Atmospheric production of nitrous oxide from excited ozone and its significance

Chemosphere ± Global Change Science 2 (2000) 235±245

Atmospheric production of nitrous oxide from excited ozone and its signi®cance Sheo S. Prasad a,*, Edward C. Zipf b,1 a

b

Creative Research Enterprises, P.O. Box 174, Pleasanton, CA 94566, USA Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh, PA 15260, USA Received 1 November 1999; accepted 9 March 2000

Importance of this paper: We report a new development of importance to all those who, for the sake of the global environment, care about the origin and evolution of atmospheric greenhouse gases. In an easy-to-read style, we have collected evidences converging on the signi®cant atmospheric production of nitrous oxide from extremely highly-excited ozone. This production implies that the current IPCC methodology may be overestimating nitrous oxide emission from biogenic and anthropogenic activities and/or there are missing sinks. These implications need attention since nitrous oxide is a greenhouse gas with a rising atmospheric loading and an understanding of its sources and sinks is essential for making sound regulatory policy. Abstract Today our understanding of the sources and sinks of nitrous oxide (N2 O) may be at a turning point. Currently, it is believed that there are no atmospheric photochemical sources of N2 O and that microbial activity at the earth's surface (soil, lake, ocean, etc.) is the major source of atmospheric N2 O. Anthropogenic activities are thought to release N2 O into the atmosphere, but their magnitude is uncertain and probably minor. Here we present estimates of atmospheric production of N2 O from excited ozone (O3 ) based on comprehensive laboratory experiments. These experiments covered a large range of pressures from 1 to 1000 torr to distinguish between the various possibilities on the basis of their pressure dependencies, and used two reaction vessels of widely varying surface-to-volume ratios to distinguish between surface and gas phase reactions. Never before in the history of the experimental studies of N2 O under atmospherically signi®cant conditions has such a comprehensive coverage of the parameter space been attempted. From this data, the atmospheric production is substantial, being around 40% of its ``classical'' source strength. In order to put the atmospheric production in proper perspective, we also present those considerations that led us to look into the atmospheric sources. If we accept the IPCCÕs 1990 position on the N2 O source-sink inventory, then the atmospheric production of N2 O bridges the source de®cits. On the other hand, if the later IPCC positions of a nearly balanced inventory is accepted, then the new source means that either the post-1990 IPCC methodology for establishing national inventories of greenhouse gas emissions overestimates N2 O emissions or there exists some hitherto unrecognized sinks of N2 O. Ó 2000 Elsevier Science Ltd. All rights reserved. Keywords: Nitrous oxide; Greenhouse gases; Excited ozone; IPCC

*

Corresponding author. Tel.: +1-925-426-9341; fax: +1-925426-9417. E-mail addresses: [email protected] (S.S. Prasad), [email protected] (E.C. Zipf). 1 Tel.: +1-412-624-9263; fax: +1-412-624-4928.

1. Introduction In recent year evidences, from at least three di€erent directions, have been converging on the incompleteness of the currently believed sources and sinks of nitrous

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 3 7 - 4

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oxide (N2 O). We need to seriously consider this new development since N2 O is a very important atmospheric constituent. For example, it is almost 300 times more powerful than CO2 as a greenhouse gas on per molecule basis and its atmospheric concentration is increasing at the rate of 0.2±0.3 yrÿ1 (NRC, 1993; WMO, 1995). Currently it is believed that atmospheric N2 O is produced by microbiological activities in the soil and aqueous environment, and that the production by anthropogenic activities is minor and of uncertain magnitude. It is also believed that atmospheric N2 O destruction occurs in the stratosphere only by photodissociation and reactions with O(1 D). Within the limits of the rather large uncertainties, the source-sink budget of N2 O seems to be in balance (IPCC, 1997) although some other budgets (IPCC, 1990) found an important source de®ciency. Here we present evidences showing that this simple picture of the N2 O sources and sinks is incomplete and that production of N2 O occurs in the troposphere and the stratosphere. From the data presented here it will be obvious that the atmospheric production is substantial and it adds a new perspective to the N2 O budget emerging from IPCC e€orts (IPCC, 1990, 1995, 1997). 2. Evidence based on anomalous isotopic enrichment in N2 O Several features of the isotopic chemistry of N2 O raise question about the currently accepted sources and sinks of N2 O (Cli€ et al., 1999; Thiemens, 1999). For instance, the isotopic enrichments of the heavier 15 N and 18 O in stratospheric N2 O (Kim and Craig, 1993; Rahn and Wahlen, 1997) call for atmospheric processes that either preferentially destroy the light oxygen (O) and nitrogen (N) or preferentially produce the heavier O and N isotopes in N2 O. This is so since the isotopic composition of N2 O from the microbiological sources determined by various studies is mostly opposite. For example, the measured enrichment of the heavier atomic oxygen isotope in the tropospheric N2 O are 20±24& (& ˆ per mil or per thousand) relative to the atmospheric molecular oxygen (O2 ) (Wahlen and Yoshinari, 1985; Yoshinari and Wahlen, 1985; Kim et al., 1992). This enrichment is an anomaly, since the enrichment of the heavier O-atom in the N2 O from rain forest and cultivated soils are 4±12& and 11±19&, respectively (Yoshinari and Wahlen, 1985; Kim et al., 1992). The isotopic enrichment in N2 O from ocean is relatively closer to the tropospheric value (Kim and Craig, 1992). However, this may be only a partial, likely minor, solution of the isotopic anomaly, since the estimated contribution of the oceans to the atmospheric N2 O has already decreased from 70 Tg yrÿ1 (Hahn and Junge, 1977) to only 2 Tg yrÿ1 (McElroy and Wofsy, 1986) as a

