Photo-induced isotopic fractionation of stratospheric N2O

Chemosphere ± Global Change Science 2 (2000) 255±266

Photo-induced isotopic fractionation of stratospheric N2O Charles E. Miller a,*, Yuk L. Yung b a

b

Department of Chemistry, Haverford College, Haverford, PA 19041-1392 USA Division of Geological and Planetary Sciences, California Institute of Technology, Mail Stop 150-21, Pasadena, CA 91125 USA Received 15 June 1999; accepted 28 October 1999

Importance of this paper: Recent measurements of the nitrous oxide (N2 O) isotopic fractionation in the atmosphere have shown that the stratospheric 15 N, 17 O and 18 O isotopomers are enriched relative to their abundances in the troposphere. Since all N2 O is assumed to enter the stratosphere from the troposphere, these results have prompted several authors to propose ``exotic'' chemical mechanisms for signi®cant production of N2 O in the middle atmosphere. We reconcile the stratospheric N2 O isotopic fractionation paradox within the framework of a standard photochemical model by demonstrating that N2 O photodissociation preferentially depletes 14 N14 N16 O and leaves stratospheric N2 O enriched in 15 N, 17 O and 18 O isotopomers. Abstract Context Abstract: N2 O has been identi®ed in the Kyoto Protocol as one of the six greenhouse gases for which anthropogenic emissions should be regulated, however, regulation procedures may not be implemented until a wellde®ned N2 O budget has been established. The measurement of N2 O isotopic fractionation provides a potential means for constraining the global budget since biological and anthropogenic sources have distinctly di€erent isotopic signatures. Main Abstract: This paper shows that N2 O isotopic fractionation in the stratosphere may be understood within the limits of the standard photochemical models if mass-dependent photodissociation rates for the various N2 O isotopomers are incorporated. Thus, we conclude that there is no demonstrable reason to invoke a signi®cant chemical source of N2 O in the middle atmosphere. This paper presents a general theory for isotopomer dependent photodissociation rates that accounts for the isotopic fractionation observed in stratospheric N2 O and how photodissociations appear to be both a source and a sink of N2 O in the middle atmosphere. Photo-induced isotopic fractionation e€ects (PHIFE), explain the distinct fractionation signatures found for 15 N/14 N and 18 O/16 O ratios in both laboratory and remote sensing measurements. Furthermore, PHIFE predicts substantially di€erent isotopic fractionations in the stratosphere for the isotopomers 15 N14 N16 O and 14 N15 N16 O, which have identical molecular weights but di€erent isotopic substitution sites. Modeling results based on this theory suggest that there is no demonstrable reason to invoke a signi®cant chemical source of N2 O in the middle atmosphere and that N2 O multi-isotope correlations should prove a useful measure of stratospheric air parcel history. Ó 2000 Elsevier Science Ltd. All rights reserved. Keywords: N2 O; Isotopic fractionation; Photodissociation; Multi-isotope correlations

1. Introduction *

Corresponding author. Tel.: +1-610-896-1388; fax: +1-610896-4904. E-mail address: [email protected] (C.E. Miller).

The Framework Convention on Climate Change has targeted nitrous oxide (N2 O) as one of the greenhouse gases for which anthropogenic emissions must be

1465-9972/00/$ - see front matter Ó 2000 Elsevier Science Ltd. All rights reserved. PII: S 1 4 6 5 - 9 9 7 2 ( 0 0 ) 0 0 0 1 1 - 8

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controlled. This task cannot be accomplished until an accurate budget is established (Khalil and Rasmussen, 1992). Isotopic fractionation may prove one of the most powerful methods for resolving disputes over sources and sinks of N2 O since distinctly di€erent fractionation patterns are observed for soils, oceans, the troposphere and the stratosphere (Kim and Craig, 1993). Fig. 1 shows that in global fractionation 15 N isotopomers are enhanced or depleted depending on the environment while 18 O isotopomers are enhanced regardless of the environment. If the fractionation signature of all sources and sinks can be determined, then isotopic sampling results may be used to constrain the global budget. Paradoxically, the same fractionation data that provides tremendous detail on the fate of stratospheric N2 O has thrown the stratospheric N2 O budget into doubt. The traditional picture of the N2 O budget assumes that all N2 O forms in the biosphere and that di€usion and transport through the troposphere constitute the only sources of stratospheric N2 O. However, tropospheric N2 O is isotopically light relative to stratospheric N2 O (Kim and Craig, 1993; Cli€ and Thiemens, 1997; Rahn and Wahlen, 1997). It has been proposed that this fractionation anomaly indicates the presence of signi®cant non-standard N2 O sources in the middle atmosphere (Prasad, 1994, 1997; Prasad et al., 1997; Zipf and Prasad, 1998). Such sources would generate the isotopically heavy N2 O required to match the observed fractionations, but the introduction of new N2 O sources in the middle atmosphere contradicts some of the most fundamental constraints in the standard photochemical model and, therefore, mandates a detailed investigation.

