Source fractionation

Relative to the isotopic signature of atmospheric nitrogen and oxygen, the troposphere is enriched in heavy N2O. Typical values for S 15Nbulk and S448 are 5-8%0 and 16-22%. respectively (Moore, 1974; Yoshida and Matsuo, 1983; Yoshida et al., 1984; Wahlen and Yoshinari, 1985; Kim and Craig, 1990; Johnston et al., 1995; Cliff and Thiemens, 1997; Rahn and Wahlen 1997; Cliff et al., 1999; Stein and Yung, 2003). Proximity to local sources most likely produces these variations (Cliff and Thiemens, 1997) and enrichments in the boundary layer tend to be lower than in the free troposphere.

We expect that the isotopic composition of N2O in the troposphere will reflect the fractionation of its sources. However, we find that emissions of N2O from its major sources are isotopically light relative to mean tropospheric air (Yoshida et al., 1984; Wahlen and Yoshinari, 1985; Yoshinari and Wahlen, 1985; Kim and Craig, 1990; Kim and Craig, 1993; Cliff and Thiemens, 1997; Dore et al., 1998; Naqvi et al., 1998). The isotopic enrichment of 15Nbulk and 448 in soil-derived N2O ranges from -7% to -25% and 3% to 19% respectively (Kim and Craig, 1993). The composition of the other major N2O source, the ocean, is more complex. The ocean is enriched in heavy N2O below ~600 m (S 15Nbulk = 7-10% and S448 = 22-32%), but is slightly depleted in heavy N2O near the surface (S 15Nbulk = 6-7% and S 448 = 18-19%). Since gas exchange occurs mainly in the surface waters, the ocean appears to be mainly a source of light N2O, although some heavy N2O may escape during upwelling of deep water at the continental margins (Kim and Craig, 1993).

Was this article helpful?

0 0

Post a comment