result of more accurate studies. The anomaly is deeper in the stratospheric N2 O, where the enrichment is in the range 29±35& (Kim et al., 1992). Wavelength-dependent N2 O photodissociation may be the one of the various possible enrichment processes (Yung and Miller, 1997), which seems to have withstood an experimental test (Rahn et al., 1998). However, the Yung±Miller process is easily seen to be mass-dependent 2 since it depends upon the change in the absorption cross-section leading to N2 O photodissoction and since the variations in the absorption cross-sections are based on the di€erences in the zero-point energy of the isotopically substituted species (Cli€ et al., 1999). The mass-dependent process cannot explain the mass-independent part of the observed O-atom enrichment in N2 O (Cli€ and Thiemens, 1997). The mass-independent heavy O-atom enrichment in N2 O requires atmospheric production preferably from excited ozone (O3 ) since atmospheric O3 is already similarly enriched (Cli€ and Thiemens, 1997; Cli€ et al., 1999; Thiemens, 1999).

3. Hints of excited O3 as a source of N2 O in past laboratory experiments Concurrently with these developments, N2 O production from excited O3 was being suggested also by an analysis of several laboratory experiments done over the past 46 years. Since the details are in a previous paper by one of us (Prasad, 1994), here we give simply the gist for the sake of putting a complete picture at one place for the bene®t of the diverse N2 O research community. Thus, Table 1 lists eight pertinent experiments. The four set A experiments are amenable to interpretation suggestive of N2 O production in the atmosphere from excited O3 . In contrast, four Set B experiments appear to contradict this suggestion. To see the possibility that excited O3 may be a source of N2 O, consider the experiments A1 and A2. The former suggests a large barrier of about 26 kcal/mol in the reaction ÿ
ÿ
ÿ
O 3 1 A 1 ‡ N2 ! N 2 O 1 R ‡ O 2 3 R ; …1† which is understandable since the reaction (1) does not conserve spin. The reaction 2 A mass-dependent enrichment is indicated when d17 O  (0.5)d18 O on a three-isotope plot with d17 O on the ordinate and d18 O on the abscissa. In contrast, a massindependent enrichment is indicated when d17 O 6 …0:5†d18 O on the same plot. The relative abundance, d, of an isotope in a molecule, i.e., d18 O or d15 N in N2 O, is de®ned as: d18 O or d15 N ˆ ‰Rsample =Rstandard ÿ 1Š
1000:0, where R is either 18 O/16 O or 15 N/14 N or any other heavier isotope that may be under consideration.

S.S. Prasad, E.C. Zipf / Chemosphere ± Global Change Science 2 (2000) 235±245

237

Table 1 List of laboratory experiments bearing on N2 O formation via gas phase reactions Set A: experiments suggestive of N2 O production from excited O3 reacting with N2 A1. Goody and Walshaw (1953) O3 mixed with N2 left in a reaction vessel for prolonged time period (days). A rate coecient for N2 O formation from O3 in thermodynamical equilibrium at the room temperature was deduced to be about 10ÿ29 cm3 sÿ1 A2. Harteck and Dondes (1954) Pyrolysis of O3 diluted with O2 and N2 at 568 K. High N2 O yield, 10ÿ4 O3 converted to N2 O A3. DeMore and Raper (1962) O3 dilutely mixed in liquid N2 was photolyzed by radiation in the wavelength region 248 to 320 nm. The observed quantum yields were: U…N2 O† ˆ 0:014 at 248 6 k 6 300 nm; U…N2 O† ˆ 0:014  0:007 at k > 3000 nm A4. Norrish and Wayne (1965) Photolysis of 20 torr of O3 in 1 atm of N2 . N2 O was detected after 6 h of irradiation. But the yield was considered of marginal signi®cance Set B: suggestive of no such production B1. Katakis and Taube (1962) B2. Simonaitis et al. (1972) B3. Kajimoto and Cvetanovic (1976) B4. Maric and Burrows (1992)

ÿ
ÿ
O3 …1 A1 † ‡ N2 ! N2 O 1 R ‡ O2 1 R

Photolysis of 10±100 torr of O3 in 300±500 torr of N2 at 253.7 nm. No N2 O formation detected. U…N2 O† < 10ÿ4 5±12 torr of O3 was photolysis at 253.7 nm in 42±115 torr of O2 and about 800 torr of N2 . N2 O below detectability limit Typically about 10 torr of O3 , 100 torr of O2 mixed in 20±100 atm of N2 was photolyzed by 254 nm from 500 W Hanovia Hg lamp. U…N2 O† ˆ 3  107 at 1 atm. U…N2 O† varies as n2 (N2 ) A large amount of O3 (3350 ppmv) mixed with synthetic air at 1 atm was photolyzed by a ®ltered Hg-lamp. Little N2 O production was found

