Reduction Of Nitrogen Dioxide Research Articles

Gaussian process-based analysis of the nitrogen dioxide at Madrid Central Low Emission Zone

Abstract Concern about air-quality in urban areas has led to the implementation of Low Emission Zones as one of many other initiatives to control it. Recently in Spain, the enactment of a law made this mandatory for cities with a population larger than 50k inhabitants. The delimitation of these areas is not without controversy because of possible negative economic and social impacts. Therefore, clear assessments of how these initiatives decrease pollutant concentrations are to be provided. Madrid Central is a major initiative of Madrid city council for reducing motor traffic and the associated air pollution in the city centre. This Low Emission Zone starts at the end of 2018, but the first fully-operational period corresponds to the second quarter of 2019. In this work, a methodology based on the Gaussian Process to analyse the evolution of Nitrogen Dioxide inside Madrid Central is undertaken. A Gaussian Process is a stochastic process suitable for interpretable model selection and predictions. Due to its probabilistic nature it provides error estimation at predictions. After the activation of Madrid Central, a relevant reduction of Nitrogen Dioxide has been observed. However, the role of the meteorology during this period must be ascertained to correctly evaluate the role of the activation of the Low Emission Zone against a prone weather. In this work, a model based on the Gaussian Process is trained with meteorological information to predict the concentration of Nitrogen Dioxide at Madrid Central, $[NO_{2}]$. This probabilistic description allows extracting statistical information on the reduction affected by the meteorological scenario and separately by the Madrid Central activation.

Machine Learning for Transport Policy Interventions on Air Quality

Air pollution reduction is a major objective for transport policy makers. This paper considers interventions in the form of clean air zones, and provide a machine learning approach to assess whether the objectives of the policy are achieved under the designed intervention. The dataset from the Newcastle Urban Observatory is used. The paper first tackles the challenge of finding datasets that are relevant to the policy objective. Focusing on the reduction of nitrogen dioxide (NO2) concentrations, different machine learning algorithms are used to build models. The paper then addresses the challenge of validating the policy objective by comparing the NO2 concentrations of the zone in the two cases of with and without the intervention. A recurrent neural network is developed that can successfully predict the NO2 concentration with root mean square error of 0.95.

An ANN-PSO-Based Method for Optimizing Agricultural Tractors in Field Operation for Emission Reduction

Aiming at the serious problem of agricultural tractor emission pollution, especially the limitation of nitrogen dioxide (NOx) and soot emissions, we took an agricultural diesel engine as the research object, and a diesel engine combustion chamber model was established for both simulated calculations and experimental verification analysis. The in-cylinder pressure and heat release obtained from the combustion chamber model simulation calculations were within 6% error of the experimental data. The overall trend of change was basically consistent. The established model can simulate the working conditions of the experimental engine relatively well. An artificial neural network (ANN) was also established as an agent model based on the indentation rate, tab depth, and combustion chamber depth as the input, and NOx and soot as the output. The decision coefficients of the ANN model were R2 = 0.95 and 0.93, with corresponding Mean Relative Error (MRE) values of 10.13 and 8.18%, respectively, which are within the generally required interval, indicating that the obtained ANN model has good adaptability and accuracy. On the basis of the general particle swarm optimization (PSO) algorithm, an improved PSO algorithm was proposed, in which the inertia factor is continuously adjusted with the help of the skip line function in the optimization process so that the inertia factor adapts to different rates and adjusts the magnitude of the corresponding values in different periods. The improved PSO algorithm was used to optimize the optimal input parameter matching of the agent model to form a new combustion chamber structure, which was imported into CONVERGE CFD software for emission simulation and comparison with the emissions of the original combustion chamber. It was found that the NOx reduction was about 1.21 g·(kW·h)−1, and the soot reduction was about 0.06 g·(kW·h)−1 with the new combustion chamber structure. The ANN + PSO optimization method proved to be effective in reducing the NOx and soot emissions of diesel engine pollutants, and it may also provide a reference and ideas for the design and development of relevant agricultural engine combustion chamber systems.

