N2O emission from cropland in China

Based on the regionalization of uplands and paddy fields in China, the crop intensity in each region and the available field measurements, N₂O emission from cropland in China in 1995 was estimated to be 398 Gg N, in which, 310 Gg N was from uplands, accounting for 78% of the total. 88 Gg N–N₂O was emitted from paddy fields with 35 Gg N emitted during the rice growing season and 53 Gg N emitted during upland crop season. N₂O emission from upland and from paddy field during upland land crop season accounted for 91% of the total emission.     

Catalytic reduction of NH4NO3 by NO: Effects of solid acids and implications for low temperature DeNOx processes

Ammonium nitrate is thermally stable below 250 °C and could potentially deactivate low temperature NO_x reduction catalysts by blocking active sites. It is shown that NO reduces neat NH₄NO₃ above its 170 °C melting point, while acidic solids catalyze this reaction even at temperatures below 100 °C. NO₂, a product of the reduction, can dimerize and then dissociate in molten NH₄NO₃ to NO⁺ + NO₃⁻, and may be stabilized within the melt as either an adduct or as HNO₂ formed from the hydrolysis of NO⁺ or N₂O₄. The other product of reduction, NH₄NO₂, readily decomposes at ≤100 °C to N₂ and H₂O, the desired end products of DeNO_x catalysis. A mechanism for the acid catalyzed reduction of NH₄NO₃ by NO is proposed, with HNO₃ as an intermediate. These findings indicate that the use of acidic catalysts or promoters in DeNO_x systems could help mitigate catalyst deactivation at low operating temperatures (<150 °C).     

A kinetic study of NOx reduction over Pt/SiO2 model catalysts with hydrogen as the reducing agent

Flow reactor experiments and kinetic modeling have been performed in order to study the mechanism and kinetics of NO_x reduction over Pt/SiO₂ catalysts with hydrogen as the reducing agent. The experimental results from NO oxidation and reduction cycles showed that N₂O and NH₃ are formed when NO_x is reduced with H₂. The NH₃ formation depends on the H₂ concentration and the selectivity to NH₃ and N₂O is temperature dependent. A previous model has been used to simulate NO oxidation and a mechanism for NO_x reduction is proposed, which describes the formation/consumption of N₂, H₂O, NO, NO₂, N₂O, NH₃, O₂ and H₂. A good agreement was found between the performed experiments and the model.     

Modeling bio-atmospheric coupling of the nitrogen cycle through NOx emissions and NOy deposition

The tropospheric and terrestrial nitrogen cycles are connected to one another through the emissions of NO_x and NH_x from soils and vegetation and the subsequent redeposition of these compounds and their products elsewhere. These connections play an important role in the Earth system influencing tropospheric concentrations of NO_x, O₃, and CO₂. Estimates of the biogenic sources of NO_x, soil emissions and biomass burning, are amongst the most variable terms in the global budget of NO_x and are eclipsed only by lightning. A 3-D chemistry transport model, IMAGES, was used to examine how soil emissions and biomass burning influence tropospheric concentrations of NO_x and O₃ as well as NO_y deposition. Soil and biomass burning emissions of NO_x contributed the most to atmospheric NO_x concentrations closest to the surface and south of 30°N. The influence of these emissions on tropospheric O₃ and NO_x concentrations dissipated with height suggesting that these surface emissions are most important to surface ozone concentrations. The removal of either the soil or biomass burning source resulted in a 5-20% difference in tropospheric O₃ concentrations over large regions of the atmosphere. Both sources are also important contributors to N deposition, particularly south of 30°N which, in turn, can generate significant carbon storage. These exercises demonstrate both the importance and complexity of the connections between atmospheric chemistry and the terrestrial biosphere.     

