NO Yield Research Articles

Important yet Overlooked HONO Source from Aqueous-phase Photochemical Oxidation of Nitrophenols.

Nitrites (NO2-/HONO), as the primary source of hydroxyl radicals (•OH) in the atmosphere, play a key role in atmospheric chemistry. However, the current understanding of the source of NO2-/HONO is insufficient and therefore hinders the accurate quantification of atmospheric oxidation capacity. Herein, we highlighted an overlooked HONO source by the reaction between nitrophenols (NPs) and •OH in the aqueous phase and provided kinetic data to better evaluate the contribution of this process to atmospheric HONO. Three typical NPs, including 4-nitrophenol (4NP), 2-nitrophenol (2NP), and 4-nitrocatechol (4NC), underwent a denitration process to form aqueous NO2- and gaseous HONO through the •OH oxidation, with the yield of NO2-/HONO varied from 15.0 to 33.5%. According to chemical composition and structure analysis, the reaction pathway, where the ipso addition of •OH to the NO2 group on 4NP generated hydroquinone, can contribute to more than 61.9% of the NO2-/HONO formation. The aqueous photooxidation of NPs may account for HONO in the atmosphere, depending on the specific conditions. The results clearly suggest that the photooxidation of NPs should be considered in the field observation and calculation to better evaluate the HONO budget in the atmosphere.

Reactive oxygen species play key roles in the nitrite formation by the inorganic nitrate photolysis in the presence of urban water-soluble organic carbon

Inorganic nitrates were considered to be a potential source of atmospheric NO2−/HONO during the daytime. To better evaluate the contribution of nitrate photochemistry on NO2−/HONO formation, the photolysis of nitrates in the real atmospheric environment needs to be further explored. Here, the NO2− generation by the photolysis of inorganic nitrates in the presence of total water-soluble organic carbon (WSOC) was quantified. The physicochemical properties of WSOC were measured to understand the underlying mechanism for the photolysis of inorganic nitrates with WSOC. WSOC enhanced or suppressed the photochemical conversion of nitrates to NO2−, with the quantum yield of NO2− (ΦNO2−) varying from (0.07 ± 0.02)% to (3.11 ± 0.04)% that depended on the light absorption properties of WSOC. Reactive oxygen species (ROS) generated from WSOC, including O2−/HO2 and OH, played a dual role in the NO2− formation. Light-absorbing substances in WSOC, such as N-containing and carbonyl aromatics, produced O2−/HO2 that enhanced the secondary conversion of NO2 to NO2−. On the other hand, OH deriving from the WSOC photochemistry inhibited the nitrate photodegradation and the NO2− formation. HONO source strength by the aqueous photolysis of nitrates with WSOC was estimated to be lower than 100 ppt h−1, which may partly contribute to the atmospheric HONO source in some cases.

Multi-physical field simulation calculation and analysis of simulated high-level waste liquid spray calcination.

Aiming at the independent research and development of a simulated high-level waste liquid spray calcination transformation treatment test device, a three-dimensional multi-physical field model of spray calcination was established by means of finite element analysis method. In this paper, the simulated high-level waste liquid is a mixed solution of nitrate solution and sucrose. The main chemical components of nitrate dissolution are HNO3 and NaNO3. The process of evaporation and calcination of high-level waste liquid to form oxides is also called the pretreatment of high-level waste liquid or the conversion of high-level waste liquid. In this experiment, the atomized droplets sprayed at high speed are evaporated, dried and calcined in turn in the calciner to obtain the calcined product. The distribution law of temperature flow field and chemical reaction state and results inside the test device were revealed by simulation calculation. The results show that under the condition of multi-physical field coupling, the chemical reaction temperature has an effect on the yield of the product. The temperature is positively correlated with the product concentration, and the effect of temperature on the yield of NO2 is greater than that of Na2O. At the same time, in this chemical reaction, the concentration of reactants (NaNO3 and HNO3) had a positive correlation with the concentration of main products (NO2 and Na2O). However, the rate of increase in the concentration of the main products (NO2 and Na2O) decreased with the increase of the concentration of the reactants (NaNO3 and HNO3).