Atomically Dispersed Copper Sites in a Metal-Organic Framework for Reduction of Nitrogen Dioxide.

Metal–organic framework (MOF) materials provide an excellent platform to fabricate single-atom catalysts due to their structural diversity, intrinsic porosity, and designable functionality. However, the unambiguous identification of atomically dispersed metal sites and the elucidation of their role in catalysis are challenging due to limited methods of characterization and lack of direct structural information. Here, we report a comprehensive investigation of the structure and the role of atomically dispersed copper sites in UiO-66 for the catalytic reduction of NO2 at ambient temperature. The atomic dispersion of copper sites on UiO-66 is confirmed by high-angle annular dark-field scanning transmission electron microscopy, electron paramagnetic resonance spectroscopy, and inelastic neutron scattering, and their location is identified by neutron powder diffraction and solid-state nuclear magnetic resonance spectroscopy. The Cu/UiO-66 catalyst exhibits superior catalytic performance for the reduction of NO2 at 25 °C without the use of reductants. A selectivity of 88% for the formation of N2 at a 97% conversion of NO2 with a lifetime of >50 h and an unprecedented turnover frequency of 6.1 h–1 is achieved under nonthermal plasma activation. In situ and operando infrared, solid-state NMR, and EPR spectroscopy reveal the critical role of copper sites in the adsorption and activation of NO2 molecules, with the formation of {Cu(I)···NO} and {Cu···NO2} adducts promoting the conversion of NO2 to N2. This study will inspire the further design and study of new efficient single-atom catalysts for NO2 abatement via detailed unravelling of their role in catalysis.

Relationship between air quality and the number of acute respiratory disease infections in Mexico City

In this study, an analysis of the daily averages of Nitrogen Dioxide and Tropospheric Ozone is done because they are produced mainly during or because of the combustion of fossil fuels. Two time periods were studied (January 1 to March 14, and March 15 to December 31) for 2019 and 2020 in order to determine the relationship between the regulatory measures imposed by the government to contain COVID-19 and acute respiratory diseases in Mexico City. Using Google Sheets’s tools, the percentages of change in the atmospheric concentration for each pollutant in each period and year were calculated and then compared with the reduction in the number of infections from acute respiratory diseases in 2020. The reduction of Nitrogen Dioxide was enough to conclude that the general air quality in Mexico City improved, which resulted in fewer people hospitalized for acute respiratory diseases in 2020 compared to 2019. So, a conclusion can be made that the contingency period caused by the recent COVID-19 pandemic resulted in a reduction in the atmospheric concentrations of the pollutants studied and in a reduction in the number of people hospitalized for acute respiratory diseases related to these pollutants.

Room-temperature reduction of NO2 in a Li-NO2 battery: a proof of concept

Although considerable effort has been devoted to purifying nitrogen oxides (NOx), it is still challenging to effectively reduce NOx at room temperature and ambient pressure without catalysts. In this study, as a proof-of-concept, we have for the first time demonstrated the room-temperature reduction of nitrogen dioxide (NO2) using a rechargeable lithium-nitrogen dioxide (Li-NO2) battery. The battery shows a capacity of 884 mAh g−1 at 50 mA g−1 (an actual energy density of 666 Wh kg−1) and a promising electrochemical Faraday efficiency (FE) of 67%. The unique properties of Li-NO2 rechargeable batteries not only provide a way to reduce and recycle NO2 but also highlight the potential of oxidative air pollutants as energy sources for next-generation electrochemical energy storage (EES) systems.

