The other sources that contribute significantly to atmospheric N2O are biomass burning (0.5TgN/yr) and human industry (nylon and nitric acid production, fossil fuel combustion, 1.3TgN/yr) (Ehhalt et al, 2001). Very little work has been performed to determine the N and O isotopic content produced from these sources, leading to the postulation (Rahn and Wahlen, 2000) that it is reasonably similar to atmospheric N2 and O2 (or 518Osmow ~22.5%o) in isotopic composition. One sample with ~12nmol/mol excess N2O purported to be due to a pollution point source(s), was depleted in both 15N and lsO (Kaiser et al, 2003c). Extrapolation of this result in a two-end-member mixing model would indicate that it arises from a source with 515N = —20%o and <5180 = — 31 %o, which is more in the range of N2O produced in agricultural systems. The error in this two-point analysis is quite large, however, and it cannot be ruled out that the result does not arise from some source of contamination other than industrial pollution.
One additional source of N2O in the atmosphere is in-situ production via the reaction
and may account for as much as 5% of the total N2O sources (Dentener and Crutzen, 1994; Kohlmann and Poppe, 1999). Odd nitrogen (defined as the total of the reactive, oxygenated nitrogen species, e.g., NO, N02, no3, nitric acid, etc., gas phase species not to be confused with the dissolved ionic species) is actively involved in the catalytic cycling of ozone via the following reactions
and since ozone is known to have excess 170, it has been proposed (Rockmann et al, 2001c) that N20 production from Eq. 15.1 might account for the small but extant A170 anomaly that has been observed in atmospheric N2O samples (Cliff etal., 1999; Rockmann etal, 2001c). Calculations have shown that a source of ~0.4TgN/yr from Eq. 15.1 may be sufficient to account for the ~l%o A170 anomaly (Rockmann et al, 2001c). It should also be noted that the catalytic cycle that enriches ozone and the nitrogen oxides in 170 and 180 can result in the production of atmospheric no3 which becomes a sink from the odd nitrogen cycle when it is aerosolized and deposited at Earth's surface. Nitrate deposited in this manner is then available for consumption by denitrifying bacteria and any A17() anomaly that is present in the no3 deposited should be at least partially conserved in N2O produced (Michalski etaL, 2003). It has not yet been definitively shown which of these two pathways is responsible for the observed atmospheric A170 anomaly or whether it may be some combination of the two.
Other gas phase reaction sources of N2O have been suggested (Zipf and Prasad, 1998; Estupinan et al, 2002) but they are expected to be minor in terms of their mass flux and the details of their isotopic systematics have yet to be elucidated.
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