G. Tinarelli

A New High Performance Version of the Lagrangian Particle Dispersion Model Spray, Some Case Studies

Lagrangian particle models are powerful tools to simulate the atmospheric dispersion of gaseous releases. Although having a quite complex mathematical basis (Thomson, 1987, Rodean, 1994), their practical implementation is generally simple and intuitive, allowing to easily take into account complex situations such as the presence of the topography or terrain inhomogeneities, low wind speeds, spatial and temporal variations of meteorological fields. In these models the atmospheric dispersion is simulated by the motion of fictitious particles splitted in a mean part due to the mean wind, and a stochastic fluctuation related to the statistical characteristics of the turbulent flow. It is quite clear that the model accuracy is strongly dependent on the number of emitted particles and the computer time often limits the kind of simulations that can be performed. For this reason, the earlier version of these models were mainly devoted to reproduce the dispersion of a limited number of emissions at local scale. The recent wide and rapid diffusion of very fast computational tools lead to the development of more sophisticated codes, able to take into account more general situations. SPRAY (Tinarelli et al., 1992) is a Lagrangian stochastic particle model designed to perform dispersion simulations in complex terrain. The version 1 of the code, based on a three dimensional form of the Langevin equation for the random velocity with coupled non-gaussian random forcing following Thomson (1984, T84 in the following) and subsequently improved (Tinarelli et al., 1992), was able to satisfactorily reproduce local to regional scale dispersion both over flat (Brusasca et al., 1989, 1992) and complex terrain (Brusasca et al., 1995, Nanni et al., 1996) taking into account the emission from single or multiple sources. The development of a better based theory (Thomson, 1987) and the further demand of more complex regional scale simulations able to cover longer periods with a variety of emissions of different kinds (i.e. main roads, industrial or urban area) called for a new version of the code. The new version 2 of SPRAY code contains some improvements regarding the theoretical approach, turbulence parametrizations and time response characteristics. In this paper we describe these new developments, comparing model performances with those of the previous version through simulations performed both in theoretical and real cases.

Simulations of atmospheric dispersion in an urban stable boundary layer

The results of numerical simulations of tracer dispersion in an urban stable boundary layer are presented. Both Gaussian approximation and fourth order closure are considered and a Gram-Charlier probability density function is used in our Lagrangian stochastic model. Different parameterizations for the boundary-layer height and different values of kurtosis are tested. The model is based on the generalized Langevin equation, whose coefficients are solution of the Fokker-Planck equation and plume rise is taken into account. The model accuracy is assessed by means of a model evaluation based on the predicted and observed crosswind integrated concentrations, maximum on the arc and standard deviation of the crosswind concentration distribution on the arc.

Evaluation of different wind field modeling techniques for wind energy applications over complex topography

Abstract The possibilities offered by a mass-consistent model and by a non-hydrostatic meteorological model to reconstruct the wind field features at local scale, for wind energy applications in complex terrain, have been verified. The mass-consistent model has been applied to compute the space distribution of wind speeds and their variation with height to identify the areas suitable to wind turbine siting. The model has been initialized on the basis of a data set similar to the one available from a standard experimental campaign. Even though some subgrid scale characteristics of the flow are missing, the modeled field has been judged to give a rather satisfactory description of the winds over the studied region. The non-hydrostatic flow model has been applied to investigate the possibility to describe the local scale wind field characteristics starting from synoptic meteorological information. Upper air stratification proved to be important in determining the local scale wind speed and distribution even in near-neutral ground level stratification. The model results have been compared with both measurements and mass-consistent model.

Application of a Lagrangian particle model to the source apportionment for primary macropollutants in Taranto area (South Italy)

A modelling system has been applied to estimate the annual contribution to the total concentrations of different pollutant sources in Taranto, one of the most industrialized areas in Italy. Industrial sources, traffic, domestic heating and harbour emissions have been taken into account. Modelling system includes 3-dimensional meteorological models SWIFT-SURFPRO with the Lagrangian particle dispersion model SPRAY. The air emissions inventory was partially established using measured data, local activity indicators and emission factors. The meteorology was reconstructed by the SWIFT model from the products supplied, for the year 2007,by the national MINNI project. The annual simulation led to the identification of the main emitting sources for primary pollutants such as NOx, SO 2 , PM10, PM2.5 and C 6 H 6 at receptor sites. In addition, a more refined source apportionment was achieved for industrial primary PM10, providing a useful preliminary identification of the main industrial sources emitting dangerous micropollutants, such as POPs and heavy metals.

Comparison of atmospheric modelling systems simulating the flow, turbulence and dispersion at the microscale within obstacles

Three different modelling techniques to simulate the pollutant dispersion in the atmosphere at the microscale and in presence of obstacles are evaluated and compared. The Eulerian and Lagrangian approaches are discussed, using RAMS6.0 and MicroSpray models respectively. Both prognostic and diagnostic modelling systems are considered for the meteorology as input to the Lagrangian model, their differences and performances are investigated. An experiment from the Mock Urban Setting Test field campaign observed dataset, measured within an idealized urban roughness, is used as reference for the comparison. A case in neutral conditions was chosen among the available ones. The predicted mean flow, turbulence and concentration fields are analysed on the basis of the observed data. The performances of the different modelling approaches are compared and their specific characteristics are addressed. Given the same flow and turbulence input fields, the quality of the Lagrangian particle model is found to be overall comparable to the full-Eulerian approach. The diagnostic approach for the meteorology shows a worse agreement with observations than the prognostic approach but still providing, in a much shorter simulation time, fields that are suitable and reliable for driving the dispersion model.

