Modeling atmospheric δ¹³CH₄ and the causes of recent changes in atmospheric CH₄ amounts

Abstract

Inclusion of kinetic isotope effects (KIEs) of methane (CH4) sinks (gaseous OH and C1, and soil microbes) has a significant effect on modeled distributions of 613C of atmospheric CH 4. For a given scenario of surface sources and corresponding 6•3C values of individual CH 4 sources, the KIE due to soil uptake enriched 613C by 1.18%o in the model's northern hemisphere (NH) (40øN) and 1.16%o in the southern hemisphere (SH) (40øS) under steady state conditions in January. The KIE due to CH 4 oxidation by stratospheric C1 radicals further enriched these 6•C values at the surface by 0.99%• and 1.03%•, respectively. In the vertical direction, during January at 50øN, inclusion of a KIE due to C1 enriched 6•C at 18 km by 0.95%• compared to the corresponding surface value, whereas the enrichment was only 0.31%• when this KIE was omitted. These results suggest that modeling of 6•3C distributions should include KIEs due to CH 4 oxidation by soil and stratospheric chlorine radicals. It is shown that possible oxidation of CH 4 in marine boundary layer by C1 radicals can significantly enrich 6•3C. However, if a recent theoretical value for the KIE of the C1 and CH 4 reaction is correct, then the impact of this reaction is less than the figures quoted above. In the model, monthly variations in OH concentration and interhemispheric exchange transport cannot reproduce the observed seasonal amplitude variation of 6•3C in either the NH or SH. It is argued that seasonal variations in individual CH 4 fluxes are primarily responsible for this discrepancy. We show that increasing C1 radical concentrations due to continued release of anthropogenic chlorocarbons enrich the 6•3C values. The effects of an increase in tropospheric OH concentration due to stratospheric ozone depletion and a cooling of the troposphere due to the eruption of Mt. Pinatubo, with a lowering of water vapor concentration and reduction in isoprene emissions, on 6•C and surface CH 4 mixing ratios are investigated. Other model simulations with adjusted surface CH 4 fluxes have been performed to study the postulated explanations for recent changes in CH 4 surface mixing ratios and •3C values. A modified version of the Oslo two-dimensional global tropospheric photochemical model was used for all simulations. Introduction The importance of atmospheric methane (CH4) is well established [see Ehhalt, 1974; Cicerone and Oreroland, 1988], for example, its role as a radiative trace gas in global warming [Ramanathan et al., 1985; Dickinson and Cicerone, 1986] and its importance in tropospheric and stratospheric chemistry [Crutzen and Schmailzl, 1983; Thompson and Cicerone, 1986; Isaksen and Hov, 1987; Crutzen, 1987]. Measurements of atmospheric CH 4 in air trapped in ice cores show that during the last 150 years CH 4 surface concentration has increased by a factor of 2 [Etheridge e! al., 1992]. Continuous direct surface measurements since 1978 show a consistent upward trend in CH 4 concentration of about 1% per year at least through 1984 [Rasmussen and Khalil, 1981; Steele et al...

Methane Emissions Deduced from a Two-Dimensional Atmospheric Transport Model and Surface Measurements

“…It is obvious from this examination that the kinetic fractionation effect of soil absorption is effective for the atmospheric 613C, though the CH4 loss by itself is relatively small. This is also pointed out by Gupta et al (1996).…”

“…Recently, Gupta et al (1996) suggested that the chemical reaction of CH4 with Cl radicals is especially important for the o13C value in the stratosphere and the marine boundary layer, because of its large kinetic fractionation. If this is the case, the atmospheric S13C would be increased by taking the reaction with Cl into consideration.…”

The D/H content of methane emitted from biomass burning

“…This simple comparison only approximates the global CH4 budget as it does not account for the nonsteady state situation for CH4 concentration and f•3C, nor any possible nonsteady state conditions for the fD of atmospheric CH4. Nor does this analysis account for other factors, such as the spatial distribution of CH4 sinks and sources, which also affect the isotopic composition of atmospheric CH4 [Gupta et al, 1996]. This analysis is useful to illustrate that, within uncertainties, there is good agreement between the isotopic composition of the global CH4 source derived from (1) the flux-weighted sum of the isotopic compositions of the individual CH4 sources, using current estimates of relative source strengths and the experimental determination of fDcll4(bb) presented here, and (2) …”

Atmospheric Methane: Trends and Impacts

“…Comparison of regional mixing ratios with δ 13 C isotopic measurements minimize the role of atmospheric chemistry or changes in wetland emissions. Instead, they support a decrease in δ 13 C-enriched sources such as biomass burning in the Southern Hemisphere or fossil fuel use in the Southern Hemisphere (Lowe et al, 1994(Lowe et al, , 1997Gupta et al, 1996). However, using isotopic measurements from six sites, Quay et al (1999) were unable to distinguish between the possible impacts of changes in biogenic sources, fossil sources, or OH concentration over this time.…”

“…Although other isotopic signatures exist (Conny and Currie, 1996), 13 C is the main isotope used to differentiate between atmospheric methane originating from biogenic sources that is 13 C-depleted relative to atmospheric methane (Tyler, 1986), as opposed to the 13 C-enriched methane resulting from fossil fuel or biomass burning. Observed 13 C abundances are therefore indicative of the relative strength of biogenic vs. fossil sources of CH 4 , and have been used to infer the causes behind observed changes in atmospheric methane, as well as to constrain the global budget (e.g., Lassey et al, 1993Lassey et al, , 1999Lowe et al, 1994Lowe et al, , 1997Lowe et al, , 1999Gupta et al, 1996;Quay et al, 1999).…”