Nitrous oxide formation potential of various humid tropic soils of Malaysia: A laboratory study

Nitrous oxide (N₂O) is formed mainly during nitrification and denitrification. Inherent soil properties strongly influence the magnitude of N₂O formation and vary with soil types. A laboratory study was carried out using eight humid tropic soils of Malaysia to monitor NH₄ ⁺ and NO₃ ^– dynamics and N₂O production. The soils were treated with NH₄NO₃ (100 mg N kg^–1 soil) and incubated for 40 days at 60% water-filled pore space. The NH₄ ⁺ accumulation was predominant in the acid soils studied and NO₃ ^– accumulation/disappearance was either small or stable. However, the Munchong soil depicted the highest peak (238 g N₂O-N kg^–1 soil d^–1) at the beginning of the incubation, probably through a physical release. While the Tavy soil showed some NO₃ ^– accumulation at the end of the study with a maximum N₂O flux of 206 g N₂O-N kg^–1 soil d^–1, both belong to Oxisols. The other six soils, viz. Rengam, Selangor, Briah, Bungor, Serdang and Malacca series, formed smaller but maximum peaks in an decreasing order of 116 to 36 g N₂O-N kg^–1 soil d^–1. Liming the Oxisols and Ultisols raised the soil pH, resulting in NO₃ ^– accumulation and N₂O production to some extent. As such the highest N₂O flux of 130.2 and 77.4 g N₂O-N kg^–1 soil d^–1 was detected from the Bungor and Malacca soils, respectively. The Selangor soil, belonging to Inceptisol, did not respond to lime treatment. The respective total N₂O formations were 3.63, 1.92 and 1.69 mg N₂O-N kg^–1 soil from the Bungor, Malacca and Selangor soils, showing an increase by 49 and 99% over the former two non-limed soils. Under non-limed conditions, the indigenous soil properties, viz. Ca⁺⁺ content, %clay, %sand and pH of the soils collectively could have influenced the total N₂O formation.     

Dynamics and selectivity of NOx reduction in NOx storage catalytic monolith

Several nitrogen compounds can be produced during the regeneration phase in periodically operated NO_x storage and reduction catalyst (NSRC) for conversion of automobile exhaust gases. Besides the main product N₂, also NO, N₂O, and NH₃ can be formed, depending on the regeneration phase length, temperature, and gas composition. This contribution focuses on experimental evaluation of the NO_x reduction dynamics and selectivity towards the main products (NO, N₂ and NH₃) within the short rich phase, and consequent development of the corresponding global reaction-kinetic model. An industrial NSRC monolith sample of PtRh/Ba/CeO₂/ -Al₂O₃ type is employed in nearly isothermal laboratory micro-reactor. The oxygen and NO_x storage/reduction experiments are performed in the temperature range 100–500 °C in the presence of CO₂ and H₂O, using H₂, CO and C₃H₆ as the reducing agents.The spatially distributed NSRC model developed earlier is extended by the following reactions: NH₃ is formed by the reaction of H₂ with NO_x and it can further react with oxygen and NO_x deposited on the catalyst surface, producing N₂. Considering this scheme with ammonia as an active intermediate of the NO_x reduction, a good agreement with experiments is obtained in terms of the NO_x reduction dynamics and selectivity. A reduction front travelling in the flow direction along the reactor is predicted, with the NH₃ maximum on the moving boundary. When the front reaches the reactor outlet, the NH₃ peak is observed in the exhaust gas. It is assumed that the ammonia formation during the NO_x reduction by CO and HCs at higher temperatures proceed via the water gas shift and steam reforming reactions producing hydrogen. It is further demonstrated that oxygen storage effects influence the dynamics of the stored NO_x reduction. The temperature dependences of the outlet ammonia peak delay and the selectivity towards NH₃ are correlated with the effective oxygen and NO_x storage capacity.     