Quantitation and evaluation of NO2−, NO3−, and H2O2 in the sonolysis of aqueous NaOH solution under air and air-Ar mixture: Effects of solution temperature, ultrasonic power, and ratio of gas mixture

When an aqueous solution containing dissolved air is sonicated, H2O2, HNO2, and HNO3 are formed. This is a result of the formation of active bubbles with extremely high-temperature and high-pressure. The yields of H2O2, NO2−, and NO3− are representative indexes for understanding the chemical effects of ultrasonic cavitation in water. However, these yields often vary under the acidic conditions caused by sonication. In this study, we measured the yields of H2O2, NO2−, and NO3− in the presence of NaOH, which suppresses the reaction between NO2− and H2O2 and prevents the formation of NO3− in a bulk solution. Therefore, the yields obtained should correspond to the actual yields just after bubble collapse, directly reflecting the chemical effects of the active bubbles themselves. It was confirmed that the yields of NO2− and NO3− decreased, while the ratio of [NO3−] to [NO2−] ([NO3−]/[NO2−] ratio) increased with increasing solution temperature, suggesting that the temperature and pressure in collapsing bubbles decreased with an increase in the solution temperature. Ultrasonic power clearly affected the yields of NO2− and NO3−, but it did not affect the [NO3−]/[NO2−] ratio, suggesting that 1) the quality of the active bubbles did not change largely with increasing ultrasonic power, and 2) the quantity related to the number and/or size of active bubbles increased with increasing ultrasonic power up to a certain power. Additionally, the effects of the ratio of air to Ar on the yields of NO2−, NO3−, and H2O2 were investigated. These yields could be affected not only by the bubble temperature but also by the concentration of reactants and intermediates inside the collapsing bubbles. The chemical reactions are quite complex, but these yields could be valuable analytical tools for understanding the quantity and quality of active bubbles.

Tuning the Microstructures of ZnO To Enhance Photocatalytic NO Removal Performances

Effective removal of kinetically inert dilute nitrogen oxide (NO, ppb) without NO2 emission is still a challenging topic in environmental pollution control. One effective approach to reducing the harm of NO is the construction of photocatalysts with diversified microstructures and atomic arrangements that could promote adsorption, activation, and complete removal of NO without yielding secondary pollution. Herein, microstructure regulations of ZnO photocatalysts were attempted by altering the reaction temperature and alkalinity in a unique ionic liquid-based solid-state synthesis and further investigated for the removal of dilute NO upon light irradiation. Microstructure observations indicated that as-tuned photocatalysts displayed unique nucleation, diverse morphologies (spherical nanoparticles, short and long nanorods), defect-related optical characteristics, and enhanced carrier separations. Such defect-related surface-interface aspects, especially Vo″-related defects of ZnO devoted them to the 4.16-fold enhanced NO removal and 2.76 magnitude order decreased NO2 yields, respectively. Improved NO removal and toxic product inhabitation in as-tuned ZnO was disclosed by mechanistic exploitations. It was revealed that regulated microstructures, defect-related charge carrier separation, and strengthened surface interactions were beneficial to active species production and molecular oxygen activation in ZnO, subsequently contributing to the improved NO removal and simultaneous avoidance of NO2 formation. This investigation shed light on the facile regulation of microstructures and the roles of surface chemistry in the oxidation of low concentration NO in the ppb level upon light illumination.

