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Introduction
Metropolitan cities all over the world are characterized by their economic development and large population resulting in increased human activity that, in turn, can lead to high pollution levels. Consequently, the air quality in these cities is severely affected. The ventilation coefficient, which is the product of mixing depth and the average wind speed, is an atmospheric condition which gives an indication of the air quality and pollution potential, i.e., the ability of the atmosphere to dilute and disperse the pollutants over a region. The higher the coefficient, the more efficiently the atmosphere is able to dispose the pollutants and better is the air quality. On the other hand, low ventilation coefficients lead to poor dispersal of pollutants causing stagnation and poor air quality leading to possible pollution related hazards. The ventilation coefficient is a function of mixing depth and the average wind speed through the mixing layer. A fluctuation in the values of either/both these quantities causes a variation in the ventilation coefficient.
Several studies have documented the variations in ventilation coefficient over different regions in India as well as in other parts of the world. Various methods have been employed to calculate/derive the mixing depths (required to compute ventilation coefficients) from lidar observations (Ernest Raj and Devara 1992) or sodar (Goyal et al.2006) or wind profiler (Krishnan and Kunhikrishnan 2004) or by following the profile intersection technique proposed by Holzworth (1967) (Padmanabhamurthy and Mandal 1979; Padmanabhamurthy and Tangirala 1990). Radiosondes or pilot balloons have been used for wind observations in most of the studies. In India, it was generally observed that ventilation coefficients were higher in pre-monsoon months and lower during southwest monsoon and winter months. Diurnal variation of ventilation coefficient showed that assimilative capacity (determined in terms of the ventilation coefficient) of the atmosphere is high during afternoon hours and it gets reduced during evening and morning hours in summer and winter. The season having the poorest assimilative capacity is winter. Due to low mixing depths and low wind speeds, the ventilation coefficients in winter are much lower which increases the ground level pollutant concentration as compared to other months. Low mixing heights are observed in India during monsoon and winter seasons (Devara and Ernest Raj 1993; Krishnan and Kunhikrishnan...