This study was primarily initiated to understand the consistently low levels of SO2 in cities in India in spite of rising SO2 emissions. A literature review suggested the possibility of chemical transformation of SO2 to SO4. Thus, the objective of research was to understand the formation of SO4 as a function of SO2, and other constituents of atmosphere such as PM10, water content, Ca (in aerosol) and pH (of aerosol). For this purpose, air quality monitoring was conducted at five locations in the city of Kanpur, India (during October 1999–April 2000) representing various land-use patterns. It was found that the SO4 levels were considerably high (2.8–43.6μgm−3) compared to levels in cities in the US (1.9–3.6μgm−3, Sandberg et al. (J. Air Pollut. Control Assoc. 26 (1976) 559; 15.7–18.4μgm−3, Altshuller (J. Air Pollut. Control Assoc. 26 (1976) 318; 4.0–14.0μgm−3 Dockery et al. (J. Med. 329 (1993) 1753), UK (3.2–9.0μgm−3, QUARG, 1996 http://www.environment.detr.gov.uk/airq/consult/naqs/index.htm), and 25 locations in Europe (0.34–1.68μgm−3 (1991–1999), EMEP website), but were comparable to the levels observed at Agra (14.67μgm−3, Kulshrestha et al. Indian J. Radio Space Phys. 24 (1995) 178) and at New Delhi (12.9−27.5μgm−3, Shandilya, M.E. Thesis, Malaviya National Institute of Technology, Jaipur, India, 2000). The high SO4 levels could provide a plausible explanation for low SO2 concentration levels in the city of Kanpur. It was concluded that high levels of PM10, Ca and high pH of aerosols in atmosphere provide a conducive environment for conversion of SO2 to SO4. It was also concluded that the important migration pathway in study area for the transformation of SO2 to SO4 appears to be oxidation of SO2 on the surfaces (of particulate) available in the ambient atmosphere.