Pulsed Potential Amperometric Electrochemical Gas Sensors

Pulsed electrochemical techniques are well known and are well reviewed. The various discussions and derivations of the theory of pulsed voltammetry or polarography extant make clear the motivations behind the development of these techniques: increasing the analytical sensitivity of these methods by separating, in time, the charging current and the Faradaic current. There are at least three critical criteria for the success of pulsed voltammetric methods: The potential pulse should be small: > 0.059/n V @ 25C.The time between pulses should be long: for electrode area of 1 cm2, several seconds.The electrode area should be minimized: < 1 cm2, down to 1 µm2. All of these efforts had one goal in mind; minimizing the charging current with respect to the Faradaic current. Pulsed voltammetric techniques lowered the typical useful concentration range of electroanalytical methods from parts-per-thousand (10-3) to parts per million (ppm, 10-6) and lower. The above discussion applies to classic solution-oriented electroanalytical techniques. Amperometric electrochemical gas sensors differ in several important ways which should limit or eliminate the usefulness of pulse techniques in this application. Amperometric electrochemical gas sensors are ubiquitous in the modern industrial workplace, commonly deployed for worker protection against toxic and noxious gases. The heart of any good electrochemical gas sensor is the gas diffusion electrode, which is typically the working electrode in sensors. The gas diffusion electrode allows the easy confluence of three things, the electrocatalyst of which the electrode is comprised, usually a metal, the internal electrolyte of the sensor, usually a liquid solution, and the gas of interest. This confluence is often referred to as the “triple point.” To maximize the sensitivity of a sensor, maximize the number of triple points. This is commonly accomplished by using finely divided, high surface area catalytic metals, such as platinum (Pt) or iridium (Ir) “blacks.” As gas diffusion electrodes commonly use such high surface area metal particles, their actual, usable electrochemically accessible surface area, and hence, electrical capacitance often far exceed their geometric area, and vary greatly from sensor (electrode) type to sensor type, and may approach 0.5 m2. We have recently applied pulsed potential voltammetric techniques to the high surface area electrodes commonly employed in electrochemical gas sensors with great utility. The accompanying graph depicts the results of a typical experiment. This sensor utilized a rough gold working electrode with an electrochemically accessible surface area of approximately 100 cm2. The potential was switched between -150 and +250 mV (vs an internal Pt|air pseudo-reference electrode) every 0.5 sec. The traces in the figure are offset for clarity. The upper trace in the figure is the raw response recorded with a Bio-Logic VMP3 potentiostat. The data was collected every 0.1 sec. Ten (10) ppm nitrogen dioxide (NO2), in air, was applied to the sensor at a flow rate of 0.25 l/min, beginning at the 5 minute time and switched back to clean air at the same flow rate at the 10 minute mark. The bottom traces depict the results of current sampling the sensor response just before the potential of the working electrode was switched to either -150 mV or 250 mV (a 400 mV pulse). The pulse rate was 0.5 seconds; therefore, the current was sampled approximately 0.49 sec after the application of the potential pulse. As can be seen in the lower traces, very serviceable sensor responses were obtained in this manner for the application of 10 ppm nitrogen dioxide (NO2) in air (open circles, reduction) or 10 ppm nitric oxide (NO) in air (open diamonds, oxidation). The response time, noise, other figures of merit are very similar to data obtained from this same sensor operated at a single potential of -150 mV (for the reduction of nitrogen dioxide) or at +250 mV (for the oxidation of nitric oxide). Using this technique, we have produced a single sensor that allows the simultaneous detection of both nitric oxide and nitrogen dioxide at concentrations important for worker protection. We have extended this technique to the detection of other gases, including oxygen at and near ambient conditions (20.8 vol-% at sea level and 25 C). Figure 1

Ozone-free nitric oxide production using an atmospheric pressure surface discharge – A way to minimize nitrogen dioxide co-production

Effects of temperature on nitric oxide (NO) production and on byproduct nitrogen dioxide (NO2) and ozone (O3) in a non-equilibrium sliding discharge in atmospheric pressure air are described. Heating the electrodes/dielectric surfaces in contact with the discharge plasma from 20°C to 420°C caused an increase in energy per pulse by more than an order of magnitude. More importantly, heating to a few hundred degrees above room temperature caused the destruction of ozone (O3) and the reduction of nitrogen dioxide (NO2) but promoted NO formation. At 20°C, 400–4500ppm O3 with an energy cost of 2–4MJ/mol, and 12–180ppm NO2 with an energy cost of 75–140MJ/mol were generated from dry air flowing at 1lpm. At 220°C, however, NO and NO2 were produced in about equal proportion while O3 was below detection limits. The efficiency of NO generation increased with an increase of temperature. At the maximum temperature used in this study, i.e., 420°C, 160–1040ppm NO was produced from dry air at an energy cost of 24–67MJ/mol, with a NO/NO2 ratio of more than 4 and free of O3.