Assessment of trace gases,carbon and nitrogen emissions from field burning of agricultural residues in India

Field burning of crop residue (FBCR) is becoming a growing environmental concern in developing countries. In this instance, a comprehensive crop-wise and spatially distributed study on the FBCR emissions from India for the period 1980 through 2010 have been undertaken, that covers: residue generation, its types, use pattern, and estimates of carbon, nitrogen, CH₄, CO, N₂O and NO_X emissions; along with associated uncertainties. FBCR contributed about 44 and 14% of the non-biofuel biomass and total biomass burning, respectively in India in the year 2000. The total dry residue generated are estimated as 217, 239 and 253 Tg, of which 45, 60 and 63 Tg of dry biomass are estimated to be subjected to FBCR in the years 1994, 2005 and 2010, respectively. Wheat and rice crops together accounted for about 76% of this. Burning of such huge amount of biomass is estimated to emit 22.4, 24.4 and 26.1 Tg of carbon; 0.30, 0.33 and 0.35 Tg of nitrogen; 4.18, 4.59 and 4.86 Tg carbon dioxide equivalent of greenhouse gases (GHG, viz., CH₄ and N₂O; which is over 1% of the Indian agriculture sector GHG emissions); 2951, 3,240 and 3,431 Gg of CO; and 120.8, 132.9 and 140.6 Gg NO_x emissions in 1994, 2005 and 2010, respectively. Further, the Indian states of U.P, Punjab, Haryana, M.P, Maharashtra, T.N, Karnataka, Andhra Pradesh, Bihar and W.B have been found to contribute maximum to the Indian FBCR emissions. FBCR avoidance and optimum utilization of crop-residue resource is urgently required for agro-ecosystem sustainability in the region.     

Nitrous oxide emissions for 6 years from a gray lowland soil cultivated with onions in Hokkaido, Japan

We studied nitrous oxide (N₂O) emissions every growing season (April to October) for 6 years (19952000), in a Gray Lowland soil cultivated with onions in central Hokkaido, Japan. Emission of N₂O from the onion field ranged from 0.00 to 1.86 mgN m^–2 h^–1. The seasonal pattern of N₂O emission was the same for 6 years. The largest N₂O emissions appeared near harvesting in August to October, and not, as might be expected, just after fertilization in May. The seasonal patterns of soil nitrate (NO₃ ^–) and, ammonium (NH₄ ⁺) levels and the ratio of N₂O to NO emission indicated that the main process of N₂O production after fertilization was nitrification, and the main process of N₂O production around harvest time was denitrification. N₂O emission was strongly influenced by the drying–wetting process of the soil, as well as by the high soil water content. The annual N₂O emission during the growing season ranged from 3.5 to 15.6 kgN ha^–1. The annual nitrogen loss by N₂O emission as a percentage of fertilizer-N ranged from 1.1 to 6.4%. About 70% of the annual N₂O emission occurred near harvesting in August to October, and less than 20% occurred just after fertilization in May to July. High N₂O fluxes around the harvesting stage and a high proportion of N₂O emission to total fertilizer-N appeared to be probably a characteristic of the study area located in central Hokkaido, Japan.     

Low Activation Energy Pathway for the Catalyzed Reduction of Nitrogen Oxides to N<Subscript>2</Subscript> by Ammonia

A low activation energy pathway for the catalytic reduction of nitrogen oxides to N₂, with reductants other than ammonia, consists of two sets of reaction steps. In the first set, part of the NO _x is reduced to NH₃; in the second set ammonium nitrite, NH₄NO₂ is formed from this NH₃ and NO + NO₂. The NH₄NO₂ thus formed decomposes at ~100 °C to N₂ + H₂O, even on an inert support, whereas ammonium nitrate, NH₄NO₃, which is also formed from NH₃ and NO₂ + O₂, (or HNO₃), decomposes only at 312 °C yielding mainly N₂O. Upon applying Redhead's equations for a first order desorption to the decomposition of ammonium nitrite, an activation energiy of 22.4 is calculated which is consistent with literature data. For the reaction path via ammonium nitrite a consumption ratio of 1/1 for NO and NO₂ is predicted and confirmed experimentally by injecting NO into a mixture of NH₃ + NO₂ flowing over a BaNa/Y catalyst. This leads to a yield increase of one N₂ molecule per added molecule of NO. Little N₂ is produced from NH₃ + NO in the absence of NO₂.     

