Methane (CH4) is a colorless, odorless gas that occurs in large volumes in sediments. We tap those sediments and use CH4 (natural gas) for heating and cooking. The well-known odor of natural gas is not the methane itself, but an additive that alerts us to the presence of unburned natural gas. As part of the carbon cycle, methane and other small hydrocarbons are converted to water and HOx (OH and HO2), species that affect stratospheric ozone concentrations. While a full discussion of the carbon cycle is well beyond the scope of this chapter, we will focus here on just one part of that cycle, the production of methane (CH4) and its oxidation.
The complete oxidation of methane leads to one CO2 and two H2O molecules. The oxidation process consists of many reactions, some of which produce hydrogen oxides such as OH and HO2 (HOx) as intermediates. Thus, methane oxidation consumes an OH molecule in its initial step but can produce additional HOx during subsequent oxidation steps. Another byproduct of methane (and other hydrocarbons) oxidations is ozone. This occurs in the presence of nitrogen oxides (NOx). In cities it is the oxidation of hydrocarbons in the presence of high levels of nitrogen oxides that leads to photochemical smog production.
Atmospheric carbon dioxide is "fixed" by the process of photosynthesis into compounds (carbohydrates) useful for plants. This fixed carbon is eventually returned to the atmosphere as CO2 during respiration. A fraction of this carbon may be converted to methane in oxygen-free (anaerobic) conditions. These conditions occur in swamps, rice paddies, and in the stomachs of ruminant animals such as cows and sheep. Methane forms in deposits from decay of very old plant and animal material. Some of the methane thus produced will escape to the atmosphere.
In very cold temperatures, mixtures of methane and water can form an ice-like frozen structure called a clathrate. It has been proposed that such deposits exist in the Arctic and may be a significant source of methane. Their quantitative importance is not known at this time.
Atmospheric methane reacts with OH to begin an oxidation chain which eventually leads to the formation of water and carbon dioxide. This is the equivalent of burning the methane very slowly in the atmosphere. The atmospheric lifetime of methane is about 10 years. Thus some of the methane escapes the troposphere and makes it to the stratosphere where its oxidation provides a source for atmospheric hydrogen oxides and eventually water.
CH4 concentrations are increasing in the atmosphere. The current concentration is about 1.8 ppmv which is more than twice that deduced to have existed thousands of years ago from analyses of air trapped in Arctic and Antarctic ice. The rate of increase over the last couple of decades has been in the range of 0.5 to 1 %/year. This rate of increase has slowed in the last few years. The reasons for this slowing are not entirely understood at this time.
Nonmethane hydrocarbons (ethane, C2H6, propane, C3H8, propene, C3H6, etc.) are generally of importance only to the troposphere. They are generally more reactive than methane, and thus are oxidized before they can reach the stratosphere. They participate in smog-type reactions that lead to the formation of tropospheric ozone. They have a similar but small impact on the lower stratosphere.
The amount of CO2 is observed to be increasing due to the burning of fossil fuels. The increasing amount of CO2 is most important in terms of its contribution to global warming. However, CO2 in the stratosphere leads to cooling by absorbing infrared radiation coming up from the ground or troposphere and reradiating it. The reradiation in the upward direction can escape to space, resulting in a net loss of energy. The resultant cooling of the stratosphere has several potential impacts. One is an increase in ozone since temperature-dependent ozone loss rates are slower at colder temperatures. An exception is at the poles where polar stratospheric clouds completely change the chemical balance (see Chapter 11). Another effect of increased CO2 may be the formation of more polar stratospheric clouds (PSCs) through increased water vapor concentrations.