Towards a fully printed electrochemical NO2 sensor on a flexible substrate using ionic liquid based polymer electrolyte

An electrochemical amperometric nitrogen dioxide sensor with solid polymer electrolyte was fabricated by means of screen printing technology on both rigid and flexible substrates. The sensor concept is based on a semi-planar, three-electrode topology that enables low power, high performance, thin, and selective gas sensors to be produced on poly(ethylene terephthalate) (PET) and/or Kapton foil. The sensor response mechanism, i.e. the reduction of nitrogen dioxide at the boundary of a solid polymer electrolyte/working electrode/gas analyte, was predicted by using a Langmuir adsorption isotherm. It was demonstrated that a new platform for the electrochemical NO2 sensor can be completely manufactured by screen printing, which allows the fabrication of a flexible and low cost device suitable for mass production. Further, it was demonstrated that the fully printed sensor can be fabricated without using metal-based printing pastes, which is important from the point of view of the environment. The fully-printed, metal-free electrochemical sensor exhibited a linear response in the range of 0–10ppm, fast response/recovery times (70/60s, respectively), a resolution of 0.2ppm, and a sensitivity of 590nA/ppm, which enables the sensor to be used for both the monitoring of NO2 exposure in the workplace as well as environmental air pollution.

Theoretical study of the hydrogen abstraction of substituted phenols by nitrogen dioxide as a source of HONO.

The mild yet promiscuous reactions of nitrogen dioxide (NO2) and phenolic derivatives to produce nitrous acid (HONO) have been explored with density functional theory calculations. The reaction is found to occur via four distinct pathways with both proton coupled electron transfer (PCET) and hydrogen atom transfer (HAT) mechanisms available. While the parent reaction with phenol may not be significant in the gas phase, electron donating groups in the ortho and para positions facilitate the reduction of nitrogen dioxide by electronically stabilizing the product phenoxy radical. Hydrogen bonding groups in the ortho position may additionally stabilize the nascent resonantly stabilized radical product, thus enhancing the reaction. Catechol (ortho-hydroxy phenol) has a predicted overall free energy change ΔG(0) = -0.8 kcal mol(-1) and electronic activation energy Ea = 7.0 kcal mol(-1). Free amines at the ortho and para positions have ΔG(0) = -3.8 and -1.5 kcal mol(-1); Ea = 2.3 and 2.1 kcal mol(-1), respectively. The results indicate that the hydrogen abstraction reactions of these substituted phenols by NO2 are fast and spontaneous. Hammett constants produce a linear correlation with bond dissociation energy (BDE) demonstrating that the BDE is the main parameter controlling the dark abstraction reaction. The implications for atmospheric chemistry and ground-level nitrous acid production are discussed.

Reduction of nitrogen dioxide from etching vent gases by scrubbing with caustic sodium sulfide solution

