Visibility impairing aerosols in the urban atmosphere of Delhi.

To study the visual air quality of Delhi, size fractionated aerosols - coarse and fine fractions of PM10 - were collected and analysed for SO4(-2), NO(-3), NH4+ and EC at three sites with different background activities. The analysed species constitute a smaller portion of coarse fraction (39%) but a larger portion of fine fraction (69%). The sampling was performed from June 2003 to November 2003 which covers monsoon and post monsoon seasons. Aerosol data was used to describe the spatial variation of Visibility Range as a function of chemical composition of visibility impairing aerosols. During the study period, visibility was found to be poor varying between 4.7 and 13 km with an average value of 9.4 km. It is observed that visibility impairment was more due to carbonaceous aerosol followed by sulphate.