic organisms appeared. These organisms converted low energy atmospheric compounds were converted to higher energy products which supported life. The chemical reaction can be written as follows:

In this case, the products are formaldehyde, the high energy compound that supported life, and oxygen, a waste-product. Thus, oxygen began to be emitted into the atmosphere in significant quantities. Oxygen was also generated by the photodecomposition of water:

Despite the fact that there were now sources of oxygen, there was not a large increase in oxygen levels. This is because there were several reactions that consumed the generated O₂ molecules. For example, in the presence of hydrogen, oxygen is readily converted to water:

Oxygen was also rapidly consumed by reaction with dissolved iron in the oceans, as described by:

This product is known as limonite and is essentially just rust.

Hydrogen was lost by a number of processes in addition to the conversion to water listed above. First, having such a low density, it tends to rise to the upper layers of the atmosphere and is gradually lost to space. (The same thing happens to helium). In addition, it readily reacts with an oxide of iron, FeO, to form metallic iron and water as shown below:

Thus, there were many reactions, many of which were interdependent, that had the effect of decreasing the hydrogen and increasing the oxygen levels in the atmosphere.

Around 2 billion years ago atmospheric oxygen reached toxic levels for much of the anaerobic life forms which had evolved in the hydrogen-rich environment. However, new life forms that utilized oxygen in respiration processes evolved in response to the new atmosphere. Oxygen levels continued to increase until reaching current levels about 350 million years ago, as mentioned above.

The increase in molecular oxygen levels resulted in the formation of the ozone (O₃) layer by the following photochemical processes:

Ozone serves to filter out harmful ultraviolet radiation from the sun that is not blocked by either O2 or N2. The relevant process is:

The above processes, upon reaching equilibrium, result is a relatively constant level of ozone in the stratosphere. The establishment of the ozone layer was critical to the development of life on land about 500 million years ago because it served as protection from the damaging physiological effects of ultraviolet radiation to which terrestrial organisms would otherwise be exposed.

Current oxygen levels are relatively stable because of the balance between production by photosynthesis and consumption by respiratory processes; note that there is a slight imbalance in the oxygen budget due to the large extent to which it is consumed in combustion processes (see p. 111). (average lifetime of O₂ molecule is ~2,000 years before conversion back to water; yearly exchange = 6 ´ 10¹¹ tons/yr)
 

Catalytic Destruction of Ozone by chlorofluorocarbons (CFCs).

catalyst: a material that increases the rate of a chemical reaction without undergo a net change.

CFCs were widely used as refrigerants, solvents, and other applications because of a variety of advantageous properties. Unfortunately they are susceptible to degradation from ultra-violet light, the result of which are chlorine atoms, as shown below:

The wavelengths required for this process are found in the stratosphere. The resulting chlorine atoms can react with ozone as shown below:

The resulting ClO molecule can then combine with a free oxygen atom, thereby reforming the chlorine atom and a moleculue of O₂. Thus, the chlorine can go on to react again with another ozone molecule.

Exercise #1:

The carbon cycle refers to the continual exchange of carbon between the ocean, atmosphere, organic material and minerals. In the atmosphere most of the carbon is in the form of carbon dioxide so, for our purposes we can look view the carbon cycle as a system of inputs and outputs of carbon dioxide into and out of the atmosphere. Use the following information to construct the carbon dioxide budget of the atmosphere. Be clear as to what the inputs and the outputs are. Determine if the carbon dioxide "budget" is in balance and what this means for future CO₂ concentrations. (1 Gt = 10⁹ ton)

carbon used in photosynthesis: 100.2 Gt

carbon released by plant respiration: 50 Gt

carbon dissolved by ocean waters: 100.0 Gt

carbon released by ocean waters: 98 Gt

carbon released by fossil fuel combustion: 5.5 Gt

carbon released by animal respiration: 17 Gt

carbon released by rotting plant material: 37 Gt
 
 
 
 

Exercise 2 (due Friday, 2/8)

The graph on page 98 in Graedel & Crutzen shows the change in methane (CH₄) concentration in the atmosphere over the past 1,000 years.
 

Using the information in your text (from chapters 1 - 5) construct a methane budget for a pre-1800 atmosphere - what are the sources of methane and what are the means by which methane is removed from the atmosphere? What happened to the inputs and outputs in recent years to cause the obvious change in methane concentration? Be specific as to what any new inputs or lost outputs are responsible for the change.