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.     

Methane and nitrous oxide concentration and emission flux of Yangtze Delta plain river net

Methane (CH4) and nitrous oxide (N2O) saturation concentration and gas-water interface emission flux in surface water of the Yangtze Delta plain river net were investigated in summer at representative sites including the upper reaches of the Huangpu River and the rivers in the Chongming Island. The results show that the CH4 concentration in river water ranged from 0.30±0.03 to 6.66±0.14 μmol.L-1, and N2O concentration ranged from 13.8±2.33 to 435±116 nmol.L-1. River surface water had a very high satura- tio...     

A fully automatic system for underway N2O measurements based on cavity ring-down spectroscopy

N₂O is one of the most important greenhouse and ozone-depleting gases and has been the source of considerable concern in recent years. The oceans account for ~ 1/4 of the global N₂O emission budget; however, the oceanic N₂O source/sink characteristics are not well understood. To enhance the study of oceanic N₂O source/sink characteristics, our laboratory developed a fully automatic underway system for surface water N₂O concentration and atmospheric N₂O mole fraction measurements consisting of a cavity ring-down spectroscopy (CRDS) instrument and an upstream device. The developed device can be programmed to switch the CRDS measurements from the equilibrator headspace to the atmospheric sample and the reference gas sample. The surface water N₂O concentration is calculated from the equilibrium headspace N₂O mole fraction in the equilibrator. The response time of this equilibrator is ~ 3.4 min, and the estimated precision of this method for surface water N₂O measurements is better than 0.5% (relative standard deviation, RSD), which is one order of magnitude better than that of traditional gas chromatographic methods and can be further optimised. Data are acquired every 20 s, and the calibration frequency requirement of this system is approximately 7–10 days. This labor-saving underway system is a powerful tool for high-precision and high-resolution measurements of atmospheric and oceanic N₂O and can significantly improve the study of the characteristics of oceanic N₂O sources/sinks and their response to climate change.     

FFT analysis of atmospheric trace concentration of N2O continuously monitored by gas chromatography and cross-correlation to climate parameters

To investigate the characteristics of N₂O concentration, we applied several types of time series analyses such as fast Fourier transform (FFT), auto-correlation, and cross-correlation, to 2.5-year time series data of trace N₂O concentration continuously monitored by gas chromatography and meteorological data, measured in an urban area of Nagoya. It was found that there is a positive correlation between atmospheric N₂O concentration (ppbv) and, both steam pressure (hPa) and temperature (°C). In addition, negative and positive correlations in atmospheric pressure and in solar flux were also found, respectively. These findings suggest an enrichment of N₂O through environmental steam during the summer season, particularly in urban areas. On the other hand, the correlation to wind direction shows a variation with amplitude of 7 ppbv, from the north-west to the south-east, and a seasonal variation up to 12 ppbv, from winter to summer. These results support the hypothesis that atmospheric steam controls the N₂O concentration in urban areas. In addition, the correlation with wind direction suggests the existence of an emission source in the direction of seaside areas.     

N2O and NO emissions during wastewater denitrification step: Influence of temperature on the biological process

The denitrification process occurring in wastewater treatment plants (WWTPs) is responsible for nitrous oxide (N₂O) and nitric oxide (NO) emissions. These compounds indirectly lead to the global warming. In this study, we investigated the impact of the temperature on N₂O and NO emissions. Experiments were achieved at PH 7 in a batch reactor with acetate as the carbon source. The nitrogen source was nitrates (NO₃⁻) and the COD/N ratio was set to three. Results showed that NO and N₂O emissions increased when the temperature decreased. NO emissions appeared only at 10 °C and 5 °C, with respectively 8% and 18% of the total denitrified nitrogen. N₂O emissions increased from 13 to 40 then 82% of the total denitrified nitrogen, respectively at 20, 10 and 5 °C. Several hypotheses were suggested to explain these results: a general enzymatic slow down, enzymatic inhibitions, electron donor competition between the different enzymes and metabolic pathway alterations.     