Fig. 1. Global N2 O isotopic fractionations depend on the environment. Oceanic (r), stratospheric (s) and tropospheric (s) samples exhibit enhancement in both 15 N and 18 O while soil (n) samples exhibit enhanced 18 O but depleted 15 N. The axes mark the fractionation of N2 and O2 in standard air (after Kim and Craig, 1993).

The analysis presented here addresses two critical questions: 1. What processes control N2 O isotopic fractionation in the stratosphere? 2. How does one resolve these processes with experimental data and still maintain a consistent photochemistry for the entire middle atmosphere? An examination of the ``exotic'' (i.e., non-traditional) source reactions (Prasad, 1994, 1997; Prasad et al., 1997; Zipf and Prasad, 1998) suggests that this chemistry cannot simultaneously account for (a) the magnitude of the stratospheric N2 O fractionation, (b) the enhancement of 15 N, 17 O and 18 O isotopomers relative to tropospheric N2 O, (c) the constraints on NOx and NOy required by other data (WMO, 1999). The standard photochemical model, represented here by the Caltech/ JPL two-dimensional (2D) model, contains the chemical kinetics and photochemistry recommended by NASA for stratospheric modeling (DeMore et al., 1997). It includes no photochemical sink of N2 O in the troposphere. There are only two sinks in the stratosphere N2 O ‡ hm…E > 42 000 cmÿ1 † ! N2 ‡ O…1 D† … 90%† …R1† N2 O ‡ O…1 D† ! 2NO or N2 ‡ O2

… 10%†:

…R2†

If one assumes there are no N2 O sources in the troposphere or stratosphere, then one concludes that the isotopic enrichment of heavy N2 O must be due to a preferential loss of the 14 N14 N16 O isotopomer relative to the heavier N2 O isotopomers. This hypothesis implies that stratospheric fractionation of N2 O is most likely due to preferential photodissociation of 14 N14 N16 O since the photodissociation sink is an order of magnitude larger than the chemical sink. In fact, Yoshida employed an empirical version of this idea in a one-dimensional (1D) box calculation which yielded d15 N ˆ ‡24& for stratospheric N2 O (Yoshida and Matsuo, 1983). In this paper, we present a theory of photo-induced isotopic fractionation e€ects (PHIFE), which demonstrates that isotopomer dependent photodissociation rates account for all atmospheric and laboratory N2 O isotopic fractionation measurements within the cohesive framework of the standard photochemical model. A preliminary version of this theory has been reported by Yung and Miller (1997). The present work extends the treatment of N2 O PHIFE to include new experimental results and a more detailed investigation of the fractionation observed in atmospheric samples. Additionally, since the initial application of this theory to N2 O, it has become apparent that photo-induced isotopic fractionation is a completely general phenomenon and its e€ects should be observable in any photochemically driven planetary atmosphere (Miller and Yung, 2000).

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2. PHIFE principles The principle behind PHIFE is that each isotopomer of a given molecule has a unique spectroscopic signature. This is readily apparent for rotational or vibrational spectra where the transition frequencies are strongly dependent on the reduced mass of the molecule. The di€erences are less pronounced in electronic absorption spectra since the dependence on the molecular reduced mass is much smaller. However, isotopic substitution will change the molecular dipole moment, the molecular symmetry properties and the nuclear spin statistics of the molecular wave function, making the electronic absorption spectra of di€erent isotopomers distinguishable. Therefore, PHIFE treats the photodynamics of each isotopomer independently. Given that each isotopomer has a unique electronic absorption spectrum, it follows that the photodissociation rates for di€erent isotopomers are mass dependent (MD). Since J …E† ˆ r…E†I…E†;

…1†

J 0 …E† ˆ r0 …E†I…E†;

…2†

where J …E† is the photodissociation rate for photon energy E ˆ hm; r…E† the absorption cross-section, I…E† is the photon ¯ux rate. The isotopic enrichment due to an irreversible sink such as photodissociation is described by the Rayleigh distillation equation (Rahn et al., 1998) d ˆ d0 ‡ eLn…f †;