…2†

conserves spin. For ground state O3 is in complete thermodynamic equilibrium at the room temperature, however, reaction (2) also will have a barrier of almost the same amount due to the high endothermicity. So it is logical to expect that the production of N2 O from O3 may be possible if O3 is highly excited with internal energy close to the barriers discussed above. The experiment A2 con®rms this expectation for O3 excited due to high temperature (i.e., thermal excitation). The experiment A3 does the same for O3 optically pumped to the bound portions of the electronically excited predissociated 1 B2 state as discussed in detail by Prasad (1994). (Note that the spin conservation may not be any problem in the reaction O3 …1 B2 † ‡ N2 ! N2 O…1 R† ‡ O2 since there is enough energy in the 1 B2 state to let the product O2 be in the excited b 1 R state.) Unfortunately, the predissociative lifetime of the 1 B2 state is only 3 ps (Sinha et al., 1986). For this reason while the 1 B2 state can produce N2 O in condensed phase experiment A3, the N2 O production via this excited O3 cannot be signi®cant under atmospheric pressures. The lifetime in the unbound portion of the 1 B2 state (the state responsible for the Hartley Bands) is even shorter. Thus it is no wonder that neither A4 nor B1 experiment in which O3 was irradiated by 254-nm radiation, showed any unambiguous N2 O production. Like the 1 B2 state, other electronically excited states of O3 are also predissociated. So, if atmospherically signi®cant N2 O production from excited O3 is to occur it must be via translationally and/or vibrationally excited

O3 in the ground state analogous to the excited O3 in the A2 experiment. This condition can be ful®lled in the atmosphere since the nascent O3 from O, O2 three-body recombination is highly vibrationally excited with effective vibrational temperature on the order of 2000 K (Rawlins, 1984). Thus, the spin conserving reaction (4b) in the following reaction sequence (initiated by O3 and NO2 photodissociation) can be a source of N2 O in the troposphere and stratosphere ÿ
2O O 3 P ‡ O2 ‡ M ! ON ‡M 3

…3a†

ÿ
O 3 P ‡ O2 ‡ M ! O3 ‡ M

…3b†

O3N2 O ‡ N2 ! N2 O ‡ O2 …X 3 R†

…4a†

O3N2 O ‡ N2 ! N2 O ‡ O2 …b1 R†

…4b†

O3N2 O ‡ N2 ! N2 ‡ O3

…4c†

O3N2 O ‡ O2 ! O3 ‡ O2

…5†

O3N2 O ! O3

…6†

2O Here ON is O3 with extremely high degree of vibra3 tional excitation (approaching the threshold of dissociation) in the most favorable vibrational mode (or modes) so that the reaction (4b) is fast; O3 * is the rest of the vibrationally excited O3 from O ‡ O2 ‡ M recom2O bination. The model further assumes that ON may 3 change to O3 * via fast intramolecular processes in

238

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addition to collisions, i.e., k6  106 sÿ1 . The pressure dependence of N2 O quantum yield in this model can be shown to be of the form 1=…1 ‡ a=p† (Prasad et al., 1997). The entire reaction sequence and particularly reaction (4b), must have been occurring in all B2, B3, and B4 experiments. Why was reaction (4b) not recognized as a source of N2 O from these experiments? The answer is simple. The impurity levels in the B2 experiment were so high that the smaller N2 O production was masked. The B3 experiment was done at very high pressures 27 atm and more. At these pressures the contributions of the pressure squared-dependent N2 O formation from the O…1 D† ‡ N2 ‡ M ! N2 O ‡ M reaction is so high that the contributions of reaction (4b) was again masked. The B4 experiment was highly unusual, in the sense that the lamp was placed inside the reaction vessel exposing the entire surface of the reaction chamber to large amount of O3 and intense UV radiation capable of dissociating O3 . The situation in this respect was very closely the same as in Black et al. (1983) experiment in which a rapid loss of N2 O was observed. Thus, the nondetection of signi®cant N2 O formation in Marick and Burrows experiment may not contradict the reality of reactions (4a)±(4c) and may be calling attention to a hitherto unrecognized N2 O loss process that may have plagued laboratory experiments (Prasad, 2000). We therefore, may conclude that there is nothing in the published literature against the formation of N2 O via reactions (4a)±(4c).

