Improvement of the modelling of daytime nitrogen oxide oxidation in plumes by using instantaneous plume dispersion parameters

The effect of concentration fluctuations on the modelling of non-linear chemical reactions in the atmosphere will be considerable when hourly mean dispersion parameters are used. This effect can be reduced by introducing instantaneous dispersion parameters, as will be shown in this paper for the modelling of the oxidation of NO emited into an O₃ containing atmosphere.We used the results of atmospheric tracer gas experiments to derive the dimensions of the instanteneous plume. With these results model calculations were carried out for the NOO₃ reaction. Results were compared with airborne measurements of NO oxidation in the plumes of Dutch power plants.The introduction of the instantaneous plume leads to better agreement between model calculations and measurements. It is also found that the deviation from photochemical equilibrium in the plume is considerably smaller when instantaneous, instead of hourly, mean dispersion parameters are used. The dimension of the instantaneous plume did not show a dependence on atmospheric stability.     

Large-eddy simulation of a turbulent reacting plume

The results of large-eddy simulations (LES) of a chemically reacting, dispersing plume in a turbulent boundary layer are presented. The detailed simulations of the dispersion mechanisms are used to investigate the effects of turbulent fluctuations on nonlinear chemical reactions. The idealized single reaction between ozone and nitrogen oxides, NO+O₃→NO₂+O₂, is used as a representative simple, irreversible binary reaction. Effects of different reaction rates are investigated by varying the source NO concentration, while maintaining a constant ambient ozone level. The simulations demonstrate that the rate of production of NO₂ is significantly affected by the turbulent nature of the dispersing plume. The effect of incomplete turbulent mixing of the two reactants on the reaction rate can persist for long distances downstream, particularly for the high NO levels typical of power plant stacks. The LES results also show that the important measure of the turbulent fluctuations, the turbulence segregation coefficient, may be reasonably approximated by a constant value over most of the plume cross-section. This result may prove useful for simplified plume modeling, but will depend on whether the range of constant segregation coefficient covers the whole reaction region since this takes place predominantly in the plumeedges.     

An intercomparison of semiempirical diffusion models under low wind speed,stable conditions

The performances of some diffusion models are analysed using concentration data measured at ground level up to 400 m from the emission point, in a series of diffusion tests conducted by U.S. National Oceanic and Atmospheric Administration (NOAA) under inversion conditions with light winds. All tested models are simple semiempirical formulae based on the Gaussian formulation, with different assumptions concerning dispersion parameters; each model utilizes a minimum set of information, i.e. vertical stability category, mean wind speed and standard deviation of the horizontal wind direction over the considered averaging time, σ_θ.Results show that for cases with very low wind speed and large plume spread, explicit consideration of diffusion along the mean wind direction, which is neglected in the standard plume model, significantly improves model results; moreover, when σ_θ is very large (greater than 50–60°), the analysis suggests that standard deviations of the horizontal wind speed may significantly differ from the estimates commonly found in the literature.     

Simulation of dispersion in moderately complex terrain—Part C. A dispersion model for operational use

An operational dispersion model for use in areas with complex terrain is presented. The model uses mean and turbulence quantities simulated with the fluid dynamic model presented in Part A. A large number of wind and turbulence fields are simulated with the fluid dynamic model. These simulations are put into a database and can be used in the calculations of dispersion with the operational model. To get relevant meteorological data for the model a Doppler sodar and a 10 m high mast with a temperature profile and wind and wind direction at one level are used. The model calculates a trajectory for the plume centerline from the simulated wind field, and approximates the concentration field with a bi-Gaussian distribution. For convective conditions the mixing height and the surface heat flux, used as input for the model, are being determined from the sodar measurements through relations related to the temperature structure parameter C_T² and the standard deviation of the vertical velocity. The horizontal and vertical standard deviations for the plume are determined by using the simulated turbulence quantities from the dynamic model and Eulerian velocity spectra. Simulations with the model is compared with dispersion measurements performed in an area in the southern Sweden, the Vänersborg-Trollhättan region. The geographical area is characterized by topographical features on the meso-γ-scale, i.e. 2–20 km. Thus there are forested hills, a relatively flat agricultural area and an extended lake area within the model domain. The terrain height relief is typically 80 m. The simulations show, in general, good agreement with the measured data both for unstable and stable stratifications.     

