Composition Of N2O Research Articles

Tracing sources and fate of nitrate in multilayered karstic hydrogeological catchments using natural stable isotopic composition (δ15N-NO3− and δ18O-NO3−). Application to the Toulon karst system (Dordogne, France)

• Nitrate contents and isotopes are measured in a large multilayered karst aquifer. • Primary and secondary sources of nitrate are determined, at the local scale, using nitrate isotopes. • Origins of nitrate, over a karstic hydrogeological catchment, are assessed by nitrate isotopes. • The fate of nitrate in a karst aquifer is specified/ elucidated/ quantified/ combining isotopes and hydrogeological approaches. In groundwater management, the determination of the origins of nitrate (NO 3 – ) endmember sources is the first step in improving the management over a hydrogeological catchment. The generally multiple source contamination by nitrate over the karst systems makes it difficult to identify the exact nitrate origins in surface water and groundwater. This work aims to assess nitrate vulnerability of a karst system fed by several karst aquifers. The Toulon karst system offers the opportunity to study the relationships between two multilayered karst aquifers. It shows an increase in mean NO 3 – concentrations over decades. However, the origins of the NO 3 – remain unclear. Natural stable isotopic composition of NO 3 – (δ 15 N -NO3 – and δ 18 O -NO3 – ) measurements combined with concentrations were used to identify the origins of nitrogen causing this increase. This information was also integrated into the local hydrogeological situation to improve knowledge of the contamination of both confined and unconfined karst aquifers. δ 15 N -NO3 − and δ 18 O -NO3 − in potential nitrate sources over the hydrogeological catchment were analysed in order to identify the range of values that can be measured in both rivers and groundwater. Over the Toulon Springs catchment, rivers are contaminated with nitrate coming from N synthetic fertilizer mineralized in agriculturalsoils. Most of the springs are also contaminated by this nitrate source, but some record an isotope composition of nitrate coming from sewage effluents. Toulon Springs are only contaminated with nitrate from N synthetic fertilizer mineralized in agricultural soils. Finally, from a hydrogeological point of view, the analysis of δ 15 N -NO3 − and δ 18 O -NO3 − values improves knowledge on the functioning of the study area and shows that the upper Jurassic karst confined aquifer has nitrate of the same origins (sources) as the upper Cretaceous unconfined aquifer. The results here highlight that the lower aquifer can be fed by the Upper Cretaceous unconfined aquifer, contrary to previous belief that the lower aquifer was confined. From a more fundamental standpoint, this work provides a set of parameters adapted for karstic hydrogeological catchment. It lays the foundation for the use of isotope biogeochemistry as a powerful tool for understanding the recharge of drinking water catchments and beyond the efficient management of water quality at the scale of karstic hydrogeological catchment.

Crystal Structure and Guest Distribution of N2O Hydrate Determined by Powder X-ray Diffraction Measurements

Research and development of control technology of N2O greenhouse gas are crucial for sustainable development. The crystallographic structural model of structure I (sI) for N2O hydrate, with a composition of N2O·6.1H2O and a density of 1.127 g/cm3 at 263 K, was obtained using powder X-ray diffraction data. In addition, the distributions of the disordered N2O molecules in 512 and 51262 host water cages of sI hydrate were explored. N2O in the large 51262 cage had an off-center distribution and it was out-of-plane to the long axis of the cage, and N2O in the small 512 cage had an isotropic distribution around its center. These crystallographic features of the N2O hydrate were identical to those of the CO2 hydrate, and an increase in cage occupancy depending on the formation pressure was observed. The hydrate composition and dissociation kinetics were discussed pertaining to the information required for the gas storage of N2O.

Recognition of binary mixture of NO2 and NO gases using a chemiresistive sensors array combined with principal component analysis

Novel room-temperature type chemiresistive gas sensor arrays consisted of two, three or four metal oxide semiconductor (MOS) gas sensors that were constructed using iron oxide/multi-walled carbon nanotubes/tungsten oxide (Fe2O3/MWCNTs/WO3), surface Au-, Pd- and Ag- modified films as sensing materials. Principal component analysis (PCA) coupled with various MOS chemiresistive gas sensor arrays were used to analyze the obtained data under testing conditions of binary mixture gases containing various composition of NO and NO2 at room-temperature. The careful pattern recognition of sensing data obtained with a chemiresistive gas sensor array consisting of three MOS sensors made it possible to qualitatively differentiate the variation in the composition of gas species in a binary mixture of NO2 and NO gases with related to their concentrations.

