Surface ozone in Karachi

Abstract

Ozone measurements from three urban and suburban sites in Karachi, Pakistan have been used to evaluate the contribution of city's urban air pollution in the ozone formation. Round the year measurements were made during 1987-1988. Karachi has shown marked afternoon ozone maximum between 12-15 hours local time, irrespective of climate conditions and site location. Diurnal ozone variations inland (downwind) sites were compared with those of upwind coastal site. Although impact of sunshine, season, and maximum temperature have been observed on the daily variation of ozone level at all the three sites, upwind ozone concentrations during the day were recorded varying between 22-24 ppb as compared to inland site-2 and site-3, where mean ozone concentrations were around 40 ppb. This excess ozone over the upwind site STE-1 clearly identifies the influence of the urban area on the downwind ozone levels. The effect of wind direction on the seasonal pattern of ozone has also been demonstrated. Minimum O, concentrations were observed during the monsoon (July-August) at a time when the number of sunshine hours/day are minimum and the relative humidity is at its maximum.