Isotopic Considerations

The stable isotopic composition of atmospheric trace gases provides information about their origin and fate that cannot be determined from mixing ratio measurements alone. Biological source and loss processes (e.g., bacterial production of N20 or ch4 and photosynthetic processing of co2) are typically accompanied by isotopic selectivity associated with the kinetics of bond formation and destruction (Rosenfeld and Silverman, 1959; Whiticar et al., 1986; Farquhar et al., 1989, 1993). Thermodynamic considerations predict isotopic differentiation between phases and/or reactants under equilibrium conditions (Urey, 1947; Richet et al, 1977).

Table 15.1 Best Estimates of the Annual Mass Fluxes of the Various Known N2O Sources and Sinks3

Mosier et al. (1999),

Kroeze et al. (1999)

Olivier 1

ital (1998)

TARt

SAR*

Base year

1994

Range

1990

Range

1980s

1990s

Sources

Ocean

3.0

1-5

3.6

2.8-5.7

3

Atmosphere (in situ)

0.6

0.3-1.2

0.6

0.3-1.2

Tropical soils

Wet forest

3.0

2.2-3.7

3

Dry savannas

1.0

0.5-2.0

1

Temperate soils

Forests

1.0

0.1-2.0

1

Grasslands

1.0

0.5-2.0

1

All soils

6.6

3.3-9.9

Natural sub-total

9.6

4.6-15.9

10.8

6.4-16.8

9

Agricultural soils

4.2

0.6-14.8

1.9

0.7-4.3

3.5

Biomass burning

0.5

0.2-1.0

0.5

0.2-0.8

0.5

Industrial sources

1.3

0.7-1.8

0.7

0.2-1.1

1.3

Cattle and feedlots

2.1

0.6-3.1

1.0

0.2-2.0

0.4

Anthropogenic sub-total

8.1

2.1-20.7

4.1

1.3-7.7

5.7

6.9

Total sources

17.7

6.7-36.6

14.9

7.7-24.5

14.7

Trend

3.9

3.1-4.7

3.9

3.8

Total sinks (stratosphere)

12.3

9-16

12.3

12.6

Implied total source

16.2

16.2

16.4

"From Ehhalt eiai, 2001.

^Intergovernmental Panel on Climate Change, Second Assessment Report, fIntergovernmental Panel on Climate Change, Third Assessment Report.

"From Ehhalt eiai, 2001.

^Intergovernmental Panel on Climate Change, Second Assessment Report, fIntergovernmental Panel on Climate Change, Third Assessment Report.

Photochemical reactions and dissociation via photolysis have also been shown to be a source of isotopic fractionation that can be useful for determining different aspects of trace-gas budgets (Rahn el al, 1998; Tyler et al, 2000; Rockmann et al, 2001b; Saueressig et al, 2001; Kaiser el al, 2002a).

Of the three important biologically mediated greenhouse gases, our understanding of the isotopic budget of N2O lags far behind that of CO2 and ch4. The reasons for this are several-fold and include, but are not limited to, the following. The long lifetime of N2O in the atmosphere means that it is not only well mixed in terms of its mixing ratio but isotop-ically; therefore, the seasonal and spatial gradients that lend insight into the isotopic budgets of these other trace gases (Mook et al, 1983; Keeling et al, 1984, 1996; Levin et al, 2002; Miller et al, 2002) are not apparent in N20; the temporal and spatial isotopic variability of the individual sources is quite

Isotopomers and Isotopologues 271

large (as will be discussed below), making it difficult to assign unique values for modeling purposes; and, the low mixing ratio leads to inherent difficulties in collection, extraction, and analysis that, in the past, has limited sample throughput as well as the overall precision of the measurements although recent advances have improved on this situation (Rockmann et al., 2003b).

