N20 Loss Processes

The destruction of atmospheric N2O occurs in the stratosphere where it is either photolysed

or oxidized as in

These processes account for the loss of ~12.6TgN/yr (Ehhalt et al, 2001) with Eq. 15.5 accounting for approximately 90% of the loss, Eq. 15.6a ~6%, and Eq. 15.6b ~4% (Eq. 15.6a is also the source of odd nitrogen in the stratosphere that initiates the catalytic cycling of o3 as illustrated in Eqs 15.2-15.4). An early enigma of the isotopic budget of N2O was the enrichment of 15N and lsO in tropospheric samples relative to any other known values other than deep oceans (Yoshida and Matsuo, 1983; Yoshida et al, 1984; Wahlen and Yoshinari, 1985; Yoshinari and Wahlen, 1985; Yoshida, 1988, 1989; Kim and Craig, 1990).

The concept of a stratosphere to troposphere return flux counterbalancing the depleted terrestrial and oceanic source terms was first proposed in 1983 (Yoshida and Matsuo, 1983) and the discovery of a significant enrichment of both 15N and 180 in stratospheric N2O apparently verified this hypothesis but it was ultimately concluded that the effect was to over-compensate for the depleted sources (Kim and Craig, 1993). Furthermore, it was concluded that photolysis and photooxidation (Eqs 15.5 and 15.6) could not account for the observed stratospheric enrichments (Johnston et al, 1995) indicating that the details of the global N2O budget, in particular its isotopic systematics, were poorly understood. An explanation for the stratospheric enrichment was provided when Yung and Miller (1997) theorized that photolysis could indeed significantly fractionate N2O while

Figure 15.2 Representation of the theoretical shift in cross-section with lsO substitution according to Yung and Miller (1997). The inset plot is a detail of the highlighted section between 184 and 187nm. From Rahn and Wahlen (1999).

Wavelength (nm)

Figure 15.2 Representation of the theoretical shift in cross-section with lsO substitution according to Yung and Miller (1997). The inset plot is a detail of the highlighted section between 184 and 187nm. From Rahn and Wahlen (1999).

not violating the results of Johnston et al (1995). Their model proposed a wavelength-dependent mechanism for the photolytic fractionation of N2O based on subtle shifts in the absorption spectrum of the different isotopologues due to variations in zero point energy. This principle is demonstrated graphically in Fig. 15.2 using the recommended absorption cross-section spectral function. The curve representing the 180-substituted species (dashed curve) is slightly blue shifted, by —27.5cm"1 (Yung and Miller, 1997), relative to the normal curve (both calculated at 300 K). Cross-sections are equal where the two curves cross near the absorption peaks but a clear separation is observed on both shoulders. The inset plot of Fig. 15.2 details the highlighted section on the higher wavelength shoulder. Illustrated are the cross-sections at A. = 185 nm for the normal curve (14.00 x 10^20 cm2) and the blue shifted N2 lsO curve (13.96 x 10"20 cm2). Analogous to determining the kinetic fractionation for a chemical reaction, the photolytic fractionation factor will be equal to the ratio of the heavy to light cross-sections or cri80/cri60(185 nm) = «135 = 0.9971. When expressed as an enrichment factor, where ex = 1000(ax — 1), £185 = —2.9%o.

In reality, the situation is complicated even further by vibrational structure in the absorption continuum. If we calculate the enrichment factors empirically using published cross-sectional data for isotopically substituted species (Selwyn and Johnston, 1981), we see that the cross-section curves cross several times, causing to change sign several times over the spectrum (Fig. 15.3). The first experiments to confirm this fractionation at discrete wavelengths and isotopic natural abundance levels (Rahn et al, 1998) are included in Fig. 15.3. The results compare favorably with the predicted £x value at 193 nm but the cross-sectional data do not extend to wavelengths greater than 197 nm, making a comparison of the 207 nm

Figure 15,3 Top two curves show absorption cross-sections for the N20 species as indicated (reproduced from Selwyn and Johnston, 1981). Bottom curve indicates the spectral enrichment factor calculated as described in the text. The two symbols at 193 and 207 m are the laboratory results from Rahn et al. (1998). From Rahn and Wahlen (1999).

Figure 15,3 Top two curves show absorption cross-sections for the N20 species as indicated (reproduced from Selwyn and Johnston, 1981). Bottom curve indicates the spectral enrichment factor calculated as described in the text. The two symbols at 193 and 207 m are the laboratory results from Rahn et al. (1998). From Rahn and Wahlen (1999).

data impossible. It is important to note here that the data presented in Fig. 15.3 represent the total loN enrichment such that the cross-section data are the average spectra of pure 14N15NO and 15N14NO while the data at discrete wavelengths are total <515N, which is not necessarily the same as |(i514N15NO + ¿15N14NO). For a complete discussion on the consequences of linear combinations of <5 values see Kaiser et al. (2003c).

The discrete wavelength data represented in Fig. 15.3 have since been verified by a number of studies that have expanded and improved our knowledge of the photolytic fractionation effects that influence stratospheric N20 (Umemoto, 1999; Rockmann et al, 2000; Turatti et al, 2000; Zhang et al, 2000; Rockmann et al, 2001a,b; Kaiser et al, 2002b; Kaiser et al, 2003a,b). Perhaps most notable has been the work of Kaiser et al. (2003b), which has demonstrated the influence of the vibrational structure near the peak of the absorption continuum by showing simultaneous enrichment of 14N15N160 and depletion of 15N14NleO and 14N2 lsO in residual N20 during photolysis at 185 nm.

All of these studies demonstrate more than adequate agreement and the results have revealed fractionations that are approximately double those predicted by the theory of Yung and Miller (1997). Several theoretical studies have subsequently been completed to account for this discrepancy (Johnson etal, 2001; Blake etal., 2003) and have met with reasonable success when applied in chemical modeling of the stratosphere (Blake et al., 2003; McLinden et al, 2003). Furthermore, McLinden et al (2003) and

Blake et al (2003) note that the predicted fractionation due to photolysis is not strictly mass dependent, particularly at longer wavelengths, and may account for as much as half of the observed atmospheric A170 anomaly discussed earlier.

The 10% loss of N20 due to Eqs 15.6a and 15.6b must also be taken into account when considering the isotopic budget. An early study of the ()(' I)) oxidation pathway determined that fractionation for oxygen isotope substitution was 18e0('d) = 1000[(k(N2 I80)/k(N2 ieO) - 1] = —6%o (Johnston et al., 1995). A subsequent study which investigated fractionation for both N and O isotopologues has determined that the O fractionation for Eq. 15.6 is ^fio^D) = ~~ 12.2%o and that the total N fractionation is 15So(!D) = —5.5%o (Kaiser et al., 2002a). The discrepancy in the O isotope fractionation is unclear although it is noted that the latter study was conducted under a wide range of conditions (i.e., varying temperature and pressure regimes in a variety of reactors) with no significant variation (Kaiser et al, 2002a).

A final note on the photochemical sinks of N20. It has been noted that the ratios of the fractionation factors for the various pathways (e.g., 15£a/18£a for photolysis) have unique values which lend insight into processes dominating in the stratosphere (Rahn and Wahlen, 1999; Rockmann et al, 2001b; Kaiser et al, 2002a). The relationships for the different 15N isotopomers in photolysis (Rockmann et al, 2000) and the difference between different fractionation ratios for photolysis and photooxidation (Kaiser et al, 2002a) show particular promise for elucidating stratospheric transport processes.

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