Dietmar Oettl

A New Method to Estimate Diffusion in Stable, Low-Wind Conditions

Abstract Sonic anemometer observations were made 10 m above ground level for a period of 1 yr. From these data, Eulerian autocorrelation functions were computed for the horizontal and vertical wind velocity fluctuations for low wind speeds. Although the autocorrelation function for the vertical velocity component exhibited the well-known exponential form, the function for the horizontal components of the wind vector showed a negative loop for all stability classes at low wind speeds. This result might be an effect of low-frequency meandering of the flow. Observations of the standard deviations of the vertical wind component confirmed the proportionality with the friction velocity, though with a slightly lower constant of proportionality than has been found by other authors. A Lagrangian dispersion model (LDM) with random time steps and a negative intercorrelation parameter ρu,υ for the horizontal wind components was used to take the first of the above-mentioned findings into account. In a simple test case...

A simple model for the dispersion of pollutants from a road tunnel portal

Abstract The dispersion of pollutants from a roadway tunnel portal is mainly determined by the interaction between the ambient wind and the jet stream from the tunnel portal. In principal, Eulerian microscale models by solving the conservation equations for mass, momentum, and energy, are thus able to simulate effects such as buoyancy etc. properly. However, for engineering applications such models need too much CPU time, and are not easy to handle by non-scientific personnel. Only a few dispersion models, applicable for regulatory purposes, have so far appeared in the literature. These models are either empirical models not always applicable for different sites, or they do not capture important physical effects like buoyancy phenomena. Here, a rather simple model is presented, which takes into account most of the important processes considered to govern the dispersion of a jet stream from portals. These are the exit velocity, the buoyancy, the influence of ambient wind direction fluctuations on the position of the jet stream, and traffic induced turbulence. Although the model contains some heuristic elements, it was successfully tested against tracer experiments taken near a motorway tunnel portal in Austria. The model requires relatively little CPU time. Current limitations of the model include the neglect of terrain, building, and vehicle effects on the dispersion, and the neglect of the horizontal dispersion arising from entrainment of ambient air in the jet stream. The latter could lead to an underestimation of plume spreads for higher wind speeds. The validation of the model will be the focus of future research. The experimental data set is also available for the scientific community.

Simulation of the air quality during a field study for the city of Graz

Abstract The research project DATE Graz (Dispersion of Atmospheric Trace Elements taking the city of Graz as an example) aimed at the investigation of mesoscale γ pollution dispersion of a city in complex terrain. The winter episode investigated here was characterised by an anticyclonic fair weather situation. Local wind systems developed together with strong temperature inversions. During such an episode air quality is dominated by emissions from within the city. The city of Graz is situated in the southeast of the Alps in the transition area of mountainous to flat land. The city itself is located in the valley of the river Mur, which forms a basin surrounded by small mountains. In order to understand the emitter–receptor relationship a mesoscale dispersion model was applied for the simulation of air quality during the winter episode. Input for the simulation was the emission inventory and extensive meteorological measurement data. Results of the simulation reflect the distinct patterns of daily variations of air quality level measured. The influence of meteorology, emission patterns and chemical reactions are evident and can be qualitatively and partly quantitatively simulated by the model. A validation attempt was made using air quality data from the monitoring network. The introduction shows the importance of anticyclonic fair weather conditions on pollution dispersion in mountainous regions. Section 2 describes the meteorological situation and the instrumentation during the winter measurement campaign. Section 3 deals with the simulation model, initial and boundary conditions and the emission inventory for the city. In Section 4 the simulation results are compared to measurements. Finally conclusions are drawn.

