emission inventory; dispersion model evaluation; calm wind condition; statistical analysis
A comparative study between predicted and observed concentrations of major pollutants SO2, suspended particulate matter (SPM) and NOx in Agra, India has been undertaken using two Gaussian plume models. A Gaussian plume model (IITLT) and climatological dispersion model (CDM) have been used to estimate long-term concentrations of non-reactive pollutants due to emissions from area and point sources. The IITLT model is designed to treat the calm wind conditions (u < 2 ms−1). The computations show that about 10% of the total concentration of each type of pollutant at each station is due to calm winds alone. Therefore, consideration of calm winds is quite important. A comparative study of various methods of treating calm winds in IITLT model has been made. It is noted that in CDM model the central wind speed of the lowest wind speed class was arbitrarily taken as 1.5 m s−1. This means that light winds or calm winds reported in the first wind speed class (0–1.54 ms−1) are assigned this value.
IITLT model has different dispersion parameters for moderate winds and calm winds, whereas CDM uses the same dispersion parameters for both the winds.
The monthly wind frequency tables and monthly mean concentrations of SO2, SPM and NOx for the period November 1975–March 1977 (Mathura Refinery Report, 1978) have been computed. The results obtained from the two models are found to be in good agreement with the observed values. A statistical error analysis of the observed and the predicted concentrations show a satisfactory performance of both models with RMSE of 1.38 for the IITLT model and 1.62 for the CDM. The value of index of agreement for IITLT model is 0.6 whereas for CDM it is 0.54.