A photochemical box model for urban air quality study

The photochemical box model (PBM) developed in the present study is based on the principle of mass conservation. It has a horizontal domain of the size of a typical city and a vertical dimension defined by the mixed-layer height. The concentration of any pollutant is determined by horizontal advection, vertical entrainment, source emissions and chemical reactions. A one-dimensional high resolution boundary layer model by Blackadar (Preprints, Third Symp. on Atmospheric Turbulence, Diffusion, and Air Quality, Raleigh, Am. Met. Soc., pp. 443–447, 1976; Advances in Environmental Sciences and Engineering, Vol. 1, No. 1 (edited by Pfafflin J. and Ziegler E.), pp. 50–85. Gordon and Breach, New York, 1979) has been incorporated in the PBM and further developed to consider the effect of urban heat islands in the simulation of mixed layer height. The predicted mixed-layer heights compare very well with observations. The gas phase chemical kinetic mechanism used in the Regional Acid Deposition Model II (RADM2) and that of an earlier version of PBM have been used to calculate the contributions of chemical reactions to the changes of pollutant concentrations. Detailed analysis and comparisons of the two chemical mechanisms have been made. The simulated pollutant concentrations using both chemical mechanisms are in very good agreement with available observations for CO, NO, NO₂ and O₃. A radiative transfer model developed by Madronich (J. geophys. Res.92, 9740–9752, 1987) has been incorporated in the PBM for the calculation of actinic flux and photolytic rate constants. Height-averaged and radiation-corrected photolytic rate constants are used for the photochemical reactions. Budget analyses conducted for CO, NO, NO₂ and O₃ have enhanced our understanding of the relative contributions of horizontal advection, vertical entrainment, source emissions and chemical reactions to the overall rate of change of their concentrations. Model predictions are not sensitive to the large number of peroxy radical-peroxy radical reactions in the RADM2 chemical mechanism under urban conditions.     

Physico-chemical study by two aircraft of a plume from a coal-fired power plant

During the experimental field program that was conducted in October 1985 ton study the physics and chemistry of the Le Havre power plant plume, joints flights were performed with two aircraft from EDF and KEMA. The two episodes with very different meteorological conditions were investigated. The first episode was characterized by fairly constant wind speed and direction with altitude, low solar irradiation and neutral vertical dispersion conditions. The second episode was characterized by low wind speed and changing wind direction, high solar irradiation and stable vertical dispersion conditions. During the latter episode, the concentrations of emitted gases, SO₂ and NO_x remained high, even at several tens of km downwind of the stock. Increased SO₄^2- and NO₃^- concentrations were detected in the power plant plume. In addition, important photochemical activity took p-lace in the plume of a nearby industrial area. The former episode showed more significantly vertical dilution and oxidation of NO but did not reveal any contribution of the power plant plume to aerosol formation.     

Effects of temperature,TSP and per cent non-exchangeable material in determining the gas-particle partitioning of organic compounds

The temperature dependence of the measured, gas-particle partitioning ratio (F_T/TSP)/A_T has been examined for the case when a constant fraction x (%) of a compound is assumed to be bound within the particulate matter, and non-exchangeable with the gas phase. The parameter F_T is the total (exchangeable + non-exchangeable) measured concentration in the atmosphere (ng m⁻³), A_T is the gaseous concentration (ng m⁻³), and TSP is the level of suspended atmospheric particulate matter (μg m⁻³). It is assumed that the true thermodynamic constant K_p depends upon 1/T according to log K_p = m_p/T + b_p where m_p depends on the enthalpy of desorption of the compound of interest, b_p depends in part on other properties of the compound as well as the specific surface area of the particulate matter, and T is the temperature (Kelvin). When K_p or TSP are low, the difference between the measured quantity (F_T/TSP)/A_T and K_p can be significant even when the non-exchangeable fraction x is as low as a few per cent. This approach has been used to examine the PAH data set of Yamasaki et al. [(1982) Env. Sci. Technol.16, 189–194]. It was found that the Yamasaki data set does not allow estimates of x that are consistent with the current understanding of the temperature dependence of log K_p values. A likely reason for this result is some dependence of m_p and b_p on the exact nature of the particulate matter and atmospheric conditions such as relative humidity. It is concluded that estimates of x values for a given compound on actual particulate matter may only be possible by the direct examination of individual particulate matter samples.     