4. Evidence from direct laboratory experiment that excited O3 is a source of N2 O Consistent with the previous conclusion, direct laboratory experiments by Zipf and Prasad (ZP) (1998) have now veri®ed the spin conserving reaction (4b) being a source of N2 O via the reaction sequence (3a)±(6). Even though the experiment is published, we describe it in some detail here clarifying those details, which could not be adequately addressed within the strict page limitation. Zipf and Prasad (1998) studied gas phase N2 O formation from highly vibrationally excited O3 produced in O, O2 recombination process. Atomic O needed to drive reactions (3a)±(6) was produced by the photodissociation of O2 in ultra-pure research grade synthetic air by Schumann±Runge continuum radiation from a EG&G Argon ¯ash lamp ®tted with ®lters to greatly reduce (if not totally eliminate) the excitations of the Bstate. We needed to reduce the excitation of the B-state in order to minimize the contamination of the N2 O formation from the excited O3 by the formation of N2 O from the reaction O2 …B3 R† ‡ N2 ! N2 O ‡ O

…7†

which was theoretically predicted by Prasad (1997) and has now been experimentally veri®ed by Zipf (Prasad and Zipf, 2000). ZP did not attempt to produce O by photolyzing O3 since previous experiments to study N2 O production in photolysis of O3 mixed with N2 and O2 have yielded uncertain and con¯icting results (Prasad, 1994, and also Section 3). Photodissociation of NO2 as a source of O was also avoided since NO2 produces N2 O heterogeneously (Estupinan et al., 2000). The initial background levels of N2 O, NO and NO2 in the unirradiated air was less than 50 pptv. To minimize the build up of secondary species, especially O3 , the irradiation time was short (10±15 min), the lowest possible light ¯ux commensurate with excellent N2 O detection was used and the gas was ¯owed through the absorption cell rapidly. For the ®rst time in the history of the experimental studies of gas phase formation of N2 O under atmospherically signi®cant conditions, pressure range 1±1000 torr was covered (to discriminate between various possibilities on the basis of their pressure dependencies) and two vessels of di€erent volume-to-surface ratios were used (to discriminate between gas phase and surface reactions). The raw data (un®lled symbols) in Fig. 1 suggest that two types of N2 O source processes occur. Type I source dominate in the low-pressure regime and become insigni®cant at p > 100 torr. Type II source emerge at the higher pressures, level o€ quickly and become almost pressure independent around 600 torr. As discussed in detail by ZP, Type I processes cannot be gas phase reactions. None of the species in ZP reaction cell can produce N2 O at the observed rate by gas phase reactions in the low-pressure regime either

Fig. 1. Pressure variation of the measured N2 O yield in a cell with 21 cm diameter and computer solutions of the coupled, non-linear continuity equations governing the microchemistry. Curves (A) and (B) show the N2 O production due to wall reactions with O(3 P) and vibrationally excited O3 (low m). Curve …N O† (C) shows N2 O formation when excited O3 2 reacts with N2 .

S.S. Prasad, E.C. Zipf / Chemosphere ± Global Change Science 2 (2000) 235±245

because of endothermicity constraints, or a high activation energy barrier, or an incompatible pressure dependence. For example, neither thermalized O(3 P) nor fast O(3 P) from O2 photodissociation have the center-of-mass energy needed to reach the point, where the repulsive N2 , O(3 P) potential energy (PE) curve crosses the attractive N2 , O(1 D) PE curve. O(1 D) atoms can form N2 O in three-body reactions with N2 . But this process is too slow to account for the N2 O observed in this experiment in either the Type I or Type II region. The translational energy possessed by the O(1 D) atoms does not alter this situation. With the exception of O3 none of the molecular species in our cell can produce N2 O if they are in their ground state due to endothermicity constraints. The reaction O3 ‡ N2 ! N2 O ‡ O2 is exothermic, but has a high activation energy based on the Goody and Walshaw (1954) experiment who found that the rate coecient for this process at room temperature was <10ÿ29 cm3 sÿ1 . Most of the excited molecular species expected in our cell (e.g., O2 (a 1 D), O2 (b 1 R)) do not have the required energy. Goody and Walshaw's experiments also show that O3 in the lowest vibrational levels is not a factor. O2 (A 3 R) and O2 (B 3 R) are not expected in any signi®cant amount due to the spectral intensity distribution of the ®ltered light source. But even if they were, their role was minor since the production of N2 O from these species will have a linear pressure dependence, which is not observed. Hence, ZP concluded that Type I processes are chie¯y due to surface reactions. At the lowest pressure Type I processes are attributed to surface reactions of atomic oxygen. This process saturates at p < 10 torr because nearly all of the O atoms from the O2 dissociation reach the wall to drive the heterogeneous formation of N2 O. As the pressure increases, the contribution from O decreases because the increased transit time from the production region in the center of the vessel to the wall …/ p† facilitates conversion of O to O3 …/ p2 †. In the transition range (10±100 torr) the observed Type I production rate decreases more slowly than predicted for O surface reactions alone. ZP attribute this behavior to the emergence of surface reactions involving an excited molecular species because none of the ground-state molecular species expected in the photolysis cell can form N2 O via a surface reaction. For example, O3 and N2 can remain mixed with for several days in a vessel without producing any signi®cant N2 O when both are in complete thermodynamic equilibrium. The ultimate decrease of this surface source with increasing pressure is attributed to the increasing quenching of the excited molecules while in transit to the wall. Due to the exothermicity of the reaction, the surface reaction of non-thermally populated O3 (low v) is the most likely source of N2 O in the transition pressure range. Type II processes must be gas phase reactions involving excited molecular species in order to be consis-

