Showing papers on "NOx published in 1997"

Tropospheric air pollution: ozone, airborne toxics, polycyclic aromatic hydrocarbons, and particles.

Abstract: Tropospheric air pollution has impacts on scales ranging from local to global. Reactive intermediates in the oxidation of mixtures of volatile organic compounds (VOCs) and oxides of nitrogen (NOx) play central roles: the hydroxyl radical (OH), during the day; the nitrate radical (NO3), at night; and ozone (O3), which contributes during the day and night. Halogen atoms can also play a role during the day. Here the implications of the complex VOC-NOx chemistry for O3 control are discussed. In addition, OH, NO3, and O3 are shown to play a central role in the formation and fate of airborne toxic chemicals, mutagenic polycyclic aromatic hydrocarbons, and fine particles.

Tropospheric Ozone in a Global-Scale Three-Dimensional Lagrangian Model and Its Response to NOX Emission Controls

Abstract: A three-dimensional Lagrangian tropospheric chemistry modelis used toinvestigate the impact of human activities on the tropospheric distributionofozone and hydroxyl radicals. The model describes the behaviour of 50 speciesincluding methane, carbon monoxide, oxides of nitrogen, sulphur dioxide andnineorganic compounds emitted from human activities and a range of other sources.Thechemical mechanism involves about 100 chemical reactions of which 16 arephotochemical reactions whose diurnal dependence is treated in full. The modelutilises a five minute chemistry time step and a three hour advection timestepfor the 50,000 air parcels. Meteorological data for the winds, temperatures,clouds and so on are taken from the UK Meteorological Office global model for1994 onwards. The impacts of a 50% reduction in European NOXemissions onglobal ozone concentrations are assessed. Surface ozoneconcentrations decrease in summertime and rise in wintertime, but to differentextents.

Method and apparatus for maintaining catalyst efficiency of a NOx trap

Abstract: An engine control computer estimates the cumulative NOx storage in a NOx trap based on engine operating conditions and enters a NOx purge mode of operation when a maximum storage capacity limit, based on trap temperature, is exceeded. The purge mode is terminated when NOx storage level decreases below a minimum storage capacity limit. The rate of NOx dissipation is also based on trap temperature.

Global inventory of NO x sources

Abstract: Nitrogen oxides are key compounds for the oxidation capacity of the troposphere. NOx concentrations depend on the proximity of sources because of their short atmospheric lifetime. An accurate knowledge of the distribution of their sources and sinks is therefore crucial. At global scale, the dominant sources of nitrogen oxides - combustions of fossil fuel (∼50%) and biomass burning (∼20%) - are basically anthropogenic. Natural sources, including lightning and microbial activity in soils, represent therefore less than 30% of total emissions. Fertilizer use in agriculture constitutes an anthropogenic perturbation to the microbial source. The methods to estimate the magnitude and distribution of these dominant sources of nitrogen oxides are discussed. Some minor sources which may play a specific role in tropospheric chemistry such as NOx emission from aircraft in the upper troposphere or input from production in the stratosphere from N2O photodissociation are also considered.

Dependence of ozone production on NO and hydrocarbons in the troposphere

Abstract: An expression for the production rate of 03, P(O 3), is derived based on a radical budget equation applicable to low and high NOx conditions. Differentiation of this equation with respect to NO or hydrocarbons (HC) gives an approximate analytic formula in which the relative sensitivity of P(O3) to changes in NO or HC depends only on the fraction of radicals which are removed by reactions with NOx. This formula is tested by comparison with results from a photochemical calculation driven by trace gas observations from the 1995 Southern Oxidants Study (SOS) campaign in Nashville, Tennessee.

