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.     

小风条件下大气扩散模式发展回顾研究

由于小风对污染事件的潜在影响,以及与中等风速和大风条件相比边界层结构、湍流和扩散的特殊性,常规的稳态高斯烟流模式在这种条件下具有局限性,建立适用于小风条件的大气扩散模式具有重要的理论和实际意义。本文回顾了近年来小风条件下大气扩散模式的建立和发展,从较简单的解析模式到复杂的数值模式,并对不同模式的参数化方案、与示踪物实验的比较结果、适用性和不足之处进行了总结。综合考虑不同模式的优劣,在应用方面有利于根据实际情况选择合适的模式,在模式开发方面,有助于建立更加完备的大气扩散模式。     

地形对日本福岛核事故放射性粒子扩散影响的敏感性研究

利用FLEXPART-WRF粒子扩散模式和Stohl等给出的福岛核事故137Cs排放数据,对日本福岛核事故放射性粒子扩散情况进行了数值模拟和有、无地形的敏感性试验;同时假定核污染物排放源位于日本高崎市的RN38站(36.3°N、139.1°E),对向东开口的V型特殊地形进行了敏感性试验. 福岛核事故数值模拟结果表明:放射性污染物的扩散路径、范围和强度既与天气形势有关,还与地形密切相关,不同天气或不同盛行风向条件下,地形的绕流和抬升作用对核污染物扩散输送态势的影响具有明显差异;在偏东气流影响下,由于地形作用,相比无地形时核污染物扩散偏西1个经度左右,偏高约0.5km. V型特殊地形敏感性试验结果表明:在合适的环流形势下(边界层为偏东风,中、高层为偏西风),由于V型地形的阻挡作用,可形成强烈上升运动区,核污染物主要向上、向东扩散输送,向上可扩散至3.0km以上,向东可扩散至145°E左右;而无地形时核污染物向上、向东影响范围减小,向东扩散至140°E,向上扩散至1.5km,但由于无地形阻挡和偏东风影响,向西扩散范围有所增加. 讨论了天气和地形条件对核电站选址的影响,不同天气形势下地形对核污染物扩散的影响并不相同,因此核电站选址应首先考虑选址地的大气环流背景和常见的天气系统,在此基础上再考虑地形的影响.      

液氨泄漏事故的大气环境危害性模拟预测研究

以液氨储罐泄漏事故对大气环境的影响为研究对象,分析了典型化学品泄漏的大气扩散特征,简要介绍了高斯烟团模型,并使用基于该模型的RiskSystem1.2软件对某假定一次液氨储罐发生灾害性事故状态下的泄漏进行模拟。预测出在一定稳定度、不同风速不同时刻下液氨的半致死浓度、重度及轻度伤害范围等的下风向距离和不同风速条件下液氨的最大落地浓度,以期为使用RiskSystem1.2进行大气环境风险评价提供一定帮助,并在此基础上提出具体的防范措施及应急预案,为该类事故的应急救援工作提供一定参考。     

石油化工企业硫化氢泄漏事故环境风险分析

采用风险预测模型SEVEX,在各种不利气象条件下,对硫磺回收装置酸性气管线泄漏的硫化氢的扩散情况进行了模拟分析.结果表明:风险影响区域不仅与硫化氢泄漏的源强有关,而且与事故状态下的气象条件密切相关,在风速大、不稳定的气象条件下,硫化氢泄漏形成的高浓度区多分布在事故源的近距离处;而在静小风、稳定的气象条件下,高浓度区分布范围较大.据此,可以制定应急计划和撤离方案,减轻事故带来的不良影响.     

环渤海区域风能资源WRF同化模拟及特征分析

利用WRF模式及其3DVAR同化模块,结合环渤海区域风能资源观测网测风塔观测数据,设计并构建了多种3DVAR 同化方案,用来提高环渤海区域风能资源数值模拟初始场质量。以2010 年4 月12 日至13 日一次大风过程为研究对象,利用该同化方案进行了一系列WRF数值模拟对比试验。选择较优的3DVAR同化方案对环渤海区域70 m高度风能资源进行了2009 年6 月1 日至2010 年5 月31 日一整年的数值模拟。模拟结果表明,较优的3DVAR同化方案能有效地改进区域风能资源数值模拟初始场质量,主要体现在模拟值与实测值更为接近,模拟效果得到显著改善。环渤海区域70 m高度年平均风速和年平均风功率密度呈现出南小北大的分布特征,渤海中部和北部风能较大,沿垂直于海岸线向内陆方向,风能参数急剧减小。     

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.     