Proposal of a new Lagrangian particle model for the simulation of dense gas dispersion

A Lagrangian particle dispersion model for dense gas dispersion is proposed. It is a new version of MicroSpray oriented to simulate dense gas dispersion in urban or industrial environment, and includes the treatment of obstacles, complex terrain and low wind. The dense gas descent is computed by a 3D dynamical plume rise/descent model, the gravity spreading and bottom boundary condition are accounted for by empirical formulations. The differences between simulations obtained with the new model and with the standard one are presented. The model is validated against tracer field data gathered in the Thorney Island experiment 8.

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

Abstract Dispersion from line sources in complex terrain is still a challenging task. Nevertheless in environmental impact assessments often detailed information about the expected pollution levels besides roads is required. In this work two different methods are demonstrated. One method utilized the models RAMS-MINERVE/SPRAY and a two-way nesting procedure starting at the continental scale and ending up with a final horizontal resolution for the innermost domain of 100 m for the 3D wind and concentrations fields. Seven episodes, representative of the most frequently occurring circulation systems, were simulated and from their weighted mean, the annual average concentrations were deduced. The second method used the models GRAMM/GRAL which were driven by local observations of wind and stability class. Instead of simulating episodes to recover annual mean concentrations, steady-state wind and concentration fields were calculated by establishing a simple classification of meteorological situations based on wind speed, wind direction and stability class. The horizontal resolution used in the latter approach was 100 m for the wind field computation and 10 m for the dispersion calculations. Both Lagrangian dispersion models used a new set of Langevin equations especially suited for all wind speed regimes, which have been recently developed. The results of the computed concentration fields were compared with observed NO2 and PM10 levels recorded by four air quality monitoring stations (NO2, PM10) and about 15 passive samplers (NO2). The main goal of this presentation is to outline the differences/similarities, the effectiveness and implications of the two different methods on achieving the objectives of the project, with a special emphasis on their application to the epidemiological study.

Intercomparison of Lagrangian stochastic models based on two different PDFs

In this work, two different Lagrangian stochastic models for convective turbulence are compared and tested against the Indianapolis dataset. The two models are based on the generalised Langevin equation, whose coefficients are solutions of the Fokker-Planck equation. The first model uses the bi-Gaussian probability density function (PDF) following Luhar and Britter (1989) and the second one, proposed by us (Ferrero and Anfossi, 1998), is based on the Gram-Charlier PDF. Both models are able to take into account up to the third order moment of the wind velocity fluctuation vertical component and make use of the Gaussian assumption in the horizontal directions. Basically, the two models considered here are different versions of our stochastic dispersion model for flat terrain, LAMBDA. This means that plume rise is taken into account and that the correct boundary conditions are used. The results obtained are presented and their differences discussed.

Composition and emission of VOC from biogas produced by illegally managed waste landfills in Giugliano (Campania, Italy) and potential impact on the local population

The composition in Volatile Organic Compounds (VOC) of the biogas produced by seven landfills of Giugliano (Naples, Campania, Italy) was determined and VOC emission rates assessed to verify if these compounds represent a potential threat to the population living nearby. VOC composition in the biogas could not be predicted, as heterogeneous waste was dumped from the late 1980s to the early 2000s and then underwent biological degradation. No data are available on the amount and composition of VOC in the biogas before the landfills closure as no operational biogas collection system was present. In this study, VOC composition was determined by gas chromatography-mass spectrometry (GC-MS), after collecting samples from collection pipes and from soil fractures in cover soil or capping. Individual VOC were quantified and data compared with those collected at two landfills in Latium, when they were still in operation. Relevant differences were observed, mainly due to waste aging, but no specific VOC revealing toxic waste dumping was found, although the concurrent presence of certain compounds suggested that dumping of industrial wastes might have occurred. The average VOC emission was assessed and a dispersion model was run to find out if the emitted plume could affect the health of population. The results suggested that fugitive emissions did not represent a serious danger, since the concentrations simulated at the neighboring cities were below the threshold limits for acute and chronic diseases. However, VOC plume could cause annoyance at night when the steady state conditions of the atmosphere enhance pollutants accumulation in the lower layers. In addition, some of the emitted VOC, such as alkylbenzenes and monoterpenes, can contribute to tropospheric ozone formation.

Evaluation of Local-Scale Models for Accidental Releases in Built Environments: Results of the Modelling Exercises in Cost Action ES1006

A main research task of COST Action ES1006 is the evaluation of atmospheric dispersion models by their comparison against test data from qualified field and laboratory experiments and by a model inter-comparison. The model comparison and evaluation carried out for three test cases is presented, addressing the performance of the different modelling approaches, quantifying the scatter of results when different models are applied and assessing the effect of uncertainties.