Operational Air Pollution Modelling
by Tiziano Tirabassi
Air quality management and protection require a knowledge of the state of the environment encompassing both cognitive and interpretative elements. The monitoring network, together with the inventory of emission sources, is of fundamental importance for constructing the cognitive framework but neglects the interpretative one. Efficient air pollution management must dispose of interpretative tools which are able to extrapolate in space and time the values measured where analysers are located. However, the improvements to the atmosphere can only be obtained using plans that reduce emissions ie by means of instruments (like air pollution models) able to link the causes (source emissions) of pollution to the related effects (air pollution concentrations). We describe a set of air pollution models developed at the Istituto per lo Studio dei Fenomeni Fisici e Chimici della Bassa e Alta Atmosfera (FISBAT-CNR).
In practice most of the estimates of dispersion of air pollution from continuous point sources are based on the Gaussian approach. A basic assumption for the application of this approach is that the plume is dispersed by homogeneous turbulence. However, due to the presence of the ground, turbulence is usually not homogeneous in the vertical direction.
Various organisations world-wide are currently introducing new advanced modelling techniques based on the results of recent research on the meteorological state of the boundary layer. These advanced modelling techniques contain algorithms for calculating the main factors that determine air pollution diffusion in terms of the fundamental parameters of the atmospheric boundary layer, such as the Monin-Obukhov length scale and friction velocity. Experimental work and modelling efforts have attempted to parameterize the surface fluxes of momentum, heat and moisture in terms of routinely measured meteorological parameters.
Within this framework, several model codes employing a non-Gaussian analytical solution of the advection-diffusion equation have been developed at FISBAT-CNR, Bologna:
- KAPPAG This model uses an analytical solution based on the vertical profiles of wind and diffusion coefficients that are power functions of height. The model can handle multiple sources and multiple receptors, simulating time-varying conditions in which each time interval (eg, 1 hour) is treated as a stationary case. The model output is a statistical summary of the concentrations computed at each receptor, during each time step, and due to each source. Partial and total concentrations are computed for hourly and multi-hour averages. Highest and second-highest values are also evaluated.
- KAPPAG-LT This is the climatological version of KAPPAG, insofar as it produces seasonal and/or annual mean concentrations.
- CISP This screen model provides a method for estimating maximum ground level concentrations from a single point source as a function of turbulence intensity and wind speed. It is designed for the low-cost, detailed screening of point sources in order to determine maximum one-hour concentrations and is regarded as a useful tool for a screen analysis in that it is a relatively simple estimation technique, providing a conservative estimate of the air quality impact.
- VIM This is a screening model for estimating maximum ground-level concentrations as a function of turbulence intensity, wind speed and wind direction, in an area with many emission sources. It is considered to be a useful tool for screen analysis as it constitutes a relatively simple evaluation technique that provides a conservative estimate of the air quality impact of a specific multi-source area and a model for the evaluation of the maximum ground concentrations produced by many emission sources.
- MAOC This is a model for the evaluation of pollutant concentrations in complex orography. The simulation of terrain-induced distortion of flow streamlines is accounted for by modifying the effective plume height.
- VHDM This practical model evaluates ground level concentrations from elevated sources, utilising a Fickian-type formula where the source height is a simple function of the wind velocity and eddy diffusivity vertical profiles. The model accepts experimental profiles of the above parameters, as well as the theoretical profiles proposed in the scientific literature, such as the vertical profiles of the wind and eddy diffusion coefficients predicted by similarity theory. In the latter case, the model can be applied routinely using as input simple ground level meteorological data acquired by an automatic network.
- M4PUFF A puff model where the pollutant concentration in puffs is described through the first four moments of its spatial distribution. The model is based on a general technique for solving the K-equation using the truncated Gram-Charlier expansion of the concentration field and the finite set of equations for the corresponding moments. Currently, the type A Gram-Charlier expansion is a classical method for approximating a given distribution with moments of any order, basically consisting of a truncated expansion in terms of Hermite functions, whose coefficients are chosen so as to reproduce the sequence of moments of the distribution up to a given order.
- SPM This is a practical model based on similarity theory for the dispersion of skewed puffs. It utilises approximate solutions proposed for the dispersion of a cloud of passive contaminant released from an instantaneous source near the ground and particular importance is accorded to describing the interaction between wind shear and vertical diffusion and the process that transforms shear produced skewness into diffusive variance in the wind direction.
The SPM model has been used in a project between Italy and Brazil to investigate the dispersion of radionuclides from the Brazilian Navy's industrial installation 'Centro Experimental ARAMAR' (CEA) located in Iperó, in a rural region of the State of São Paulo.
The performance of the models has been assessed with success in cases of both ground level sources, using data of the Prairie Grass experiment , as well as elevated sources, using data from the EPRI experiment at Kincaid power plant and from the Copenhagen data set. In addition, performance in the case of complex terrain has been evaluated using wind tunnel data from the Fluid Modelling Facility of the US Environmental Protection Agency, Triangle Park (North Carolina).
Tiziano Tirabassi - FISBAT (CNR)
Tel: +39 051 63 99 601