…3†

where d0 and d are the initial and ®nal fractionation values, f is the fraction of N2 O remaining from the original sample, e is a loss factor, which may be expressed in terms of the absorption cross-sections to yield  0  r …E† d…E† ˆ d0 ‡ 1000 ÿ 1 Ln…f †; …4† r…E†

257

An ideal application of the PHIFE theory to photochemical modeling entails selecting a molecule to investigate, labeling each isotopomer of interest separately, scaling the abundance of each isotopomer to the appropriate initial abundance, and providing experimentally measured UV absorption cross-sections for each isotopomer, then executing the simulation. The model runs normally in all other ways. The output provides a clear picture of the isotopic fractionation induced by photochemistry. One often ®nds that the experimental UV absorption spectrum for the isotopomer X 0 of molecule X has not been measured, but that r…E† is known. In these circumstances, one may approximate the absorption spectrum r0 …E† based on the experimental spectrum r…E† and the relative zero point energies (ZPE) of X and X 0 (Yung and Miller, 1997). In the spirit of the Born-Oppenheimer approximation, the ZPE-shift model assumes that the molecular potential functions are invariant to isotopic substitution. A simple re¯ection principle treatment of the absorption process (Schinke, 1993) illustrates the changes very nicely. Given a bound ground state and a dissociative upper state, as in Fig. 2, one generates the absorption spectrum r…E† by projecting the ground state wave packet jW0 j2 onto the upper state potential surface and then re¯ecting this projection onto the energy axis. The Gaussian form of jW0 j2 translates into a r…E† which has a nearly Gaussian contour, but is compressed at low energies and elongated at higher energies. If the isotopomer X 0 has a larger reduced mass than X , then its ZPE will be lower. Since the electronic potential surfaces are the same for both isotopomers, the contours of W00 and r0 …E† will be nearly identical to W0 and r…E†; however, the lower energy of W00 results in a blue shift or energy increase in W00 relative to r…E†. The absorption spectrum for the isotopomer X 0 is thus r0 …E ‡ DZPE† ˆ r…E†;

…7†

where r0 …E† is the cross-section for the substituted isotopomer and d…E† has units of per mil. Eq. (4) shows that the mass dependence in PHIFE arises naturally as a consequence of isotopomers having distinguishable spectra. An atmospheric isotopic fractionation measurement will represent the in¯uence of the total integrated photodissociation rate and should be determined from R 0 R 0 J …E† dE r …E†I…E†dE R ˆR ; …5† J …E† dE r…E†I…E† dE R 0  r …E†I…E† dE d ˆ d0 ‡ 1000 R ÿ 1 Ln…f †: r…E†I…E† dE

…6†

Eq. (6) thus predicts isotopic fractionation if photodissociation is a signi®cant sink for the molecule under consideration.

Fig. 2. A schematic representation of the re¯ection principle (Schinke, 1993). A photon promotes the ground state wave packet onto the excited state potential surface. The absorption spectrum is the projection of the excitation process onto the energy axis.

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where 0

DZPE ˆ ZPE…X † ÿ ZPE…X †:

…8†

Further detail may be found elsewhere (Miller and Yung, 2000).

3. PHIFE treatment of N2 O The N2 O absorption spectrum exhibits a broad Gaussian-shaped continuum with a maximum near 55 000 cmÿ1 (Selwyn and Johnston, 1981; Yoshino et al., 1984; Merienne et al., 1990) (see Fig. 3). The contour and breadth of r…E† suggest a transition to a repulsive electronic state. This is supported by photodissociation studies which have measured unit quantum yield for O(1 D) production, highly rotationally excited N2 fragments (20 6 J 6 90) and anisotropy parameters consistent with a 21 A0 …1 D† 11 A0 …1 R‡ † transition (Hopper, 1984; Hanisco and Kummel, 1993; Springsteen et al., 1993; Neyer et al., 1999). Stratospheric photodissociation of N2 O occurs primarily in the 47 500±50 000 cmÿ1 atmospheric window between the O2 Schumann±Runge and O3 Hartley absorption bands. The N2 O absorption cross-sections are not very large in this region, but the convolution of the absorption cross-sections with the atmospheric UV ¯ux produces a J …E† function which maximizes for these photon energies. Therefore, it will be crucial to examine the PHIFE e€ects in the 47 500±50 000 cmÿ1 window since stratospheric fractionation of N2 O should be dominated by photodissociation at these photon energies. An assessment of the photo-induced isotopic fractionation for N2 O requires accurate UV absorption cross-sections for all of the isotopomers to be considered. Several groups have reported high quality 14 14 16 N N O (abbreviated 446) cross-sections (Selwyn and

