Observational study of surface ozone at New Delhi, India

Surface ozone has been measured over New Delhi, an urban site, a region of intensive anthropogenic activity since 1997. Seasonal variations in ozone concentration show pronounced maxima in the summer and autumn seasons and minima in monsoon and winter seasons. Diurnal patterns in ozone concentration show daytime in situ photochemical production throughout the year. The high ozone episode days were associated with meteorological parameters such as sunny and warm weather, stagnant wind patterns and low relative humidity. The monthly average maximum concentration in summer was found to be in the range of 62-95 ppb whereas, it was found to be 50-82 ppb in the autumn (October and November). The analysis of hourly averaged surface ozone data illustrates that on a large number of days the surface ozone values at Delhi exceed the World Health Organization (WHO) ambient air quality standards (hourly average of 80 ppb) for ozone. On some occasions, night-time increases of surface ozone concentration have been observed under stable boundary layer conditions and during thunderstorms.