Effective and selective electrocatalytic nitrate reduction to ammonia on urchin-like and defect-enriched titanium oxide microparticles

This work reported a facile approach to surface oxygen vacancy (OV)-enriched urchin-like TiO2 microparticles (U-TiO2), which were highly effective and durable in catalyzing selective nitrate reduction to ammonia (NO3RR). Specifically, the U-TiO2 delivered a mass activity of 1.15 min−1 mg−1catalyst, a low yield of toxic NO2−-N intermediate (≤0.4 mg/L) and an exceptional high NH3-N selectivity of 98.1% in treating 22.5 mg/L of NO3−-N under a potential of -0.60 V vs. RHE, outperforming most of the reported oxide-based catalysts. When comparing the performance of U-TiO2 with that of the solid amorphous TiO2 counterpart (A-TiO2) that had close particle size but more OV on surfaces, we identified that the OV was the reactive sites, but rather than its content, the NO3RR kinetics were primarily limited by the electron and mass transfer at U-TiO2/water interfaces. Accordingly, the superior performance of U-TiO2 to A-TiO2 could be ascribed to the hierarchical urchin-like structure in U-TiO2. The in-situ DEMS test revealed that the NO3RR on U-TiO2 followed a pathway of *NO3− → *NO2−→ *NO → *N → *NH → *NH2 → *NH3. We also demonstrated that the U-TiO2 could keep its robust performance under a wide NO3−-N concentration range and in the presence of some co-existing ions (such as Ca2+, Cl−, Mg2+). However, the presence of humic acid and CO32− in water slowed down the NO3RR on U-TiO2. This work provides a more fundamental insight into the OV-driven NO3RR process on TiO2, which should benefit for the development of efficient TiO2-based catalysts.

Construction of S-scheme heterojunction for enhanced photocatalytic conversation of NO over dual-defect CeO2−x/g-C3N4−x

An increasing amount of nitric oxide (NOx) is being discharged into the atmosphere due to the development of industry and combustion of fossil fuels. Photocatalysis, as a green and renewable technology, has attracted increasing attention from global researchers to address environmental pollution caused by excessive NOx in air. g-C3N4 is a metal-free organic polymer photocatalyst that has received widespread attention due to its unique physical and chemical properties. However, the photocatalytic activity of g-C3N4 under visible light irradiation is limited by high photocarrier recombination, poor conductivity, and low visible light utilization. The construction of novel S-scheme heterojunction semiconductors based on g-C3N4 is a promising strategy to enhance the photocatalytic activity. In this study, S-scheme CeO2−x/g-C3N4−x (Ce/CN) photocatalysts were synthesized by the thermal polymerization of melamine and Ce(NO3)4. The photocatalytic activity of the as-prepared photocatalysts was investigated for the removal of NO with visible-light irradiation. The photocatalytic efficiency of 4Ce/CN was about 1.7 times higher than that of g-C3N4 with a low NO2 yield. Material characterization and DFT analysis demonstrated that the enhanced photocatalytic activity was attributed to N and O dual defects, the excellent conductivity of CeO2, and the in-built field of the Ce/CN S-scheme heterojunction.

Evolution behaviors of the nitrogen-containing species and SO2 from coal fast pyrolysis

The nitrogen-containing species and SO2 generated during coal pyrolysis lead to environmental problems. This paper investigated the evolution behaviors of four identified nitrogen-containing species including hydrogen cyanide (HCN), ammonia (NH3), nitric oxide (NO), nitrogen dioxide (NO2), and sulfur dioxide (SO2) from fast pyrolysis of four different types of coal in a fixed bed reactor in the 600–1200 °C temperature range. Results show that HCN yield increases as the temperature increasing while the NH3 maximum yield exists a transition temperature ranging from 800 to 1000 °C that depends on coal category. The impact of pyrolysis temperature on the SO2 yield is not linear. To further quantify the NH3 formation source, a mathematical method was proposed to divide the NH3 evolution profile into gas-gas reaction and gas-solid reaction two categories. The gas-solid reaction predominates the NH3 formation with a ratio up to 75% in 1200 °C high temperature condition. In low temperature condition, which path is predominant depends on the coal category. In addition, the oxynitride including both NO and NO2 were observed in pyrolysis process. Moreover, enhancing temperature can boost NO2 yield and restrain NO yield. The NO and NO2 come from the combination of nitrogen and oxygen functional group which are both directly split off the parent coal. In addition, the elements liberation sequence from fast to slow is carbon > sulfur > nitrogen. To be further, the liberation sequence of pyrolysis species from fast to slow is CO > SO2>NO > NO2>HCN > NH3, which can supply guides to remove pollutants in industry.