Selective Catalytic Reduction of NOx over H-ZSM-5 Under Lean Conditions Using Transient NH3 Supply

The selective catalytic reduction (SCR) of NO _x over zeolite H-ZSM-5 with ammonia was investigated using in situ FTIR spectroscopy and flow reactor measurements. The adsorption of ammonia and the reaction between NO _x , O₂ and either pre-adsorbed ammonia or transiently supplied ammonia were investigated for either NO or equimolar amounts of NO and NO₂. With transient ammonia supply the total NO reduction increased and the selectivity to N₂O formation decreased compared to continuous supply. The FTIR experiments revealed that NO _x reacts with ammonia adsorbed on Brønsted acid sites as NH₄ ⁺ ions. These experiments further indicated that adsorbed -NO₂ ^– is formed during the SCR reaction over H-ZSM-5.     

Reduction of NO2 stored in a commercial lean NO x trap at low temperatures

Isothermal storage of NO₂ and subsequent reduction with different reducing agents (H₂, CO or H₂ + CO) in a lean NO _x trap catalyst was tested by Temperature Programmed Desorption (TPD) and Temperature Programmed Reduction (TPR) experiments at temperatures representative of automotive “cold-start” conditions (<200 °C) using a commercial NO _x trap catalyst. Results from the TPR experiments revealed that no reduction of stored NO₂ to N₂ was observed at 100–180 °C, and at 200 °C 10% reduction only of NO₂ to N₂ was measured. A special affinity of H₂ to form NH₃ was observed during the reduction of stored NO₂. The formation of NH₃ increases with increasing amount of stored NO₂ and decreases with increasing storage temperature. Direct relation exists between the amount of adsorbed and/or stored NO₂ and the formation of H₂O and NH₃.     

Preparation of Ceria-Zeolite Catalysts by Different Techniques and Its Effect on Selective Catalytic Reduction of NO with NH3 at High Space Velocities

Several zeolite-based catalysts containing Ce³⁺ and/or CeO₂ were prepared by a variety of catalyst preparation techniques like ion exchange, solid-state ion exchange, impregnation and physical mixing and are characterised. Selective catalytic reduction was evaluated using simulated exhaust gas containing NO _x , NH₃, O₂ and H₂O at high space velocities (>180000 h^–1) in the temperature window 150–600 °C. The activity and selectivity in NO _x reduction was found to strongly depend on the charge compensating ions, crystallite size of the zeolite and CeO₂ content in the catalyst. CeO₂ mixed with zeolite having H⁺ or Ce³⁺ co-cations showed benificial effect and increased the NO _x conversion and selectivity. Among the different zeolite materials studied, the structure and the strength and amount of Brønsted acidity did not influence the NO _x conversion.     

NOx storage behavior and sulfur-resisting performance of the third-generation NSR catalysts Pt/K/TiO2–ZrO2

The NO_x storage and reduction (NSR) catalysts Pt/K/TiO₂–ZrO₂ were prepared by an impregnation method. The techniques of XRD, NH₃-TPD, CO₂-TPD, H₂-TPR and in situDRIFTS were employed to investigate their NO_x storage behavior and sulfur-resisting performance. It is revealed that the storage capacity and sulfur-resisting ability of these catalysts depend strongly on the calcination temperature of the support. The catalyst with theist support calcined at 500 °C, exhibits the largest specific surface area but the lowest storage capacity. With increasing calcination temperature, the NO_x storage capacity of the catalyst improves greatly, but the sulfur-resisting ability of the catalyst decreases. In situ DRIFTS results show that free nitrate species and bulk sulfates are the main storage and sulfation species, respectively, for all the catalysts studied. The CO₂-TPD results indicate that the decomposition performance of K₂CO₃ is largely determined by the surface property of the TiO₂–ZrO₂ support. The interaction between the surface hydroxyl of the support and K₂CO₃ promotes the decomposition of K₂CO₃ to form –OK groups bound to the support, leading to low NO_x storage capacity but high sulfur-resisting ability, while the interaction between the highly dispersed K₂CO₃ species and Lewis acid sites gives rise to high NO_x storage capacity but decreased sulfur-resisting ability. The optimal calcination temperature of TiO₂–ZrO₂ support is 650 °C.     