Building wake dispersion at an Arctic industrial site: Field tracer observations and plume model evaluations

Ten multi-hour atmospheric dispersion SF₆ tracer experiments were conducted during October and November of 1987 near a large oil gathering facility in the Prudhoe Bay, Alaska, oilfield reservation. The purpose of this study was to investigate dispersion under arctic conditions and in situations where building-generated airflow disturbances dominate downwind distributions of ground level pollutant concentrations. This was accomplished with a network of micrometeorological instruments, portable syringe tracer samplers, continuous tracer analyzers, and infrared visualization of near source plume behavior.Atmospheric stability and wind speed profiles at this arctic site are influenced by the smooth (surface roughness = 0.03 cm), snow covered tundra surface which receives negligible levels of solar isolation in winter. The dispersion of pollutants emitted from sources within the oil gathering facility, however, is dominated by the influence of nearby buildings when high winds generate elevated ground level concentrations. An order of magnitude increase in maximum ground level concentration was observed as wind speeds increased from 5 to 8 m s⁻¹ and another order of magnitude increase was observed as winds increased from 8 to 16 m s⁻¹. Variation in maximum concentrations was also observed with changes in wind direction. Vertical plume diffusion (σ_z) near the buildings was a factor of 2–3 greater than that observed in open terrain and was dependent on both wind speed and the projected building width and location of nearby buildings. Wind tunnel tracer distributions for east winds agree with field observations but also indicate that a significant increase in plume downwash occurs with other wind directions. Concentration distributions were calculated using several versions of the Industrial Source Complex (ISC) model. Model estimates of ground level concentrations were within a factor of three depending on wind direction. The model predictions are extremely sensitive to the ratio of plume height to vertical plume diffusion which is significantly influenced by a complex aerodynamic wake in the field.     

Dispersion modelling of nitrogen oxides in the United Kingdom

This paper describes the construction and application of an Eulerian-dispersion model for the calculation of NO₂ and NO_x concentrations in the U.K. Model results are compared with measurements from 44 U.K. sites. The sensitivity of the model results to chemical and dispersion parameterisations is assessed. The model is also used to verify the validity of ‘instantaneous mixing’ assumptions used in trajectory models. Finally, the distribution of ground-level NO_x concentrations from power stations, motor vehicles and other stationary sources is calculated across the U.K., including a preliminary assessment of the possible contribution from natural sources.     