BACKGROUND Acids and nitrogen dioxide (NO2) in gases that are vented from Al-etching operations for the production of thin-film transistor-liquid crystal displays (TFT-LCD) are harmful to the environment. Scrubbing the gases using traditional caustic liquids with high salinities is ineffective in removing NO2. This work develops practices for improving the removal of NO2 by adding reducing agents to the scrubbing liquids. RESULTS Field results reveal that Na2SO3 is a less effective agent for reducing NO2 than is Na2S, because it has a stronger reduction ability. In this work, when the scrubber parameters were a packing height of 1.0–2.0 m, a liquid-to-gas ratio L/G of 0.48–2.64 L m−3 and a superficial gas velocity U of 0.66–2.08 m s−1, and for influent NO2 concentrations of several tenths of ppm, the NO2 removal depended more on the sulfide dose than on the mass-transfer rate of NO2 to the liquid. CONCLUSIONS A sodium sulfide dose of 1.53±0.22 kg/(kg NO2 removed) over influent NO2 at a concentration 30–50 ppm was obtained. This value was much higher than the stoichiometric value for the reduction of NO2 to nitrite. Sulfide addition using an ORP-controlled mode with an ORP of −400 ± 10 mV and a pH of 12.5 ± 0.1 in the scrubbing liquid is suggested for reducing chemical costs and obtaining the desired NO2 concentration of exhaust gas. © 2013 Society of Chemical Industry

Abstract 9779: Nitric Oxide Generated Through Ascorbic Acid Reduction of Nitrogen Dioxide Reduces Pulmonary Vascular Resistance in Hypoxemic Swine with Minimal Inhaled and Exhaled Nitrogen Dioxide

Background: Inhaled nitric oxide (NO) has the capacity to selectively dilate pulmonary blood vessels and improve oxygenation in patients with pulmonary diseases. NO in the presence of oxygen spontaneously converts to nitrogen dioxisw (NO 2 ), a toxic byproduct with multiple adverse pulmonary effects. The potential for co-inhalation of NO 2 drives delivery system complexity, gas monitoring and cost. We demonstrate a new method for generating inhaled NO based on reducing NO 2 just prior to inhalation, using ascorbic acid/silica gel reactor cartridges (GeNO, LLC). Ascorbic acid protonates NO 2 , yielding NO, water and dehydroascorbic acid. This reaction occurs within the ventilator hoses, leaving little time for NO 2 reformation. Methods: In vivo efficacy was assessed through the inhalation of NO (1, 5, 20, and 80 PPM) in hypoxemic swine (FiO 2 15%). NO 2 in N 2 was mixed with O 2 prior to entry into the hoses of a portable ventilator upstream of two ascorbic acid cartridges in series. Inhaled and exhaled NO and NO 2 were sampled using in-line electrochemical detectors (step response time &lt; 0.8 s, GeNO, LLC). Results: Inspired and expired NO 2 was always under 0.3 PPM in 50% and 0.9 PPM in 100% O 2 (Fig. 1). Inhaled NO significantly reduced the hypoxemia elevated mean pulmonary artery pressure and pulmonary vascular resistance at all doses (Fig. 2). Conclusion: Inspired and even expired NO 2 levels were low using ultra-fast electrochemical methods. NO supplied by this novel system was capable of reversing hypoxemia induced pulmonary hypertension. The lack of co-delivered toxic NO 2 may allow for a substantial reconsideration of the therapeutic role of inhaled NO in a wide range of respiratory diseases.

Use of ultra pure nitric oxide generated by the reduction of nitrogen dioxide to reverse pulmonary hypertension in hypoxemic swine

Inhaled nitric oxide (NO) has the capacity to selectively dilate pulmonary blood vessels, and thus enhance the matching of ventilation and perfusion, improve oxygenation and decrease pulmonary hypertension. However, existing approaches for the administration of inhaled NO are associated with the co-delivery of potentially toxic concentrations of nitrogen dioxide (NO2) due to the oxidation of NO in oxygen rich environments. We tested the ability of a novel methodology for generating highly purified NO through the reduction of NO2 by ascorbic acid to reverse pulmonary hypertension. In vitro testing demonstrated that the NO output of the novel device is ultrapure and free of NO2. An in vivo hypoxemic swine model of pulmonary hypertension was used to examine the dose response to NO in terms of pulmonary pressures and pulmonary vascular resistance. Pulmonary hypertension was induced by lowering inspired oxygen to 15% prior to treatment with inhaled ultra purified NO (1, 5, 20, and 80PPM). Hypoxemia increased mean pulmonary artery pressures and pulmonary vascular resistance. Inhaled ultra purified NO doses (down to 1PPM) show a marked reduction of hypoxemia-induced pulmonary vascular resistance. These experiments demonstrate a simple and robust method to generate purified inhaled NO that is devoid of NO2 and capable of reversing hypoxemia induced pulmonary hypertension.

