The objective of this study is to describe the atmospheric dispersion in summer of the emissions from a power plant situated on very complex terrain. The power plant selected, with a 343-meter-tall chimney, is located in Teruel (Spain), near the city of Andorra, Northeast of the Iberian mountain range. By experimentation and modelling, the study attempts to characterise both the advection (through the reconstruction of 3-D wind fields) and the turbulent dispersion present during the period of analysis. Systematic SO2 plume tracking was carried out for 3 days in summer, by means of a vehicle equipped with a COSPEC (COrrelation SPECtrometer). This passive remote sensor utilises solar radiation to obtain S02-concentration distribution measurements aloft and around the emission focus. In addition, the study used a non-hydrostatic mesoscale meteorological model MM5 coupled to a Lagrangian Particle Dispersion (LPD) Model FLEXPART.
Simulated dispersion results are generally checked against measurements of tracer-pollutant surface concentrations, with the dispersion analysis limited to the impact areas. The availability of measurements aloft enables us to verify the patterns of advection and turbulent diffusion which govern atmospheric pollutant dynamics in the area as a previous step to analysis of the cause-effect relation between the emission source and the ground-level concentration.
The mesoscale model uses a nested-grid configuration with 5 domains (100x100 grids spaced 108, 36, 12, 4 and 1.3 km, respectively) centred over the power plant. The model predicts the wind field and turbulence parameters. The LPD model takes into account wind velocity variances and the Langrangian autocorrelations, considering the time step as a function of the Langrangian time scale.
* From the point of view of advection (whole-body advection+differential advection), the model was able to reproduce the typical stationary-dispersion scenarios (experimentally characterised with the COSPEC), although a significant temporal delay was detected between the simulation and the experimental measurements ( figure ) of the plume dispersion.
* From the turbulent dispersion (differential advection+turbulent diffusion) point of view, contrary to what occurs in stationary periods, during the transition between dispersion scenarios (figure) there is a significant discrepancy between experimental values of plume concentration horizontal distribution (Sigma-y, defined from the transversal axe to the average transport direction) and the values obtained from the model. In these kinds of situations, with no defined transport direction, classical dispersion parameters lose their physical meaning.
In conclusion, this study shows: (A) that the availability of measurements aloft (obtained with an easily-moved instrumented vehicle) facilitates the comparison between experimental and simulated dispersion parameters, which represents a distinct advantage with respect to the information provided by fixed networks for ground-level air pollution control, and (B) that in transitory situations between different dispersion scenarios it is not appropriate to use dispersion parameters that assume a defined transport direction since this concept makes sense only under a sufficiently stationary wind field.
Figure: Experimental measurements of the distribution of the plume concentration, obtained with the COSPEC over the road network around the power plant and particle distribution simulated with the LPD, FLEXPART.
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