Rapid Conversion of Added Nitrate to Nitrous Oxide and Dinitrogen in Northern Forest Soil

ABSTRACTThe aims of this study were to simulate wet deposition of atmospheric nitrate (NO3−) onto forest soils and trace its fate via conversion spatially and temporally into gaseous products nitrous oxide (N2O) and dinitrogen (N2). The most likely mechanism is microbial denitrification, but an intermediate product nitrite (NO2−) can fuel N2O production via a chemical pathway involving reactions with iron and/or organic matter referred to as chemodenitrification. During summer months, we applied tracer amounts of 15N-labeled NO3− onto forest soils (pH ∼ 4) at three sites in the White Mountain Region of New Hampshire, USA. We recovered 15N as N2O in 210 of 504 measurements (42%) versus 15N as N2 in 51 of 504 measurements (10%), suggesting partial microbial denitrification and/or chemodenitrification. When recovery occurred, the mean percent recovery of added 15N was just 1.1% as N2O, although N2 recovery was 33%. A site with old-growth trees had a larger percentage recovery as N2 (48%), whereas a site that had burned 100 years ago had a small percentage recovery as N2O (0.24%). The 15N composition of N2O in ambient air, collected before addition of the label, was markedly enriched in 15N. Since flux measurements were made 2 h after the addition, the results suggest that denitrification enzymes and conditions for chemodenitrification are present throughout the summer months but account for small amounts of NO3− conversion into N2O and N2.

Technical considerations for the use of passive samplers to quantify the isotopic composition of NOx and NO2 using the denitrifier method

The isotopic composition of nitrogen oxides (NOx = NO + NO2) hold potential to trace emissions sources, as well as chemistry in the atmosphere. Passive samplers that collect NOx and NO2 as nitrite could provide a simple and inexpensive means by which to collect samples in a variety of environments, and we report here on several methodological considerations when using Ogawa passive collectors of NOx and NO2 for concentration analysis via colorimetric methods and isotopic analysis via the denitrifier method. Using Ogawa samplers, NOx and NO2 were collected in Providence, RI, USA (41.8°N, 71.4°W) with exposure times of seven, fourteen, twenty-one, and twenty-eight days during summer and winter in an area dominated by vehicle emissions, particularly diesel delivery trucks. Average values were −9.7 ± 0.7‰ for δ15N-NOx and −8.3 ± 0.9‰ for δ15N-NO2 based on duplicate seven, fourteen, twenty-one, and twenty-eight day exposures (n = 8) in summertime, and −11.9 ± 0.7‰ for δ15N-NOx and −6.4 ± 1.4‰ for δ15N-NO2 based on seven, fourteen, twenty-one, and twenty-eight day exposures (n = 7) in wintertime. The oxygen isotopic composition (δ18O, Δ17O) was also determined with averages of: 36.3 ± 2.4‰ for δ18O-NOx (n = 8), 50.5 ± 3.2‰ for δ18O-NO2 (n = 8), 6.2 ± 0.8‰ for Δ17O-NOx and 10.8 ± 0.6‰ for Δ17O-NO2 (n = 4) in summer; and 38.7 ± 2.9‰ for δ18O-NOx and 47.4 ± 1.2‰ for δ18O-NO2 (n = 7), 7.7 ± 1.5‰ for Δ17O-NO2 (n = 4) and 5.3 ± 1.2‰ for Δ17O-NOx (n = 5), in winter. Our isotopic results differ from previous studies that utilized passive samplers and suggest specific nitrogen and oxygen isotopic signatures associated with vehicular emissions. To collect sufficient sample for multiple isotopic analyses (e.g., 10 nmol N for δ15N and δ18O; 50 nmol N for Δ17O) the exposure time for the samplers, even in urban areas, is at least days. This necessarily results in collection of a mixture of local emissions sources, as well as background air that can represent multiple NOx sources. Thus, passive samplers cannot be used to diagnose isotopic signatures unless only exposed to a single emissions source. The oxygen isotopic composition of nitrite (the collection analyte) results determined via the denitrifier method must be corrected in comparison to nitrate isotopic reference materials, resulting in an increase of ∼25‰. Even with this correction, the oxygen isotopic results are lower than what would be expected for ozone-dominated atmospheric chemistry in an urban area, and the oxygen isotopic results for NOx were consistently lower than that found for NO2. We suggest that the oxygen isotopic composition of NO and NO2 were modified upon capture and conversion to nitrite on the passive sampler pads, and does not directly reflect abundances in the atmosphere.

