Ozone distribution in the urban environment of Delhi during winter months

Measurements of ozone (O3) and oxides of nitrogen (NOx) were carried out at six sites in Delhi during the winter months of 1993. Concentrations of ozone and NOx varied between 34 and 126 ppbv and 32 and 272 ppbv, respectively. Interestingly, ozone exhibited initial high levels during morning hours which subsequently declined to lower levels around noon. However, during evening hours a delayed build up of ozone was observed which is a departure from earlier studies. The micro-meteorological parameters like mixing height and ventilation coefficient were observed to favour dispersion of the pollutants, predominantly between 1100 and 1800 hours, whereas stable atmospheric conditions between late evening to early morning hours seem to favour accumulation of the pollutants. The paper reports some unusual winter diurnal ozone trends and speculates the possible reasons.