Variation of ambient SO2 over Delhi

The temporal variation of ambient SO2 and the chemical composition of particulate matters (PM2.5 and PM10) were studied at National Physical Laboratory (NPL), New Delhi (28A degrees 38'N, 77A degrees 10'E). Spatial variation of SO2 at seven air quality monitoring stations over Delhi was also studied simultaneously. Wide range of ambient SO2 was recorded during winter (2.55 to 17.43 ppb) compare to other seasons. SO2 mixing ratio was recorded significantly high at industrial sites during winter and summer; however, no significant spatial difference in SO2 mixing ratio was recorded during monsoon. SO (4) (2-) /(SO2+SO (4) (2-) ) ratio was recorded high (0.74) during winter and low (0.69) during summer. Monthly variation of PSCF was analyzed using HYSPLIT seven days backward trajectories and daily average SO2 data. PSCF analysis suggests that, during winter (December, January, February) ambient SO2 at the study site might have contributed from long distance sources, located towards west and southwest directions; during monsoon (July, August, September) marine contribution was noticed; whereas, during summer (April, May and June) it was from regional sources (located within few 100 km of study site). During winter there was significant contribution from the long distance sources located in western Asia, northwestern Pakistan, Rajasthan and Punjab provinces of India. Coal used in thermal power plants at Panipat (in the northwestern side) and Faridabad (in the southeastern side), local industries, soil erosion and biomass burning may be major contributing factors for SO2 during summer. The study establishes that the transport sector may not be the major source of ambient SO2 in Delhi.