239

tent with the observed pressure dependence, because the surface reactions possible for excited species in our axially irradiated system must decrease monotonically with pressure. Gas phase reactions of O(1 D), O2 (a1 D), O2 (b 1 R), O2 (high v), O2 (A 3 R), O2 (B 3 R) and O3 (low v) species have been discounted already. The possibility that N2 (A 3 Ru ) and N2 (a 1 Pg ) molecules might produce N2 O in our experiment via their reactions with O2 was tested directly by using the Zeeman ®lter ®lled with N2 to a column density of 1024 molecules cmÿ2 to suppress the optical pumping of these metastables. Since no e€ect on the N2 O formation rate was observed, no electronically excited state of N2 was involved in the production of the N2 O observed in our experiments. These devel2O opments leave ON as the most likely species responsi3 ble for the Type II production of N2 O via the chemical model embodied by the set of reactions (3a)±(6). Various key elements of the above model are amply supported by independent experiments. The formation of highly vibrationally excited O3N2 O in three-body O, O2 recombination is supported directly by the COCHISE experiment and its interpretation in terms of either a statistical model or an e€ective vibrational temperature of 2000 K (Rawlins, 1984). The formation of N2 O with very high vibrational excitation, exceeding 63 kcal/mol, in the three-body N2 , O(1 D) association (Kajimoto and Cvetanovic, 1976) provides additional corroboration, albeit indirectly for reaction (3a). Our assumption that O3N2 O relaxes to O3 by intermolecular energy rearrangement, i.e., reaction (6), parallels the Kajimoto and Cvetanovic (1976) assumed relaxation of N2 O to N2 O by fast intramolecular processes. As stated earlier, reaction (4a) does not conserve spin. But reaction (4b) does. Curves A, B, and C shown in the Fig. 1 were obtained by numerical simulation of the above discussed surface (Type I) and gas phase reactions (Type II) involved in the observed N2 O production. They represent, respectively, the contributions of the surface reaction of O, the surface reactions of O3 (low v) and the gas phase reactions of O3N2 O . The numerical simulation used a three-dimensional continuity equation solver. This approach was necessary due to the complexity of the experiment. For example, the beam ®lling factor for each cell was quite di€erent: the small diameter cell was illuminated nearly uniformly across its cross-section area while only a small region of the large chamber along its longitudinal axis was uniformly illuminated by the collimated UV beam (dia. 3.6 cm). It is important to note that photon deposition along the longitudinal axis of the photolysis cell varies strongly with the wavelength of the argon ¯ash lamp spectrum, position, pressure, and gas composition. This complex source supports a chemistry that is characterized by strong local gradients and a competition between multi-mode di€usion, volume recombination, quenching, and excited O3 formation that

240

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varies throughout the photolysis cell. The bulk measurements re¯ect averages over this non-linear and spatially sensitive photochemical environment. In order to obtain meaningful quantum yields it was necessary to solve the time-dependent, non-linear partial di€erential equations governing the di€usion and the coupled chemistry of O, O3 , O3 and N2 O accurately. Curve C represented an asymptotic yield …p ! 1†; u ˆ 4  10ÿ5 N2 O per O, O2 recombination and with a ˆ 500 torr in the form factor ‰1=…1 ‡ a=p†Š. The assignment of the observed N2 O production in the Curve C region to gas phase reactions (4a), (4b), and (4c) was tested further by comparing experimental data from reaction cells with smaller and larger volume-to-surface ratios. The latter strongly discriminated against surface reactions. As expected, the role of the gas phase reactions (curve C) stood out more, relative to the surface reactions, in the high-pressure regime in the larger cell. 5. Remaining issues Although ZP covered a large parameter space, many issues remain to be solved which is quite natural when the chemistry of highly-excited short-lived species is involved. Here we discuss two issues that should be addressed soon. The most important issue is the possibility that reaction (4b) might be endothermic. The e€ective endothermicity cannot be large, since otherwise the reaction would have been too slow to matter at the room temperature. The e€ective endothermicity depends upon 2O the function f(e) de governing the number of ON with 3 energy lying between e and e ‡ de kcal/mol for which reaction (4b) is endothermic by 23:5 ÿ e kcal/mol. With 2000 K as the experimentally determined e€ective vibrational temperature of the nascent O3 from the threebody O, O2 recombination (Rawlins, 1984), 0.3% of nascent O3 will have e P 23:5 kcal/mol. For these, reaction (4b) is either thermoneutral or exothermic. Even so, there is no guarantee that there will not be a small intrinsic barrier in the reaction (4b). For the rest of the nascent O3 , reaction (4b) will be endothermic. For example, for 5.7% of the nascent O3 , the 20 kcal/mol 6 e 6 23:5 kcal/mol. For these, the endothermicity driven barrier in the reaction (4b) will be in the range of 3.5 to 0.0 kcal/mol. This is consistent with our expectation that the barrier is small for a non-negligible component of the nascent O3 . Only experiments at different temperatures can decide what the actual e€ective barrier is, and these experiments are needed since the temperature in the troposphere and the stratosphere is smaller than the room temperature. The other issue concerns the role of water vapor which is an ecient quencher of vibration and is also quite abundant (with volume mixing ratio on the order of 1%) in the planetary boundary layer. If water vapor