An evaluation of the mechanism of nitrous acid formation in the urban atmosphere

Nitrous acid (HONO) has been observed to build in the atmosphere of cities during the nighttime hours and it is suspected that photolysis of HONO may be a significant source of HO radicals early in the day. The sources of HONO are poorly understood, making it difficult to account for nighttime HONO formation in photochemical modeling studies of urban atmospheres, such as modeling of urban O₃ formation. This paper reviews the available information on measurements of HONO in the atmosphere and suggest mechanisms of HONO formation. The most extensive atmospheric measurement databases are used to investigate the relations between HONO and potential precursors. Based on these analyses, the nighttime HONO concentrations are found to correlate best with the product of NO, NO₂ and H₂O concentrations, or possibly the NO, NO₂, H₂O, and aerosol concentrations. A new mechanism for nighttime HONO formation is proposed that is consistent with this precursor relationship, namely, reaction of N₂O₃ with moist aerosols (or other surfaces) to form two HONO molecules. Theoretical considerations of the equilibrium constant for N₂O₃ formation and the theory of gas-particle reactions show that the proposed reaction is a plausible candidate for HONO formation in urban atmospheres. For photochemical modeling purposes, a relation is derived in terms of gas phase species only (i.e., excluding the aerosol concentration): NO + NO₂ + H₂O → 2 HONO with a rate constant of 1.68 x 10^-17 e^6348/T (ppm^-2 min^-1). This rate constant is based on an analysis of ambient measurements of HONO, NO, NO₂ and H₂O, with a temperature dependence from the equilibrium constant for formation of N₂O₃. Photochemical grid modeling is used to investigate the effects of this relation on simulated HONO and O₃ concentrations in Los Angeles, and the results are compared to two alternative sources of nighttime HONO that have been used by modelers. Modeling results show that the proposed relation results in HONO concentrations consistent with ambient measurements. Furthermore, the relation represents a conservative modeling approach because HONO production is effectively confined to the model surface layers in the nighttime hours, the time and place for which ambient data exist to show that HONO formation occurs. The empirical relation derived here should provide a useful tool for modelers until such time as knowledge of the HONO forming mechanisms has improved and more quantitative relations can be derived.     

Some aspects of plasma copolymerization of acetylene with N2 and/or water

Plasma polymerizations of mixture of acetylene-N₂, acetylene-H₂O, were investigated by using an electrodeless glow discharge from a 13.5-MHz radiofrequency source. Properties of plasma polymers were examined as functions of mole ratios of N₂ and/or H₂O to acetylene. The concentration of trapped free radicals and the internal stress in a plasma polymer decreased as the mole ratio of N₂ or H₂O increased. Water showed the most pronounced effects in those properties at the mole ratio of 0.3. Gas permeabilities increased by the copolymerization of N₂ and/or H₂O. Surface energies were also investigated by analysis of contact angles of liquids. Copolymerization of N₂ caused a remarkable increase in polar contribution of surface energy. Some fundamental aspects of flow-rate pressure relationship of mixed gases are presented.     

Rotating Gliding Arc Assisted Water Splitting in Atmospheric Nitrogen

In this study, hydrogen production from water splitting in N₂ using an atmospheric pressure rotating gliding arc plasma was investigated. The effect of input H₂O concentration and total flow rate on the performance of the plasma water splitting process (e.g., H₂ and O₂ yield, H₂ production rate, and energy yield of H₂) was investigated. N₂ showed a pronouncedly facilitating effect on the H₂O splitting and H₂ production process due to the reactions of the excited N₂ species [e.g., electronically excited metastable N₂(A)] with the H₂O molecules. The maximum H₂ production rate reached up to 41.3 μmols^?1, which is much higher than that of other typical non-thermal plasmas (e.g., ~0.2 μmols^?1 for a dielectric barrier discharge). Optical emission diagnostics has shown that in addition to the NO, N₂, and N₂ ⁺ that were observed in the pure N₂ spectra, strong OH and NH emission lines also appeared in the H₂O/N₂ spectra. OH radical is considered as a key intermediate species that could contribute to the formation of H₂, O₂, and H₂O₂. The increase of the H₂O concentration could lead to a continuous enhancement of the OH intensity. The rotational temperature of N₂ ⁺ dropped drastically from 2875 ± 125 to 1725 ± 25 K with the addition of 1 % (mol/mol) H₂O into the N₂ plasma.     

Nitrous oxide emissions from waste incineration

EU energy and environmental policy in waste management leads to increasing interest in developing methods for waste disposal with minimum emissions of greenhouse gases and minimum environmental impacts. From the point of view of nitrous oxide (N₂O) emissions, waste incineration and waste co-combustion is very acceptable method of waste disposal. Two factors are important for attaining very low N₂O emissions from waste incineration, particularly for waste with higher nitrogen content (e.g. sewage sludge, leather, etc.): temperature of incineration over 900°C and avoiding selective noncatalytic reduction (SNCR) de-NO_x method based on urea. For reduction of N₂O emissions retrofitting such plants to ammonia-based SNCR is recommendable. The modern selective catalytic reduction facilities for de-NO_x at waste incineration plants are only negligible source of N₂O.     

