Seasonal variation in ambient air concentration of nitrate and sulfate aerosols in a tropical city, Varanasi

This study reports the seasonal variation in the ambient air concentration of sulfate and nitrate aerosols in the urban atmosphere of Varanasi, India, during the years 1991–1993. The city was hypothetically divided into six monitoring zones (I–VI), with five sampling points in each zone. Sulfate and nitrate concentrations (24-h average) ranged from 0.1 to 22.4 μg m−3 and 0.06 to 28.4 μg m−3 respectively. The maximum concentrations of sulfate and nitrate were observed at zones II and VI, respectively. However, the minimum concentrations of both pollutants were observed at zone I. Highest concentrations of sulfate and nitrate in the winter season followed by summer and rainy season revealed the effect of various seasons on the distribution pattern of sulfate and nitrate. Variation in NO3−/SO42− ratio indicates that ambient aerosol concentration depends on quality and quantity of primary pollutants released at the source.