Atmospheric Environment Vol. 24B, No. 3, pp. 443~-48, 1990 0957 1272/90 $3.00+0.00 Printed in Great Britain. Pergamon Press plc ESTIMATION OF A QUANTITATIVE AIR QUALITY IMPACT ASSESSMENT SCORE FOR A THERMAL POWER PLANT S. B. PATIL Air Pollution Control Wing, CIDCO Bhavan, New Bombay-400 614, India and R. S. PATIL Centre for Environmental Sc. & Engg., IIT Powai, Bombay-400076,India (First received 10 June 1987 and in final form 30 May 1990) Abstraet--A case study on air quality impact assessmentfor a proposed expansion of a thermal power plant situated in Bombay is presented. The major aim was to finally estimate a total impact assessment score, which is easy for interpretation and communication. The assessment variables chosen for the power plant are concentration levelsof the pollutants SO2, NO x and total suspended particulate matter (TSP), as wellas visibility,episode potential and noise. The changes in these parameters for future years, 'with' and 'without' the power plant, have been estimated. The parameter estimates are then normalized in commensurate units to get a final impact assessment score. Key word index: Air quality impact assessment, prediction modelling, environmental evaluation system, thermal power plant, mixing height, episode potential. 1. INTRODUCTION It is now well recognized that before a developmental project is implemented its Air Quality Impact Assess- ment (AQIA) exercise should be carried out. However, the procedures for formulation of methodologies and project evaluation are still not very clear. The AQIA method should be as comprehensive, objective and quantitative as possible so that rational decisions for various courses of actions can be made. Most of the case studies reported in the literature give in the final form a large number of parameters, on which impacts are defined qualitatively. This makes it difficult for the user to trade-off between different parameters and get a total integrated picture. Keeping this in view, this paper provides a very simple and practical approach to AQIA based on well- known techniques and formulae. The main objective of the work is to develop a method for quantitative estimation of a final total impact assessment score, which represents the net quantitative change in air quality due to the project under consideration. This objectively brings out the environmental feasibility of the project, and can have immense practical applica- tion. A final single measure facilitates communication to users, law enforcing agencies, decision makers etc. The method is demonstrated through a case study on a proposed expansion of a thermal power plant. The power plant project is located in the Chembur region of the city of Bombay, which is one of the most industrialized and heavily populated cities in India. The Chembur region is predominantly an industrial area housing heavy industries like refineries, fertilizers, chemicals etc. It is well established that pollution levels in this region have reached alarming levels. In spite of tremendous concern expressed by the general public and environmentalists for the expansion of the power plant, it has been difficult to convince the concerned authorities. It is felt that a quantitative environmental impact assessment score would be of great help in making rational decisions. 2. METHODOLOGY The three well-known steps for AQIA, viz. identification, prediction and evaluation have been followed and are ex- plained in the flow chart in Fig. 1. As per convention, the impacts are analyzed on a 10 km area around the power plant. In the identification step, the assessment parameters are selected and also input data on emissions and meteoro- logical parameters are collected. The values of the assessment parameters are predicted for the future year 1990 using mathematical models.This is done for the two cases, viz. 'with' and 'without' the project. The quantitative differencebetweenthe two is finallyevaluated as explained in the following sections. 3. IDENTIFICATION 3.1. Assessment parameters The assessment variables are judiciously chosen, such that they can be used to describe the baseline environmental setting and upon which impacts can occur. Several have been suggested by various authors (Rau, 1980). For the project under study six assess- ment parameters are found relevant, viz. SO 2, NOx, 443