Methane concentration atmosphere

The large number of individual hydrocarbons in the atmosphere and the many different hydrocarbon classes make ambient air monitoring a very difficult task. The ambient atmosphere contains an ubiquitous concentration of methane (CH4) at approximately 1.6 ppm worldwide (9). The concentration of all other hydrocarbons in ambient air can range from 100 times less to 10 times greater than the methane concentration for a rural versus an urban location. The terminology of the concentration of hydrocarbon compounds is potentially confusing. Hydrocarbon concentrations are referred to by two units—parts per million by volume (ppmV) and parts per million by carbon (ppmC). Thus, 1 fx of gas in 1 liter of air is 1 ppmV, so the following is true ... 

This means that the observed change in M mainly reflects a change in the source flux Q or the sink function. As an example we may take the methane concentration in the atmosphere, which in recent years has been increasing by about 0.5% per year. The turnover time is estimated to be about 10 years, i.e., much less than Tobs (200 years). Consequently, the observed rate of increase in atmospheric methane is a direct consequence of a similar rate of increase of emissions into the atmosphere. (In fact, this is not quite true. A fraction of the observed increase is probably due to a decrease in sink strength caused by a decrease in the concentration of hydroxyl radicals responsible for the decomposition of methane in the atmosphere.)... 

The role of carbon dioxide in the Earth s radiation budget merits this interest in atmospheric CO2. There are, however, other changes of importance. The atmospheric methane concentration is increasing, probably as a result of increasing cattle populations, rice production, and biomass burning (Crutzen, 1983). Increasing methane concentrations are important because of the role it plays in stratospheric and... 

Brook, E. J., Sowers. T., and Orchardo, J. (1996). Rapid variations in atmospheric methane concentration during the past 110 000 years. Science 273, 1087-1091. 

Problems may arise when the atmospheric concentration of greenhouse gases increases. Since the beginning of the industrial revolution, atmospheric concentrations of carbon dioxide have increased nearly 30%, methane concentrations have more than doubled, and nitrous oxide concentrations have risen by about 15%. These increases have enhanced the heat-trapping capability of the earth s atmosphere. 

Although atmospheric methane concentrations appear to have stabilized over the past few decades, melting of gas hydrates in permafrost and shallow marine sediments have the potential to rapidly release large quantities of this potent greenhouse gas. As noted in... 

There were relatively few measurements of atmospheric methane concentrations prior to about 1980, except for a set from 1963 to 1970 by Stephens and co-workers (Stephens and Burleson, 1969 Stephens, 1985), which were in the 1.37-1.57 ppm range. The current global mean concentration of methane is 1.72 ppm, with higher concentrations in the Northern than... 

FIGURE 14.17 Atmospheric methane concentrations over the past 1000 years. Different symbols represent data from ice cores in Antarctica and Greenland and the Antarctic firm layer. Line from 1978 includes air measurements at Cape Grim, Tasmania (adapted from Etheridge et al., 1998). 

Figure 4.15. Block diagram for formation and transport of methane in waterlogged country. Notation FlCHi is the methane flux across the atmosphere/water body interface F2CHi is the oxidation of methane in aerobic zones FCH is the intensity of the methane source M is methane concentration.

Such a complicated interactivity of processes can both directly and indirectly affect formation of the atmospheric greenhouse effect. Derwent et al. (2001) described a global 3-D Lagrangian chemistry transport model (STOCHEM) which reproduces chemical processes including MGC transport and can be used to reproduce interrelated fields of TO and methane concentration (Johnson et al., 2002) under conditions of emission to the atmosphere of short-lived TO precursors such as CH4, CO, NOx, and hydrogen. At the same time, the radiative forcing (RF) of NOx emissions depends on the location of emissions near the surface or in the upper troposphere, in the Northern or Southern Hemisphere. For each short-lived MGC/... 

The Oxford results have recently been used by Solomon and Garcia to examine the distribution of long-lived tracers and chlorine species in the middle atmosphere. This important paper has crystallized many of the issues relating to the hydroxyl and chlorine species, particularly the relationship between the variability in methane concentration and the variability in CIO. Figure 10 summarizes the correlation between the two-dimensional model of Solomon and Garcia [18] and the Oxford CH4 maps. These results are then used to define the expected variability in local CIO concentrations reported by in situ observations. 

It is estimated that about 500 million tons of methane are being added to the air each year (Craig and Chou, 1982), largely by anaerobic production in rice paddies and wetlands as well as from the metabolism of ruminant domestic animals and, possibly, African termites (Rasmussen and Khalil, 1981 Zimmerman et d., 1982). This gas is slowly oxidized by reactions with Hydroxyl free radical. Its atmospheric content is around 5 gigatons, indicating that the residence time in the atmosphere is about 10 years. As Figure 12 shows, since 1965 the atmospheric concentration of methane has increased by about 3096. If this rate continues, the methane concentration will have doubled early in the 21st century. 

Kasting J. F., Pavlov A. A., and Siefert J. L. (2001) A coupled ecosystem-climate model for predicting the methane concentration in the Archean atmosphere. Origin Life Evol. Biosphere 31, 271-285. 

Blunier T., Chappellaz J., Schwander J., Stauffer B., and Raynaud D. (1995) Variations in atmospheric methane concentration during the Holocene epoch. Nature 374, 46-49. 

King G. M. and Schnell S. (1994b) Effect of increasing atmospheric methane concentration on ammonium inhibition of soil methane consumption. Nature 370, 282-284. 

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