During the last two winters widespread fog frequently occurred in northeastern India and Pakistan, in a region extending over 1500 km. A particularly severe fog episode lasted from mid-December, 1998 to early January, 1999. The fog caused extensive economic damage and disruptions in transport. We determined concentrations of SO42-, NO3⋅, and selected trace elements at Lahore, Pakistan during and after the fog event by collecting aerosols on Whatman 41 filters every 12 h. SO42- concentrations of up to 100 &mgr;g/m3 were observed during fog. The SO42-/Se ratios and trace element data suggest a distant source of SO42- aerosols, hundreds of kms away. Lahore was downwind of coal-burning in India during the fog. The high concentrations of SO42- observed suggest a more extensive investigation of the chemistry and transport processes in this region is necessary to delineate emission sources and develop control strategies as there are serious likely effects on human health and economy in a region populated by hundreds of millions of people, and on global climate change through direct and indirect forcing. ¿ 2000 American Geophysical Union