Heterogeneous photocatalytic reduction of NO in the presence of conjugated polymers

The reaction of NO with NH₃ is photocatalytic in the presence of poly(p-phenylene), polythiocyanogen and TiO₂, and leads to high N₂/N₂O ratios. The reduction of NO in the presence of Cu-containing paracyanogen or Cu is not a photocatalytic reaction, and yields relatively low N₂/N₂O 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 (N₂O) it is essential to understand the biochemical pathways by which it is produced. The ¹⁸O signature of N₂O is increasingly used to characterize these processes. However, assumptions on the origin of the O atom and resultant isotopic composition of N₂O that are based on reaction stoichiometry may be questioned. In particular, our deficient knowledge on O exchange between H₂O and nitrogen oxides during N₂O production complicates the interpretation of the ¹⁸O signature of N₂O. Here we studied O exchange during N₂O formation in soil, using a novel combination of ¹⁸O and ¹⁵N 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 ¹⁸O‐enriched H₂O into N₂O exceeded their maxima achievable through reaction stoichiometry. Based on the retention of the enrichment ratio of ¹⁸O and ¹⁵N of NO into N₂O, we quantified O exchange during denitrification. Up to 97% (median 85%) of the N₂O‐O originated from H₂O instead of from the denitrification substrate NO. We conclude that in soil, the main source of atmospheric N₂O, the ¹⁸O signature of N₂O is mainly determined by H₂O due to O exchange between nitrogen oxides and H₂O. This also challenges the assumption that the O of N₂O originates from O₂ and NO, in ratios reflecting reaction stoichiometry. Copyright © 2008 John Wiley & Sons, Ltd.     

废水生物脱氮工艺中N2O排放数学模型研究进展

N_2O是一种重要的温室气体,而污水生物脱氮处理过程是N_2O的潜在产生源之一。随着污水处理量和处理程度的不断提高,N_2O的排放量也将不断增大。建立N_2O排放数学模型对污水生物脱氮系统中N_2O生成机制的深入研究和污水处理行业N_2O削减工艺技术的开发具有重大的理论及实践意义。本文归纳了生物脱氮工艺的原理,系统阐述了生物脱氮工艺中N_2O的生成机理和排放数学模型的类型、建立方法及应用情况。在此基础上,对生物脱氮工艺中N_2O排放数学模型的研究现状和研究方向进行了总结和展望,以期为N_2O排放数学模型的完善、N_2O排放量的削减、污水生物脱氮工艺的优化及污水处理行业的可持续发展提供理论基础和科学依据。     

Isotopologue fractionation during N2O production by fungal denitrification

Identifying the importance of fungi to nitrous oxide (N₂O) production requires a non‐intrusive method for differentiating between fungal and bacterial N₂O production such as natural abundance stable isotopes. We compare the isotopologue composition of N₂O produced during nitrite reduction by the fungal denitrifiers Fusarium oxysporum and Cylindrocarpon tonkinense with published data for N₂O production during bacterial nitrification and denitrification. The fractionation factors for bulk nitrogen isotope values for fungal denitrification were in the range −74.7 to −6.6‰. There was an inverse relationship between the absolute value of the fractionation factors and the reaction rate constant. We interpret this in terms of variation in the relative importance of the rate constants for diffusion and enzymatic reduction in controlling the net isotope effect for N₂O production during fungal denitrification. Over the course of nitrite reduction, the δ¹⁸O values for N₂O remained constant and did not exhibit a relationship with the concentration characteristic of an isotope effect. This probably reflects isotopic exchange with water. Similar to the δ¹⁸O data, the site preference (SP; the difference in δ¹⁵N between the central and outer N atoms in N₂O) was unrelated to concentration during nitrite reduction and, therefore, has the potential to act as a conservative tracer of production from fungal denitrification. The SP values of N₂O produced by F. oxysporum and C. tonkinense were 37.1 ± 2.5‰ and 36.9 ± 2.8‰, respectively. These SP values are similar to those obtained in pure culture studies of bacterial nitrification but quite distinct from SP values for bacterial denitrification. The large magnitude of the bulk nitrogen isotope fractionation and the δ¹⁸O values associated with fungal denitrification are distinct from bacterial production pathways; thus multiple isotopologue data holds much promise for resolving bacterial and fungal production. Our work further provides insight into the role that fungal and bacterial nitric oxide reductases have in determining site preference during N₂O production. Copyright © 2008 John Wiley & Sons, Ltd.     

