Investigation of air pollution distribution in Linz: case studies to evaluate a K-type diffusion model coupled with a mass-consistent wind model

An Eulerian diffusion model coupled with a refined mass consistent wind model is developed for the operational forecasting of pollution distribution in complex terrain. The model is evaluated for a city situated in complex terrain. The study is carried out for a 20×20 km² area surrounding Linz, one of the industrial cities of Austria. The models are initialized with routinely measured meteorological parameters and topology information derived from the Geographical Information Systems (GIS). Four case studies, representative for major pollution episodes, are presented for model evaluation. The case studies include presence of a thermally induced wind system, presence of cold front an easterly southeasterly flow and a westerly–northwesterly flow. In presence of thermally induced wind systems, the flow field is most complex and existence of a shallow mixed layer with residual layer aloft enhances the pollution levels inside the city. Second case is used to study the development of pollution distribution inside the city in presence of low-level inversions and associated with low surface wind speeds. The low wind speeds in the surface layer lead to less mechanical generation of turbulence and lateral transport. The case studies of easterly and westerly flow fields are carried out to assess the capability of model under most frequently observed meteorological situations. The model is able to reproduce the pollution distribution near the slopes. There were over prediction inside the city in presence of thermally induced wind systems and is attributed to inadequate model physics during convective case. The present model setup is found to be a useful tool for the routine forecasting of pollution and could also be tested for other complex terrains.     

The influence of locally induced wind systems on the effectiveness of nocturnal dry deposition of ozone

The structure of the planetary boundary layer above a valley of the Swiss Plateau is investigated.The very high night-time loss rates of O₃ observed are mainly attributable to dry deposition at the surface. Although it can be inferred that smog-chemistry plays an important role, homogeneous chemistry accounts on average for only about 20% of the observed O₃ destruction rates.It is concluded that the dry deposition of O₃ at night is much more effective in the hilly terrain around the field site than over horizontally homogeneous terrain. This increase in efficiency is linked with a local wind system.The O₃ destruction due to dry deposition occurs mainly in layers of nocturnal downslope wind where O₃-rich air is brought to the ground during the whole night. Hence the air reaching the bottom of the valley has a very low O₃ content and the upward airflow associated with this local wind system leads to continuous O₃ depletion within the lower part of the boundary layer.It follows that the increased effectiveness of the dry deposition in hilly terrain results in a self-cleaning mechanism of the atmosphere which is not necessarily restricted to O₃.     

Numerical study of atmospheric dispersion of a substance released from an industrial complex in the southern coast of Korea

Diurnal variations of wind field and pollutant dispersion in a complex terrain with a shoreline were investigated under the insolation conditions of summer and winter. The area is located in the south of the Korean Peninsula and includes a large petrochemical industrial complex. The Regional Atmospheric Modeling System (RAMS) was used in the simulation study. Initially, horizontally homogeneous wind fields were assumed on the basis of sounding data at the nearby upper-air station for days with morning wind speeds below 2 m s⁻¹. On these days, the sea breeze prevailed in summer while the land breeze lasted for a few hours in the morning; the effect of synoptic winds was strong in winter with some inclusion of wind variations owing to the interaction between sea and land. The predicted wind direction at the location of the weather station captured an important change of the sea/land breeze of the observed one. In the morning, both in summer and winter, complicated wind fields with low wind speeds resulted in high pollutant concentrations almost all over the area. On the other hand, in the afternoon, the wind field was rather uniform and the terrain effects were not significant even in the mountainous area with the development of a mixing layer.     

Gas diffusion over an isolated hill under neutral,stable and unstable conditions

Wind tunnel experiments of gas diffusion were performed over flat terrain and over an isolated three-dimensional hill under neutral, stable and unstable (sea breeze) conditions. Conditions of airflow in the wind tunnel were determined so as to satisfy the similarity rule for the bulk Richardson number, by controlling temperature profiles and wind velocity of the thermally stratified wind tunnel. Typical characteristics were observed under each condition of atmospheric stability; reversed vortex behind the hill in neutral condition, downward slope wind in stable one and convective motion in unstable one.We compared these experiments with the results of a Direct Numerical Simulation (DNS) model for the wind velocity over the hill under neutral conditions. The numerical results showed good agreement with the experimental results.     