Organic peroxy radicals: Kinetics,spectroscopy and tropospheric chemistry

The present state of knowledge of organic, or carbon-based, peroxy radicals (RO₂) is reviewed. Data on the chemical and physical properties of peroxy radicals in the gas-phase is considered, as well as the role of peroxy radicals in tropospheric chemistry and measurements of their concentrations in the atmosphere. Where appropriate, peroxy radicals are grouped together by type (alkyl, acyl, oxygen-substituted, halogen-substituted and aromatic radicals) to facilitate comparison. Data on the hydroperoxy radical (HO₂) is included where it is directly relevant to measurements on organic peroxy radicals, eg. absorption cross-sections used in measurements of RO₂ + HO₂ rate constants. The literature data is critically reviewed and recommendations for absorption cross-sections, rate constants and branching ratios are made where considered appropriate.The laboratory experimental techniques which have been used for the generation and detection of peroxy radicals and the products of their reactions are discussed. The structure, spectroscopy and thermochemistry of the radicals are examined. Although the majority of spectroscopic data concerns the u.v. spectra much used for kinetic studies, near-infrared, infrared and electron spin resonance spectra are also considered. In many cases, peroxy radical u.v. spectra are well-fitted by a Gaussian distribution function, enabling the cross-sections to be easily calculated at any wavelength.For the purpose of this review, the chemical reactions of peroxy radicals are divided into reactions with organic peroxy radicals with HO₂, with NO and NO₂, and finally with other species. Peroxy radical abstraction and addition reactions with closed-shell species are sufficiently slow to be of negligible importance at temperatures pertinent to the atmosphere and are consequently not covered. Data on both the kinetics and mechanisms of peroxy radical reactions are considered.The role of peroxy radicals as intermediates in the atmospheric degradation of volatile organic compounds and in the production of ozone in the troposphere under both low and high [NO_x] conditions is discussed. The involvement of peroxy radicals in night-time oxidation chemistry and the oxidation of halocarbons is also indicated. The techniques used for the difficult measurement of peroxy radical concentrations in the atmosphere are described, together with the results to date.Finally, some tentative suggestions as to further avenues of research are made, based on the data reviewed here and with particular reference to the solution of outstanding problems in atmospheric chemistry. Although a great deal of progress has been made in recent years, it is clear that additional work is needed in most areas covered by this review. New, sensitive and selective laboratory techniques are required for studies of peroxy radical kinetics and high level ab initio calculations would help design laser-based detection techniques. Further product studies of photooxidation systems are needed, particularly as a function of temperature. Recent work has shown that the rate constants for RO₂ + HO₂ reactions used in modelling studies may be too low; if so, these reactions will be correspondingly more important than previously believed in tropospheric oxidation. Recent kinetic studies of the potentially important reactions of methylperoxy radicals with ClO and NO₃ need to be confirmed and mechanistic work is necessary. Although substantial progress has been made towards the monitoring of peroxy radical concentrations in the atmosphere, more work is needed, both on measurements and the development of new techniques.     

Plume rise,entrainment and dispersion in turbulent winds

This paper describes a new analytical model which combines within one theoretical framework several aspects of the phenomena of plume rise, dispersion, thermal stratification and ambient turbulence. The model is based in part on knowledge gained from recent investigations of flow within free shear layers. The observations suggest a simple model for the turbulent mixing process, which accounts for the known entrainment of air into smoke plumes by plume-generated turbulence. More importantly, the model also predicts a path by which ambient turbulence causes reverse entrainment of plume material into the surrounding fluids. This gives rise to a new ‘extrainment’ term in each of the plume momentum and buoyancy equations.These equations are solved for a turbulent atmosphere of arbitrary thermal stability, and yield plume trajectories which gradually level off at final rise heights that depend on the degree of thermal stratification and on the scale and intensity of ambient turbulence. A link between plume rise and dispersion is identified by means of the concentration species equation, which is solved to show that the plume acts along its length as a distributed source of passively dispersing material.The new theory, specialized for an adiabatic atmosphere, plus the familiarx^2/3 law and a semi-empirical final rise theory from the literature, are all compared against full-scale data on plume rise in turbulent winds. The new theory significantly improves the accuracy of estimates of plume trajectory and final plume height. The price for this improved predictive ability is the need to evaluate the air temperature and its gradient at plume level, and the corresponding intensity and scale of turbulent air movement. This is no longer a technical obstacle since recently developed SODAR and RASS remote sensors have this capability.     

Effects of irrigation,fertilization and crop straw management on nitrous oxide and nitric oxide emissions from a wheat–maize rotation field in northern China

One-year winter wheat–summer maize rotation is the most popular double cropping system in north-central China, and this highly productive system is an important source of nitrous oxide (N₂O) and nitric oxide (NO) emissions due to the high fertilizer N and irrigation water inputs. To sustain the high crop production and mitigate the detrimental impacts of N₂O and NO emissions, improved management practices are extensively applied. The aim of this study is therefore to evaluate the effects of an improved management practice of irrigation, fertilization and crop straw on grain yield and N₂O and NO emissions for a wheat–maize rotation field in northern China. Using automated and manual chamber measuring systems, we monitored N₂O and NO fluxes for the conventional (CT, 2007–2008), improved (IT, 2007–2008), straw-amended (WS, 2008–2009), straw-not-amended (NS, 2008–2009), and no N-fertilizer treatments (WS–NN, 2008–2009), respectively, for one rotation-year. The grain yields were determined for CT and IT for three rotation-years (2005–2008) and for WS, NS and WS–NN for one rotation-year (2008–2009). The improved management of irrigation and fertilization reduced the annual N fertilization rate and irrigation amount by 17% and 30%, respectively; increased the maize yield by 7–14%; and significantly decreased the N₂O and NO emissions by 7% (p < 0.05) and 29% (p < 0.01), respectively. The incorporation of wheat straw increased the cumulative N₂O and NO emissions in the following maize season by 58% (p < 0.01) and 13%, respectively, whereas the effects of maize straw application were not remarkable. The N₂O and NO emission factors of applied N were 2.32 ± 2.32% and 0.42 ± 1.69% for wheat straw and 0.67 ± 0.23% and 0.54 ± 0.15% for chemical N-fertilizers, respectively. Compared to conventional management practices using high application rates of irrigation water and chemical N-fertilizer as well as the field burning of crop straw, the improved management strategy presented here has obvious environmentally positive effects on grain yield and mitigation of N₂O and NO emissions.     

