Methane

CH is the most important greenhouse gas after CO in terms of its impact on increases in radiative forcing compared to values in the pre-industrial atmosphere (Isaksen et al., 1992). It also plays a vital role in atmospheric chemistry through its effect on tropospheric O and OH. These two species, O and OH, are important because they determine the oxidizing capacity of the atmosphere. Much of the oxidizing capacity of the troposphere is determined by its odd hydrogen content (the odd hydrogen pool is defined as the sum of OH, HO, HNO, HNO, HO, CHO, and other organic radicals and peroxides) and by the balance of species within the odd hydrogen pool, particularly the OH concentration. Thus, for example, reaction with OH is the single most important scavenger for a variety of species in the troposphere. (NO is a second important scavenger whose reactions are particularly important at night.) Methane acts as both a source and sink of odd hydrogen species (HOx) and oxidizing capacity in the troposphere. Thus, the oxidation products of CH act as a source for O and HOx. But since CH also reacts directly with OH, increases in its abundance can decrease the tropospheric OH concentration.

Methane has a current atmospheric abundance of about 1.7 ppm with about 0.1 ppm higher concentrations in the Northern Hemisphere. It has been increasing in the atmosphere by about 1%/yr although the rate of increase has been declining in recent years (Steele et al., 1992). The concentration of CH in the atmosphere represents the balance achieved between a variety of sources and its removal by chemical reaction with OH. In addition, the reactions of CH with Cl and O(1D) (electronically exited oxygen atom) in the stratosphere each represent secondary, minor sinks as does removal by soils. We have fairly good knowledge of the total source strength for CH because the sum of the sources of CH must equal the sum of the removal by chemical reaction and soils plus its estimated growth rate. The reaction rate coefficient for the reaction of CH with OH was measured by Vaghjianai and Ravishankara (1991). They found that the rate coefficient was approximately 25% slower than the rate used in previous analyses. With the new rate approximately 430 Tg (Tg = 1012 g) of CH/yr is removed by reaction with OH (Fung et al., 1991). The removal of CH by microbes in soils is estimated to be 30 15 Tg/yr, though such removal may be reduced when soils are cultivated or fertilized (Ojima et al., 1993). The estimated growth rate for CH in the atmosphere is approximately 45 5 Tg/yr. The total source must therefore equal approximately 505 Tg CH/ yr. The range of uncertainty for this estimate in the total source strength is 400-610 Tg CH/yr.

Various studies have attempted to better quantify the sources of CH. The largest estimated source category for methane is wetlands (approximately 115 Tg CH/yr) with recent work aimed at quantifying the source from tropical and northern wetlands (Bartlett and Harris, 1993) and from tundra (Whalen and Reeburgh, 1992). Emissions from rice paddies remain uncertain, though efforts to quantify emissions in Asia where cultivation practices may lead to different emission rates than elsewhere are proceeding (Khalil et al., 1991, Delwiche and Cicerone, 1993, Bachelet and Neue, 1993). Work to quantify the fossil fuel related anthropogenic sources of CH has led to more detailed categories and estimates of emissions including emissions from coal burning (Khalil et al. 1993) and from coal mines (Beck, 1993; Kirchgessner et al., 1993). Quantification of the 14C content of atmospheric CH (Quay et al., 1991) can be used to estimate the total source strength from fossil fuels, about 100 Tg/yr. The 13C content has been used to try to constrain the source from biomass burning (Quay et al., 1991).

The decline in the rate of increase of CH has been most notable in the latitude range from 30 to 90ûN (Steele et al., 1992; Khalil et al., 1993, Dlugokencky et al., 1994). The northern hemisphere change in 1992 was much smaller than that during the 1980's, less than 2 ppb. The decline in growth rates may be related to a variety of causes, including increases in tropospheric OH (Prinn et al., 1992) which may be related to decreases in stratospheric ozone (Madronich and Granier, 1992), though Dlugokencky et al. (1994) suggest that the more recent dramatic decline in rates is most likely due to a decrease in natural gas leaks associated with production particularly in the former Soviet Union. It remains to be seen whether the smaller rates of increase remain in the future.