Nox reduction from biodiesel exhaust by plasma induced ozone injection supported by lignite waste adsorption

This paper explores the technique of ozone injection to induce reactions leading to NO conversion in a diesel engine exhaust and subsequent adsorption by lignite ash, an industrial waste from lignite coal fired power plants. Experimental investigations were carried out using a diesel engine fuelled by biodiesel, prepared from pongamia pinnata, a native Indian plant. Ozone is produced from dry air in a Dielectric Barrier Discharge (DBD) reactor by the application of high voltage. The choice of the high voltage was made after a comparative study between high frequency AC and unipolar repetitive pulse source, with the latter consuming lesser power for given ozone production. By injecting ozone, the higher composition of NO in biodiesel exhaust gets oxidized to higher oxides of nitrogen in general and NO2 in particular. This NO2 can be adsorbed in the lignite fly ash pellets as successfully demonstrated in this paper. This combined ozone injection-lignite ash adsorption technique has resulted in an overall removal efficiency of oxides of nitrogen (NOx) as high as 95% even under loaded conditions. A comparison has also been made between the ozone injection technique and direct plasma treatment of exhaust for NOx removal.

Sources and fate of nitrate in a groundwater‐fed estuary elucidated using stable isotope ratios of nitrogen and oxygen

We used δ15N‐nitrate (NO−3) and δ18O‐NO−3 to unravel the provenance and fate of NO−3 in a groundwater‐fed estuary. A total of 13 monthly and two time series surveys were undertaken in the Werribee River estuary near Melbourne, Australia. The different survey timescales provided a comprehensive evaluation of the hydrological effects on the biogeochemistry of NO−3, which accounted for tidal variability and episodic runoff events. The distribution of δ15N and δ18O of the estuarine NO−3 along a mixing line between shallow and deep groundwater provided strong evidence for the predominance of groundwater‐derived NO−3 in the estuary. During dry periods when water residence time in the estuary was extended, shallow groundwater contributed 60% to 76% of the NO−3 (calculated from δ15N‐NO−3 and δ18O‐NO−3) to the estuary, and assimilation removed ∼ 70% of this groundwater‐derived NO−3. During wet periods, deep groundwater provided more (62%) NO−3 than shallow groundwater, and there was no indication of NO−3 consumption. Occasional sources of NO−3, which were also reflected by their δ15N‐NO−3 and δ18O‐NO−3 values, included NO−3 from nitrification and sewage‐derived NO−3 particularly at the bay entrance to the estuary. Greater emphasis should be placed upon the role of groundwater as a substantial NO−3 end member when assessing NO−3 biogeochemistry using methods relying on the dual isotopic composition of NO−3.

Infiltration Mechanism Controls Nitrification and Denitrification Processes under Dairy Waste Lagoon

Earthen waste lagoons are commonly used to store liquid wastes from concentrated animal feeding operations. The fate of ammonium (NH) and nitrate (NO) was studied in the vadose zone below earthen-clay dairy farm waste lagoons using three independent vadose zone monitoring systems. The vadose zone was monitored from 0.5 to 30 m below land surface through direct sampling of the sediment porewater and continuous measurement of the sediment profile's water content variations. Four years of monitoring revealed that wastewater infiltration from the lagoon is controlled by two mechanisms: slow (mm d), constant infiltration from the lagoon bed; and rapid (m h) infiltration of wastewater and rainwater via preferential flow in desiccation cracks formed in the unsaturated clay sediment surrounding the lagoon banks. The preferential flow mechanism is active mainly during wastewater-level fluctuations and intensive rain events. The vadose zone below the waste sources remained unsaturated throughout the monitoring period, and all infiltrating NH was oxidized in the upper 0.5 m. The NH oxidation (nitrification) was coupled with NO reduction (denitrification) and depended on the sediment water content, which was controlled by the infiltration mechanism. Coupled nitrification-denitrification (CND) resulted in 90 to 100% reduction in the total nitrogen mass in the vadose zone, with higher removal under high water content (∼0.55 m m). Mass balance of nitrogen and isotopic composition of NO indicated that CND, rather than cation exchange capacity, is the key factor regulating nitrogen's fate in the vadose zone underlying earthen waste lagoons.

