Sources And Sinks

Usually the global emission rate from a source is estimated by using a measured emission factor (grams of CH4 emitted/day/unit source) and multiplying it by the number of such units in the world (units of source) and the time of year when emissions take place (days/year), which results in the grams of CH4/year emitted by the source. The complexity of the estimate varies depending on the information available. Once the budget is assembled, it must comply with the constraints discussed earlier.

Over the years many budgets have been proposed. Most of them were not entirely independent of previous estimates but tended to improve the estimates of emissions for one source or another. Two recent budgets are shown in Table 1. Both are "consensus"-type budgets in which several types of estimates by different researchers are put together. The first is from a NATO-sponsored Advanced Research Workshop (Khalil and Shearer, 1993). One of the goals of this project was to improve the budget based on direct measurements of emission factors and data on their global extrapolation. The second budget is from an assessment of the Intergovernmental Panel on Climate Change (IPCC) (Prather et al., 1995). The two budgets show one measure of the level of uncertainty that currently exists in the estimates of emissions from individual sources. Both these budgets are generally consistent with the known constraints, including the total emissions of around 500 Tg/yr discussed earlier. The budgets satisfy the constraints of the ratio of natural to anthropogenic emissions required by the ice core data. The budgets also agree on the major sources.

These budgets, like earlier ones, show that there are a few major sources. The major natural source is the wetlands, as has been known for a long time, since

TABLE 1 Comparison of Two Recent Budgets of Methane Sources

Source

NATO-ARW (1993)

IPCC (1994)

Natural sources (Tg)

Tg

Tg

Wetlands

110

115 (55-150)

Termites

20 (15-35)"

20 (10-50)

Open ocean

4

10 (5-50)

Marine sediments

(8-65)

Geological

10 (1-13)

Wild fire

2 (2-5)

Other

15 (10-40)

Natural total

150

160 (110-210)

Anthropogenic source (Tg)

Tg

Tg

Rice agriculture

65 (55-90)

60 (20-100)

Animals

79

85 (65-100)

Manure

15

25 (20-30)

Landfills

22 (11-33)

40 (20-70)

Wastewater treatment

25 (12-38)

25 (15-80)

Biomass burning

50

40 (20-80)

Coal mining

46

30 (15-45)

Natural gas

30 (25-50)

40 (25-50)

Other anthropogenic

13 (7-30)

15 (5-30)

Low-temperature fuels

17

? (1-30)

Anthropogenic total

360

375 (300—450)

Total

510

535 (410-660)

a Numbers in parentheses show estimated range of source values.

a Numbers in parentheses show estimated range of source values.

4 SOURCES AND SINKS 99

methane has been called "marsh gas." Other natural sources are generally small but not well constrained. These include termites, oceans, and lakes. Most of the current sources are "anthropogenic." While these emissions are not directly from stacks and other easily identifiable icons of man-made pollution, they are a result of human activities nonetheless. These sources may be classified mostly as agricultural and from use of energy. Of these, rice agriculture, cattle, waste management, biomass burning, coal mining, and use of natural gas are the largest contributors. There are some moderate sized sources of a few teragrams/year that include transportation and fossil fuel combustion. There are perhaps many small sources that together fall within the range of uncertainty of the global emission rate and are therefore not included.

Methane is removed from the atmosphere by a number of processes. The most effective is reaction with hydroxyl radicals, or OH. The main process by which hydroxyl radicals are formed in the atmosphere occurs when sunlight splits an ozone molecule into 02 and 0('/J), an excited state of the oxygen atom. A few of these 0(' D) atoms react with water vapor (H20) to form two OH radicals. OH has a lifetime of a few seconds and is removed mostly by its reaction with methane and CO. In addition to these major processes there are others that contribute to both the formation and destruction of OH radicals in the atmosphere (Thompson, 1992; DeMore et al., 1997). OH radicals are also responsible for removing many other gases from the atmosphere both man-made and natural. For example, many of the recently introduced chemicals (hydrofluorocarbons and hydrochlorofluorocarbons) that replace the chlorofluorocarbons are removed by OH radicals in the lower atmosphere. Methane is not only removed by OH, but there is enough methane in the atmosphere to control the abundance of OH and hence the oxidizing capacity of the atmosphere. Current estimates using photochemical models or proxy data suggest that the average concentration of OH is about 106 molecules/cm3. At any location, OH concentrations vary greatly depending on latitude, altitude, season, and time of day. The rate constant (K) for the reaction of methane with OH is about 2.4 x 10~15 cm3/molecule/sec 256 K (the average temperature of the atmosphere). Then, according to Eq. (1) the total loss of methane due to reactions with OH should be C/toh = AT|0H]C or 400Tg/yr after appropriate unit conversions. This corresponds to a lifetime of about 12 years due to reactions with OH alone.

Methane is also removed at Earth's surface by deposition and transport into the soils and then utilized by biological processes. This sink is estimated to be about 25 to 30 Tg/yr based on experimental field data. In the stratosphere methane is removed again by reacting with OH and also by other photolytic processes (DeMore et al., 1997). Recently, it has been suggested that there may be significant concentrations of CI atoms in the marine boundary layer produced by precursors from the oceans. The concentration of these radicals is not known at present, but various estimates put it between 103 to 106 molecules/cm3 (Singh and Kasting, 1988; Keene et al., 1990; Graedel and Keene, 1995, and references therein). At the upper limit this would constitute a significant sink for methane since it reacts 17 times faster with CI atoms than with OH at the temperature in the boundary layer. Depending on how much of the marine atmosphere contains CI radicals, this sink could be as large as 50 Tg/yr. Although we have not stated the sizable range of uncertainties in the estimates of these smaller sinks, it should be noted that these calculations provide a composite lifetime of about 50 years, which when combined with the lifetime due to OH reactions results in a total global lifetime of about 10 years. This was used to impose the first constraint discussed earlier based on Eq. (1) whereby we calculated the total emissions to be about 500 Tg/yr.

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