Theoretical Investigation of the Mechanism of the Selective Catalytic Reduction of Nitrogen Dioxide with Ammonia on H-Form Zeolites and the Role of Nitric and Nitrous Acids as Intermediates

The selective catalytic reduction (SCR) of NO2 with ammonia and the role of nitrous and nitric acids as intermediates have been investigated on a portion of the H-ZSM5, which contains 5T atoms by using the density functional theory, representing H-form zeolites. For the NO2-SCR, it is most likely that, first, NO2 reacts as as-N2O4 with ammonia, forming nitrosamine and nitric acid. The latter dehydrates in a self-reaction to N2O5, which reacts with ammonia by recycling nitric acid and producing nitramide. Nitramide decomposes in a push−pull mechanism to N2O and H2O, similar to the decomposition of nitrosamine to nitrogen and water. Nitrous and nitric acids are stable intermediates in the SCR reacting network and strongly adsorb to the active site. They can be considered to be the main surface species besides NH3 and H2O and are mainly produced from the reaction of NOx with ammonia. Their most important sink is the dehydration in a self-reaction or with each other. The hydrolysis of NOx as the source of the...

Reduction of lean NO2 with diesel soot over metal-exchanged ZSM5, perovskite and γ-alumina catalysts

The catalytic performances of metal-exchanged ZSM5, perovskite and γ-alumina catalysts for the reduction of nitrogen dioxide (NO2) by diesel soot were investigated. The reaction tests were performed through temperature-programmed reaction (TPR), in which NO2 and O2 were passed through a fixed bed of catalyst-soot mixture. On the three types of catalyst, NO2 was reduced to N2 by model soot (Printex-U) and most of the soot was converted into CO2. Pt-, Cu- and Co-exchanged ZSM5 catalysts exhibited reduction activities with conversions of NO2 into N2 of about 20%. Among the perovskite catalysts tested, La0.9K0.1FeO3 showed a 32% conversion of NO2 into N2. The catalytic activities of the perovskite catalysts were largely influenced by the number and stability of oxygen vacancies. For the γ-alumina catalyst, the peak reduction activity appeared at a relatively high temperature of around 500 °C, but the NO2 reduction was more effective than the NO reduction, in contrast to the results of the ZSM-5 and perovskite catalysts.

Numerical Study of The Effect of Traffic Restriction on Air Quality in Beijing

The Nested Air Quality Prediction Modeling System (NAQPMS), in coupling with the fifth-generation NCAR/Penn State Mesoscale Model (MM5), is employed to assess the impact of vehicle traffic restriction on air quality in Beijing within preOlympic environmental measures implemented from 17 th to 20 th August 2007. Predictions are compared against meteorological and air quality observed data and validation shows model good performance as a whole. Sensitivity experiments, including the baseline and traffic control scenarios, are designed to estimate the potential reduction of nitrogen dioxide (NO2) and particulate matter (PM10) concentrations during the traffic restriction. Results indicate that the NO2 concentration in Urban Beijing is reduced by 16%~32%, with the average of 21%, while NOx emissions are lowered within 28%; the primary PM10 concentrations is also reduced by 6%~15%, lower than the decreased percentages of NO2 concentration. The results show that the most significant reduction of air pollutants occurs in Urban Beijing where the restriction has been mainly imposed. This study demonstrates the efficiency of traffic restriction measure in air quality improvement over Beijing.

