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1984 85 86 87 88 89 90 91 92 93 94 95 96 Year

FIGURE 14.18 Globally averaged (a) CH4 concentrations and (b) growth rates from 1984 to 1996 from 82°N to 90°S latitude (adapted from Dlugokencky et al., 1998).

negative values. The increase after the eruption is attributed by Dlugokencky and co-workers (1996, 1998) to reduced removal rates for CH4 by its reaction with OH; light scattering by the volcanic aerosol particles and UV absorption by S02 decreased UV, and hence decreased OH would be expected in the troposphere. Reasons for the sharp decrease during late 1992 and 1993 are not clear but may involve such factors as smaller natural emissions from wetlands due to lower temperatures following the eruption (Hogan and Harriss, 1994; Dlugokencky et al., 1998) and/or changes in anthropogenically associated source strengths such as decreased emissions from the former U.S.S.R. and decreased biomass burning (e.g., see Dlugokencky et al., 1994b, 1998; Rudolph, 1994; and Crutzen, 1995). Increased removal by OH due to increased UV associated with stratospheric ozone destruction may also have contributed to these trends in CH4 (Bekki et al., 1994).

While quantifying the sources and sinks of CH4 has been difficult, isotopic measurements of CH4 and CHXD4 x are promising. Various sources have characteristic isotopic signatures; e.g., as mentioned previously, fossil fuel derived CH4 is depleted in l4C (Lowe et al., 1988, 1994; Wahlen et al., 1989). The sinks of CH4, e.g., reaction with OH, reaction with CI, and uptake by soil bacteria, also exhibit kinetic isotope effects and these have been used to probe the causes of the observed recent changes in CH4 growth rates (e.g., see Gupta et al., 1996, 1997). Measurements of isotopic fractionation of both 13 C and D have been used to estimate the fraction of CH4 oxidized during transport through the coversoils in landfills, for example (e.g., see Bergamaschi et al., 1998a; and Liptay et al., 1998), and to identify sources of CH4 in the troposphere (e.g., Bergamaschi et al., 1998b; Moriizumi et al., 1998). Recent studies of the isotopic composition in the upper troposphere show that the methane is enriched in ' C in a manner that is not consistent with known kinetic isotope effects for CH4 reactions (Tyler et al., 1998), again demonstrating the complexity of quantitatively defining the sources and sinks of methane in the atmosphere.

Nitrous oxide is important not only as a greenhouse gas but, as discussed in Chapter 12, as the major natural source of NOx in the stratosphere, where it is transported due to its long tropospheric lifetime (Crutzen, 1970). The major sources of N20 are nitrification and denitrification in soils and aquatic systems, with smaller amounts directly from anthropogenic processes such as sewage treatment and fossil fuel combustion (e.g., see Delwiche, 1981; Khalil and Ras-mussen, 1992; Williams et al., 1992; Nevison et al., 1995, 1996; Prasad, 1994, 1997; Bouwman and Taylor, 1996; and Prasad et al., 1997). The use of fertilizers increases NzO emissions. For pastures at least, soil water content at the time of fertilization appears to be an important factor in determining emissions of N20 (and NO) (Veldkamp et al., 1998).

Table 14.2 shows one estimate of the contribution of various sources to the NzO budget (Bouwman and Taylor, 1996). While the major source of N20 is known to be biological, there are several observations that

TABLE 14.2 Estimated Annual N20 Budget"

Source

Emissions Tgof N/year)

Soils under natural vegetation and grasslands

0 0

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