Variations in the concentration and isotopic composition of nitrate nitrogen in wet deposition and their relation with meteorological conditions in Xi’an city, Northwest China

Abstract The characterization of N isotopes in NO 3 - is an effective method to determine NO 3 - sources and NOX transformation mechanisms. Xi’an city is located in a semi-arid to arid region and is representative of most cities in the north and NW of China. Few studies have been carried out on N isotopes in wet deposition of this region. In this study, 29 wet deposition samples were collected from Xi’an city in northwestern China between March 2008 and May 2009 and analyzed for their volume-weighted NO 3 - and NH 4 + concentrations and N isotopic compositions. Volume-weighted mean concentrations were 3.9 μmol L−1 for NO 3 - and 13.5 μmol L−1 for NH 4 + . The wet deposition samples showed pronounced seasonal variation in the N isotopic composition of NO 3 - with δ 15 N – NO 3 - ranging from −2.0‰ to +9.2‰ over the course of a year, with a mean value of +2.6 ± 2.6‰. Higher δ 15 N – NO 3 - values were observed in the cool season (November–May, +3.8‰) than in the warm season (June–October, +1.0‰). In autumn and winter in particular, the mean δ 15 N – NO 3 - values were clearly different (+0.9‰ and +6.7‰, respectively). Various factors are shown to lead to variation in the δ 15 N – NO 3 - values in wet deposition. One effect is changing ratios of N derived from agricultural/soil sources (low δ 15 N – NO 3 - , dominant in the warm season) and fossil fuel-derived NOX (high δ 15 N – NO 3 - , relatively more important in the cool season). However, in the cool season δ 15 N – NO 3 - unexpectedly shows a strong negative correlation with SO2 concentration, which should correlate with fossil fuel combustion inputs. This may be explained by interactions of N-species with other atmospheric pollutants. The results also suggest that temperature may be a significant factor affecting δ 15 N – NO 3 - . The NOX reacts in different ways depending on temperature, leading to a correlation of δ 15 N – NO 3 - with temperature. Moreover, the effects of dilution cannot be neglected here, as there is clear seasonal variation in precipitation. When combined with meteorological parameters, the volume-weighted concentrations of NO 3 - and NH 4 + and air pollutant precursor data, the δ15N of NO 3 - in wet deposition can be used as a reliable tool for the precise identification of NOX transformation mechanisms in the environment and the fate of NOX emissions.

Diel cycling and stable isotopes of dissolved oxygen, dissolved inorganic carbon, and nitrogenous species in a stream receiving treated municipal sewage

Diel and synoptic studies were undertaken in Silver Bow Creek, a small, highly eutrophic stream receiving municipal sewage from the city of Butte, Montana, USA. During mid-summer baseflow conditions, oxidation of ammonium from the Butte wastewater treatment plant created a 2-km long reach marked by nightly hypoxia and extreme growth of aquatic plants and algae. Large diel cycles in the concentrations and isotopic compositions of dissolved oxygen (DO) and dissolved inorganic carbon (DIC) were inversely related, and are explained by the daily cycle of photosynthesis and respiration, modified by the effects of nitrification. Nitrification rates were higher during the day than at night due to a combination of higher water temperatures and higher DO concentrations. Changes in nitrification rate imparted diel cycles in the concentrations and N-isotopic compositions of NO 3 − and NH 4 +, whereas total dissolved inorganic nitrogen (DIN) concentrations and δ 15N-DIN values showed minimal diel variation. δ 15N-NH 4 steadily increased with distance downstream over a 5 km reach, whereas δ 15N-NO 3 showed a more complex spatial pattern. Plant assimilation caused downstream decreases in DIN and soluble reactive phosphorous: however, it was not possible to determine whether the plants assimilated NO 3 −, NH 4 +, or both. An important new finding of this study is the recognition that synoptic and diel changes in δ 18O-DO in the zone of active nitrification imparted a corresponding change in δ 18O-NO 3. Future studies examining the O-isotope composition of nitrate need to consider whether DO has a constant or shifting isotopic composition at the source of nitrification. The results of this study underscore the complex and dynamic interactions between biologically-active solutes in nutrient-rich streams, and also emphasize the importance of recognizing these cycles when collecting samples for scientific or monitoring purposes.