Johnston, 1981; Yoshino et al., 1984; Merienne et al., 1990), but only Selwyn and Johnston (1981) reported cross-sections for other N2 O isotopomers and their data have been lost. This means that any PHIFE analysis of N2 O must be made using cross-sections for the heavy isotopomers constructed using the ZPE-shift approximation described above. Cross-sections for 14 N14 N17 O (447), 14 N14 N18 O (448), 14 N15 N16 O (456) and 15 N14 N16 O (546) have been calculated using Eq. (7), the (446) crosssections of Yoshino et al. (1984) and the ZPE shifts listed in Table 1. Examination of the data in Table 1 reveals that all of the heavy N2 O isotopomers have ZPE values lower than the 446 isotopomer and will thus have blue-shifted absorption spectra. One may generate an isotopic fractionation function from the cross-sections calculated with the ZPE-shift method. The e parameter from the Rayleigh distillation function of Eq. (3) becomes dependent on the photon energy according to   dr 1 e…E† ˆ ÿDZPE  ; …9† dE r…E† where the enrichments are given relative to 446. Eq. (9) demonstrates that the 447, 448, 456 and 546 N2 O isotopomers will all appear isotopically enriched relative to the 446 isotopomer since all have ZPE which are smaller than that of 446. The e(E) functions for 456, 546, 447 and 448, plotted in Fig. 4, show several very interesting trends. The PHIFE theory predicts fractionations of ‡15 to ‡35& in the atmospheric photodissociation window for all four isotopomers. These values possess the same sign and magnitude as the fractionations observed for stratospheric air samples (Kim and Craig, 1993; Cli€ and Thiemens, 1997; Rahn and Wahlen, 1997). The enrichment factors uniformly increase as the photon energy decreases due to the increasing magnitude of the dr=dE term at lower excitation energies. Finally, the PHIFE theory predicts that the 456 isotopomer will experience nearly two times greater enrichment than the 546 isotopomer despite the identical molecular masses of these two molecules. The prediction that the substitution Table 1 N2 O isotopomer ZPEa

Fig. 3. The absorption spectrum of N2 O recorded by Yoshino et al. (1984) at 295 K.

a

Isotopomer

ZPE (cmÿ1 )

D(ZPE) (cmÿ1 )

446 456 546 448 447 556 458 548

2343.5 2304.0 2321.0 2316.0 2328.5 2282.0 2280.5 2292.0

0.0 39.5 22.5 27.5 15.0 61.5 63.0 51.5

D…ZPE† ˆ ZPE…446† ÿ ZPE…X 0 †:

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259

Subsequent experiments in several laboratories (Rahn et al., 1998; Umemoto, 1999; R ockmann et al., 2000; Zhang et al., 2000) have clearly demonstrated that N2 O fractionation depends strongly on the excitation energy, that all heavy isotopomers are enriched relative to 446 and that this enrichment increases as the photon excitation energy decreases. The di€erences between the Rahn and Yoshida results for fractionation near 48 000 cmÿ1 are not yet understood; this issue needs to be addressed. Correlation diagrams of NOy (sink) versus N2 O (source) provide a stringent test of our understanding of the processes which regulate NOy in the upper stratosphere (Prather, 1998; WMO, 1999). Similarly, the PHIFE results presented here suggest that multi-isotope correlations of N2 O will provide an excellent measure of the photochemical age of an air parcel. Fig. 5 illustrates

Fig. 4. Photo-induced isotopic fractionation values calculated for 14 N14 N17 O (447), 14 N14 N18 O (448), 14 N15 N16 O (456) and 15 14 16 N N O (546) as a function of photon excitation energy. The dotted lines mark the atmospheric photodissociation window.

position is as critical as the mass of the isotopomer awaits experimental con®rmation, but we anticipate that isotopically resolved optical detection will con®rm this conclusion. One may evaluate the calculated PHIFE for N2 O in a more quantitative manner by comparing calculated fractionations against those observed in photodissociation experiments (Table 2) (Johnston et al., 1995; Rahn et al., 1998; Umemoto, 1999; Yoshida, 1999; R ockmann et al., 2000; Zhang et al., 2000). The overall agreement is very good, especially the ability of the PHIFE to reproduce the increased fractionations at lower excitation energies. Johnston et al. (1995) argued that photodissociation could not explain the mass independent (MI) N2 O isotopic fractionation e€ects since they observed d18 O 6 0.3&; however, this measurement was made at an excitation energy of 54 100 cmÿ1 . Fig. 3 shows that this is near the maximum of the N2 O absorption spectrum where dr=dE  0. Eq. (9) and the 448 enrichment factor curve in Fig. 4 correctly predict fractionations near zero for excitation near the absorption maximum.