Ag/ZnO Heterojunction Decorated on Polypropylene as a Membrane for Photocatalytic Nitrogen Oxide Removal

We synthesized a Ag/ZnO junction coated on a polypropylene (PP) membrane toward the removal of nitrogen oxide (NO) gas under visible light for an air-filtration machine in the future. The Ag/ZnO catalyst plays an important role in the separation of electron–hole pairs, leading to enhancing the photocatalytic NO removal through the localized surface plasmon resonance effect (LSPR) of Ag nanoparticles (NPs). In our investigation, the 3 wt % Ag NPs are the best composite sample, which is achieved the best content for the PP membrane in the NO removal. The Ag/ZnO heterojunction retains an excellent performance of 40.87% even after being used repetitively for 6 cycling tests. In addition, this Ag/ZnO/PP membrane has a good trend for the green product’s conversion in the NO reaction, which expressed a very low concentration of the NO2 yield. This is the first report on a Ag/ZnO/PP membrane for NO photocatalytic removal, to advance membrane technology for air purification and treatment in the near future.

The NH[formula omitted]/NO[formula omitted]/O[formula omitted] system: Constraining key steps in ammonia ignition and N[formula omitted]O formation

Amine/NO2 interactions are important for ignition and N2O emissions in ammonia combustion. In the present work, reported results from batch reactors (580–690 K) and flow reactors (850–1350 K) for the NH3/NO2 system were re-interpreted in terms of the present understanding of the amine chemistry. Furthermore, additional flow reactor results on the impact of O2 on the NH3/NO2 reaction were presented and analyzed. Based on the experimental results and the modeling analysis, it was possible to constrain the rate constants for reactions of NH3 and NH2 with NO2 and for subsequent steps involving H2NO and HNO intermediates. The key reaction is NH2 + NO2, forming H2NO + NO (R2) and N2O + H2O (R3). The results indicate that the yield of N2O in the NH2 + NO2 reaction decreases with temperature in the 850–1350 K range, in agreement with the theoretical study by Klippenstein and coworkers. The fate of H2NO and HNO is important for the overall reactivity. In the absence of O2, formation of chain carriers is controlled by the sequence H2NO + NO2→ HNO + HONO, HNO + NO2→ NO + HONO, HONO (+M) → NO + OH (+M). At higher temperatures, in the presence of O2, the sequence H2NO + O2→ HNO + HO2, HNO + O2→ NO + HO2, NO + HO2→ NO2 + OH enhances radical formation and recycles NO2. The satisfactory agreement between experiments and modeling predictions, both without and with O2, supports the present rate constants for the reactions of H2NO and HNO with NO2 and O2, respectively.

Microwave-Induced Deep Catalytic Oxidation of NO Using Molecular-Sieve-Supported Oxygen-Vacancy-Enriched Fe-Mn Bimetal Oxides.

A novel microwave (MW) catalytic oxidation denitrification method was developed, which can deeply oxidize NO into nitrate/nitrite with little NO2 yield. A molecular-sieve-supported oxygen-vacancy-enriched Fe2O3-MnO2 catalyst (Ov-Fe-Mn@MOS) was fabricated. Physicochemical properties of the catalyst were revealed by various characterization methods. MW irradiation was superior to the conventional heating method in NO oxidation (90.5 vs 70.6%), and MW empowered the catalyst with excellent low-temperature activity (100-200 °C) and good resistance to H2O and SO2. Ion chromatography analysis demonstrated that the amount of nitrate/nitrite accounted for over 90.0% of the N products, but the main product gradually varied from nitrate to nitrite as the reaction proceeded because of the switching of the main reaction path of NO removal. Mechanism analyses clarified that NO oxidation was a non-radical catalytic reaction: (i) the chemisorbed NO on ≡Mn(IV) reacted with O2* to produce nitrate and (ii) the excited NO* due to MW irradiation reacted with the active O* generated from Ov···O2 to form nitrite. Density functional theory calculations combined with electron paramagnetic resonance tests revealed the promotional effects of Fe2O3 in (i) boosting the Ov's quantity; (ii) facilitating O2 adsorption; (iii) increasing the nitrite formation; and (iv) alleviating the suppression of SO2.