Hydrogen assisted diesel combustion

Hydrogen assisted diesel combustion was investigated on a DDC/VM Motori 2.5L, 4-cylinder, turbocharged, common rail, direct injection light-duty diesel engine, with a focus on exhaust emissions. Hydrogen was substituted for diesel fuel on an energy basis of 0%, 2.5%, 5%, 7.5%, 10% and 15% by aspiration of hydrogen into the engine's intake air. Four speed and load conditions were investigated (1800 rpm at 25% and 75% of maximum output and 3600 rpm at 25% and 75% of maximum output). A significant retarding of injection timing by the engine's electronic control unit (ECU) was observed during the increased aspiration of hydrogen. The retarding of injection timing resulted in significant NO X emission reductions, however, the same emission reductions were achieved without aspirated hydrogen by manually retarding the injection timing. Subsequently, hydrogen assisted diesel combustion was examined, with the pilot and main injection timings locked, to study the effects caused directly by hydrogen addition. Hydrogen assisted diesel combustion resulted in a modest increase of NO X emissions and a shift in NO/NO 2 ratio in which NO emissions decreased and NO 2 emissions increased, with NO 2 becoming the dominant NO X component in some combustion modes. Computational fluid dynamics analysis (CFD) of the hydrogen assisted diesel combustion process captured this trend and reproduced the experimentally observed trends of hydrogen's effect on the composition of NO X for some operating conditions. A model that explicitly accounts for turbulence–chemistry interactions using a transported probability density function (PDF) method was better able to reproduce the experimental trends, compared to a model that ignores the influence of turbulent fluctuations on mean chemical production rates, although the importance of the fluctuations is not as strong as has been reported in some other recent modeling studies. The CFD results confirm that temperature changes alone are not sufficient to explain the observed reduction in NO and increase in NO 2 with increasing H 2. The CFD results are consistent with the hypothesis that in-cylinder HO 2 levels increase with increasing hydrogen, and that the increase in HO 2 enhances the conversion of NO to NO 2. Increased aspiration of hydrogen resulted in PM, and HC emissions which were combustion mode dependent. Predominantly, CO and CO 2 decreased with the increase of hydrogen. The aspiration of hydrogen into the engine modestly decreased fuel economy due to reduced volumetric efficiency from the displacement of air in the cylinder by hydrogen.

Sources and fate of nitrate in the Illinois River Basin, Illinois

Summary We conducted a two-year investigation into the sources and fate of nitrate ( NO 3 - ) in the Illinois River from the Chicago area to the river’s confluence with the Mississippi River. Samples from waterways in the Chicago area (Des Plaines River and the Sanitary and Ship Canal) had relatively high concentrations of nitrogen (N) species and NO 3 - isotopic compositions indicative of treated wastewater (TWW). Downstream of the Brandon Road Lock and Dam, NO 3 - in tributaries discharging to the Illinois River primarily comes from tiles draining row crops. Nitrate isotopic signatures from these tributaries as well as drain tiles were indicative of synthetic fertilizer and/or soil organic matter (SOM) at various stages of denitrification. Nitrate–N concentrations generally decreased in the Illinois River with distance from the Chicago area primarily due to dilution. The decrease in NO3–N concentrations was especially conspicuous during the summer, when there is minimal discharge from drain tiles and NO3–N concentrations in the tributaries were low. In August 2005, when conditions were very dry, NO3–N concentrations decreased from 7.4 mg/L in the Chicago area to less than 1 mg/L near where the Illinois River discharges to the Mississippi River. The isotopic composition of NO 3 - in water samples from the Illinois River were a mixture of three end members: (1) fertilizer and/or SOM in drain tile water, typically showing the least amount of denitrification, (2) fertilizer and/or SOM in deeper ground water, showing the highest degree of denitrification, and (3) TWW. There was seasonal variability, depending on the volume of water flowing in the Illinois River. During high flow periods, river water samples plotted closest to those of tile drain samples; during low flow periods, a greater influence of TWW was observed in the isotopic composition. A subset of summer samples from the Chicago waterways had isotopic values plotting near and within the domain that characterizes manure and sewage. Nitrate in the Chicago area is primarily derived from TWW, with its isotopic signature evident downstream at least as far as Pekin during most of the year and all the way to the Mississippi River during periods of low flow. Denitrification occurs predominantly in groundwater between and away from drain tiles, although there is evidence that in-stream denitrification and/or biological uptake of NO 3 - occurs in the Peoria Lake reach of the Illinois River, at least during periods of low flow in the summer. We calculated that the river was losing about half of its NO3–N load in Peoria Lake in August 2005 (a period of very low flow), at a rate of about 7500 kg/day.