Comparison of the complex terrain algorithms incorporated into two commonly used local-scale air pollution dispersion models (ADMS and AERMOD) using a hybrid model

ADMS and AERMOD are the two most widely used dispersion models for regulatory purposes. It is, therefore, important to understand the differences in the predictions of the models and the causes of these differences. The treatment by the models of flat terrain has been discussed previously; in this paper the focus is on their treatment of complex terrain. The paper includes a discussion of the impacts of complex terrain on airflow and dispersion and how these are treated in ADMS and AERMOD, followed by calculations for two distinct cases: (i) sources above a deep valley within a relatively flat plateau area (Clifty Creek power station, USA); (ii) sources in a valley in hilly terrain where the terrain rises well above the stack tops (Ribblesdale cement works, England). In both cases the model predictions are markedly different. At Clifty Creek, ADMS suggests that the terrain markedly increases maximum surface concentrations, whereas the AERMOD complex terrain module has little impact. At Ribblesdale, AERMOD predicts very large increases (a factor of 18) in the maximum hourly average surface concentrations due to plume impaction onto the neighboring hill; although plume impaction is predicted by ADMS, the increases in concentration are much less marked as the airflow model in ADMS predicts some lateral deviation of the streamlines around the hill.     

Experimental Characterization of Atmospheric Diffusion in Complex Terrain with Land-Sea Interactions

The body of information presented in this paper is directed to scientists working in atmospheric dispersion research and model development. Two years of field measurements in the coastal area of Bilbao in northern Spain show that the diffusion behavior in this complex terrain can be classified into several well defined patterns, which correspond to certain meteorological conditions. The approach taken has been the systematic use of SO₂ remote sensors (COSPEC) and ground level monitors in moving platforms which are used to follow and document the flow of the air mass. Results to date show that complex reentry cycles can occur and that synoptically different flows may be indistinguishable by wind sensors at ground level (affected by channeling), and yet result in totally different observed pollution levels by a fixed monitoring network (affected by topographical effects). These results are being used to parameterize the cause-effect relationships and guide the modeling efforts in this area of complex terrain.     

A Lagrangian stochastic model for predicting concentration fluctuations in urban areas

A combined Lagrangian stochastic model with a micromixing sub-model is used to estimate the fluctuating concentrations observed in two wind tunnel experiments. The Lagrangian stochastic model allows fluid trajectories to be simulated in the inhomogeneous flow, while the mixing model accounts for the dissipation of fluctuations using the interaction by exchange with the mean (IEM) mechanism. The model is first tested against the open terrain, wind tunnel data of Fackrell, J.E. and Robins, A.E. [1982. Concentration fluctuations and fluxes in plumes from point sources in a turbulent boundary layer. Journal of Fluid Mechanics 117, 1–26] and shows good agreement with the observed mean concentrations and fluctuation intensities. The model is then compared with the wind tunnel simulation of a two-dimensional street canyon by Pavageau, M. and Schatzmann, M. [1999. Wind tunnel measurements of concentration fluctuations in an urban street canyon. Atmospheric Environment 33, 3961–3971]. Despite the limitations of the k–ε turbulence scheme and the IEM mixing mechanism, the model reproduces the fluctuation intensity pattern within the canyon well. Overall, the comparison with both sets of wind tunnel experiments are encouraging, and the simplicity of the model means that predictions in a complex, three-dimensional geometry can be produced in a practicable amount of time.     

Evaluation of a Puff Dispersion Model in Complex Terrain

California's Pacific Gas and Electric Company has many power plant operations situated in complex terrain, prominent examples being the Geysers geothermal plant in Lake and Sonoma Counties, and the Diablo Canyon nuclear plant in San Luis Obispo County. Procedures ranging from plant licensing to emergency response require a dispersion modeling capability in a complex terrain environment. This paper describes the performance evaluation of such a capability, the Pacific Gas and Electric Company Modeling System (PGEMS), a fast response Gaussian puff model with a three-dimensional wind field generator.

Performance of the model was evaluated for ground level and short stack elevated release on the basis of a special intensive tracer experiment in the complex coastal terrain surrounding the Diablo Canyon Nuclear Power Plant in San Luis Obispo County, California. The model performed well under a variety of meteorological and release conditions within the test region of 20-kilometer radius surrounding the nuclear plant, and turned in a superior performance in the wake of the nuclear plant, using a new wake correction algorithm for ground level and roof-vent releases at that location.     