Fig. 5. A comparison of the calculated and experimental multiisotope N2 O fractionations for stratospheric air samples. Experimental data from Kim and Craig (1993) and Rahn and Wahlen (1997) PHIFE fractionations were calculated assuming hm ˆ 48 800 cmÿ1 (the middle of the atmospheric photodissociation window).

Table 2 Calculated and observed photolytic fractionation in N2 O hm (cmÿ1 )

18

54 100b 51 800c 48 500d 48 170c a

PHIFEa

Experiment 15

15

18

d O&

d N&

d N/d O

d18 O&

d15 N&

d15 N/d18 O

6 0.3 14.5 ± 46.0

± 18.4 15.5 48.7

± 1.27 ± 1.05

0.65 9.11 21.12 22.94

0.74 10.27 23.80 25.86

1.13 1.13 1.13 1.13

The d15 N value represents the average of the d15 N (456) and d15 N (546) fractionations calculated for the speci®ed photon energy. Johnston et al. (1995). c Rahn et al. (1998). d Yoshida (1999), unpublished results. b

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Fig. 6. A comparison of the PHIFE and experimental d17 O/ d18 O correlation in N2 O for stratospheric air samples. Experimental data from Cli€ and Thiemens (1997). Mass dependent (MD), mass independent (MI) and PHIFE (P) correlation functions are shown. The PHIFE fractionations were calculated assuming hm ˆ 48 800 cmÿ1 (the middle of the atmospheric photodissociation window).

the linear correlation between d15 N and d18 O observed for a number of stratospheric air samples (Kim and Craig, 1993; Rahn and Wahlen, 1997). We have previously noted that the lack of agreement between the experimental data and the calculated fractionations for the 456 and 546 isotopomers is due to the fact that the experimental analysis employed mass spectroscopic detection and could not distinguish between di€erent isotopomers with identical molecular weights (Yung and Miller, 1997). The averaged contributions of the 456 and 546 fractionations produce an excellent ®t to the observed data. Experimental veri®cation relative fractionations of 456 and 546 in the atmosphere would be extremely valuable. Perhaps the most unusual N2 O isotopic fractionation e€ect observed in the atmosphere is the MI enrichment of 17 O reported by Cli€ and Thiemens (1997). Both tropospheric and stratospheric air samples were found to contain N2 O with substantial enhancement of 17 O over the standard MD fractionation. The results are

Fig. 7. Simulations of the seasonal variation in the SF6 mixing ratio using the standard photochemical model (DeMore et al., 1997). Since SF6 has no chemical decomposition channels, it acts as an excellent measure of the photochemical age of an airmass. Compare the SF6 distributions to those for N2 O and its isotopomers in Figs. 8±10.

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261

Fig. 8. Simulations of the seasonal variation in the N2 O mixing ration using the standard photochemical model (DeMore et al., 1997).

summarized in Fig. 6 where the correlation between d17 O and d18 O is displayed. The open circles represent the digitized data from Fig. 2 of Cli€ and Thiemens (1997) and the solid line originating at the origin with slope 0.515 illustrates the expected MD. A linear least squares ®t to the atmospheric sample data yields a slope of 0.543 and an intercept of 0.433 (plotted as the solid line MI in Fig. 6). This analysis agrees with the recent conclusion of Cli€ et al. (1999) that the di€erence between the observed MI data and the MD line increases as d18 O increases. The PHIFE theory predicts that the correlation between the 447 and 448 isotopomer fractionations will be given by the ratio of their ZPE shifts. The DZPE values in Table 1 produce d17 O/d18 O ˆ 15.0/27.5 ˆ 0.545; this result is plotted as the line labeled P in Fig. 6. The agreement between the slopes of the PHIFE prediction and the analysis of the atmospheric measurements is very good; however, the PHIFE 17 O/18 O correlation function passes through the origin while the atmospheric data have a de®nite positive intercept. It seems likely that this non-zero intercept is linked to O3 photochemistry since stratospheric O3 has an un-