Ammonia and nitrous oxide emissions from grass and alfalfa mulches

Ammonia (NH₃) and nitrous oxide (N_-2O) emissions were measured in the field for three months from three different herbage mulches and from bare soil, used as a control. The mulches were grass with a low N-content (1.15% N in DM), grass with a high N-content (2.12% N in DM) and alfalfa with a high N-content (4.33% N in DM). NH₃ volatilization was measured using a micrometeorological technique. N_-2O emissions were measured using closed chambers. NH₃ and N_-2O emissions were found to be much higher from the N-rich mulches than from the low-N grass and bare soil, which did not differ significantly. Volatilization losses of NH₃ and N_-2O occurred mainly during the first month after applying the herbage and were highest from wet material shortly after a rain. The extent of NH₃-N losses was difficult to estimate, due to the low frequency of measurements and some problems with the denuder technique, used on the first occasions of measurements. Nevertheless, the results indicate that NH₃-N losses from herbage mulch rich in N can be substantial. Estimated losses of NH₃-N ranged from the equivalent of 17% of the applied N for alfalfa to 39% for high-N grass. These losses not only represent a reduction in the fertilizer value of the mulch, but also contribute appreciably to atmospheric pollution. The estimated loss of N_-2O-N during the measurement period amounted to 1% of the applied N in the N-rich materials, which is equivalent to at least 13 kg N_-2O-N ha^-1 lost from alfalfa and 6 kg ha^-1 lost from high-N grass. These emission values greatly exceed the 0.2 kg N_-2O-N ha^-1 released from bare soil, and thus contribute to greenhouse gas emissions.     

Application of the thermal DeNOx process to diesel engine DeNOx: an experimental and kinetic modelling study

Diesel DeNO_x experiments have been conducted using the selective noncatalytic ‘thermal DeNO_x’ process in a diesel fuelled combustion-driven flow reactor which simulated a single cylinder (966 cm³) and head equipped with a water-cooling jacket and an exhaust pipe. NH₃ was directly injected into the cylinder to reduce NO_x emissions. A wide range of air/fuel ratios (A/F=20-40) was selected for NO_x reduction where an initial NO_x of 530 ppm was usually maintained with a molar ratio (β=NH₃/NO_x) of 1.5.The results indicate that a 34% NO_x reduction can be achieved from the cylinder injection in the temperature range, 1100-1350 K. Most of the NO_x reduction occurs within the cylinder and head section (residence time<40 ms), since temperatures in the exhaust are too low for additional NO_x reduction. Under large gas quenching rates, increasing β values (e.g. 4.0) substantially increase the NO_x reduction up to 60%, which is comparable with those achieved under isothermal conditions. Experimental findings are analysed by chemical kinetics using the Miller and Bowman mechanism including both N/H/O species and CO/hydrocarbon reactions to account for CO/UHC oxidation effects, based on practical nonisothermal conditions. Comparisons of the kinetic calculations with the experimental data are given as regards temperature characteristics, residence time and molar ratio. In addition, the effects of CO/UHC and branching ratio (α=k₁/(k₁+k₂)) for the reaction NH₂+NO=products are discussed in terms of NO reduction features, together with practical implications.     

Simulation of SCR equipped vehicles using iron-zeolite catalysts

Iron-catalysts, based on ZSM-5 (FeZSM5) and Cuban natural Mordenite (FeMORD) zeolites have been prepared by a conventional ion-exchange method and their catalytic activity in the selective catalytic reduction (SCR) of NO with ammonia was studied in the presence of H₂O and SO₂. A commercial SCR catalyst (CATCO) based on V₂O₅–WO₃–TiO₂, was also studied as a reference. This paper presents the experimental results of using these catalysts without toxic vanadium and also exploits a neural network-based approach to predict NO_x conversion efficiency of three SCR catalysts. The mathematical functions derived have been integrated into a numerical model to simulate diesel road vehicles equipped with SCR catalysts such as those studied here. The main results indicate that despite toxic vanadium and N₂O formation, CATCO shows better NO_x conversion efficiencies. However, FeMORD does not produce N₂O and performs better than the FeZSM5. The simulation results on real cycles show lower level of NO_x for heavy-duty and light-duty diesel vehicles compared with homologation load cycles.     