A complex terrain dispersion model for regulatory applications at the Westvaco luke mill

A data set for studying transport and dispersion in complex terrain was collected at the Westvaco Corporation's Luke Mill, located in the Potomac River valley in western Maryland. Meteorological analyses indicate very strong channeling of winds and the presence of strong inversions and wind shears in a shallow layer at the height of the surrounding mountaintops (300 m above the valley floor). Wind velocities observed near the valley floor are unrepresentative of wind velocities at plume height. Observed turbulence intensities at plume height are about twice as large as those observed over flat terrain. Standard stability classification schemes generally underestimate plume dispersion at this site. When high 3-h and 24-h average SO₂ concentrations are observed, winds are usually light and an inversion is present. These instances of relatively high concentrations are often associated with periods when the wind shifts direction 180° from up-valley to down-valley or vice versa, and the nearly stagnant polluted air mass blows against the mountainsides.A dispersion model was developed that is Gaussian in form but uses observed meteorological data to the maximum extent possible. For example, observed turbulence intensities at plume height are used to estimate dispersion. Plume impaction on terrain is calculated if the plume height is below a critical height dependent on the Hill Froude number. Evaluation of the model with the full 2-y data set shows that it can estimate the second highest 3-h and 24-h average concentrations (of regulatory significance) with a mean bias of less than 7%.     

A Microcomputer-based Forecasting Model: Potential Applications for Emergency Response Plans and Air Quality Studies

Most atmospheric transport and diffusion models within emergency response systems have very limited physics and are forced to rely on the assumption that wind and turbulence conditions at the time of the release will be representative over the period for which dispersion must be predicted. For releases where the principal concern is about the first few kilometers of travel, such an assumption is appropriate. However, for large accidental releases during stable conditions, the plume may travel for several hours before it is diluted to safe levels and the assumption of persistence may be inappropriate, particularly for transport in complex terrain. Under these circumstances, a model that can forecast changes in wind and turbulence conditions is required. We have installed such a model on microcomputers and tested it in complex terrain near Salt Lake City. One-hour tracer releases produced surface concentrations that remain high for much longer times than that expected based on one hour’ travel time with the mean wind at the source height. Furthermore, relatively large concentrations were found at distances of over 40 km from the source. The model was generally able to reproduce the principal features described by the measurements, although some effects of subgrid scale terrain were missed.     

冬季沈阳市典型源排放PM10浓度分布模拟分析

选取沈阳市7个典型的大气污染源2006年12月～2007年2月的PM10排放浓度资料,利用CALPUFF对PM10浓度月平均分布做模拟分析。模拟结果分析表明：冬季月平均PM10浓度分布的范围与风场、地形有直接的关系。地势平坦、风速大时,污染物扩散范围大,污染物浓度小;地势不平、风速小时,污染物扩散范围小,污染物浓度大。1月份是沈阳市冬季月平均大气污染最严重的月份,污染物分布主要集中在市区的北部、东部和南部地区,东部地区大气污染最为严重。     

Test a modified surface wind interpolation scheme for complex terrain in a stable atmosphere

This study proposes a modified scheme for surface wind interpolation respective to stable atmospheres. A potential flow concept is applied to the simple 1/r² interpolation to handle direct terrain effects. A procedure is designed to pretreat digital terrain data. Numerical tests are made for the Hong Kong area with a diagnostic wind field model to compare the influences of the modified interpolation. The effects of the adjustment factor α in the diagnostic model are also discussed.     

Verification of the shelter effect of a windbreak on coal piles in the POSCO open storage yards at the Kwang-Yang works

The use of windbreaks to reduce wind-blown coal dust at the POSCO Kwang-Yang open storage yards was studied using wind simulations on a scale model of the yards. Based on these simulation results, a full-scale wind fence was constructed on two sides of the yard. Here, we present results on the wind behavior both for the real yard and for the simulation results that guided its construction. Wind-tunnel simulations were used to study the effect of a porous wind fence of porosity ε=30% on the surface pressure and shear stress on coal piles using a 1/1200 model of the POSCO Kwang-Yang open storage yards. In addition, the shelter effects found in the model system were verified in field measurements on the full-scale system. The storage yard model was fully embedded in an atmospheric surface boundary layer over open terrain. The fence and coal pile model had the same height (12.2 mm) and Reynolds number (Re=1.6×10⁴, based on the model height). The mean and fluctuating surface-pressure distributions on the coal piles, which are closely related to the dust emission from the surface, were measured for several directions of the oncoming wind. The wind directions pertinent to the study were determined by statistical analysis of seasonal wind data over the storage yard. A porous wind fence of porosity ε=30% was found to be useful for reducing the wind speed without the formation of a recirculating bubble behind the fence. In addition, the fence caught the wind-borne particles when it was located behind the coal piles. The wind fence reduced the pressure fluctuations and surface shear stress on the coal piles to less than half of the levels observed in the no fence case. To verify the effectiveness of the porous wind fence installed around the Kwang-Yang open storage yard, the local wind speed and the concentration of suspended particles were measured directly. Full-scale porous fences installed around the Kwang-Yang open storage yard greatly decreased the turbulence intensity of the wind over the coal piles and reduced the total suspension particles by 70–80%.     

冬季沈阳市典型源排放PM_(10)浓度分布模拟分析

选取沈阳市7个典型的大气污染源2006年12月~2007年2月的PM10排放浓度资料,利用CALPUFF对PM10浓度月平均分布做模拟分析。模拟结果分析表明:冬季月平均PM10浓度分布的范围与风场、地形有直接的关系。地势平坦、风速大时,污染物扩散范围大,污染物浓度小;地势不平、风速小时,污染物扩散范围小,污染物浓度大。1月份是沈阳市冬季月平均大气污染最严重的月份,污染物分布主要集中在市区的北部、东部和南部地区,东部地区大气污染最为严重。     

Improving ozone modeling in complex terrain at a fine grid resolution: Part I – examination of analysis nudging and all PBL schemes associated with LSMs in meteorological model

Meteorological variables such as temperature, wind speed, wind directions, and Planetary Boundary Layer (PBL) heights have critical implications for air quality simulations. Sensitivity simulations with five different PBL schemes associated with three different Land Surface Models (LSMs) were conducted to examine the impact of meteorological variables on the predicted ozone concentrations using the Community Multiscale Air Quality (CMAQ) version 4.5 with local perspective. Additionally, the nudging analysis for winds was adopted with three different coefficients to improve the wind fields in the complex terrain at 4-km grid resolution. The simulations focus on complex terrain having valley and mountain areas at 4-km grid resolution. The ETA M–Y (Mellor–Yamada) and G–S (Gayno–Seaman) PBL schemes are identified as favorite options and promote O₃ formation causing the higher temperature, slower winds, and lower mixing height among sensitivity simulations in the area of study. It is found that PX (Pleim–Xiu) simulation does not always give optimal meteorological model performance. We also note that the PBL scheme plays a more important role in predicting daily maximum 8-h O₃ than land surface models. The results of nudging analysis for winds with three different increased coefficients' values (2.5, 4.5, and 6.0 × 10^?4 s^?1) over seven sensitivity simulations show that the meteorological model performance was enhanced due to improved wind fields, indicating the FDDA nudging analysis can improve model performance considerably at 4-km grid resolution. Specifically, the sensitivity simulations with the coefficient value (6.0 × 10^?4) yielded more substantial improvements than with the other values (2.5 and 4.5 × 10^?4). Hence, choosing the nudging coefficient of 6.0 × 10^?4 s^?1 for winds in MM5 may be the best choice to improve wind fields as an input, as well as, better model performance of CMAQ in the complex terrain area. As a result, a finer grid resolution is necessary to evaluate and access of CMAQ results for giving a detailed representation of meteorological and chemical processes in the regulatory modeling. A recommendation of optimal scheme options for simulating meteorological variables in the complex terrain area is made.     

电除尘器进气烟箱参数对气流分布影响的仿真研究

电除尘器进气烟箱结构参数不仅对除尘器内部气流分布的均匀性产生重要影响,而且还会影响除尘效率和排放指标。针对电除尘器进气烟箱结构参数选取与气流分布均匀性问题,采用欧拉-拉格朗日多相流模型,对进气烟箱扩散角,气流分布板孔形、开孔率和导流板角度等参数,进行了三维流场模拟仿真计算,获得各参数对气流均匀性的影响关系规律。仿真结果表明:方形孔的气流分布板能获得较佳的气流分布均匀性;合理调节进气烟箱扩散角、开孔率和导流板角度可有效地改善电场内气流分布状况。研究结果可为电除尘器进气烟箱的设计与改进提供依据。     

Evaluation of low wind modeling approaches for two tall-stack databases

The performance of the AERMOD air dispersion model under low wind speed conditions, especially for applications with only one level of meteorological data and no direct turbulence measurements or vertical temperature gradient observations, is the focus of this study. The analysis documented in this paper addresses evaluations for low wind conditions involving tall stack releases for which multiple years of concurrent emissions, meteorological data, and monitoring data are available. AERMOD was tested on two field-study databases involving several SO₂ monitors and hourly emissions data that had sub-hourly meteorological data (e.g., 10-min averages) available using several technical options: default mode, with various low wind speed beta options, and using the available sub-hourly meteorological data. These field study databases included (1) Mercer County, a North Dakota database featuring five SO₂ monitors within 10 km of the Dakota Gasification Company’s plant and the Antelope Valley Station power plant in an area of both flat and elevated terrain, and (2) a flat-terrain setting database with four SO₂ monitors within 6 km of the Gibson Generating Station in southwest Indiana. Both sites featured regionally representative 10-m meteorological databases, with no significant terrain obstacles between the meteorological site and the emission sources. The low wind beta options show improvement in model performance helping to reduce some of the overprediction biases currently present in AERMOD when run with regulatory default options. The overall findings with the low wind speed testing on these tall stack field-study databases indicate that AERMOD low wind speed options have a minor effect for flat terrain locations, but can have a significant effect for elevated terrain locations. The performance of AERMOD using low wind speed options leads to improved consistency of meteorological conditions associated with the highest observed and predicted concentration events. The available sub-hourly modeling results using the Sub-Hourly AERMOD Run Procedure (SHARP) are relatively unbiased and show that this alternative approach should be seriously considered to address situations dominated by low-wind meander conditions.

Implications: AERMOD was evaluated with two tall stack databases (in North Dakota and Indiana) in areas of both flat and elevated terrain. AERMOD cases included the regulatory default mode, low wind speed beta options, and use of the Sub-Hourly AERMOD Run Procedure (SHARP). The low wind beta options show improvement in model performance (especially in higher terrain areas), helping to reduce some of the overprediction biases currently present in regulatory default AERMOD. The SHARP results are relatively unbiased and show that this approach should be seriously considered to address situations dominated by low-wind meander conditions.     

An Eulerian transport/transformation/removal model for SO2 and sulfate—I. Model development

A combined transport/chemistry model which simulates the regional distribution of SO₂ and sulfate within the lower troposphere is described. The mathematical analysis is based on the coupled three-dimensional advection-diffusion equations for SO₂ and sulfate, and incorporates chemical transformations as well as the physical phenomena of dry deposition at the surface. The analysis also considers spatial variations in topography and spatial and temporal variations in both the mixing layer heights and the wind field. Based on the results from a series of numerical experiments, the dynamic model employs a Galerkin method for the numerical solution of the partial differential equations.A SO₂ photochemical oxidation mechanism is incorporated into the transport model. The SO₂ photochemical oxidation rate is based on a set of 27 reactions used to estimate the hydroxyl and peroxyl radical concentrations. The kinetic mechanism has been tested in simulations of smog chamber studies and yields realistic concentrations and conversion rates in model simulations of both urban and natural tropospheres.Other major facets treated in the formulation of the model include the interpretation and use of data available on dry deposition and the development of procedures to calculate meteorological model inputs (e.g., eddy diffusivities, dry deposition velocities, the three components of wind velocity, etc.) from routinely measured meteorological data. Simulations using the analysis are presented in a companion paper.     

A directional passive air sampler for monitoring polycyclic aromatic hydrocarbons (PAHs) in air mass

A passive air sampler was developed for collecting polycyclic aromatic hydrocarbons (PAHs) in air mass from various directions. The airflow velocity within the sampler was assessed for its responses to ambient wind speed and direction. The sampler was examined for trapped particles, evaluated quantitatively for influence of airflow velocity and temperature on PAH uptake, examined for PAH uptake kinetics, calibrated against active sampling, and finally tested in the field. The airflow volume passing the sampler was linearly proportional to ambient wind speed and sensitive to wind direction. The uptake rate for an individual PAH was a function of airflow velocity, temperature and the octanol-air partitioning coefficient of the PAH. For all PAHs with more than two rings, the passive sampler operated in a linear uptake phase for three weeks. Different PAH concentrations were obtained in air masses from different directions in the field test.     

Impact of using prognostic and objective wind fields on the photochemical modeling of Athens, Greece

A three-dimensional Eulerian photochemical model is used to follow the dynamics of ozone, NO_x, and CO over the Athens area, for 25 May 1990, the day considered in the APSIS project. A unique aspect of this work lies in the study of the impacts of the wind field preparation methods on the concentrations predicted by the model. Three sets of wind fields are developed. The first one used is derived from a prognostic meteorological model. The second one is calculated from available wind observations using objective: methods. For these two cases, a previous day is simulated, using the same conditions, to develop preconditioned initial conditions for the following day. For the third simulation, again two days are simulated, this time using the observed winds for each of the two days modeled. The predictions using the prognostically derived and the objective analysis wind fields are significantly different, particularly for the primary pollutants. Comparing predictions to the observations did not favor any particular method of wind field preparation. In this case, when using the prognostically derived field, the simulations are very sensitive to boundary conditions. In contrast, when using the wind fields constructed by objective methods, the simulations became most sensitive to emissions and initial conditions. This comes directly from the different residence times in the domain, which are governed by the wind speed.