Evaluation of GRAL for the pollutant dispersion from a city street tunnel portal at depressed level

Abstract A new data set for evaluation of atmospheric dispersion models, designed to treat road tunnel portals, is presented. Further, the performance of GRAL 3.5 (Graz Lagrangian model Version 3.5) using this data is discussed. One main drawback of the model in its current version is that two empirical parameters have to be adjusted for each tunnel site separately in dependence on average traffic volume and construction of the tunnel portal. As the model has been tested for five different tunnel sites by now, which differ significantly in traffic volume, terrain, and construction, model users should be able to choose reasonable values for those empirical parameters for other tunnel sites as well. GRAL has not been tested against field data, where the tunnel air is colder than ambient air. Here, further research is necessary.

Lagrangian dispersion modeling of vehicular emissions from a highway in complex terrain

Abstract Transit traffic through the Austrian Alps is of major concern in government policy. Pollutant burdens resulting from such traffic are discussed widely in Austrian politics and have already led to measures to restrict traffic on transit routes. In the course of an environmental assessment study, comprehensive measurements were performed. These included air quality observations using passive samplers, a differential optical absorption spectroscopy system, a mobile and a fixed air quality monitoring station, and meteorological observations. As was evident from several previous studies, dispersion modeling in such areas of complex terrain and, moreover, with frequent calm wind conditions, is difficult to handle. Further, in the case presented here, different pollutant sources had to be treated simultaneously (e.g., road networks, exhaust chimneys from road tunnels, and road tunnel portals). No appropriate system for modeling all these factors has so far appeared in the literature. A prognostic wind field model coupled with a Lagrangian dispersion model is thus presented here and is designed to treat all these factors. A comparison of the modeling system with results from passive samplers and from a fixed air quality monitoring station proved the ability of the model to provide reasonable figures for concentration distributions along the A10.

Analysis of the Daily Variations of Wintertime Air Pollution Concentrations in the City of Graz, Austria

Measured air pollution concentrations in a city reflect the influence of different kinds of sources as well as varying meteorological conditions. In the city of Graz in southern Austria, frequent stagnant meteorological conditions can cause elevated levels of air pollution although emission levels are not exceptionally high. With the aid of a detailed emission inventory and an array of sodars and tethersondes as well as lidar systems supplementing the routine meteorological and air chemistry network during a field experiment in January 1998, the daily variations of air pollution concentrations of selected components within the complex topography of the city of Graz are explained. Main results show the almost linear dependence of the morning maximum concentrations on the predicted emission rates. Throughout the day the rising of the well mixed layer reduces concentrations considerably. Concerning NOX the fast reaction from NO to NO2 is important due to the down-mixing of O3 from the residual layer. The maximum in the afternoon is influenced by emission rates and pollution transport due to the mountain wind.

An inter-comparison exercise of mesoscale flow models applied to an ideal case simulation

Abstract An exercise is described aiming at the comparison of the results of seven mesoscale models used for the simulation of an ideal circulation case. The exercise foresees the simulation of the flow over an ideal sea–land interface including ideal topography in order to verify model deviations on a controlled case. All models involved use the same initial and boundary conditions, circulation and temperature forcings as well as grid resolution in the horizontal and simulate the circulation over a 24-h period of time. The model differences at start are reduced to the minimum by the case specification and consist mainly of the parameterisation and numerical formulation of the fundamental equations of the atmospheric flow. The exercise reveals that despite the reduction of the differences in the case configuration, the differences in model results are still remarkable. An ad hoc investigation using one model of the original seven identifies the treatment of the boundary conditions, the parameterisation of the horizontal diffusion and of the surface heat flux as the main cause for the model deviations. The analysis of ideal cases represents a revealing and interesting exercise to be performed after the validation of models against analytical solution but prior to the application to real cases.

Evaluation of the Revised Lagrangian Particle Model GRAL Against Wind-Tunnel and Field Observations in the Presence of Obstacles

A revised microscale flow field model has been implemented in the Lagrangian particle model Graz Lagrangian Model (GRAL) for computing flows around obstacles. It is based on the Reynolds-averaged Navier–Stokes equations in three dimensions and the widely used standard

Chapter 7.2 Lagrangian particle model simulation to assess air quality along the Brenner transit corridor through the Alps

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