Unlike in USA, the concentration of trace elements in urban air is still high enough in South Asian cities to study the impact of trace elements on climate and human health. Therefore, continuous sampling of PM_2.5 (particulate matter of <2.5 µm aerodynamic diameter) was carried out using low volume sampler in winter (2005-2006) in Lahore, the second largest city of Pakistan highly impacted by urban and agricultural emissions, which has remained unexplored in terms of atmospheric chemistry. During sampling from December 2005 to January 2006, it was possible to collect several samples during brief fog episodes. Samples were analyzed for 27 elements using inductively coupled plasma mass spectrometry. High pollutant concentrations were observed throughout the study, for instance, Cr concentrations up to 1.2 µgm^-3, As, 70.5 ngm^-3, Cd, 83 ngm^-3, Sn, 988 ngm^-3 and Sb, 93 ngm^-3. Pb and Zn concentrations up to 6.5 µgm^-3 were observed. Calculation of enrichment factor and crustal correction illustrate the attribution of Cr, Co, Ni, Zn, As, Se, Mo, Ag Cd, Sn, Sb, Tl and Pb to non-crustal sources. Air parcel back trajectories, interelemental relationships and meteorological observations have been used to explain the sources and the impacts of fog chemistry and mixing heights on atmospheric processing of trace elements in PM_2.5. Source apportionment will be done using positive matrix factorization. