Oxygen isotope ratios in N2O from different environments

Identifying the origin of nitrous oxide dissolved in deep ocean by concentration and isotopocule analyses

Nitrous oxide (N₂O) contributes to global warming and stratospheric ozone depletion. Although its major sources are regarded as bacterial or archaeal nitrification and denitrification in soil and water, the origins of ubiquitous marine N₂O maximum at depths of 100–800 m and N₂O dissolved in deeper seawater have not been identified. We examined N₂O production processes in the middle and deep sea by analyzing vertical profiles of N₂O concentration and isotopocule ratios, abundance ratios of molecules substituted with rare stable isotopes ¹⁵N or ¹⁸O to common molecules ¹⁴N¹⁴N¹⁶O, in the Atlantic, Pacific, Indian, and Southern oceans. Isotopocule ratios suggest that the N₂O concentration maxima is generated by in situ microbial processes rather than lateral advection or diffusion from biologically active sea areas such as the eastern tropical North Pacific. Major production process is nitrification by ammonia-oxidizing archaea (AOA) in the North Pacific although other processes such as bacterial nitrification/denitrification and nitrifier-denitrification also significantly contribute in the equatorial Pacific, eastern South Pacific, Southern Ocean/southeastern Indian Ocean, and tropical South Atlantic. Concentrations of N₂O below 2000 m show significant correlation with the water mass age, which supports an earlier report suggesting production of N₂O during deep water circulation. Furthermore, the isotopocule ratios suggest that AOA produce N₂O in deep waters. These facts indicate that AOA have a more important role in marine N₂O production than bacteria and that change in global deep water circulation could affect concentration and isotopocule ratios of atmospheric N₂O in a millennium time scale.

Biochar amendment suppresses N2 O emissions but has no impact on 15 N site preference in an anaerobic soil

RATIONALE

Biochar amendments often decrease N₂ O gas production from soil, but the mechanisms and magnitudes are still not well characterized since N₂ O can be produced via several different microbial pathways. We evaluated the influence of biochar amendment on N₂ O emissions and N₂ O isotopic composition, including ¹⁵ N site preference (SP) under anaerobic conditions.

METHODS

An agricultural soil was incubated with differing levels of biochar. Incubations were conducted under anaerobic conditions for 10 days with and without acetylene, which inhibits N₂ O reduction to N₂ . The N₂ O concentrations were measured every 2 days, the SPs were determined after 5 days of incubation, and the inorganic nitrogen concentrations were measured after the incubation.

RESULTS

The SP values with acetylene were consistent with N₂ O production by bacterial denitrification and those without acetylene were consistent with bacterial denitrification that included N₂ O reduction to N₂ . There was no effect of biochar on N₂ O production in the presence of acetylene between day 3 and day 10. However, in the absence of acetylene, soils incubated with 4% biochar produced less N₂ O than soils with no biochar addition. Different amounts of biochar amendment did not change the SP values.

CONCLUSIONS

Our study used N₂ O emission rates and SP values to understand biochar amendment mechanisms and demonstrated that biochar amendment reduces N₂ O emissions by stimulating the last step of denitrification. It also suggested a possible shift in N₂ O-reducing microbial taxa in 4% biochar samples.

Isotopocule analysis of biologically produced nitrous oxide in various environments

Natural abundance ratios of isotopocules, molecules that have the same chemical constitution and configuration, but that only differ in isotope substitution, retain a record of a compound's origin and reactions. A method to measure isotopocule ratios of nitrous oxide (N₂ O) has been established by using mass analysis of molecular ions and fragment ions. The method has been applied widely to environmental samples from the atmosphere, ocean, fresh water, soils, and laboratory-simulation experiments. Results show that isotopocule ratios, particularly the ¹⁵ N-site preference (difference between isotopocule ratios ¹⁴ N¹⁵ N¹⁶ O/¹⁴ N¹⁴ N¹⁶ O and ¹⁵ N¹⁴ N¹⁶ O/¹⁴ N¹⁴ N¹⁶ O), have a wide range that depends on their production and consumption processes. Observational and laboratory studies of N₂ O related to biological processes are reviewed and discussed to elucidate complex material cycles of this trace gas, which causes global warming and stratospheric ozone depletion. © 2015 Wiley Periodicals, Inc. Mass Spec Rev 36:135-160, 2017.

Proper interpretation of dissolved nitrous oxide isotopes, production pathways, and emissions requires a modelling approach

Stable isotopes ([Formula: see text]15N and [Formula: see text]18O) of the greenhouse gas N2O provide information about the sources and processes leading to N2O production and emission from aquatic ecosystems to the atmosphere. In turn, this describes the fate of nitrogen in the aquatic environment since N2O is an obligate intermediate of denitrification and can be a by-product of nitrification. However, due to exchange with the atmosphere, the [Formula: see text] values at typical concentrations in aquatic ecosystems differ significantly from both the source of N2O and the N2O emitted to the atmosphere. A dynamic model, SIDNO, was developed to explore the relationship between the isotopic ratios of N2O, N2O source, and the emitted N2O. If the N2O production rate or isotopic ratios vary, then the N2O concentration and isotopic ratios may vary or be constant, not necessarily concomitantly, depending on the synchronicity of production rate and source isotopic ratios. Thus prima facie interpretation of patterns in dissolved N2O concentrations and isotopic ratios is difficult. The dynamic model may be used to correctly interpret diel field data and allows for the estimation of the gas exchange coefficient, N2O production rate, and the production-weighted [Formula: see text] values of the N2O source in aquatic ecosystems. Combining field data with these modelling efforts allows this critical piece of nitrogen cycling and N2O flux to the atmosphere to be assessed.

Nylon production: an unknown source of atmospheric nitrous oxide

Nitrous oxide in the earth's atmosphere contributes to catalytic stratospheric ozone destruction and is also a greenhouse gas component. A precise budgetary accounting of N(2)O sources has remained elusive, and there is an apparent lack of source identification. One source of N(2)O is as a by-product in the manufacture of nylon, specifically in the preparation of adipic acid. Characterization of the reaction N(2)O stoichiometry and its isotopic composition with a simulated industrial adipic acid synthesis indicates that because of high rates of global adipic acid production, this N(2)O may account for approximately 10 percent of the increase observed for atmospheric N(2)O.

Isotopomeric analysis of N2O dissolved in a river in the Tokyo metropolitan area

River water has been suggested as a potential source of nitrous oxide (N2O), which is a greenhouse gas that is accumulating rapidly in the troposphere and which is a precursor to stratospheric NOx that depletes ozone. Previous studies on freshwater N2O sources have specifically examined estuaries where sedimentary N2O production might be important and a few points near anthropogenic nitrogen sources such as agricultural or municipal wastewater areas. Here we present the first observation of a temporal and horizontal distribution of N2O and its isotopomers between the midstream and estuary of an urban river. Surface water was supersaturated (100-6800%) with N2O at all stations during the study period. The average or maximum saturation value was greater than described in most previous reports. High N2O concentrations were observed near sewage plants and the unique signature of isotopomer ratios implied direct N2O addition from the plants. The isotopomer ratios also suggested N2O production/consumption at the sediment-water interface. Fluxes and isotopomer ratios of N2O, from the river to the atmosphere, estimated from our observations, indicated that the urban river is indeed a source of atmospheric N2O and that its production could be distinguished from other natural or anthropogenic sources using isotopomer ratios.

The 18O signature of biogenic nitrous oxide is determined by O exchange with water

To effectively mitigate emissions of the greenhouse gas nitrous oxide (N(2)O) it is essential to understand the biochemical pathways by which it is produced. The (18)O signature of N(2)O is increasingly used to characterize these processes. However, assumptions on the origin of the O atom and resultant isotopic composition of N(2)O that are based on reaction stoichiometry may be questioned. In particular, our deficient knowledge on O exchange between H(2)O and nitrogen oxides during N(2)O production complicates the interpretation of the (18)O signature of N(2)O.Here we studied O exchange during N(2)O formation in soil, using a novel combination of (18)O and (15)N tracing. Twelve soils were studied, covering soil and land-use variability across Europe. All soils demonstrated the significant presence of O exchange, as incorporation of O from (18)O-enriched H(2)O into N(2)O exceeded their maxima achievable through reaction stoichiometry. Based on the retention of the enrichment ratio of (18)O and (15)N of NO(3)(-) into N(2)O, we quantified O exchange during denitrification. Up to 97% (median 85%) of the N(2)O-O originated from H(2)O instead of from the denitrification substrate NO(3)(-).We conclude that in soil, the main source of atmospheric N(2)O, the (18)O signature of N(2)O is mainly determined by H(2)O due to O exchange between nitrogen oxides and H(2)O. This also challenges the assumption that the O of N(2)O originates from O(2) and NO(3)(-), in ratios reflecting reaction stoichiometry.

Stable isotope natural abundance of nitrous oxide emitted from Antarctic tundra soils: effects of sea animal excrement depositions

Nitrous oxide (N2O), a greenhouse gas, is mainly emitted from soils during the nitrification and denitrification processes. N2O stable isotope investigations can help to characterize the N2O sources and N2O production mechanisms. N2O isotope measurements have been conducted for different types of global terrestrial ecosystems. However, no isotopic data of N2O emitted from Antarctic tundra ecosystems have been reported although the coastal ice-free tundra around Antarctic continent is the largest sea animal colony on the global scale. Here, we report for the first time stable isotope composition of N2O emitted from Antarctic sea animal colonies (including penguin, seal and skua colonies) and normal tundra soils using in situ field observations and laboratory incubations, and we have analyzed the effects of sea animal excrement depositions on stable isotope natural abundance of N2O. For all the field sites, the soil-emitted N2O was 15N- and 18O-depleted compared with N2O in local ambient air. The mean delta values of the soil-emitted N2O were delta15N = -13.5 +/- 3.2 per thousand and delta18O = 26.2 +/- 1.4 per thousand for the penguin colony, delta15N = -11.5 +/- 5.1 per thousand and delta18O = 26.4 +/- 3.5 per thousand for the skua colony and delta15N = -18.9 +/- 0.7 per thousand and delta18O = 28.8 +/- 1.3 per thousand for the seal colony. In the soil incubations, the isotopic composition of N2O was measured under N2 and under ambient air conditions. The soils incubated under the ambient air emitted very little N2O (2.93 microg N2O--N kg(-1)). Under N2 conditions, much more N2O was formed (9.74 microg N2O--N kg(-1)), and the mean delta15N and delta18O values of N2O were -19.1 +/- 8.0 per thousand and 21.3 +/- 4.3 per thousand, respectively, from penguin colony soils, and -17.0 +/- 4.2 per thousand and 20.6 +/- 3.5 per thousand, respectively, from seal colony soils. The data from in situ field observations and laboratory experiments point to denitrification as the predominant N2O source from Antarctic sea animal colonies.

Estimation of pathways of the production of greenhouse gases in the tropical swamp forest in Thailand by stable isotope investigation

Dynamics of greenhouse gases (N(2)O and CH(4)) with the dry-wet cycle along with the variation of oxidation-reduction boundaries were investigated in the tropical wetland in monsoon Asia. It was clarified that the production of N(2)O and CH(4) was closely related to the development of a redox boundary in the Bang Nara River systems. An intermittent increase in N(2)O was observed at the beginning of the rainy season, when a large amount of easily decomposable organic matter was introduced into the river. After 10 days, when dissolved oxygen was consumed completely at the middle reaches, the emission of CH(4) became maximal due to the possible occurrence of denitrification. The distribution of stable isotope ratios in N(2)O clearly demonstrated that nitrification is the major process for its production. Furthermore, the production of N(2)O in this study area was found to vary in time and space with changes in the redox boundary along the water flow.

A review of stable isotope techniques for N2O source partitioning in soils: recent progress, remaining challenges and future considerations

Nitrous oxide is produced in soil during several processes, which may occur simultaneously within different micro-sites of the same soil. Stable isotope techniques have a crucial role to play in the attribution of N(2)O emissions to different microbial processes, through estimation (natural abundance, site preference) or quantification (enrichment) of processes based on the (15)N and (18)O signatures of N(2)O determined by isotope ratio mass spectrometry. These approaches have the potential to become even more powerful when linked with recent developments in secondary isotope mass spectrometry, with microbial ecology, and with modelling approaches, enabling sources of N(2)O to be considered at a wide range of scales and related to the underlying microbiology. Such source partitioning of N(2)O is inherently challenging, but is vital to close the N(2)O budget and to better understand controls on the different processes, with a view to developing appropriate management practices for mitigation of N(2)O. In this respect, it is essential that as many of the contributing processes as possible are considered in any study aimed at source attribution, as mitigation strategies for one process may not be appropriate for another. To aid such an approach, here the current state of the art is critically examined, remaining challenges are highlighted, and recommendations are made for future direction.

Isotopomeric characterization of N2O produced, consumed, and emitted by automobiles

Fossil fuel combustion is the second largest anthropogenic source of nitrous oxide (N2O) after agriculture. The estimated global N2O flux from combustion sources, as well as from other sources, still has a large uncertainty. Herein, we characterize automobile sources using N2O isotopomer ratios (nitrogen and oxygen isotope ratios and intramolecular site preference of 15N, SP) to assess their contributions to total global sources and to deconvolute complex production/consumption processes during combustion and subsequent catalytic treatments of exhaust. Car exhaust gases were sampled under running and idling state, and N2O isotopomer ratios were measured by mass spectrometry. The N2O directly emitted from an engine of a vehicle running at constant velocity had almost constant isotopomer ratios (delta15Nbulk = -28.7 +/- 1.2 per thousand, delta18O = 28.6 +/- 3.3 per thousand, and SP = 4.2 +/- 0.8 per thousand) irrespective of the velocity. After passing through catalytic converters, the isotopomer ratios showed an increase which varied with the temperature and the aging of the catalysts. The increase suggests that both production and consumption of N2O occur on the catalyst and that their rates can be comparable. It was noticed that in the idling state, the N2O emitted from a brand new car has higher isotopomer ratios than that from used cars, which indicate that technical improvements in catalytic converters can reduce the N2O from mobile combustion sources. On average, the isotopomeric signatures of N2O finally emitted from automobiles are not sensitive to running/idling states or to aging of the catalysts. Characteristic average isotopomer ratios of N2O from automobile sources are estimated at -4.9 +/- 8.2 per thousand, 43.5 +/- 13.9 per thousand, and 12.2 +/- 9.1 per thousand for delta15Nbulk, delta18O, and SP, respectively.

Oxygen exchange between (de)nitrification intermediates and H2O and its implications for source determination of NO3- and N2O: a review

Stable isotope analysis of oxygen (O) is increasingly used to determine the origin of nitrate (NO(3)-) and nitrous oxide (N(2)O) in the environment. The assumption underlying these studies is that the (18)O signature of NO(3)- and N(2)O provides information on the different O sources (O(2) and H(2)O) during the production of these compounds by various biochemical pathways. However, exchange of O atoms between H(2)O and intermediates of the (de)nitrification pathways may change the isotopic signal and thereby bias its interpretation for source determination. Chemical exchange of O between H(2)O and various nitrogenous oxides has been reported, but the probability and extent of its occurrence in terrestrial ecosystems remain unclear. Biochemical O exchange between H(2)O and nitrogenous oxides, NO(2)- in particular, has been reported for monocultures of many nitrifiers and denitrifiers that are abundant in nature, with exchange rates of up to 100%. Therefore, biochemical O exchange is likely to be important in most soil ecosystems, and should be taken into account in source determination studies. Failing to do so might lead to (i) an overestimation of nitrification as NO(3)- source, and (ii) an overestimation of nitrifier denitrification and nitrification-coupled denitrification as N(2)O production pathways. A method to quantify the rate and controls of biochemical O exchange in ecosystems is needed, and we argue this can only be done reliably with artificially enriched (18)O compounds. We conclude that in N source determination studies, the O isotopic signature of especially N(2)O should only be used with extreme caution.

Nitrous oxide nitrification and denitrification 15N enrichment factors from Amazon forest soils

The isotopic signatures of 15N and 18O in N2O emitted from tropical soils vary both spatially and temporally, leading to large uncertainty in the overall tropical source signature and thereby limiting the utility of isotopes in constraining the global N2O budget. Determining the reasons for spatial and temporal variations in isotope signatures requires that we know the isotope enrichment factors for nitrification and denitrification, the two processes that produce N2O in soils. We have devised a method for measuring these enrichment factors using soil incubation experiments and report results from this method for three rain forest soils collected in the Brazilian Amazon: soil with differing sand and clay content from the Tapajos National Forest (TNF) near Santarém, Pará, and Nova Vida Farm, Rondônia. The 15N enrichment factors for nitrification and denitrification differ with soil texture and site: -111 per thousand +/- 12 per thousand and -31 per thousand +/- 11 per thousand for a clay-rich Oxisol (TNF), -102 per thousand +/- 5 per thousand and -45 per thousand +/- 5 per thousand for a sandier Ultisol (TNF), and -10.4 per thousand +/- 3.5 per thousand (enrichment factor for denitrification) for another Ultisol (Nova Vida) soil, respectively. We also show that the isotopomer site preference (delta15Nalpha - delta15Nbeta, where alpha indicates the central nitrogen atom and beta the terminal nitrogen atom in N2O) may allow differentiation between processes of production and consumption of N2O and can potentially be used to determine the contributions of nitrification and denitrification. The site preferences for nitrification and denitrification from the TNF-Ultisol incubated soils are: 4.2 per thousand +/- 8.4 per thousand and 31.6 per thousand +/- 8.1 per thousand, respectively. Thus, nitrifying and denitrifying bacteria populations under the conditions of our study exhibit significantly different 15N site preference fingerprints. Our data set strongly suggests that N2O isotopomers can be used in concert with traditional N2O stable isotope measurements as constraints to differentiate microbial N2O processes in soil and will contribute to interpretations of the isotopic site preference N2O values found in the free troposphere.

Vertical gradients of delta15N and delta18O in soil atmospheric N2O--temporal dynamics in a sandy soil

The greenhouse gas nitrous oxide (N(2)O) can be both formed and consumed by microbial processes in the soil. As these processes fractionate strongly in favour of (14)N and (16)O, delta(15)N and delta(18)O gradients of N(2)O in the soil profile may elucidate patterns of N(2)O formation, consumption or emission to the atmosphere. We present the first in situ data of such gradients over time for a mesic typic Haplaquod seeded with potatoes (Solanum tuberosum L.). On two adjacent fields in 2002 and 2003, topsoil N(2)O fluxes were measured and the soil atmosphere was regularly sampled for N(2)O concentrations, delta(15)N and delta(18)O signatures of N(2)O at depths of 18, 48 and 90 cm during approximately 400 days. During the entire sampling period, the N(2)O concentrations were the highest and the delta(15)N signatures the lowest in the subsoil (48 or 90 cm depth) as compared with the topsoil, indicating production of N(2)O in the subsoil. For delta(15)N, differences greater than 30 per thousand between topsoil and subsoil on the same date were regularly observed. The highest N(2)O concentration of 100385 microL m(-3) at 90 cm depth on 1 July 2003, was preceded by the lowest delta(15)N value of -43.5 per thousand one week earlier. This was followed by a 150-day general decrease of N(2)O concentrations at 90 cm depth to 1723 microL m(-3) and a simultaneous enrichment of delta(15)N to +7.1 per thousand, mostly without a significant topsoil flux. There was a negative logarithmic relationship between N(2)O concentration at 90 cm depth and its delta(15)N signature. This relationship indicated a delta(15)N signature of -40 to -45 per thousand during the production of N(2)O in the subsoil, and a subsequent enrichment during the consumption of N(2)O. We conclude that the isotopic signature of the N(2)O topsoil flux is the result of various processes of consumption and production at different depths in the soil profile. It is therefore not a reliable estimator for the overall delta(15)N signature of N(2)O in the soil atmosphere, nor for indirect losses of N(2)O to the environment. Therefore, these findings will pose a further challenge to ongoing efforts to draw up a global isotopic budget for N(2)O.

Production of nitrogen oxide and dinitrogen oxide by autotrophic nitrifiers

Autotrophic nitrifiers have been shown to produce nitrogen oxide and dinitrogen oxide under oxic conditions. Dinitrogen oxide is produced mainly during nitrite reduction (i.e. aerobic denitrification) whereas nitrogen oxide is produced during both aerobic denitrification and as a result of chemodenitrification. Oxygen is the single most influential environmental factor affecting the production of nitrogen and dinitrogen oxides; a decrease in oxygen can result in a several-fold increase in nitrogen oxide and dinitrogen oxide production. Emission of nitrogen oxide and dinitrogen oxide from wastewater treatment plants and fertilized soils is well documented; however, only recently have the contributions from such environments to the global nitrogen and dinitrogen oxide budget been considered.

Tree species of the Central Amazon and soil moisture alter stable isotope composition of nitrogen and oxygen in nitrous oxide evolved from soil

The use of stable isotopes of N and O in N2O has been proposed as a way to better constrain the global budget of atmospheric N2O and to better understand the relative contributions of the main microbial processes (nitrification and denitrification) responsible for N2O formation in soil. This study compared the isotopic composition of N2O emitted from soils under different tree species in the Brazilian Amazon. We also compared the effect of tree species with that of soil moisture, as we expected the latter to be the main factor regulating the proportion of nitrifier- and denitrifier-derived N2O and, consequently, isotopic signatures of N2O. Tree species significantly affected delta15N in nitrous oxide. However, there was no evidence that the observed variation in delta15N in N2O was determined by varying proportions of nitrifier- vs. denitrifier-derived N2O. We submit that the large variation in delta15N-N2O is the result of competition between denitrifying and immobilizing microorganisms for NO3(-). In addition to altering delta15N-N2O, tree species affected net rates of N2O emission from soil in laboratory incubations. These results suggest that tree species contribute to the large isotopic variation in N2O observed in a range tropical forest soils. We found that soil water affects both 15N and 18O in N2O, with wetter soils leading to more depleted N2O in both 15N and 18O. This is likely caused by a shift in biological processes for 15N and possible direct exchange of 18O between H2O and N2O.

Infrared laser spectrometer with balanced absorption for measurement of isotopic ratios of carbon gases

Measurement of the isotopic compositions of carbon dioxide and methane is a powerful tool for quantifying their atmospheric sources and sinks, which is especially important considering the dramatic increase in these greenhouse gases during the industrial era. Laser absorption spectroscopy is a technique which has demonstrated the high sensitivity needed for isotopic measurement. A significant problem in the spectroscopic measurement of isotopic abundances is the large difference in concentrations of the major and minor isotopic constituents. The measurement of two isotopic species using lines of similar strength but very unequal concentrations leads to low precision, with either the minor constituent having too small an absorption depth, or the major constituent having too great an absorption depth. If lines with unequal strength are chosen to compensate for the absorption depth imbalance, then precision tends to suffer due to the greater temperature sensitivity of the weaker line strength. We describe the development of a compact instrument for isotopic analysis CO2 and CH4 using tunable infrared laser absorption spectroscopy which combines novel optical design and signal processing methods to address this problem. The design compensates for the large difference in concentration between major and minor isotopes by measuring them with pathlengths which differ by a factor of 72 within the same multipass cell. We have demonstrated the basic optical design and signal processing by determining delta13C (CO2) isotopic ratios with precision as small as 0.2/1000 using a lead salt diode laser based spectroscopic instrument.