usually large d17 O (Thiemens, 1999) and O3 is the principal source of O(1 D) in the stratosphere. We note that the chemical N2 O sink, O(1 D) + N2 O, has been reported to produce a fractionation of d18 O ˆ 6 ‹ 1& in laboratory experiments (Johnston et al., 1995). Estimating the contribution of the O(1 D) + N2 O sink to be 10% of the total stratospheric N2 O loss leads to a fractionation on the order of d18 O ˆ 0.6& from chemical loss. A similar order of magnitude enrichment in d17 O or the exchange of 17 O between O3 and N2 O seems the likely source of the remaining di€erences between the PHIFE predictions and the atmospheric measurements for the d17 O/d18 O correlation. Further experimental and theoretical explorations of these processes are underway. Multi-isotope correlations of N2 O should also provide an excellent measure of the photochemical age of an air parcel. The advantage of using N2 O isotopomers over comparisons of several long-lived tracers, e.g., N2 O, SF6 , CH4 , etc., is that the N2 O isotopomers provide an internally consistent set of markers: each isotopomer has the identical chemical behavior but

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Fig. 9. Simulations of the seasonal variation in N2 O d18 O using the standard photochemical model (DeMore et al., 1997) and ZPEshifted cross-sections for (448).

di€ers in its photodecomposition rate. Figs. 7±10 show a set of seasonal predictions which provide a guide for future measurement expeditions. The SF6 mixing ratios (Fig. 7) provide a measure of the photochemical age of the local airmass. The N2 O mixing ratios are presented in Fig. 8 and the predicted d18 O and d15 N fractionations are shown in Figs. 9 and 10, respectively. Fig. 11 provides the mixing ratio of SF6 as a function of time in the simulations. Fractionations of 448 in the 20±40 km altitude range are expected to be on the order ‡5 to ‡60& with a uniform distribution across all latitudes for all seasons. Slightly larger enrichments should be observed in d15 N. Model predictions for altitudes above 40 km produce fractionations that must be interpreted with great care since the N2 O mixing ratio approaches zero at these altitudes (Fig. 8). We note that the stratospheric fractionations predicted by these simulations are even larger than the highest values measured to date (Cli€ and Thiemens, 1997; Kim and Craig, 1993; Rahn and Wahlen, 1997). In situ mea-

surements made at altitudes above 25 km would be extremely valuable. Data from these higher altitudes can be obtained from balloon-borne experiments or from space-based platforms. For instance, the FTIR spectra in the Mark IV and ATMOS data sets should already contain records of the upper stratospheric N2 O fractionation. The correlation of N2 O fractionation with photochemical age, as determined from the SF6 mixing ratio, is presented for d18 O and d15 N in Fig. 12. One notes minimal fractionation for air aged < 3±4 years, but that after approximately four years the fractionation increases dramatically, as does the distribution of fractionation values for air of the same nominal age. The changes observed after year four re¯ect the di€ering histories of the air parcels, the in¯uence of atmospheric mixing, transport, etc. These model simulations clearly produce isotopic enrichment of the heavier N2 O isotopomers as a function of photochemical age. The mean age of the air producing fractionations in the 20±30& range is 5±7 years, which is in good agreement

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263

Fig. 10. Simulations of the seasonal variation in N2 O d15 N using the standard photochemical model (DeMore et al., 1997) and ZPEshifted cross-sections for the average of (456) and (546).

Fig. 11. The mixing ratio of SF6 in the atmosphere plotted as a function of time. This curve provides a photochemical clock for the simulations (see Fig. 7).

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photodissociation of the 446 isotopomer occurs due to a blue shift of the absorption cross-sections of the heavier isotopomers. This enriches the 447, 448, 456 and 546 isotopomers relative to 446 in the stratosphere. The present analysis resolves N2 O isotopic fractionation in the stratosphere within the standard photochemical model and without the need to invoke exotic N2 O sources in the middle atmosphere. Model simulations of N2 O fractionation suggest that enhancements in d18 O of 30±60& should be observable at altitudes near 40 km during the hemispheric winter. The PHIFE model also indicates the potential power of using N2 O multi-isotope correlations as a tracer of the photochemical age of an air parcel.

5. Added in proof Since the submission of this paper there have been a number of experimental measurements which support the predictions of di€erential N2 O PHIFE enrichments in 456 and 546. Zhang et al. (2000) used FTIR spectroscopy to measure enrichments of 72  5 and 40  10& in 546 and 456, respectively, after 213 nm photolysis. R ockmann et al. (2000) photolyzed N2 O samples at 193 nm and observed 11:0  2 and 36:7  0:8& enrichments in 546 and 456, respectively, as well as an enrichment of 17:6  0:5& in 448. Umemoto (1999) observed enrichments of 100  30& in 456 and 30  20& in 546 by monitoring the O(1 D) photoionization signal after photolyzing a molecular beam of N2 O at 205.47 nm. All of these results are compared with PHIFE predictions in Table 3. Additionally, Cli€ et al. (1999) have reported a large number of new atmospheric N2 O observations which augment the d17 O/d18 O correlation measurements reported by Cli€ and Thiemens (1997). The ¯urry of experimental and theoretical work on N2 O isotopic fractionation produced since the Tsukuba Conference clearly illustrates the importance of continued research in this area.