Catalysts Optimization of WO3‐SiO2 Supported Iridium for NOx Reduction by CO under Excess Oxygen Conditions

AbstractA series of Ir‐based catalysts were prepared by ultrasonic‐assisted equal‐volume impregnation to evaluate the catalytic performance of selective catalytic reduction of NOx by CO (CO‐SCR) with excess O2 in simulated flue gas. We investigated the effects of different carrier, Ir loading, and composite carrier ratio (W/Si) on the CO‐SCR activities. The results showed that WO3 and SiO2 were effective carriers for the CO‐SCR reaction under oxygen‐rich conditions. Results of XRD and H2‐TPR studies indicated that metal Ir strongly interacted with carrier WO3 and SiO2, respectively, which improved the surface dispersion of active component Ir species and was conducive to the removal of NOx and CO. When W/Si molar ratio was 1/9 and Ir loading was 2 wt%, the Ir/WO3‐SiO2 catalyst reached 49 % NOx conversion and 76 % CO conversion at 225°C, while NO2 yield was also only 5 ppm. The effects of O2, SO2, and H2O on the structure of Ir/WO3‐SiO2 were analyzed by XRD and XPS characterization methods. It was found that O2 could oxidize Ir species to IrO2, thereby inhibiting NOx reduction. A low concentration of SO2 can weaken the process of Ir oxidation, promote NOx reduction, and improve catalytic activity. H2O can reduce the NOx conversion rate and inhibit the CO‐SCR reaction, but the inhibition effect is reversible.

Nitrogen Oxide Production in Laser‐Induced Breakdown Simulating Impacts on the Hadean Atmosphere

AbstractThe high‐energy‐density synthesis of NxOy species is simulated in gas mixtures representing an O2‐free early‐Earth atmosphere by terawatt‐kilojoule‐class laser‐induced dielectric breakdown (LIDB). These experiments differ from previous LIDB experiments due to the 100 times greater energy delivered per pulse and sensitive analysis of products by high‐resolution infrared spectroscopy. The measured yields of NO, N2O, and NO2 are 0.08–8 × 1015, 5 × 1012, and 0.03–7 × 1014 molec J −1. The high N2O yield is above the upper‐limit constraint of previous tabletop LIDB experiments and the expected yield of a thermochemical freeze‐out at any temperature between 2000 and 5000 K, while the NO and NO2 yields are in broad agreement with freeze‐out models. Using a one dimensional chemical model of the Hadean atmosphere and a simple model of late bombardment, we compute the source flux of N2O assuming the same high production yield as measured experimentally and find the steady‐state partial pressure of N2O is insufficient to warm the climate.

Comprehensive Study about the Photolysis of Nitrates on Mineral Oxides.

Nitrates formed on mineral dust through heterogeneous reactions in high NOx areas can undergo photolysis to regenerate NOx and potentially interfere in the photochemistry in the downwind low NOx areas. However, little is known about such renoxification processes. In this study, photolysis of various nitrates on different mineral oxides was comprehensively investigated in a flow reactor and in situ diffuse reflectance Fourier-transform infrared spectroscopy (in situ DRIFTS). TiO2 was found much more reactive than Al2O3 and SiO2 with both NO2 and HONO as the two major photolysis products. The yields of NO2 and HONO depend on the cation basicity of the nitrate salts or the acidity of particles. As such, NH4NO3 is much more productive than other nitrates like Fe(NO3)3, Ca(NO3)2, and KNO3. SO2 and water vapor promote the photodegradation by increasing the surface acidity due to the photoinduced formation of H2SO4/sulfate and H+, respectively. O2 enables the photo-oxidation of NOx to regenerate nitrate and thus inhibits the NOx yield. Overall, our results demonstrated that the photolysis of nitrate can be accelerated under complex air pollution conditions, which are helpful for understanding the transformation of nitrate and the nitrogen cycle in the atmosphere.