Constraining the atmospheric N2O budget from intramolecular site preference in N2O isotopomers

Nitrous oxide (N2O) is an important trace gas in the atmosphere. It is an active greenhouse gas in the troposphere and it also controls ozone concentration in the stratosphere through nitric oxide production. One way to trace the geochemical cycle of N2O is by measuring the natural abundance of stable isotopes, namely 15N and 18O (refs 2-15). Here we report the intramolecular distribution of 15N within the linear NNO molecule, determined by measuring molecular and fragment ions of N2O on a modified mass spectrometer. This revealed a preference for 15N at the central N position, or alpha-site, within N2O isotopomers (isotope-containing molecules). Moreover, this preference varied significantly throughout the atmosphere. In the troposphere, low alpha-site preference indicates local emission of N2O from soils and fossil-fuel combustion, each with distinct isotopomer signatures, which then mixes with background N2O. In the stratosphere, on the other hand, loss of N2O is observed as enhanced alpha-site preference for 15N, due to fractionation during ultraviolet photolysis of N2O. We have constructed an atmospheric mass balance of N2O, incorporating isotopomer abundance, which shows that the intramolecular distribution of 15N is a parameter that has the potential to increase significantly the resolution with which sources and sinks of N2O can be identified and quantified in the atmosphere.

Mass-independent isotope effects in planetary atmospheres and the early solar system

A class of isotope effects that alters isotope ratios on a mass-independent basis provides a tool for studying a wide range of processes in atmospheres of Earth and other planets as well as early processes in the solar nebula. The mechanism for the effect remains uncertain. Mass-independent isotopic compositions have been observed in O3, CO2, N2O, and CO in Earth's atmosphere and in carbonate from a martian meteorite, which suggests a role for mass-independent processes in the atmosphere of Mars. Observed mass-independent meteoritic oxygen and sulfur isotopic compositions may derive from chemical processes in the presolar nebula, and their distributions could provide insight into early solar system evolution.

Stable isotope enrichment in stratospheric nitrous oxide

Nitrous oxide is a greenhouse gas that also plays a role in the cycling of stratospheric ozone. Air samples from the lower stratosphere exhibit 15N/14N and 18O/16O enrichment in nitrous oxide, which can be accounted for with a simple model describing an irreversible destruction process. The observed enrichments are quite large and incompatible with those determined for the main stratospheric nitrous oxide loss processes of photolysis and reaction with excited atomic oxygen. Thus, although no stratospheric source needs to be invoked, the data indicate that present understanding of stratospheric nitrous oxide chemistry is incomplete.

The 18O/16O and 17O/16O ratios in atmospheric nitrous oxide: A mass-independent anomaly

Measurements of the oxygen isotope ratios (18O/16O and 17O/16O) in atmospheric nitrous oxide (N2O) from La Jolla, Pasadena, and the White Mountain Research Station (elevation, 3801 meters) in California and the White Sands Missile Range in New Mexico show that N2O has a mass-independent composition. These data suggest the presence of a previously undefined atmospheric process. The La Jolla samples can be explained by a mixing between an atmospherically derived source of mass-independent N2O and biologically derived mass-dependent N2O. Possible origins of the mass-independent anomaly in N2O are discussed.

High-precision direct measurements of (13)CH(4)/(12)CH(4) and (12)CH(3)D/(12)CH(4) ratios in atmospheric methane sources by means of a long-path tunable diode laser absorption spectrometer

Measurements of (13)CH(4)/(12)CH(4) and (12)CH(3)D/(12)CH(4) ratios in atmospheric methane (CH(4)) sources provide important information about the global CH(4) budget as well as about CH(4) production and consumption processes occurring within the various sources. As an alternative to the conventional mass spectrometer (MS) technique, which requires conversion of CH(4) to CO(2) and H(2), we have developed a tunable diode laser absorption spectrometer (TDLAS), which permits rapid direct measurements of the (13)CH(4)/(12)CH(4) and (12)CH(3)D/(12)CH(4) ratios. An intercomparison between TDLAS and MS techniques for samples from natural wetlands, landfills, and natural gas sources resulted in a mean deviation of Δδ(13)C = 0.44‰ and ΔδD = 5.1‰. In the present system the minimum mixing ratios required are 50 parts in 10(6) by volume (ppmv) CH(4) (sample size 2 µmol CH(4)) for direct δ(13)C measurements and 2000 ppmv (sample size 80 µmol CH(4)) for direct δD measurements. These mixing-ratio limits are adequate for most CH(4) source characterization studies without requiring sample preconcentration.