Fig. 12. Correlations of d18 O and d15 N plotted against the photochemical age of the airmass as determined from the SF6 mixing ratio. The variations in the fractionation values for any given photochemical age re¯ect the distinct histories of di€erent airmasses. Note that the enrichments increase with time and that the fractionations measured for stratospheric air correspond to airmasses with mean ages of 5±7 years.

with the mean age of the mid-latitude stratosphere (WMO, 1999).

4. Conclusions We have presented a theory for PHIFE and shown that MD photodissociation rates can account for the isotopic fractionation of stratospheric N2 O. Preferential

Table 3 Experimental and theoretical PHIFE for ÿ1

hm (cm ) d (456)&

d (546)&

51 800a 48 670b 47 000c

36:7  08 100  30 72  5

R ockmann et al. (2000). Umemoto (1999). c Zhang et al. (2000). b

N15 N16 O and

15

N14 N16 O

Experiment d (456)&

a

14

PHIFE d (546)&

d …456†=d …546† d …456†=d …546†

11:0  2 30  20 40  10

3.34 3.33 1.80

13.08 30.33 41.10

7.45 17.28 23.41

1.76 1.76 1.76

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Acknowledgements The authors wish to thank M. Thiemens, T. Rahn and G.A. Blake for interesting discussions as well as N. Yoshida, T. R ockmann, H. Zhang, and H. Umemoto for sharing their data prior to publication. CEM thanks the N2 O Conference organizers for the invitation to present the PHIFE paper in Tsukuba. YLY was supported in part by NSF ATM-9903790. References Cli€, S.S., Brenninkmeijer, C.A.M., Thiemens, M.H., 1999. First measurement of the 18 O/16 O and 17 O/16 O ratios in stratospheric nitrous oxide: a mass-independent anomaly. J. Geophys. Res. 104, 16171±16175. Cli€, S.S., Thiemens, M.H., 1997. The 18 O/16 O and 17 O/16 O ratios in atmospheric nitrous oxide: A mass-independent anomaly. Science 278, 1774±1776. DeMore, W.B., Sander, S.P., Golden, D.M., Hampson, R.F., Kurylo, M.J., Howard, C.J., Ravishankara, A.R., Kolb, C.E., Molina, M.J., 1997. Chemical Kinetics and Photochemical Data for Use in Stratospheric Modeling. Evaluation No. 12, NASA JPL, Pasadena, CA. Hanisco, T.F., Kummel, A.C., 1993. State-resolved photodissociation of N2 O. J. Phys. Chem. 97, 7242±7246. Hopper, D.G., 1993. Ab initio multiple root optimization MCSCF study of the C1v and Cs excitation spectra and potential energy surfaces of N2 O. J. Chem. Phys. 80, 4290± 4316. Johnston, J.C., Cli€, S.S., Thiemens, M.H., 1995. Measurement of multioxygen isotopic (d18 O and d17 O) fractionation factors in the stratospheric sink reactions of nitrous oxide. J. Geophys. Res. 100, 16801±16804. Khalil, M.A.K., Rasmussen, R.A., 1992. The global sources of nitrous oxide. J. Geophys. Res. 97, 14651±14660. Kim, K.R., Craig, H., 1993. Nitrogen-15 and oxygen-18 characteristics of nitrous oxide: a global perspective. Science 262, 1855±1857. Merienne, M.F., Coquart, B., Jenouvrier, A., 1990. Temperature e€ect on the ultraviolet absorption of CFCl3 , CF2 Cl2 and N2 O,. Planet. Space Sci. 38, 617±625. Miller, C.E., Yung, Y.L., 2000. Photo-induced isotopic fractionation. J. Geophys. Res., submitted. Neyer, D.W., Heck, A.J.R., Chandler, D.W., 1999. Photodissociation of N2 O: J- dependent anisotropy revealed in N/ sub 2/ photofragment images. J. Chem. Phys. 110, 3411± 3417. Prasad, S.S., 1994. Natural atmospheric sources and sinks of nitrous oxide. 1. An evaluation based on 10 laboratory experiments. J. Geophys. Res. 99, 5285±5294. Prasad, S.S., 1997. Potential atmospheric sources and sinks of nitrous oxide. 2. Possibilities from excited O2 , embryonic O3 , and optically pumped excited O3 . J. Geophys. Res. 102 (D17), 21527±21536. Prasad, S.S., Zipf, E.C., Xuepeng, Z., 1997. Potential atmospheric sources and sinks of nitrous oxide. 3. Consistency with the observed distributions of the mixing ratios. J. Geophys. Res. 102, 21537±21541.

