Methane also plays a major role in the background chemistry of the troposphere. It is important as a source or sink of O3 depending on the ambient levels of NOx in the atmosphere. It is oxidised to CO and the oxidation source represents a large fraction of the CO budget (see Table 2.1).
2.2.12 Methane Sources
The sources of CH4 are many and varied, and although the total emission
budget seems to be known to within about 40% (e.g. Warneck, 1988; Cicerone and
Oremland, 1988) (based on the loss budget), many of the individual components of the
budget are not known to better than a factor of two. An estimate of the sources of CH4
taken from Cicerone and Oremland (1988) is shown in Table 2.2. It can be seen that biological sources are dominant, with emissions from ruminants, natural wetlands and rice
paddies contributing more than 60%.
The magnitudes of the unknown sources are estimated by performing a complex budget analysis of methane in the atmosphere, using the total burden and its gradient and the known sources. The unknown sources can be characterised by the amounts required to balance the CH4 budget. A recent study of this nature has been supplied by Inez Fung (Fung et al, 1991)
As noted above the individual components in the budget are much more uncertain than the global figure. For example, recent studies of the northern wetlands (N. Roulet, private communication, 1990; Aselmann and Crutzen, 1989), suggest that previous estimates of emissions may have been overestimated. The emission process is complex depending on water table level, temperature etc., and a recent study by King (1990) points out that CH4 emission is also light sensitive, being roughly inversely proportional to the intensity. This is possibly due to the increased oxidation by increased dissolved O2 which in turn is due to enhanced photosynthetic activity. Similarly, the emission due to termites is quite uncertain and has been the subject of extensive debate (e.g. Cicerone and Oremland, 1988). A recent paper in Nature (Vaghjiani and Ravishankara, 1991) suggests that the methane sink by hydroxyl (OH) radicals may be over-estimated due to the use of an optimistic rate constant for that reaction by the community.
Likewise, the contribution to the CH4 budget from anthropogenic non- biologically related sources such as coal mining and gas release is uncertain, with estimates ranging from less than 13% (Quay et al., 1988) to as much as 21% (Whalen et al., 1989). Some serious questions regarding the magnitude of the methane source from natural gas operations, particularly in Eastern Europe and the USSR has been put forward in a recent paper (Crutzen, 1991), suggesting that the source distribution may be different from the traditional view.
The role played by emission of CH4 stored in the permafrost in the form of hydrates is quite uncertain but, like CH4 emissions from wetlands, they are potentially very important from the point of view of climate change. This results from a positive feedback cycle which may be set up with CH4 emissions and increasing temperatures: if global temperature continues to rise this could result in a release of CH4 from the permafrost and also possibly from the wetlands (e.g. Kvenvolden, 1988).
2.2.13 Methane loss and mixing ratios
The main loss of CH4 in the atmosphere is reaction with the OH radical
with loss to the stratosphere contributing about 12%. There is also a possible loss of order
10% due to uptake by dry soils. With current estimates of OH densities and the measured
abundance this leads to CH4 lifetime of about 7 years and means than CH4 is almost mixed
in the troposphere since the time constant for mixing is much shorter than the chemical
lifetime. The diffuse nature of most known sources, combined with the long lifetime of
methane mean that deviations of CH4 from its mean atmospheric abundance are quite small.
There is, however, an interhemispheric difference of about 8% as is shown in Figure 2.9. In addition, measurements also indicate a
seasonal station variation that may be as high as 5% (cf. Figure 2.10).
There also may be a build up of CH4 in the boundary layer in regions of active and strong sources which lead to temporary increases in mixing ratio of order 20-40% (Ehhalt, 1974). The same measurements also suggest that rapid convective transport from the boundary layer with high CH4 abundances could also be exhibited in the CH4 profile (Figure 2.11). Model calculations (Taylor et al., 1990) suggest a 4-5% variation with height reflecting intense local sources in the northern hemisphere. Their vertical variation appears to be much less than the numbers quoted above. However, they did not include a boundary layer in their model, and likewise did not allow for convective transport by clouds. On an average basis these are probably not important, but they will likely be important for a more detailed comparison with measurements.
2.2.14 Satellite Observations of Methane
The fact that such a large fraction of the global emission budget of CH4
is related to the biosphere and that it is not well defined makes it challenging to try and
refine our knowledge of the budget. The fact that CH4 is increasing and is also an important
IR active gas makes it imperative to understand the details of its budget better than we do
at present.
The interest in the potential for methane measurements from space centres on three main issues:
Evidence of trends can be cheaply and well monitored by ground stations. However, this will not assist in the determination of the details of the budget. Thus it is relevant to consider whether details of the CH4 distribution can be monitored sufficiently accurately from space in order to derive some information concerning the strength of emission sources. The identification and quantification of the sources will also need dynamical input in order to assess the effects of boundary layer, regional and global transport since the chemical time constant is so long.
From the figures that we have discussed in the previous section it is apparent that it will be necessary to have an instrument measuring CH4 with a precision of better than 1% in order to obtain useful synoptic data. If this were available on MOPITT, then together with the spatial and temporal resolution of MOPITT (612 km swath and global coverage every 5 days) suggests that it may be possible to track large continuous or pulsed sources of methane dispersed by meteorology and as a result obtain quantitative data on CH4 sources.
2.2.15 Summary of CH4 Measurement Potential
The first and most obvious implication from the science data is that the variations in
CH4 concentration are small and therefore a high precision is required from the
MOPITT measurements in order to make a meaningful contribution to the science issues.
It would also be desirable to have an accuracy commensurate with the precision in order to
identify long-term variations over the lifetime of the POP. For global measurements 1%
seems to be a minimum specification, higher precision being desirable. For the source
measurements it has been suggested that 0.1% (in column) may be only marginally
sufficient.
The construction of global maps on a monthly or seasonal basis is a task which is ideally suited to space-based instrumentation. More problematic is the generation of high time resolution snapshots of particular locations. This is difficult for a number of reasons. Firstly, the overpass of the satellite at a particular location is infrequent - of the order of days even with MOPITT's sidescan activated. Secondly, even if the satellite is passing over, the surface and atmospheric conditions may not permit a measurement - it might be night, the sun may be too low, there might be a cloud layer.
The issue of column measurements vs surface layer measurements is also significant. It would be preferable for source characterisation to measure the concentration of methane in the boundary layer (or at least the lowest 1-2km) rather than a column average. 75% of the atmospheric column is above an altitude of 2km. For global concentrations, the column amount is probably preferable.
There is also an issue of accuracy. In order to generate global maps of absolute CH4 concentration, it will be necessary to have an accuracy commensurate with the resolution. In order to measure temporally varying sources, it will be necessary to be able to accurately assess the column at a particular point in varying conditions over a long time scale.
Although a methane measurement cannot give height resolution such as is obtained by a CO instrument, nevertheless it may be possible to extract useful biogeochemical data from the column abundances if analyzed in concert with a 3-D tracer model. One must also state that satellite coverage almost invariably produces unexpected results and new insights: in the atmospheric sciences, observations usually play the major role in new discoveries.