that have taken place in recent years such as reduction of sulphur in diesel, use of cleaner fuel such as CNG in Delhi. Other measures include implementation of Bharat Stage-III emission norms for new vehicles and commensurate fuel quality. Also there has been a change in domestic fuel used from coal to LPG which may have contributed to reduction in ambient levels of SO 2 .

NO 2 levels are within the prescribed National Ambient Air Quality Standards in residential areas of most of the cities. The reasons for low levels of NO 2 may be various measures taken such as banning of old vehicles, better traffic management etc. Fluctuating trends have been observed in NO 2 levels. Various measures such as implementation of Bharat Stage-III norms etc have been taken to mitigate ambient NO 2 levels but at the same time number of vehicles have increased exponentially.

RSPM levels exceed prescribed NAAQS in residential areas of many cities. Fluctuating trends have been observed in RSPM levels. Various measures such as implementation of Bharat Stage-III norms etc have been taken to mitigate ambient RSPM levels but at the same time number of vehicles have increased exponentially. The reason for high particulate matter levels may be vehicles, engine gensets, small scale industries, biomass incineration, resuspension of traffic dust, commercial and domestic use of fuels, etc.

High levels of CO might be attributed to increase in vehicular population especially passenger cars in Delhi. Despite an increase in number of vehicles, CO levels have reduced during last few years. The decrease may be attributed to measures such as conversion of three wheelers of CNG in Delhi.

The RSPM concentrations are higher in winter season and are lower during monsoon months. A plausible explanation for these results may be found by examining meteorological conditions. During the winter season, average mixing height is lower as compared to other seasons and atmospheric dispersion is typically at a minimum and therefore the pollutants will not be as widely dispersed. Lower average mixing height in winter season results in less volume of troposphere available for mixing and hence higher concentrations. Calm conditions in winter season result in less dispersion of pollutants resulting in building up their levels. The monsoons results in large amount of precipitation, high wind velocities and changes in general wind direction. The large amounts of precipitation reduce atmospheric pollution via associated wet deposition processes. Further wind velocities will allow for pollutant transport away from sources and increase mixing processes, thereby resulting in lower levels.