Nitrous oxide emissions from agricultural soils

This paper addresses three topics related to N₂O emissions from agricultural soils. First, an assessment of the current knowledge of N₂O emissions from agricultural soils and the role of agricultural systems in the global N₂O are discussed. Secondly, a critique on the methodology presented in the OECD/OCDE (1991) program on national inventories of N₂O is presented. Finally, technical options for controlling N₂O emissions from agricultural fields are discussed.The amount of N₂O derived from nitrogen applied to agricultural soils from atmospheric deposition, mineral N fertilizer, animal wastes or biologically fixed N, is not accurately known. It is estimated that the world-wide N₂O emitteddirectly from agricultural fields as a result of the deposition of all the above nitrogen sources is 2–3 Tg N annually. This amounts to 20–30% of the total N₂O emitted annually from the earth's surface. An unknown, but probably significant, amount of N₂O is generated indirectly in on and off farm activities associated with food production and consumption.Management options to limitdirect N₂O emissions from N-fertilized soils should emphasize improving N-use efficiency. Such management options include managing irrigation frequency, timing and quantity; applying N only to meet crop demand through multiple applications during the growing season or by using controlled release fertilizers; applying sufficient N only to meet crop needs; or using nitrification inhibitors. Most of these options have not been field tested. Agricultural management practices may not appreciably affect indirect N₂O emissions.     

Spectroscopic Evidence for a Nitrite Intermediate in the Catalytic Reduction of NOx with Ammonia on Fe/MFI

The mechanism of selective catalytic reduction (SCR) of NO_x with NH₃ over Fe/MFI was studied using in situ FTIR spectroscopy. Exposing Fe/MFI first to NH₃ then to flowing NO + O₂ or using the reversed sequence, invariably leads to the formation of ammonium nitrite, NH₄NO₂. In situ FTIR results in flowing NO + NH₃ + O₂ at different temperatures show that NH₃ is strongly adsorbed and reacts with impinging NO_x. The intensity of the NH₄NO₂ bands initially increases with temperature, but passes through a maximum at 120 °C because the nitrite decomposes to N₂ + H₂O. The mechanistic model rationalizes that the consumption ratio of NO and NH₃ is close to unity and that the effect of water vapor depends on the reaction temperature. At high temperature H_2O enhances the rate because it is needed to form NH₄NO₂. At low temperature, when adsorbed H₂O is abundant it lowers the rate because it competes with NO_x for adsorption sites.     

Estimates of the interannual variations of N2O emissions from agricultural soils in Canada

The DNDC model was used to estimate direct N₂O emissions from agricultural soils in Canada from 1970 to 1999. Simulations were carried out for three soil textures in seven soil groups, with two to four crop rotations within each soil group. Over the 30-year period, the average annual N₂O emission from agricultural soils in Canada was found to be 39.9 Gg N₂O–N, with a range from 20.0 to 77.0 Gg N₂O–N, and a general trend towards increasing N₂O emissions over time. The larger emissions are attributed to an increase in N-fertilizer application and perhaps to a trend in higher daily minimum temperatures. Annual estimates of N₂O emissions were variable, depending on timing of rainfall events and timing and duration of spring thaw events. We estimate, using DNDC, that emissions of N₂O in eastern Canada (Atlantic Provinces, Quebec, Ontario) were approximately 36% of the total emissions in Canada, though the area cropped represents 19% of the total. Over the 30-year period, the eastern Gleysolic soils had the largest average annual emissions of 2.47 kg N₂O–N ha^–1 y^–1 and soils of the dryer western Brown Chernozem had the smallest average emission of 0.54 kg N₂O–N ha^–1 y^–1. On average, for the seven soil groups, N₂O emissions during spring thaw were approximately 30% of total annual emissions. The average N₂O emissions estimates from 1990 to 1999 compared well with estimates for 1996 using the IPCC methodology, but unlike the IPCC methodology our modeling approach provides annual variations in N₂O emissions based on climatic differences.