Fractionation of Nitrous Oxide

Much has been learned about N2O's loss processes through the analysis of its iso-topic composition in the troposphere and stratosphere. The reactions and processes that produce, remove and transport N2O

throughout the atmosphere, fractionate N2O into various ratios of heavy and light N2O. The influence of each process on the isotopic composition depends on its contributing flux and its enrichment capacity, which can be quantified through a parameter called a fractionation constant. By measuring the fractionation constant for each production and removal process, and by sampling the atmospheric isotopic composition, we can constrain the fluxes of the relevant processes.

The most abundant naturally occurring species of N2O is 14N14N16O (446). When the major oxygen and nitrogen isotopes (15N,

Table 14.1. Global loss rates and lifetime of nitrous oxide (N2O).

Global loss rate (Tg N/year)

Global inventory (Tg N) (reference year)

Lifetime (years)

Minschwaner et al. (1993)

12.2

1500 (1980)

123

Prather and Ehhalt (2001)

12.6

1519 (1998)

120

McLinden et al. (2003)

13.1

1410a

114

aUsing 310 ppbv for N2O concentration at lower boundary of stratosphere.

aUsing 310 ppbv for N2O concentration at lower boundary of stratosphere.

17O, and 18O) are isotopically substituted into the base molecule, a number of isotopo-logues can be created - the most abundant of these are 14N15N16O, 15N14N16O, 14N14N17O and 14N14N18O (abbreviated here as 456, 546, 447 and 448, respectively). Other isoto-pologues consisting of two or more isotopes are rare and only found in insignificant quantities. As analytic methods to separate the two isotopomers 456 and 546 are difficult, some instrumental methods fail to distinguish them, but instead measure the aggregate, abbreviated as 15Nbulk.

The isotopic fraction of an air sample relative to the abundant light isotope is often reported with respect to a standard. For example, a permil value for the heavy isotopologue 448 can be calculated as:

X1000

R448 Rsm 448

X1000

where the values within brackets [] denote concentration, sm is the sample of interest, and st is the standard. With this definition, S will be positive when the sample is enriched in the isotopologue and negative when it is depleted. The chosen standard is typically atmospheric nitrogen and oxygen, though standard mean ocean water is often used for oxygen.

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