Simplified preparation for the δ15N-analysis in soil [formula omitted] by the denitrifier method

We present a simplification of the denitrifier method proposed by Christensen and Tiedje [1988. Sub-parts-per-billion nitrate method—use of an N 2O-producing denitrifier to convert NO 3 - or ( NO 3 - )-N-15 to N 2O. Applied and Environmental Microbiology 54, 1409–1413] and Sigman et al. [2001. A bacterial method for the nitrogen isotopic analysis of nitrate in seawater and freshwater. Analytical Chemistry 73, 4145–4153] for δ 15N analysis of nitrate ( NO 3 - ) and test the method for 15 NO 3 - analysis in soil extracts. A denitrifying strain without N 2O reductase ( Pseudomonas chlororaphis) was used to quantitatively reduce NO 3 - to nitrous oxide (N 2O), which was analysed for δ 15N on an isotope ratio mass spectrometer (IRMS) with a cryo-concentration unit for N 2O. The produced N 2O was collected in dry bottles, thus enabling storage and transportation of N 2O for later isotope analysis. We show both theoretically and by measurement that sampling N 2O from the headspace above the denitrifier culture does not result in a significant loss of precision and accuracy when using bottles with a headspace to culture ratios above 11 (vol/vol). We further tested the effect of P. chlororaphis cell numbers on the conversion and isotopic composition of NO 3 - ; P. chlororaphis from a 6 day culture effectively converted 1–5 μg NO 3 - within a reasonable time window, thus making tedious concentration of cells unnecessary. When applying the denitrifier method to NO 3 - extracted from soil, caution must be exercised to avoid N 2O conversion to N 2 by indigenous denitrifiers. We found this to be a problem in extracts from a soil with high N 2O reductase activity (induced by anaerobic preincubation with N 2O as the sole electron acceptor). However, a brief heating of the soil extracts to 80 °C prior to the addition of P. chlororaphis cells eliminated any indigenous activity from the samples while having no measurable effect on the isotopic composition of the measured NO 3 - . The method has a precision (SD) better than ±0.2‰ for soil extracts with 1–5 μg NO 3 - -N sample −1.

Solute Sources in Stream Water during Consecutive Fall Storms in a Northern Hardwood Forest Watershed: A Combined Hydrological, Chemical and Isotopic Approach

Understanding the effects of climate change including precipitation patterns has important implications for evaluating the biogeochemical responses of watersheds. We focused on four storms in late summer and early fall that occurred after an exceptionally dry period in 2002. We analyzed not only the influence of these storms on episodic chemistry and the role of different water sources in affecting surface water chemistry, but also the relative contributions of these storms to annual biogeochemical mass balances. The study site was a well studied 135-ha watershed in the Adirondack Park of New York State (USA). Our analyses integrated measurements on hydrology, solute chemistry and the isotopic composition of NO� (d 15 N and d 18 O) and SO2� 4 (d 34 S and d 18 O) to evaluate how these storms affected surface water chemistry. Precipitation amounts varied among the storms (Storm 1: Sept. 14-18, 18.5 mm; Storm 2: Sept. 21-24, 33 mm; Storm 3: Sept. 27-29, 42.9 mm; Storm 4: Oct. 16-21, 67.6 mm). Among the four storms, there was an increase in water yields from 2 to 14%. These water yields were much less than in studies of storms in previous years at this same watershed when antecedent moisture conditions were higher. In the current study, early storms resulted in relatively small changes in water chemistry. With progressive storms the changes in water chemistry became more marked with particularly major changes in Cb (sum of base cations), Si, NO � , and SO 2� 4 , DOC and pH. Analyses of the relationships between Si, DOC, discharge and water table height clearly indicated that there was a decrease in ground water contributions (i.e., lower Si concentrations and higher DOC concentrations) as the watershed wetness increased with storm succession. The marked changes in chemistry were also reflected in changes in the isotopic composition of SO 2� 4 and

Routes of formation and composition of NO x complexes adsorbed on palladium-promoted tungstated zirconia

Surface species obtained during the adsorption of NO and NO/O 2 coadsorption at room temperature on Pd-free (WZ) and Pd-promoted tungstated zirconia (Pd/WZ) are identified by means of in situ FT-IR spectroscopy. The WZ and Pd/WZ samples have a tetragonal structure and contain randomly distributed mesoporous phase. Dispersed palladium(II) species are present in two different environments: (i) Pd 2+ ions, which have only Zr 4+ ions in their second coordination sphere and (ii) Pd 2+ ions, which are linked to both zirconium and tungsten ions via oxygen bridges. On the Pd/WZ sample, NO undergoes oxidation to various NO x species depending on the temperature. The compounds formed at room-temperature oxidation are adsorbed N 2O 3 and products of its self-ionization, NO + and NO 2 −. In this process W(VI) is involved, being reduced to W(IV). At high temperature N 2O 3 decomposes, restoring the W O species. Under these conditions, NO undergoes oxidation to NO 2 by the Pd(II) ions, which are reduced to Pd(I). The nitrosyls of Pd(I) display high thermal stability and do not disappear upon evacuation at 623 K. During NO/O 2 coadsorption on the Pd/WZ catalyst at room temperature, the amounts of surface nitrates and NO 2/N 2O 4 formed in the gas phase are significantly lower than those observed under identical conditions in the presence of tungstated zirconia. It is concluded that promotion of tungstated zirconia with palladium(II) suppresses the oxidation of NO by molecular oxygen at room temperature due to the elimination of acidic protons involved in the process.