Effect of Several Oxygenates on Regulated Emissions from Heavy-Duty Diesel Engines

Abstract: Oxygenates produce a significant reduction in emissions of particulate matter (PM-10) from diesel engines but in most cases also cause the nitrogen oxide emissions to increase. In this work, several oxygenates having a wide range of properties were blended with no. 2 diesel at the 1 and 2 wt % oxygen level. Emissions were measured using the hot start portion of the U.S. Heavy-Duty Transient Test (40 CFR, Part 86, Subpart N) in both a 2-stroke and a 4-stroke engine. It was found that at this oxygen level PM reductions on the order of 10−15% were obtained regardless of oxygenate chemical structure. The oxygenates affected the integrated NOx emissions differently. Methyl esters of soybean oil increased NOx 2−3%, decanoic acid had no effect, and octanol may have slightly decreased NOx. Examination of real time NOx concentration data for octanol and soy esters indicates that both oxygenates increase NOx during portions of the cycle where the engine is generating high torque at low speed with little or no turbo...

Urea pyrolysis chamber and process for reducing lean-burn engine NOx emissions by selective catalytic reduction

Abstract: Urea is pyrolyzed in a chamber designed to facilitate gasification of the urea by pyrolysis with conversion of urea to ammonia and isocyanic acid (HNCO) with water vapor and carbon dioxide The product gases are introduced into exhaust gases from a lean-burn engine, preferably upstream of a turbocharger The exhaust gases are then contacted with an SCR catalyst

Reducing NOx emissions from an engine by selective catalytic reduction utilizing solid reagents

Abstract: Urea or other solid NO x -reducing reagent is employed in a selective catalytic reduction process on emissions from diesel and lean-burn gasoline engines. The solid reagent is fed to a gas generator that produces a reactant gas by heating. In one embodiment the reactant gas is maintained at elevated temperatures to prevent condensation products from forming. The reactant gas contains ammonia and is fed to the exhaust on an as-needed basis.

Exhaust gas purifying catalyst

Abstract: PROBLEM TO BE SOLVED: To prevent the lowering of specific surface area of a carrier at the time of high temperature and keep high the NOx occlusion power of an NOx occlusion material. SOLUTION: An exhaust gas purifying catalyst is constituted of an amorphous multiple oxide composed of an NOx occlusion material and aluminum, micropore crystals having solid acid and a catalyst noble metal. The lowering of specific surface area at the time of high temperature is prevented by using the amorphous multiple oxide as a carrier, and sulfur toxication is prevented by the oxidization in the presence of micropore crystals.

Satellite measurements of atmospheric ozone profiles, including tropospheric ozone, from ultraviolet/visible measurements in the nadir geometry: a potential method to retrieve tropospheric ozone

Abstract: Numerical studies of a new method for the retrieval of ozone profile information from nadir-observing satellite measurements in the ultraviolet and visible are presented. The method combines information from back scattered radiation in the Hartley band down to the Or concentration peak, lower atmospheric information from the temperature structure of the Huggins bands, and a constraint on the total column from the Chappuis bands. The Huggins bands' temperature structure provides altitude information on the ozone distribution that includes clear distinction between stratospheric and tropospheric ozone. Studies presented here include dependence of the retrieved 0, profiles on surface albedo, tropospheric aerosol, and tropospheric 01 content for a range of atmospheric conditions. Published by Elsevier Science Ltd The importance of the stratospheric ozone layer to terrestrial life and the need for precise long-term monitoring of the stratospheric ozone climatology are well established. Ozone also plays a significant role in the chemistry of the troposphere, where its photolysis leads to the production of OH, the most important tropospheric oxidizing agent.' Haagen-Smit et al2 showed that the production of tropospheric ozone is dependent on the presence of hydrocarbons, oxides of nitrogen, and light. Crutzen3 elucidated the mechanism for the HO,- and NO,-catalysed photochemical generation of ozone in the troposphere. The first experimental evidence that tropospheric ozone may be increasing was reported from balloon-sonde observations made between 1967 and 1982 at the meteorological station on Hohenpeissenberg in southern Germany.4 An analysis of ozone measurements made at the clean air stations of Mauna Loa, Hawaii and Point Barrow, Alaska agreed with this finding and indicated an average increase of ozone of 0.5.-1.5% per year during the period 1974-1989 with the largest increases occurring during summer.5 Volz and Kley6 investigated the ozone measurement technique used by Albert-Levy and co-workers at Montsouris, near Paris, in the late 19th and early 20th centuries and concluded that the method is reliable.7,8 These data indicate that the ozone mixing ratio in clean air was about 10 ppbv at the end of the last century near Paris. Present day surface measurements are a factor of 2-4 larger. As a result of such observations, concern about the impact of tropospheric ozone pollution is rapi'dly growing. To accurately assess the importance of these changes, global measurements of the distribution of tropospheric ozone are required.