eciently deactivates O3N2 O , then quenching by water rather than intra-molecular energy transfer is the main O3N2 O loss process and the N2 O production from O3N2 O will be correspondingly reduced in this region. However, the region where this reduction might take place is very limited in extent and for this region the total e€ect may not be signi®cant. Nevertheless, these issues must be investigated. Production of N2 O from excited triplet states of O3 that lie above the dissociation threshold totally avoids the endothermicity problem since it is spin allowed for the product O2 in its ground X 3 R state. It is therefore, quite possible that this production might o€set the adverse e€ects (if any) of water vapor and/or the endothermicity in reaction (4b). Consequently, it is important to experimentally investigate production of N2 O from excited triplet states of O3 . Unfortunately, the advantage of the spin conservation and the exothermicity in N2 O formation from excited triplet O3 has a price. These states are not only highly predissociated but also dicult to access. The latter diculty arises since these triplet states cannot be accessed in the normal atmosphere by any means other than the spin forbidden optical pumping from the singlet ground state. Experiments to study the production of N2 O from excited triplet states of O3 must be designed with these diculties in mind, and must also avoid O2 in the system in order to eliminate interference from N2 O production via reaction (4b). 6. Estimates of N2 O production from O3N2 O Approximate production rates of N2 O, p(N2 O), via reaction (4b) will now be estimated for the least and the most favorable scenarios. From the foregoing and the following discussions it should be obvious that we cannot do any better at this time. A simpli®ed equation for p(N2 O) can be written as follows: ÿ
p…N2 O† ˆ p…O3 †f O3N2 O k4b n…N2 †  : kq;M n…M† ‡ kq;H2 O n…H2 O† ‡ k4b n…N2 † ‡ kintra …8† f …O3N2 O †

In the above equation, is the fraction of nascent O3 with internal (vibrational) energy P the threshold energy (ethresh ) in the most favorable vibrational mode (or modes) for driving reaction (4b) with any meaningful eciency, k4b is the rate coecient for reaction (4b), kq;M and kq;H2 O are the temperature-dependent rate coecients for the quenching of O3N2 O by, respectively, air (N2 , O2 ) and water (H2 O), and kintra is the rate at which the O3N2 O is deactivated to ordinary vibrationally-excited O3 by intramolecular energy redistribution in the various vibrational modes. At this time, nothing is known

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241

with any reliability about the either f or the various rate coecients (ks). Also, whatever ``lumped'' information about them that we have is for dry air. We therefore, make another simpli®cation of temporarily ignoring water vapor-related term in order to proceed further on the basis of the available information for dry air. With this simpli®cation, Eq. (8) can be written as    ÿ
k4b 1 p…N2 O† ˆ p…O3 †f O3N2 O k4b =kq;M ‡ 1:28 kq;M 1  :  …9† RT 1 ‡ kintra kq;M …1‡0:78…k4b =kq;M † p In Eq. (9), 1.28 ˆ n(M)/n(N2 ), 0.78 ˆ n(N2 )/n(M), p is the pressure in torr, and R is the gas constant in unit of cm3 torr degÿ1 . As of this time all that is known experimentally is that    ÿ
k4b 1 f O3N2 O …10† ˆ 4  10ÿ5 ; kq;M k4b =kq;M ‡ 1:28

Fig. 2. Altitude pro®le of N2 O production rate assuming that there is no activation energy barrier in the reaction leading to its 2O formation from highly excited ON (i.e., reaction (4b) in the 3 text). The dashed and the continuous lines are, respectively, for with and without deactivation of O3N2 O by water.

and kintra p



RT kq;M …1 ‡ 0:78…k4b =kq;M †

 ˆ 500=p

…11†

at the room temperature. To proceed further with atmospheric application, we must therefore make approximate but educated guesses about the temperature dependencies of these two terms in Eq. (9). As per the discussion of Section 5, k4b ˆ Ar exp…ÿa=T † with 0 K 6 a 6 2000 K. Due to ignorance, kq;M will be assumed to be same as that for O3 with m3 ! 8. Thus kq;M ˆ 1:7  10ÿ14 T 0:5 cm3 sÿ1 . Thus the temperature dependence of k4b =kq;M is that of T ÿ0:5 exp …ÿa=T †. The k4b =kq;M term is most probably less than unity at the room temperature and becomes increasingly smaller at the lower temperatures in the atmosphere. We may therefore, neglect the temperature dependence of the (k4b =kq;M ‡ 1:28) and the (1 ‡ 0:78 k4b =kq;M ) terms in Eq. (9). The term T =kq;M will have a mild T0:5 temperature dependence. For simplicity we will ignore this temperature dependence too. With these approximations, the atmospheric N2 O production rate via the excited O3 (i.e., the O3N2 O ) mechanism can be rewritten as p 0:544 exp …ÿ2000=T †= T p…N2 O† ˆ p…O3 † ; 1 ‡ 500=p