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Prather, M.J., 1998. Time scales in atmospheric chemistry: coupled perturbations to N2 O, NOy , and O3 . Science 279, 1339±1341. Rahn, T., Wahlen, M., 1997. Stable isotope enrichment in stratospheric nitrous oxide. Science 278 (5344), 1776±1778. Rahn, T., Zhang, H., Wahlen, M., Blake, G.A., 1998. Stable isotope fractionation during ultraviolet photolysis of N2 O. Geophys. Res. Lett. 25, 4489±4492. R ockmann, T., Brenninkmeijer, C.A., Wollenhaupt, M., Crowley, J.N., Crutzen, P.J., 2000. Measurement of the isotopic fractionation of 15 N14 N16 O, 14 N15 N16 O and 14 N14 N18 O in the UV photolysis of nitrous oxide. Geophys. Res. Lett., submitted. Schinke, R. (Ed.), 1993. Photodissociation Dynamics: Spectroscopy and Fragmentation of Small Polyatomic Molecules. Cambridge University Press, Cambridge, pp. 417. Selwyn, G.S., Johnston, H.S., 1981. Ultraviolet absorption spectrum of nitrous oxide as a function of temperature and isotopic substitution. J. Chem. Phys. 74, 3791±3803. Springsteen, L.L., Satyapal, S., Matsumi, Y., Dobeck, L.M., Houston, P.L., 1993. Anisotropy and energy disposal in the 193-nm N2 O photodissociation measured by VUV laserinduced ¯uorescence of O(1 D). J. Phys. Chem. 97 (28), 7239±7241. Thiemens, M.H., 1999. Atmosphere science ± Mass-independent isotope e€ects in planetary atmospheres and the early solar system. Science 283, 341±345. Umemoto, H., 1999. 14 N/15 N isotope e€ect in the UV photodissociation of N2 O. Chem. Phys. Lett., 314, 267±272. World Meteorological Organization (WMO), 1999. Scienti®c assessment of ozone depletion: 1998, Global ozone research and monitoring project, Report No. 44, Geneva, Switzerland. Yoshida, N., 1999. Private communication. Yoshida, N., Matsuo, S., 1983. Nitrogen isotope ratio of atmospheric N2 O as a key to the global cycle of N2 O. Geochem. J. 17 (5), 231±239. Yoshino, K., Freeman, D.E., Parkinson, W.H., 1984. High resolution absorption cross-section measurements of N2 O at 295±299 K in the wavelength region 170±222 nm. Planet. Space Sci. 32, 1219±1222. Yung, Y.L., Miller, C.E., 1997. Isotopic fractionation of stratospheric nitrous oxide. Science 278, 1778±1780. Zhang, H., Wennberg, P.O., Wu, V.H., Blake, G.A., 2000. An FTIR study of isotopic fractionation in 14 N15 N16 O and 15 14 16 N N O during photolysis at 213 nm. Geophys. Res. Lett., in press. Zipf, E.C., Prasad, S.S., 1998. Experimental evidence that excited ozone is a source of nitrous oxide. Geophys. Res. Lett. 25 (23), 4333±4336. Dr. Charles E. Miller is an Assistant Professor of Chemistry at Haverford College, a prestigious undergraduate institution in suburban Philadelphia, USA. After obtaining his Ph.D. in Physical Chemistry from the University of California Berkeley, he worked at Rice University and the NASA Jet Propulsion Laboratory. His research focuses on the photochemistry and spectroscopy of atmospheric free radicals and he continues to collaborate with the Atmospheric Chemistry groups at JPL and Caltech. Dr. Yuk Ling Yung is a Professor of Planetary Science at the Division of Geological and Planetary Sciences, California

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Institute of Technology. He received his Ph.D. degree in Physics at Harvard University. He came to Caltech as an assistant professor to build up a modeling group specializing in the chemical composition of Earth and planetary atmospheres. His modeling interests include radiation, dynamics, chemistry,

planetary evolution and biosphere. He is the author of two books and more than 100 professional papers. Dr. Yung was a member of the Voyager ultraviolet experiment and is a co-investigator on the Galileo photopolarimeter experiment and the Cassini orbiter ultraviolet spectrometer investigation.