Process for generating power including a combustion process

Abstract: The present invention relates to a process for generating power and/or heat in a combustion process where a fuel is combusted with an oxidant which is an O2/CO2/H2O-containing gaseous mixture which is supplied from a mixed conducting membrane which is capable of separating oxygen from oxygen-containing gaseous mixtures at elevated temperatures. The oxygen is picked up from the permeate side of the membrane by means of a sweep gas. The sweep gas is the product or part of the product from at least one combustion process upstream the membrane. The sweep gas can be formed by mixing a recycled part of the exhaust gas containing oxygen and fuel which are passed over a catalytic or non-calalytic burner or combuster. The process produces an exhaust gas with a high concentration of CO2 and a low concentration of NOx making the exhaust gas stream suitable for direct use in different processes or for injection in a geological formation for long term deposition or for enhanced oil and natural gas recovery.

The effect of zeolitic protons on NOx reduction over Pd/ZSM-5 catalysts

Abstract: Formation of PdO particles is observed in ion-exchanged Pd/HZSM-5 andPd/NaZSM-5 after reduction and reoxidation at 500°C. Formation of Pd2+ ions is not observed inPd/HZSM-5 in the absence of NOx, but in an atmorphere of NO2 + O2 the PdO particles are transformed into Pd2+ ions. Even inPd/NaZSM-5, roughly 20% of the PdO particles are converted to ions in NOx; the protons needed for this process are mainly formed during reduction of Pd2+ to Pd0. When a reduced and reoxidizedPd/HZSM-5 catalyst is exposed to CH4 + NO2 + O2, the rate of catalytic NO2 reduction to N2 slowly increases over 24 h from 20 to 50% while PdO is converted to Pd2+, but the rate of CH4 oxidation remains constant with time on-stream. Apparently, Pd2+ ions are needed for SCR of NOx, but for methane oxidation with O2, Pd2+ ions and PdO particles have similar activity.Pd/HZSM-5 is active even when Pd is initially present as PdO particles, butPd/NaZSM-5 remains inactive also when part of the Pd is present as Pd2+ ions. This shows that protons are not only required to convert PdO into Pd2+, but also thatPd/HZSM-5 acts as a bifunctional catalyst. Possible routes for CH4 and C3H8 activation are discussed.

Effects of diet on measurement of nitric oxide metabolites

Abstract: 1. The present study investigated whether a low nitrate/nitrite diet could minimize variability in the measurement of endogenous plasma and urine nitric oxide (NO) metabolites, nitrate and nitrite (NOx) in normal subjects. 2. Nitrate and nitrite concentrations were measured in plasma and urine as indicators of NO production in six subjects during a free diet and then during a low nitrate/nitrite diet for 6 days. 3. The plasma concentration and 24 h urine NOx/creatinine ratio were significantly lower on the low nitrate/nitrite diet than on the free diet (P < 0.01). Nitric oxide production appeared to vary greatly within and between subjects, but these variations were substantially decreased by the fourth day of a low nitrate/nitrite diet. 4. Human plasma and urine NOx measurements should be determined after a low nitrate/nitrite diet for at least 4 days.