The effect of freezing on the composition of supercooled droplets—II. Retention of S(IV)

S(IV) dissolved in droplets is partially evolved as SO₂ during freezing. A spray of droplets with average diameter 39μm, produced by an ultrasonic transducer, was let to fall through a controlled atmosphere with known SO₂ concentration, at varying temperatures between −8 and −23°C, attaining thermal and chemical equilibrium. In a first arrangement, the droplets fell by gravitation on an ice surface. Two other arrangements simulated the riming ventilation conditions: in one series of experiments, the droplets were projected by a gas jet at several m s⁻¹ against a target; in another, the droplets were caught by rotating rods. The fraction of S(IV) retained in the ice Γ was determined by analysis of the samples and comparison with the equilibrium concentration in the liquid droplets. Samples collected by gravitation showed a retention coefficient Γ = 0.25 + 0.012 T_s (T_s = supercooling); rime samples showed large dispersion in the results, the retention coefficient being best represented by an average value Γ = 0.62, independent of temperature.     

Studying high ozone concentrations by using the Danish Eulerian model

The long-range transport of air pollutants (LRTAP) over Europe is studied by a mathematical model based on a system of partial differential equations (PDEs). The number of PDEs is equal to the number of species studied and the model contains 35 species at present. Among the species are NO, NO₂, NO₃⁻, HNO₃, NH₃, NH₄⁺, O₃, PAN, SO₂, SO₄²⁻ and may hydrocarbons. Most of the 70 chemical reactions involved in the model are nonlinear (including here many photochemical reactions).The model requires large sets of input data. Emissions of SO₂, NO_x, NH₃ and both natural and anthropogenic volatile organic compounds (VOC) are needed in the model. The meteorological data consist of fields of wind velocities, precipitation, surface temperatures, temperatures of the boundary layer, relative humidities and cloud cover, which are read in the beginning of every 6-h interval. Both daytime and nighttime mixing heights are used in the model.Many of the species in the model vary on a diurnal basis. An investigation of the main mechanisms that determine the diurnal variation of the ozone concentrations is performed. One of the important conditions that is necessary if one wants to represent correctly the diurnal variations of the concentrations is to have access to meteorological data that vary diurnally. This is especially true for the temperature and the mixing height.The use of modern numerical algorithms (which are combined with vectorization of the most time-consuming numerical procedures) allows one to perform long-term runs with the model on several high-speed computers. Results obtained in runs with meteorological data for July 1985 and August–October 1989 are discussed. The computed concentrations and depositions are compared with measurements taken at stations located in different European countries. The agreement between calculated concentrations and measurements is reasonably good.Results obtained with several scenarios, in which the NO_x emission and/or the anthropogenic VOC emissions are varied, are presented. Several main conclusions are drawn by studying the results obtained during the comparisons. Some plans for future development of the models are discussed.     