What can we learn from N2O isotope data? – Analytics,processes and modelling

The isotopic composition of nitrous oxide (N₂O) provides useful information for evaluating N₂O sources and budgets. Due to the co-occurrence of multiple N₂O transformation pathways, it is, however, challenging to use isotopic information to quantify the contribution of distinct processes across variable spatiotemporal scales. Here, we present an overview of recent progress in N₂O isotopic studies and provide suggestions for future research, mainly focusing on: analytical techniques; production and consumption processes; and interpretation and modelling approaches. Comparing isotope-ratio mass spectrometry (IRMS) with laser absorption spectroscopy (LAS), we conclude that IRMS is a precise technique for laboratory analysis of N₂O isotopes, while LAS is more suitable for in situ/inline studies and offers advantages for site-specific analyses. When reviewing the link between the N₂O isotopic composition and underlying mechanisms/processes, we find that, at the molecular scale, the specific enzymes and mechanisms involved determine isotopic fractionation effects. In contrast, at plot-to-global scales, mixing of N₂O derived from different processes and their isotopic variability must be considered. We also find that dual isotope plots are effective for semi-quantitative attribution of co-occurring N₂O production and reduction processes. More recently, process-based N₂O isotopic models have been developed for natural abundance and ¹⁵N-tracing studies, and have been shown to be effective, particularly for data with adequate temporal resolution. Despite the significant progress made over the last decade, there is still great need and potential for future work, including development of analytical techniques, reference materials and inter-laboratory comparisons, further exploration of N₂O formation and destruction mechanisms, more observations across scales, and design and validation of interpretation and modelling approaches. Synthesizing all these efforts, we are confident that the N₂O isotope community will continue to advance our understanding of N₂O transformation processes in all spheres of the Earth, and in turn to gain improved constraints on regional and global budgets.     

Radiolytic degradation of malathion and lindane in aqueous solutions

Degradation of malathion and lindane pesticides present in an aqueous solution was investigated on a laboratory scale upon gamma-irradiation from a ⁶⁰Co source. The effects of pesticide group, presence of various additives and absorbed dose on efficiency of pesticide degradation were investigated. Gamma-irradiation was carried out in distilled water solutions (malathion and lindane) and in combination with humic solution (HS), nitrous oxide (N₂O) and HS/N₂O (lindane) over the range 0.1–2 kGy (malathion) and 5–30 kGy (lindane). Malathion was easily degraded at low absorbed doses compared to lindane in distilled water solutions. Absorbed doses required to remove 50% and 90% of initial malathion and lindane concentrations in distilled water solutions were 0.53 and 1.77 kGy (malathion) and 17.97 and 28.79 kGy (lindane), respectively. The presence of HS, N₂O and HS/N₂O additives in aqueous solutions, significantly improved the effectiveness of radiolytic degradation of lindane. Chemical analysis of the pesticides and the by-products resulted from the radiolytic degradation were made using a gas chromatography associated with mass spectrometry (GC–MS). Additionally, the final degradation products of irradiation as detected by ion chromatography (IC) were acetic acid and traces of some anions (phosphate and chloride).     