The microscopic oxidation mechanism of NH3 on CuO(111): A first-principles study

Understanding the oxidation of ammonia (NH3) over CuO surface and then the formation routes of N2 and NOx is rather crucial to provide a favorable direction for the rational design of high-performance Cu-based oxygen carriers in chemical looping combustion (CLC) and CuO-containing catalysts in selective catalytic reduction (SCR). This study aims to investigate the reaction mechanisms of nitrogen-containing species using density functional theory (DFT) calculations. The potential dehydrogenation pathway is identified as NH3* → NH2* + H* → NH(1)* + 2H* → N(2)* + 3H*, and the rate-determined step is the NH2* dehydrogenation. Additionally, we consider 10 dominating elementary reactions for the formation of N2, NO, NO2 and N2O; two skeletal schemes of the NH3 oxidation under low or high temperature conditions are then proposed. Under the low temperature condition of SCR, the majority of gaseous N2 comes from the Eley–Rideal reaction between NH2* fragment and gaseous NO, while the lateral recombination of N* to form N2 might play a more crucial role under the high temperature condition of CLC. The high temperature and surface adsorbed oxygen provide positive impacts on the yield of gaseous NO and NO2, respectively. Finally, the effects of O2 and H2O on the fate of nitrogen during heterogeneous reactions have also been determined.

Byproducts Generation Characteristics of Non-thermal Plasma for NO Conversion: Effect of Reaction Conditions

Over the years, the combined Non-thermal Plasma (NTP) denitrification process which is used for decomposition of NOx has have become an application technology with great potential. And the formation of byproducts, such as N2O and O3, which is an important factor that interfere with NO removal efficiency, has a high research value. In present study, HZSM-5 molecular sieve was selected as the packed material in dielectric barrier discharge reactor to investigate the effect of reaction conditions, such as with/without packed bed, discharge power, inner electrode diameter, electrode shape and operation time, on formation and conversion characteristics of byproducts (N2O and O3). Results showed that the presence of HZSM-5 pellets in discharge zone increases the discharge power, NO removal efficiency and the concentration of the byproducts in the outlet. The optimization of electrode diameter and the utilization of screw thread electrode increased the NO removal efficiency and the yield of N2O and O3. The pellets temperature increases with the operation time and the enhancement of input energy, which has different effects on O3 and N2O. These results provide some new methods for improving the NTP-assisted catalytic denitrification process.

Nitrogen conversion from ammonia to trichloronitromethane: Potential risk during UV/chlorine process

In this study, the potential formation of trichloronitromethane (TCNM) from model organic compounds in ammonia-containing water treated by UV/chlorine process was evaluated. Monochloramine generated from the reaction of chlorine and ammonia can be photolyzed to produce NO2− and reactive nitrogen species (RNS), which play important roles in the formation of TCNM during the subsequent chlorination. The results showed that increase of nitrogen to chlorine molar ratio (from 0 to 1.0) and pH (from 6.5 to 8.0) enhanced the formation of TCNM, mainly due to the increased yield of NO2− and RNS from the photolyzed monochloramine. The formation of TCNM was interestingly found to be linearly correlated with Hammett constants of the model precursors, which is theoretically related to the rate constants of RNS with model compounds. Enhanced formation of TCNM was also observed during the treatment of natural organic matter by UV/chlorine process in ammonia-containing water. The toxicity assessment showed that TCNM significantly increased the genotoxicity of formed DBPs. Furthermore, the electrophilic substitution reaction of •NO2 was proved to more likely occur on the ortho and para position of phenol according to the calculation of Gaussian program, and a possible reaction pathway of phenol and •NO2 was proposed based on the calculated results.