Measurements of wind speed and plume rise with a rapid-scanning Lidar

A series of 30-min means of plume rise and dispersion parameters were measured at Fawley power station on Southampton Water between May and December 1989 using a rapid-scanning Lidar. Where possible, plume parameters were determined at three distances, typically 250, 750 and 1500 m, from the stack. Simultaneous measurements of temperature profiles were made using model aircraft and of wind speed and direction using a cup anemometer and wind vane at a height of 44 m on a transmission tower. By matching time series of plume heights at different distances, absolute measurements of wind speed at plume height could be made. These measurements gave a more reliable estimate of plume rise than did surface wind speeds. It was found that, for travel distances out to 1.5 km, the Briggs formula gave a robust prediction of plume heights with C₁ = 1.38±0.07 (95% confidence limits). At Fawley, the standard error in the predicted plume height at 500 m was then 21.0 m, or 19% of the 30-min mean plume rise. For plume-level winds of less than 20 m s⁻¹, analysis of the dependence of plume height on distance, wind speed and power-station load gave a virtual source height of 195.2±10.3 m (95% confidence limits). This is identical with the physical stack height of 198.1 m.     

Sodar in air pollution meteorology

Information concerning atmospheric stability and turbulence is basic in studies related to air pollution meteorology. In this context a monostatic sodar has been operating at the National Physical Laboratory, New Delhi, for many years to probe the thermal structure of the lower atmosphere in real time and space.Various observed thermal structures viz. nocturnal inversion, thermal plumes, formation and break-up of inversion, orographical mixing, multilayered structures and waves etc., are the result of the different atmospheric stabilities and turbulence conditions. These structures have been studied in the light of surface measurements of wind speed and direction for organized indexing of the structural details to infer meteorological conditions concerning Pasquill stability classifications.The height of the ground-based thermal structure has been correlated with the Richardson number, Ri, for quantitative estimation of the turbulence parameter. Estimated values of Ri have been further used to determine the cross wind dispersion coefficient, σ_y. The results obtained are realistic and can be used as input parameters for air pollution modelling.     

北京城区夏季O3化学生成过程

选取2007年7月1日—8月31日中的21个晴空日,利用观测资料和光化学箱模式计算了北京城区测点的O₃生成速率G(O₃)和O₃生成效率OPE.结果表明,21个晴空日中G(O₃)日最高小时值分布在(18~82)×10^-9h^-1之间;在O₃污染和非污染日G(O₃)最高值的平均水平无显著差异,且与O_x浓度之间不存在一致的对应关系,表明O₃化学生成过程不能全面解释地面O₃浓度的累积,物理传输过程对测点O₃实测浓度有显著作用;各个化学过程对G(O₃)的贡献率对比结果显示,HO₂ 在 NO向NO₂的转化中贡献最大;OPE值分布在2.8~5.8之间,总体水平为4.1±0.1;OPE值与NO_x浓度之间为非线性关系,OPE值随NO_x浓度的增加而减少,表明消减测点附近VOCs排放能有效降低O₃浓度.     

Effects of synthesis methods on the performance of Pt + Rh/Ce0.6Zr0.4O2 three-way catalysts

The 0.7 wt% Pt + 0.3 wt% Rh/Ce0.6Zr0.4O2 catalysts were fabricated via different methods, including ultrasonic-assisted membrane reduction(UAMR) co-precipitation, UAMR separation precipitation, co-impregnation, and sequential impregnation. The catalysts were physico-chemically characterized by N2 adsorption, XRD, TEM, and H2-TPR techniques, and evaluated for three-way catalytic activities with simulated automobile exhaust. UAMR co-precipitation- and UAMR separation precipitationprepared catalysts exhibited a high surface area and metal dispersion, wide λ window and excellent conversion for NOx reduction under lean conditions. Both fresh and aged catalysts from UAMRprecipitation showed the high surface areas of ca. 60–67 m2/g and 18–22 m2/g, respectively, high metal dispersion of 41%–55%, and small active particle diameters of 2.1–2.7 nm. When these catalysts were aged, the catalysts prepared by the UAMR method exhibited a wider working window(Δλ = 0.284–0.287) than impregnated ones(Δλ = 0.065–0.115) as well as excellent three-way catalytic performance, and showed lower T50(169.C) and T90(195.C) for NO reduction than the aged catalysts from impregnation processes, which were at 265 and 309.C, respectively. This implied that the UAMR-separation precipitation has important potential for industrial applications to improve catalytic performance and thermal stability. The fresh and aged 0.7 wt% Pt + 0.3 wt% Rh/Ce0:6Zr0:4O2 catalysts prepared by the UAMR-separation precipitation method exhibited better catalytic performance than the corresponding catalysts prepared by conventional impregnation routes.