MODELLING OF ATMOSPHERIC DISPERSION OF AIR POLLUTANTS IN LOW WINDS

Abstract

Various models proposed in the literature for pollutants dispersion under low-wind conditions have been critically reviewed. Sharan et al. (1996a,b) have solved the advection-diffusion equation using constant K, and variable K; and the parameterisation of K. proposed by Rao (1983) for constant K-model, and those of Kaimal et al. (1976) for turbulence parameters in the variable K-model. New parameterisations have . been proposed for the present study for eddy diffusivity and turbulence parameters by using sigma as proposed by Briggs (1973) for different stability classes. These formulations with the new parameterisations have been tested against the experimental data for both unstable and stable conditions of atmosphere. The experimental data of Singh et al. (1991) and those of Segendorf and Dickson (1974) have been used in this study for unstable and stable atmosphere respectively. For the validation of proposed parameterisations in unstable condition, four models have been discussed. Model-a and model-b represent the constant K-model with new K, parameterisation and variable K-model with parameterisation for turbulence parameters a,,8, and y respectively. These two models have been compared with the models proposed by Sharan et al. (1996a). The statistical and graphical analyses expose the better performance of the present parameterisations over those by Sharan et al., (1996a). For justification of proposed parameterisations under stable atmosphere, a total of 11 different schemes have been studied. Sigma-split method, AT/Az ii method, segmented plume model (Yanskey et al. 1966), kinematic heat flux (Kaimal et al., 1976) and wind direction fluctuation 6B (Cirillo and Poli, 1992) have been used with constant K and variable K models. New parameterisations gave very good results as indicated by the agreement between the observed concentrations and those calculated using the model solutions. Results showed that proposed new parameterisation with sigma-split method gave the best performance followed by the segmented plume method for the parameterisation of the eddy diffusivity as well as turbulence parameters.