Assimilative capacity and pollutant dispersion studies for Gangtok city

The assimilative capacity of Gangtok's atmosphere has been estimated by two different approaches. First approach is based on ventilation coefficient, which is directly proportional to the assimilative capacity of the atmosphere and has been computed through micro-meteorological parameters for two different seasons (winter and summer) of the year 2003–2004 represented by December, 2003 and April, 2004. Similar type of analysis has been done for all the seasons. But winter and summer have only been chosen to limit the presentation to a manageable size. Diurnal variation of ventilation coefficient shows that the assimilative capacity of atmosphere is high during the afternoon hours which get reduced during the evening and morning hours in both the seasons. Among both the season winter has the poorest assimilative capacity throughout the day. Second approach is through pollution potential, which has been estimated through dispersion models in terms of concentration of pollutants, and is inversely proportional to the assimilative capacity of the atmosphere. Dispersion models for line sources have been used to predict the spatial and temporal distribution of three pollutants namely sulphur dioxide (SO2), suspended particulate matter (SPM) and oxides of nitrogen (NOx). These models have been validated using the observed data. Statistical evaluation of the models indicates satisfactory performance. The model's predictions also indicate that winter is the most critical season having maximum pollution. It has also been observed that the model predicted values are higher in December i.e. in winter than in April i.e., summer. The results of both the approaches show a good correlation and indicate that the assimilative capacity of the study region is not good especially during winter season.