First-Principles Molecular Dynamics Study of the Heterogeneous Reduction of NO2 on Soot Surfaces

The heterogeneous reduction of nitrogen dioxide on the surface of atmospheric soot particles was thoroughly investigated using pristine and defective graphene layers as simple models of the soot surface. The metadynamics method, based on Car−Parrinello molecular dynamics simulations, was used to reproduce atomistic details of the reaction paths, as well as to obtain estimations of the free energy barriers of the processes. The relative energies, geometries, and electronic structures of reactants, products, and intermediates observed during the trajectories were analyzed, and the catalytic activity of the carbon vacancies concerning the reduction of nitrogen dioxide is discussed. Our study predicts the release of nitrogen monoxide and the formation of stable nitro groups on the surface of soot particles in good agreement with experiments.

Effect of selected organic and inorganic snow and cloud components on the photochemical generation of nitrite by nitrate irradiation

The effect of selected organic and inorganic compounds, present in snow and cloudwater was studied. Photolysis of solutions of nitrate to nitrite was carried out in the laboratory using a UVB light source. The photolysis and other reactions were then modelled. It is shown that formate, formaldehyde, methanesulphonate, and chloride to a lesser extent, can increase the initial formation rate of nitrite. The effect, particularly significant for formate and formaldehyde, is unlikely to be caused by scavenging of hydroxyl radicals. The experimental data obtained in this work suggest that possible causes are the reduction of nitrogen dioxide and nitrate by radical species formed on photooxidation of the organic compounds. Hydroxyl scavenging by organic and inorganic compounds would not affect the initial formation rate of nitrite, but would protect it from oxidation, therefore, increasing the concentration values reached at long irradiation times. The described processes can be relevant to cloudwater and the quasi-liquid layer on the surface of ice and snow, considering that in the polar regions irradiated snow layers are important sources of nitrous acid to the atmosphere. Formate and (at a lesser extent) formaldehyde are the compounds that play the major role in the described processes of nitrite/nitrous acid photoformation by initial rate enhancement and hydroxyl scavenging.

Photosensitized reduction of nitrogen dioxide on humic acid as a source of nitrous acid

Nitrous acid is a significant photochemical precursor of the hydroxyl radical, the key oxidant in the degradation of most air pollutants in the troposphere. The sources of nitrous acid in the troposphere, however, are still poorly understood. Recent atmospheric measurements revealed a strongly enhanced formation of nitrous acid during daytime via unknown mechanisms. Here we expose humic acid films to nitrogen dioxide in an irradiated tubular gas flow reactor and find that reduction of nitrogen dioxide on light-activated humic acids is an important source of gaseous nitrous acid. Our findings indicate that soil and other surfaces containing humic acid exhibit an organic surface photochemistry that produces reductive surface species, which react selectively with nitrogen dioxide. The observed rate of nitrous acid formation could explain the recently observed high daytime concentrations of nitrous acid in the boundary layer, the photolysis of which accounts for up to 60 per cent of the integrated hydroxyl radical source strengths. We suggest that this photo-induced nitrous acid production on humic acid could have a potentially significant impact on the chemistry of the lowermost troposphere.

Reduction of nitrogen dioxide by propene over acidic mordenites: influence of acid site concentration, formation of by-products and mechanism

The reduction of nitrogen dioxide to nitrogen by propene was studied over a variety of acidic mordenite zeolites differing in their Si : Al ratio and thus, in their concentration of acid sites. The formation of by-products was monitored applying an ion–molecule reaction (IMR) mass spectrometer. It was found that at fixed conditions the yield of nitrogen increases with increasing concentration of acid sites, confirming that acid sites are the active catalytic centres in the reaction. Apart from nitrogen and nitric oxide, acrylonitrile and ammonia are formed as nitrogen containing gas-phase products in the reaction. In separate experiments, it was shown that acrylonitrile is hydrolysed by water over the acidic zeolites to yield ammonia and acrylic acid. When acrylonitrile is used as reducing agent for nitrogen dioxide, formation of nitrogen is strongly enhanced in the presence of water. Water also has a promoting effect on the formation of nitrogen in the reaction between nitrogen dioxide and propene. Acrylonitrile and its product of hydrolysis, ammonia, are considered to be intermediates of nitrogen dioxide reduction to nitrogen by propene over acidic zeolites.