The isotopic signature of N2O in the free troposphere has been reported by a number of investigators since the mid-1970s (Moore, 1974; Yoshida andMatsuo, 1983; Wahlen andYoshinari, 1985; Yoshinari and Wahlen, 1985; Kim and Craig, 1990; Rahn and Wahlen, 1997; Cliff et al, 1999; Yoshida and Toyoda, 2000; Kaiser et al, 2003c) with inter-laboratory averages, and in some cases, intra-laboratory ranges varying by several per mil. It is important to note that these studies employed different methodologies and/or reference materials with no inter-laboratory comparison and therefore, the true variability of 15N/14N and 180/160 in N2O remained uncertain. More recently, a study of the variability in the stable isotopic content of tropo-spheric N20 at six European stations over a 2-year period has shown that the average <515N and <5180 are 6.72 ± 0.12%o (relative to atmospheric N2) and 44.62 ± 0.21 %o (relative to standard mean ocean water, or SMOW), respectively, with no temporal (over the period of measurement) or spatial trends within the precision of the measurements taken (Kaiser et al., 2003c). These relatively constant values are as would be expected given the atmospheric lifetime discussed above and lend credence to the conclusion that earlier reported variabilities were the result of differences in reference material and lower precision with earlier methodologies.

Isotopomers and Isotopologues

In addition to the lsO and bulk 15N content, there are two additional measures of N2O isotopic content that are reported in the literature, the '15N position' and the 'mass independent' oxygen isotope anomaly (the deviation of 170 from a strict covariation relative to 180). Because N2O is linearly asymmetric, N isotopic substitution can be at either the central or terminal position, leading to what is referred to as 15N position dependent enrichment. Analytical techniques employ either traditional mass spectrometry with the twist that one not only measures the isotopic content of the ionized parent molecule (N20+, masses 44, 45, 46) but the ionic fragment, NO+ (masses 30 and 31) (Brenninkmeijer and Rockmann, 1999; Toyoda and Yoshida, 1999), or Fourier transform infra-red spectroscopy (FTIR), which is used to compare the absorption spectra of the three different N isotopomers, 14N14N160, 15N14N160 and 14N15N160 (Esler et al., 2000; Griffith et al, 2000; Turatti et al, 2000; Zhang et al,

2000). The former requires that a correction be applied for the recombination of free atomic N and O to form NO and that there be a reference gas for which the distribution of 15N is very precisely known. The FTIR requires a large sample size, which renders the method prohibitive for routine atmospheric sampling. The lack of a common reference gas and inter-laboratory calibration is undoubtedly responsible for the large discrepancies between published data sets of 15N position dependence in the free troposphere (compare ¿loN^ir ofYoshida and Toyoda [2000] with XS15N of Kaiser et al. [2003c]). While there is potential for this measurement to provide additional insight into the budget of N2O, until there is an established universal standard it will apparently not be possible to make meaningful comparisons of reported data. There have been several different systems of nomenclature that have been developed to report the l3N position dependence; (cf. Yung and Miller, 1997; Brenninkmeijer and Rockmann, 1999; Toyoda and Yoshida, 1999).

The other isotopologue that has received some attention is 14N14N170, which is usually compared to 14N14NlsO and is commonly reported as A170 = <5170-0.52<5180 (although a more rigorous definition, derived without approximation can be found [cf. Kaiser etal, 2003c]). The A170 anomaly is the result of photochemical processes and will be discussed in more detail in the 'Other N2O Sources' and 'N20 Loss Processes' sections.

In the following pages I will discuss the mass flux terms that determine the isotopic content of atmospheric N2O and how they may have varied over time. The discussion will concentrate on the bulk l3N/14N and the 180/160 content. Isotopic notation is given in the standard form where <5samp = (^samp/^std — 1)> where R is the heavy to light isotopic ratio of the sample or standard, results being expressed in units of per mil (%o). In the case of N, atmospheric N2 is the customary standard; in the case of O, I will use the reference standard mean ocean water (SMOW) although the reader should be aware that in the literature, N2 180 results are sometimes also reported relative to atmospheric O2. I will also describe fractionation and enrichment factors that relate reaction rates (k) and absorption cross-sections (a) of heavy to light isotopes (e.g., k' to k, respectively); this convention has been adopted in much of the geochemical literature although it is inverse to that recommended by IUPAC as well as to that used in some of the literature dealing with N20. For a thorough discussion, the reader is referred to Kaiser et al. (2002a, 2003c). Given the problems with i5N position standardization described above, I refer the reader to details of the mass spectrometric methodology and standardization (Brenninkmeijer and Rockmann, 1999; Toyoda and Yoshida, 1999) and to Kaiser et al. (2003c) for a more complete discussion. There are also multiply substituted species (e.g., 14N15N180, 15N14N1sO, 15N15N160) that have yet

A^ O from the Terrestrial Biosphere 273

to be measured and are beyond the scope of this work but that are certainly fodder for future studies (Kaiser et al, 2003a).

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