…12†

p 6:93  10ÿ4 = T p…N2 O† ˆ p…O3 † 1 ‡ 500=p

…13†

for the least and the most favorable cases, respectively, discussed earlier. Recollect that for the least favorable case, we assumed an activation energy barrier of 4 kcal/mol

Fig. 3. Altitude pro®le of N2 O production rate assuming that there is an activation energy barrier of 4 kcal/mol in the reaction leading to its formation from highly excited O3N2 O (i.e., reaction (4b) in the text). The dashed and the continuous lines are, respectively, for with and without deactivation of O3N2 O by water.

in reaction (4b). In contrast, the most favorable case was to have no barrier. Based on these two approximate equations, the continuous line plots in Figs. 2 and 3 present the altitude pro®les of N2 O production rates in a globally averaged model atmosphere for the two extreme scenarios. The pro®les of n(N2 ), n(O2 ), n(O), p and T for the assumed model atmosphere were from Brasseuer and Solomon (1986, Appendix D). The rate coef®cient for O, O2 three-body recombination was taken from DeMore et al. (1994).

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It is easily seen that the two extreme cases show very di€erent behavior with altitude in the troposphere. If reaction (4b) has no activation energy barrier, then p(N2 O) increases. This is due to the increase in the number densities of odd-oxygen with altitude. In the other extreme, e€ect of this increase is o€set by the activation energy-barrier due to which reaction (4b) becomes more and more inecient as the temperature decreases with altitude. In the upper stratosphere (>20 km) p(N2 O) decreases with altitude in both cases. This is due to the combined e€ect of kintra becoming more important and the three-body O, O2 recombination becoming less e€ective. Also shown, by dotted lines, in the ®gures are the production rates approximately corrected for the deactivation of O3N2 O by water vapor. The correction factor was assumed to be Correction factor

p 1:7  10ÿ14 T n…M† p p : ˆ 1:7  10ÿ14 T n…M† ‡ 2:89  10ÿ12 T n…H2 O† …14†

In Eq. (14), the rate coecient for the quenching of O3N2 O by H2 O, kq;H2 O , has been assumed to be more than two orders of magnitude faster than the quenching by either N2 or O2 . The temperature dependence of kq;H2 O has been taken to be the same as that of kq;M . It is obvious that water vapor can suppress the N2 O production rate, but in the lowest atmosphere only. This is so, since the n(H2 O) decreases sharply with altitude. It is noteworthy that this is also the region that would be most favorable to the formation of N2 O from the excited triplet states, if it occurs. From the two ®gures it is quite clear that the atmospheric production of N2 O via the excited O3 mechanism is quite substantial even in the most unfavorable case. The column integrated production rate in this case is 1:1  109 N2 O molecules sÿ1 cmÿ2 . Water vapor deactivation decreases the production from 1:1  109 to 8:4  108 or a reduction by about 17%. This is in accord with the fact that the water vaporÕs e€ect is limited to a small region near the surface. The lowest column-integrated production rate can be compared with the biogenic plus anthropogenic production at the surface. Assuming it to be in the 14±16 Tg yrÿ1 range (based on IPCC, 1995, 1997 as cited by Mosier et al., 1998), the biogenic production amounts to 1.8±2:1  109 N2 O molecules sÿ1 cmÿ2 . Thus, the atmospheric production amounts to (roughly speaking) 40% of the currently perceived biogenic plus anthropogenic production. The qualifying words ``roughly speaking'' are introduced in view of the limitations of extrapolating the result from a globally averaged model.

2O is consistent with the 7. Production of N2 O from ON 3 correlation of N2 O with other long-lived tracers

We have already explained that the formation of N2 O from excited O3N2 O is consistent with the published literature on experimental studies of N2 O formation from atmospherically signi®cant gas phase reactions. We will now argue that the formation of N2 O from excited O3N2 O is consistent with the observed correlation of N2 O with other long-lived tracers such as CH4 and CFC. We do this since the fact that neither CH4 nor CFCs has atmospheric sources might lead to the legitimate question: ``how can N2 O have atmospheric sources and be still correlated with species like CH4 and CFCs, which do not have such sources?'' The key point here is that for long-lived species to be correlated it is not necessary that the spatial distribution of the sources and sinks be similar. If the two species also have local chemical lifetimes, which are long compared to the quasi-vertical transport times, then the correlation plot will be linear (Plumb and Ko, 1992). The new source of N2 O does not change the local chemical lifetime in any signi®cant way. The observed correlation will therefore remain preserved, despite the new atmospheric source of N2 O. Other complementary reasons for the preservation of the correlation have been discussed earlier by Prasad et al. (1997). 8. Signi®cance of the new source The new source has a considerable signi®cance for the source-sink inventory of N2 O, in the construction of which it has both a direct and an indirect role. Considering the direct signi®cance ®rst, if we accept the IPCC (1990) position on the N2 O sourcesink inventory, then the new N2 O source bridges the source de®cits. If the later IPCC (1997) positions of a nearly balanced inventory is accepted, then the new source means that either the post-1990 IPCC methodology for establishing national inventories of greenhouse gas emissions overestimates N2 O emissions or there exists some hitherto unrecognized surface sinks of N2 O. The suggested possibility of overestimation is in accord with independent conclusions of others. For example, Nevison (2000) ®nds that even with corrective adjustments ``the 1997 IPCC total N2 O source is dicult to reconcile with the observed atmospheric increase. The simplest explanation for this discrepancy is that methodology may be overestimating the N2 O''. Estimation of N2 O emissions associated with agricultural leaching and runo€ underscores this situation. One of the key parameter in this emission is the emission factor for leaching/ runo€ usually abbreviated as EF5. The current (IPCC, 1997) default value of this factor is 0.025.