Combined use of 15N and 18O of nitrate and 11B to evaluate nitrate contamination in groundwater

Isotopic composition of NO 3 ( δ 15N NO 3 and δ 18O NO 3 ) and B ( δ 11B) were used to evaluate NO 3 contamination and identify geochemical processes occurring in a hydrologically complex Basin and Range valley in northern Nevada with multiple potential sources of NO 3. Combined use of these isotopes may be a useful tool in identifying NO 3 sources because NO 3 and B co-migrate in many environmental settings, their isotopes are fractionated by different environmental processes, and because wastewater and fertilizers may have distinct isotopic signatures for N and B. The principal cause of elevated NO 3 concentrations in residential parts of the study area is wastewater and not natural NO 3 or fertilizers. This is indicated by some samples with elevated NO 3 concentrations plotting along δ 15N NO 3 and NO 3 mixing lines between natural NO 3 from the study area and theoretical septic-system effluent. This conclusion is supported by the presence of caffeine in one sample and the absence of samples with elevated NO 3 concentrations that fall along mixing lines between natural NO 3 and theoretical percolate below fertilized lawns. Nitrogen isotopes alone could not be used to determine NO 3 sources in several wells because denitrification blurred the original isotopic signatures. The range of δ 11B values in native ground water in the study area (−8.2‰ to +21.2‰) is large. The samples with the low δ 11B values have a geochemical signature characteristic of hydrothermal systems. Physical and chemical data suggest B is not being strongly fractionated by adsorption onto clays. δ 11B values from local STP effluent (−2.7‰) and wash water from a domestic washing machine (−5.7‰) were used to plot mixing lines between wastewater and native ground water. In general, wells with elevated NO 3 concentrations fell along mixing lines between wastewater and background water on plots of δ 11B against 1/B and Cl/B. Combined use of δ 15N and δ 11B in the study area was generally successful in identifying contaminant sources and processes that are occurring, however, it is likely to be more successful in simpler settings with a well-characterized δ 11B value for background wells.

Fraction and composition of NOy transported in air masses lofted from the North American continental boundary layer

Five field studies have included research aircraft flights over the continental United States and the western North Atlantic Ocean from 1996 through 2000 in spring, summer, and fall seasons. The major source of NOx in this region is fossil fuel combustion, which is localized within the continental boundary layer (CBL). We use CO as a tracer of these anthropogenic emissions to estimate the fraction of the emitted NOx that is exported to the free troposphere (FT), either as NOx itself or as its oxidation products. This export was identified as plumes enhanced in CO above an estimated background by at least 30 ppbv, which account for 20–31% of the air parcels sampled in the FT during the five field studies. These plumes were encountered throughout the FT up to the 8 km ceiling of the aircraft but were primarily located just above the CBL with average altitudes of 3.0–4.1 km above ground level. In the summer over the continent, only 20 ± 5% of the originally emitted nitrogen oxides was transported in those plumes. This fraction is in reasonable accord with model results, but the models include only deep convection and not the shallow CBL venting mechanisms responsible for the observed plumes. During the two field studies in the early fall and in the spring over the western North Atlantic, we find that 9 ± 4% of the NOy was transported, although Li et al. [2004] suggest that this is an underestimate and that 15 ± 11% is more accurate. Both of these numbers indicate that model results in the literature overestimate the amount of NOy transported from the CBL to the FT. In these five field studies, HNO3 generally accounted for one‐half to two‐thirds of the NOy, which is in contrast to the dominance by NOx and organic nitrates suggested by models. Over the North Atlantic, this difference is likely due to further photochemical processing of the NOy species within the FT and over the continent due to the different transport mechanism considered in the models.

