Oxygen Isotope Anomaly

There has been considerable debate about the role of photolysis in causing the observed oxygen isotope anomaly observed in 517O and 518O record of N2O (Cliff and Thiemens,

Table 14.3. N2O isotopic fluxes from the stratosphere to the troposphere.

446

456

546

(456 + 546)/2

448

447

Reference

Lifetime (years)

113.8

116.0

114.3

-

115.4

114.6

McLinden et al. (2003)

Stratospheric

13.1

-

-

-

-

-

McLinden et al.

loss (Tg N/year)

(2003)

Fractionation

-

-19.1

-4.0

-15.0

-13.9

-7.3

McLinden et al.

constant (%o)

-22.5 ± 1.2

-14.9 ± 0.5

-13.3 ± 0.5

(2003) Park et al. (2004)

Isotopic flux

-

-

196 ± 39

182 ± 3 7

-

McLinden et al.

(% Tg N/year)

290 ± 74

192 ± 48

172 ± 43

(2003) Park et al. (2004)

1997; Cliff et al., 1999; Röckmann et al., 2001b; McLinden et al., 2003; Kaiser et al., 2004; Yung et al., 2004). The oxygen anomaly is a measure of the isotopic composition departure from pure mass dependence. Processes that fractionate isotopologues unevenly based on mass are considered mass-dependent. For N2O, these processes fractionate oxygen according to S 17O = ß x S18O where ß has been experimentally measured to be 0.515 (Cliff and Thiemens, 1997), though it can vary over a small range (Kaiser et al., 2004). Deviations from mass dependence can be quantified by defining an oxygen anomaly, which can be written in its simplest form as:

Observations show that the anomaly is nonzero (~1%) in both the troposphere and stratosphere, and increases in magnitude with altitude (Cliff and Thiemens, 1997; Cliff et al., 1999; Röckmann et al., 2001b). This is surprising since the major sources and sinks of N2O are generally thought to be mass-dependent and should not produce the anomaly. The exact origin of the mechanism is yet to be clearly identified, but it may be a combination of new atmospheric sources (McElroy and Jones, 1996; Röckmann et al., 2001b; see McLinden et al., 2003 for a discussion), chemical pathways that transfer heavy oxygen from ozone to N2O (Röckmann et al., 2001b; Yung et al., 2004) and fractionation differences in mass-dependent processes (Kaiser et al., 2004). While some modelling efforts have shown that photolysis contributes significantly to the oxygen anomaly (Johnson et al., 2001; McLinden et al., 2003), laboratory experiments indicate that photolysis is strictly mass-dependent over the relevant wavelengths of the stratosphere (Johnston et al., 1995; Röckmann et al., 2001b).

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