Simulation of dispersion in moderately complex terrain—Part B. The higher order closure dispersion model

A three-dimensional higher-order closure dispersion model is presented. The model is used to simulate dispersion from point sources in complex terrain. The model uses mean and turbulence quantities simulated with the fluid dynamic model presented in Part A to simulate dispersion in a polar coordinate system with its origin in the point source. Different turbulent length scales are used for the vertical and horizontal fluxes. Simulation results are compared with data from tracer experiments performed in southern Sweden, the Vänersborg-Trollhättan region. The tracer experiments were performed during convective atmospheric conditions as well as during very stable conditions. The geographical area has terrain features that exert forcing on the meso-γ-scale. Within the area there is a relatively flat agricultural area, forested hills, a river valley and an extended lake area. The terrain height relief is typically 80 m.The simulations with the dispersion model performed in the Vänersborg-Trollhättan region show good agreement with measured data in the region for convective atmospheric conditions as well as for very stable conditions.     

秸秆焚烧影响南京空气质量的成因探讨

综合利用卫星遥感的火点和云覆盖信息,结合气团后向轨迹分析,探讨了由秸秆焚烧造成的空气污染物的区域尺度输送和本地源对城市空气质量的影响.结果表明,在一定气象条件下,污染物可以发生区域尺度的输送,上风火点与下风城市的污染有明显的相关,将空气污染分为局地型(如,2006年5月31日、2009年11月8日)、区域型(如,2008年10月28日),以及局地区域相结合型(如,2006年6月14日、2007年6月5日、2008年6月2日)3种.应用本文的方法,在有云时,可以通过部分火点和气团后向轨迹分析推测污染物源地.空气污染气象条件分析表明,秸秆焚烧若伴随高空(500hPa)有槽(或位于槽前),低空存在弱切变,气流由周边向中心辐合;同时,若在均压场控制下,等压线稀疏,风速较小或静风,污染物则易积聚而不易输送;逆温层的形成将污染物禁锢在混合层以下,不利于垂直扩散;再加上较大的相对湿度,有利于霾的形成,造成严重空气污染.     

Study on the wind field and pollutant dispersion in street canyons using a stable numerical method

A stable finite element method for the time dependent Navier-Stokes equations was used for studying the wind flow and pollutant dispersion within street canyons. A three-step fractional method was used to solve the velocity field and the pressure field separately from the governing equations. The Streamline Upwind Petrov-Galerkin(SUPG) method was used to get stable numerical results. Numerical oscillation was minimized and satisfactory results can be obtained for flows at high Reynolds numbers. Simulating the flow over a square cylinder within a wide range of Reynolds numbers validates the wind field model. The Strouhal numbers obtained from the numerical simulation had a good agreement with those obtained from experiment. The wind field model developed in the present study is applied to simulate more complex flow phenomena in street canyons with two different building configurations. The results indicated that the flow at rooftop of buildings might not be assumed parallel to the ground as some numerical modelers did. A counter-clockwise rotating vortex may be found in street canyons with an inflow from the left to right. In addition, increasing building height can increase velocity fluctuations in the street canyon under certain circumstances, which facilitate pollutant dispersion. At high Reynolds numbers, the flow regimes in street canyons do not change with inflow velocity.     

街道峡谷型交叉口内气态污染物扩散的数值模拟

对大气边界层内大气湍流和建筑物对道路交叉口处机动车排放气态污染物扩散的影响进行了研究.在计算区域内建立了三维街道峡谷型道路交叉口及其内部机动车排放的模型,并在中性层结条件下,采用CFD(Computational Fluid Dynamics)稳态κ-ε湍流模型和被动标量的输运方程模拟了模型内外的流场和CO浓度场.结果表明:① 在相同高度条件下,交叉口处与处于下风向的街区内CO浓度明显高于其他街区;②风向对污染物的输运起决定性作用,在不同高度平面内CO浓度最大值均出现在平行风向的街区内;③外部大气湍流的驱动使得垂直风向的街区内产生强烈涡旋,涡旋的输运作用导致相同高度下上风向CO浓度较高;④交叉口处气流的掺混导致气流速度降低,使得平行风向的街区内CO可以向两侧垂直风向的街区内扩散,起到了稀释交叉口处地面附近CO浓度的作用.模拟结果与风洞实验结果符合较好,验证了方法的可靠性.      

我国现行大气环境影响评价预测模型的若干问题及其改进

我国现行大气环境影响预测评价模型存在以下几个问题:大气混合层高度求算偏低;稳定条件下大气扩散计算结果不合理;模型未考虑非扩散因素影响(干湿沉积、化学转化);非正常排放模型在小风、静风条件下计算结果偏大;熏烟模型计算的不可操作性等。本文针对这些问题提出相应的改进方法,并进行了实例计算。     

Comparison of three dry deposition models applied to field measurements in the Southern Bight of the North Sea

Dry deposition velocities have been calculated using three different approaches. Turbulent wind profile theory has been used to predict the drag coefficient, wind speed and friction velocity at 10 m height when the wind speed is measured at a higher altitude. The resulting parameters were introduced in a two-layer deposition model. The second approach was the well-known model of Slinn and Slinn (1980, Atmospheric Environment14, 1013–1016), whereas the third corresponded to the model published by Williams (1982, Atmospheric Environment16, 1933–1938). Results point to clear differences. However, in a field experiment carried out at the Southern Bight of the North Sea, all three approaches show relatively comparable results. The role played by the size distribution of atmospheric particulate matter is essential. In our case any of the three models could have given satisfactory outcomes taking into account the wide spread of the experimental results cited in the literature for the same airshed.     

