OZONE - CONCENTRATION VARIABILITIES IN A SEASONALLY DRY TROPICAL CLIMATE

This paper reports seasonal and diurnal variations in the ambient air concentrations of ozone in Varanasi city, India for the years 1989 and 1990. Different monitoring zones were established in the city on the basis of pollution sources, traffic densities, and structure of built-up areas. At most of the monitoring stations, two-hourly ozone concentrations frequently exceeded 60-mu-g m-3 (0.03 ppm) during summer months. However, the zone dominated by residential colonies, offices, and cultivated land showed maximum O3 concentrations less than 60-mu-g m-3 (0.03 ppm). Higher concentrations of ozone were mainly recorded in warmer months between 11 am to 4 pm. The highest concentration (2-h mean) of 160-mu-g m-3 (0.08 PPM) O3 was recorded in the month of April at Zone I which was mainly due to emissions from heavy-duty automobiles. Monthly average ozone concentrations were significantly correlated with temperature (r = 0.69), relative humidity (r = -0.81), wind speed (r = 0.76), and atmospheric pressure (r = -0.46). The frequency distribution of 2-h mean ozone concentrations was close to the logarithmic normal. Maximum 2-h mean O3 concentrations in some areas of Varanasi city were above the limit that has been shown to cause significant vegetation damage. The temporal variation in ozone concentrations was mainly due to the variations in meteorological conditions.