Reducing no emissions from an engine by on-demand generation of ammonia for selective catalytic reduction

Abstract: A safe, reliable SCR system for reducing NOx emissions from an internal combustion engine hydrolyzes urea or like reagent under sufficient pressure to assure generation of ammonia, without production of solids that could foul injectors or catalysts. The heat for hydrolysis can be provided by the exhaust or an auxiliary means.

The difference between alkanes and alkenes in the reduction of NO by hydrocarbons over Pt catalysts under lean-burn conditions

Abstract: The activities of Pt/SiO2 and Pt/Al2O3 samples with comparable Pt dispersions for the reduction of NOX under lean-burn conditions using C3H8 and C3H6 as the reductant is reported. C3H6 is more effective than C3H8 at reducing NOX with both catalysts. With C3H6, Pt/SiO2 exhibits greater deNOX activity than Pt/Al2O3, while with C3H8 the opposite was observed. Kinetics measurements show that at steady state during the C3H6-NO-O2 reaction the surface is dominated by carbonaceous species, while during the C3H8-NO-O2 reaction adsorbed oxygen is the main surface species. As a consequence, the surface of the catalyst is quite different for the two reactions (reduced versus oxidised) and this provides a rational explanation for apparent discrepancies in the reaction mechanisms which have been proposed.

Effect of aqueous phase cloud chemistry on tropospheric ozone

Abstract: The sensitivity of tropospheric O3 to aqueous HOx (OH + HO2) phase chemistry in clouds is examined using photochemical model calculations of the O3 production efficiency per unit NOx and the chemical lifetime of O3, combined with estimates for the residence time of air in clouds. It is found that the maximum perturbation to O3 from cloud chemistry in the tropics and midlatitudes summer is less than 3%. This result is supported by calculations using a three-dimensional, continental-scale model for North America.

Peroxy radicals from photostationary state deviations and steady state calculations during the Tropospheric OH Photochemistry Experiment at Idaho Hill, Colorado, 1993

Abstract: Concentrations of peroxy radicals and a number of other trace gases were measured during the Tropospheric OH Photochemistry Experiment in August and September, 1993. The trace gas concentrations were used to derive two estimates of peroxy radical levels: deviations from the photostationary state, and theoretical calculations with the assumption of steady state radical concentrations. As in many previous studies, the photostationary state method yielded midday peroxy radical levels about twice those measured. Calculations were performed to assess the expected uncertainty in the photostationary state derived peroxy radical concentrations from the uncertainty of the input parameters. The agreement between measurements and steady state determinations was better than the photostationary state estimates, with the measurements slightly higher on average. Radical formation and destruction rates for clear sky conditions with low and high NOx were derived from the steady state calculations and the results demonstrate the processes that control peroxy radical levels in clean and polluted continental atmospheres.

A study on selective reduction of NOx by propane on Co-Beta

Abstract: Selective catalytic reduction of NOx by propane was investigated on Co-Beta to clarify the loaded states of cobalt and their role in catalytic activity. At low ion exchange levels less than 100%, loaded cobalt is highly dispersed, which has a high selectivity for NOx reduction. At ion exchange levels much higher than 100%, Co3O4 appears as identified by Raman spectroscopy, and it contributes to propane oxidation by oxygen and lowers the selectivity especially at high temperatures.

Tropospheric Chemistry and Transport

Abstract: Oxidation rates of trace gases in the troposphere depend on species-specific rate coefficients and are predominantly governed by the concentration of the hydroxyl radical (OH), the most important oxidizing molecule in the troposphere. The concentration of OH is in local photochemical steady state; it is, however, dependent on the concentration of trace gases such as ozone, water vapor, volatile organic compounds, and oxides of nitrogen. Diffusive and advective transport processes influence and change the concentrations of these trace gases. Oxidative and transport time scales are often of similar magnitude, which leads to coupling between tropospheric chemistry and transport.