连续点源烟气空间扩散的模拟

空气质量模式一直是处理大气环境监测问题的关键之一。以连续点源烟气的高斯扩散模式为例,实现其空间分布特性的模拟,比较了不同大气稳定状况、不同风速以及不同点源高度对扩散模式的影响,并在此基础上进行多点源叠加污染模拟,从而有助于空气质量的监测与评价。     

广州地区灰霾与清洁天气变化特征及影响因素分析

利用2006~2010年广州气象站逐时地面风场、湿度、能见度观测资料和2006~2009年珠江三角洲其他33个地面观测站风场观测资料,通过对灰霾与清洁天气过程的风速风向进行统计分析与矢量和分析,研究了广州地区灰霾与清洁天气过程的变化特征及影响因素.结果表明:灰霾天气过程在干季发生较频繁,主导风向为东南及东南偏南风,风速一般在1.5m/s以下,静风频率大于多年静风频率;灰霾天气过程风矢量和较小,扩散条件较差.清洁天气过程主要出现在湿季,风速普遍较大且主导风为东南及东南偏南风,干季也可以出现清洁天气过程,主要为较强的北风及偏北风;清洁天气过程风矢量和较大,有较强的平流输送.     

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.     

城郊道路交通污染个例分析

为建立和研究适合地面道路的宽路幅、高等级、支路多等特点的大城市城郊道路交通大气污染扩散模式,选取了受环境外来源影响较小的上海市沪青平公路的徐泾段,作交通大气污染实测和相关气象因子项目的观测。在获得详细实测结果的基础上,以NOx为例分析了大气稳定度、风向、风速、交通量对城郊地面道路交通污染扩散的影响。     

A three-dimensional steady-state atmospheric dispersion-deposition model for emissions from a ground-level area source

An analytical solution to the steady-state three-dimensional atmospheric dispersion equation has been developed for the transport of non-buoyant emissions from a continuous ground-level area source. The model incorporates power law profiles for the variation of wind speed and vertical eddy diffusivity with height, represents the lateral eddy diffusivity as a function of wind speed and the crosswind dispersion coefficient, and includes dry deposition as a removal mechanism. The model is well suited for accurate prediction of emission concentration levels in the vicinity of an area source, as well as farther downwind, under neutral or stable atmospheric conditions. The impact of the important model parameters on contaminant dispersion is examined. The results from several simulations, compared with point and line sources of equivalent source strength, indicate that at short downwind distances, predictions of contaminant concentrations emitted from area sources may be unacceptably inaccurate unless the structure of the source is properly taken into account.     

京津渤地区二氧化硫浓度分布模式

本文介绍一种适用于区域污染并考虑了二氧化硫在输送过程中转化为硫酸盐和被地面吸收等过程的烟团模式。根据北京、天津和渤海地区六氟化硫实验提供的扩散参数及该地区气象资料(风向、风速)和污染源资科,用模式计算出该地区在西北风,东南风为主的气象条件下二氧化硫浓度分布,并与实测值做了比较。     

建筑扰动条件下大气流动与扩散的CFD模拟

用FLUENT模式对中性大气、单个建筑的气流扰动情况进行模拟,并以风洞试验数据检验模拟效果;将模拟方法应用于类似城市建筑阵列条件的大气污染扩散问题,并且与现场示踪试验比较. 结果表明:FLUENT对建筑扰动条件的平均风场模拟效果良好,FAC2(模拟值与试验值之比在0.5~2之间的比例)在水平与垂直风速下分别达到77.9%与61.0%;对湍流特征量的模拟偏差稍大,K(湍流动能)虽总体偏小,但FAC2仍达到了54.6%. 选择湍流闭合的标准K-ε(ε为湍流动能耗散率)方案、重整化群K-ε方案和雷诺应力模型方案对结果的影响均不大. 采用FLUENT模拟了类似城市街区建筑阵列条件的大气扩散个例, 模拟结果反映了建筑扰动导致的扩散烟流轴线相对于平均风向的非常规偏移,并且扩散浓度与示踪试验结果相符较好,下风向32与63m处的侧向模拟浓度峰值的相对误差分别为72.5%与36.9%. 相比于高斯模式ISC3,FLUENT对复杂建筑阵列条件的扩散模拟结果更符合实际,如污染物向上风向扩散以及在建筑物周围堆积与绕流的现象. FLUENT扩散模拟还显示:近源处相邻建筑街道峡谷中的最大浓度沿下风向“阶跃”式减小,排放源所在街道峡谷中的最大浓度可比相邻街谷中的高几倍甚至1个数量级以上.