Isotopologue enrichment factors of N2O reduction in soils

Isotopic signatures can be used to study sink and source processes of N₂O, but the success of this approach is limited by insufficient knowledge on the isotope fractionation factors of the various reaction pathways. We investigated isotope enrichment factors of the N₂O‐to‐N₂ step of denitrification (ε) in two arable soils, a silt‐loam Haplic Luvisol and a sandy Gleyic Podzol. In addition to the ε of ¹⁸O (ε_18O) and of average ¹⁵N (ε_bulk), the ε of the ¹⁵N site preference within the linear N₂O molecule (ε_SP) was also determined. Soils were anaerobically incubated in gas‐tight bottles with N₂O added to the headspace to induce N₂O reduction. Pre‐treatment included the removal of NO to prevent N₂O production. Gas samples were collected regularly to determine the dynamics of N₂O reduction, the time course of the isotopic signatures of residual N₂O, and the associated isotope enrichment factors. To vary reduction rates and associated fractionation factors, several treatments were established including two levels of initial N₂O concentration and anaerobic pre‐incubation with or without addition of N₂O. N₂O reduction rates were affected by the soil type and initial N₂O concentration. The ε_18O and ε_bulk ranged between ?13 and ?20‰, and between ?5 and ?9‰, respectively. Both quantities were more negative in the Gleyic Podzol. The ε of the central N position (ε_α) was always larger than that of the peripheral N‐position (ε_β), giving ε_SP of ?4 to ?8‰. The ranges and variation patterns of ε were comparable with those from previous static incubation studies with soils. Moreover, we found a relatively constant ratio between ε_18O and ε_bulk which is close to the default ratio of 2.5 that had been previously suggested. The fact that different soils exhibited comparable ε under certain conditions suggests that these values could serve to identify N₂O reduction from the isotopic fingerprints of N₂O emitted from any soil. Copyright © 2009 John Wiley & Sons, Ltd.     

Effect of Water Vapor on Benzene Decomposition Using a Nonthermal-Discharge Plasma Reactor

The effect of water vapor on benzene decomposition in air was investigatedusing a nonthermal-discharge plasma reactor packed with ferroelectricmaterials. The conversion of benzene was found to decrease with an increaseof water concentration. On the other hand, the selectivity to CO₂ in thedecomposition products increased with an increase of water concentration. Acomparison between the benzene conversion to CO and CO₂ suggested that COformation was suppressed by water to a greater extent than was CO₂formation. N₂O formation also decreased with an increase of waterconcentration. These results suggest that the activity of oxygen speciesresponsible for the formation of CO and N₂O is reduced by water.     

Determination of Sulfamerazine in River Water Using Thermoresponsive Modified Silica for Solid-Phase Extraction with High-Performance Liquid Chromatography Detection

Four thermoresponsive silica-poly(N-isopropylacrylamide-co-butyl methacrylate) materials were prepared by grafting (N-isopropylacrylamide-co-butyl methacrylate) at different ratios on multimodal porous silica via surface-initiated atom transfer radical polymerization. The thermoresponsive materials were employed as the adsorbent for the rapid determination of sulfamerazine in river water by solid-phase extraction. The properties of silica-poly(N-isopropylacrylamide-co-butyl methacrylate) were characterized by scanning electron microscopy and Fourier transform infrared spectroscopy. Static adsorption measurements showed that the silica-poly(N-isopropylacrylamide-co-butyl methacrylate)₃ material had the highest adsorption characteristics (8.72?mg?g^?1) at 35°C. The solid-phase extraction conditions were optimized, including the elution solvent and its volume used. The thermoresponsive silica-poly(N-isopropylacrylamide-co-butyl methacrylate)₃ material provided satisfactory results for solid-phase extraction, with a recovery of 90.06%, allowing the rapid purification of sulfamerazine in river water.     

Chemical Reactivity on Gas‐Phase Metal Clusters Driven by Blackbody Infrared Radiation

We report the observation of chemical reactions in gas‐phase Rh_n(N₂O)_m⁺ complexes driven by absorption of blackbody radiation. The experiments are performed under collision‐free conditions in a Fourier transform ion cyclotron resonance mass spectrometer. Mid‐infrared absorption by the molecularly adsorbed N₂O moieties promotes a small fraction of the cluster distribution sufficiently to drive the N₂O decomposition reaction, leading to the production of cluster oxides and the release of molecular nitrogen. N₂O decomposition competes with molecular desorption and the branching ratios for the two processes show marked size effects, reflecting variations in the relative barriers. The rate of decay is shown to scale approximately linearly with the number of infrared chromophores. The experimental findings are interpreted in terms of calculated infrared absorption rates assuming a sudden‐death limit.     

Slow proton exchange in water in the gas phase

To study proton exchange in water the technique of quantitative measurements of water content at very low concentrations of water in solutions or at low vapor pressure in the gas phase (for water content of 1–10 μg/ml) has been developed. Under these conditions the rate of proton exchange slows down significantly. In particular, in mixtures of H₂O and D₂O vapors under the pressure of a 10–17 mm Hg proton life-time during the exchange process reaches the values of 10–30 min. The lifetime was measured using the gradual slow growth of the signal intensity of the mixed isotopomer HDO.