S.S. Prasad, E.C. Zipf / Chemosphere ± Global Change Science 2 (2000) 235±245

The groundwater component (EF5-g) is the major component of EF5 with a value EF5-g ˆ 0.015. In contrast, as suggested by Nevison (2000), the preferred value should be as low as 0.001 ± a factor of 6 lower indeed! In general terms, the overestimation is not dicult to understand, since estimations of the global emission rate from measurements that are too sparse in both the space and the time domain can be intrinsically very uncertain. It can be especially so, since the N2 O emission depends upon many factors (e.g., soil temperature, humidity, availability of oxygen etceteras) which vary dramatically from one spatial domain to another and from one season to another. The possibility that there may indeed be unrecognized surface and atmospheric sinks of N2 O is as easy to appreciate as the possibility of IPCCÕs overestimation discussed in the previous paragraph. As a matter of fact, suggestion of possible surface sinks of N2 O has indeed been in the literature for quite sometime now. For example, Cicerone (1989, p. 18,270) pointed out: ``it is possible that in some ®eld experiments to date, when N2 O sources near zero were reported, small sinks may have been operating in reality''. The fact that these unrecognized sinks are small does not imply their unimportance, since as pointed out by Cicerone (1983, p. 18,265) ``relatively small soil sinks would decrease the atmospheric residence time of N2 O to values below those that are calculated from stratospheric removal alone''. Hint of a hitherto unrecognized atmospheric sink is more direct and compelling compared to the abovementioned hints of soil sinks. The experimental veri®cation of reaction (4b) as a source of N2 O is also direct experimental evidence for the existence of the reaction O2 …b1 R† ‡ N2 O ! N2 ‡ O3

…15†

on the basis of the principle of detailed balancing or chemical reversibility. The fact that this sink can be non-negligible in the stratosphere has been already discussed (Prasad, 1997). It will therefore, not be repeated here. Let us now consider the indirect signi®cance of the new source. The fact that the direct determination of the N2 O sources and sink by methods such as those used in IPCC methodology can be very uncertain indeed (for the intrinsic reasons already stated above), has led to the development of inverse estimations (Prinn et al., 1990). In this technique, highly precise stratospheric loss rates and long-term measurements of N2 O at globally distributed sites (i.e., ALE, AGAGE and OGI/PSU data sets) are used two- and three-dimensional inverse modeling to determine the surface emissions. The new source needs to be considered in the inverse modeling to ensure

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reliable results, especially when the new source can be quanti®ed with precision paralleling that of the stratospheric sinks (photodissociation and reaction with O(1 D)).

9. Summary and concluding remarks Evidences from three di€erent directions have recently converged on the incompleteness of the classical sources and sinks of the atmospheric N2 O, and the importance of the atmospheric production of N2 O from highly-excited O3 . Given the great importance of possible activation energy barrier in the new atmospheric source (i.e., the O3N2 O ‡ N2 ! N2 O ‡ O2 (b) reaction), it is extremely important to study this reaction at di€erent temperatures in order to determine the barrier. At present, there are considerable uncertainties in the N2 O source-sink inventories based on IPCC protocols. If we accept the IPCC, 1990 position, then the new atmospheric source bridges the source de®cit. If the IPCC, 1997 estimation is assumed correct, then there are hitherto unrecognized surface and atmospheric sinks. It is therefore, equally important to begin searches for missing soil sinks and identify those elements of the IPCC protocols that may be leading to the overestimation of the anthropogenic and biogenic emissions.

Acknowledgements One of us (SSP) acknowledges support of the National Science Foundation (ATM-9526510).

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S.S. Prasad, E.C. Zipf / Chemosphere ± Global Change Science 2 (2000) 235±245 Sheo S. Prasad is currently the sole-proprietor of Creative Research Enterprises and an Adjunct Professor at the University of Southern California and Portland State University. He specializes in theoretical and modeling studies of chemistry in the EarthÕs and other planetary atmospheres, and in the astrochemistry of celestial molecules in quiescent and shocked interstellar clouds.

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Edward C. Zipf is currently a Professor of Physics at the University of Pittsburgh and President of Innovative Science and Technology headquartered in Pittsburgh. He specializes in low energy plasma physics and ozone/global warming experiments, and sounding rocket and satellite studies of aurora, airglow, and ionospheric physics.