Emission of gaseous nitrogen oxides from an extensively managed grassland in NE Bavaria, Germany - II. Stable isotope natural abundance of N2O

We analysed the stable isotope composition of emitted N2O in a one-year field experiment (June 1998 to April 1999) in unfertilized controls, and after adding nitrogen by applying slurry or mineral N (calcium ammonium nitrate). Emitted N2O was analysed every 2–4 weeks, with additional daily sampling for 10 days after each fertilizer application. In supplementary soil incubations, the isotopic composition of N2O was measured under defined conditions, favouring either denitrification or nitrification. Soil incubated for 48 h under conditions favouring nitrification emitted very little N2O (0.024 μmol gdw−1) and still produced N2O from denitrification. Under denitrifying incubation conditions, much more N2O was formed (0.91 μmol gdw−1 after 48 h). The isotope ratios of N2O emitted from denitrification stabilized at δ15N = −40.8 ± 5.7‰ and δ18O = 2.7 ± 6.3‰. In the field experiment, the N2O isotope data showed no clear seasonal trends or treatment effects. Annual means weighted by time and emission rate were δ15N = −8.6‰ and δ18O = 34.7‰ after slurry application, δ15N = −4.6‰ and δ18O = 24.0‰ after mineral fertilizer application and δ15N = −6.4‰ and δ18O = 35.6‰ in the control plots, respectively. So, in all treatments the emitted N2O was 15N-depleted compared to ambient air N2O (δ15N = 11.4 ± 11.6‰, δ18O = 36.9 ± 10.7‰). Isotope analyses of the emitted N2O under field conditions per se allowed no unequivocal identification of the main N2O producing process. However, additional data on soil conditions and from laboratory experiments point to denitrification as the predominant N2O source. We concluded (1) that the isotope ratios of N2O emitted from the field soil were not only influenced by the source processes, but also by microbial reduction of N2O to N2 and (2) that N2O emission rates had to exceed 3.4 μmol N2O m−2 h−1 to obtain reliable N2O isotope data.

Determination of the sources of nitrate contamination in karst springs using isotopic and chemical indicators

The sources of nitrate (NO 3 −) in groundwater of the shallow karst aquifer in southwestern Illinois' sinkhole plain were investigated using chemical and isotopic techniques. The groundwater in this aquifer is an important source of potable water for about half of the residents of the sinkhole plain area. Previous work has shown that groundwater from approximately 18% of the wells in the sinkhole plain has NO 3 − concentrations in excess of the USEPA's drinking water standard of 10 mg N/l. Relative to background levels, the NO 3 − concentrations in water from 52% of the wells, and probably all of the springs in the study area, are anomalously high, suggesting that sources other than naturally occurring soil organic matter have contributed additional NO 3 − to groundwater in the shallow karst aquifer. This information, and the dominance of agriculture in the study area, suggest that agrichemical contributions may be significant. To test this hypothesis, water samples from 10 relatively large karst springs were collected during four different seasons and analyzed for inorganic constituents, dissolved organic carbon, atrazine, and δ 15N and δ 18O of the NO 3 − ions. The isotopic data were most definitive and suggested that the sources of NO 3 − in spring water are dominated by N-fertilizer with some possible influence of atmospheric NO 3 − and, to a much lesser extent, human and/or animal waste. Differences in the isotopic composition of NO 3 − and some of the chemical characteristics were observed during the four consecutive seasons in which spring water samples were collected. Isotopic values for δ 15N and δ 18O of the NO 3 − ranged from 3.2‰ to 19.1‰ and from 7.2‰ to 18.7‰, respectively. The trend of δ 15N and δ 18O data for NO 3 − also indicated that a significant degree of denitrification is occurring in the shallow karst hydrologic system (within the soil zone, the epikarst and the shallow karst aquifer) prior to discharging to springs.

Isotope identification in NO as a chemical tracer in the middle atmosphere

The nitrogen isotope ratio of middle atmosphere nitrogen oxide is predicted as a function of altitude. Nitrogen oxides originate photochemically either from stratospheric nitrous oxide reacting with O(1D) or in the mesosphere and thermosphere from direct dissociation of N2 and ionization-initiated reactions involving O2 and N2. During its formation process, N2O acquires a nitrogen isotopic composition of N isotopes different than N2. Photodissociation within the stratosphere also modifies the proportion of isotopes. Reaction of stratospheric NO with O3 produces NO2, which when photodissociated yields NO depleted in 15N relative to NO2 in laboratory air. The value of δ15NO in the stratosphere is −100‰. In the altitude region between 50 and 65 km, NO is transformed into NO2 and then returned to NO by reaction of NO2 with O and by NO2 photodissociation. These reactions determine the isotopic makeup of NO. Above 65 km, nitric oxide is produced by local ionization processes and gas phase photochemical reactions involving N2 and excited O2. These processes determine the isotopic composition of NO in the upper mesosphere and thermosphere. Here δ15NO is 0‰. Air transported into the mesosphere above 65 km will reflect the NO isotopic values of the region below, while mesospheric NO transported below 65 km will not be distinguishable from NO originating in the stratosphere.