Global kinetic model of NO oxidation on Pd/γ-Al2O3 catalyst including PdO[formula omitted] formation and reduction by CO and C[formula omitted]H[formula omitted

Pd is often used in diesel oxidation catalysts (DOC) together with Pt to ensure thermal stability and durability. Besides CO and hydrocarbon oxidation, NO oxidation is important for the function of subsequent converters in the exhaust line, such as SCR and DPF. Though Pt sites exhibit a significantly higher NO oxidation rate, Pd also provide some catalytic activity for this reaction and therefore affect the overall NO2 yield in DOC. Palladium oxides (PdOx) formation decreases the NO oxidation activity like PtOx.We present a global kinetic model for Pd/γ-Al2O3 catalyst considering standard DOC reactions extended with the PdOx formation and its influence on the NO2 yield. The dynamics of transition between the metallic Pd and oxidized PdOx state is studied in repeated temperature ramps and isothermal deactivation experiments, revealing the individual impact of O2 and NO2. The model describes well a dual NO oxidation light-off behavior observed in the experiments.

Effects of irrigation amounts on soil CO2, N2O and CH4 emissions in greenhouse tomato field.

To understand the effects of different irrigation amounts on soil CO2, N2O, and CH4 emission characteristics and tomato yield, and further put forward effective reduction measures, we carried out an experiment with three irrigation levels: full irrigation (1.0W, W1.0; W meant irrigation amount needed to provide the adequate water), 20% deficit irrigation (0.8W, W0.8) and 40% deficit irrigation (0.6W, W0.6). We used static closed chamber and gas chromatography method to measure greenhouse gas emission in two consecutive greenhouse tomato rotation cycles from April to December, 2017. The results showed that cumulative soil CO2, N2O and CH4 emissions increased with increasing irrigation amounts in the two growing seasons (W1.0＞W0.8＞W0.6), and significant difference of N2O between W0.6 and W1.0 was observed, while other treatment effects on soil gas emissions were not obvious. Compared to W1.0, cumulative soil CO2 emissions were decreased by 12.2% and 8.3%, cumulative soil N2O emissions were decreased by 19.1% and 8.0%, and cumulative soil CH4 emissions were reduced by 11.0% and 6.2% for W0.6 and W0.8, respectively. Tomato yield and global warming potential of soil N2O and CH4 emissions (GWP) increased as irrigation amount increasing. Compared with W1.0, W0.6 significantly decreased tomato yield by 17.0% and GWP by 22.9%, while the difference between the effects of W0.8 and W1.0 on these two parameters was not significant. Global warming potential per tomato yield presented an increase then a decrease as irrigation amount increasing (W0.8＞W1.0＞W0.6), but without stanificance. Irrigation water use efficiency (IWUE) showed a decrease with increasing irrigation amount. Compared with W1.0, IWUE under W0.6 and W0.8 was increased by 38.3% and 9.4%, respectively. Soil CO2 flux was nega-tively and exponentially correlated with soil moisture. The dependence of soil CH4 flux on soil moisture showed a significantly positive correlation. An exponential negative correlation was observed between the soil N2O ux and soil temperature when soil temperature was below or above 18 ℃. Irrigation increased tomato yield and soil greenhouse gas emissions, but decreased IWUE. Therefore, W0.8 was the best mode of irrigation management when synthetically considering tomato yield, IWUE, and greenhouse effect.

Low yield and abiotic origin of N2O formed by the complete nitrifier Nitrospira inopinata

Nitrous oxide (N2O) and nitric oxide (NO) are atmospheric trace gases that contribute to climate change and affect stratospheric and ground-level ozone concentrations. Ammonia oxidizing bacteria (AOB) and archaea (AOA) are key players in the nitrogen cycle and major producers of N2O and NO globally. However, nothing is known about N2O and NO production by the recently discovered and widely distributed complete ammonia oxidizers (comammox). Here, we show that the comammox bacterium Nitrospira inopinata is sensitive to inhibition by an NO scavenger, cannot denitrify to N2O, and emits N2O at levels that are comparable to AOA but much lower than AOB. Furthermore, we demonstrate that N2O formed by N. inopinata formed under varying oxygen regimes originates from abiotic conversion of hydroxylamine. Our findings indicate that comammox microbes may produce less N2O during nitrification than AOB.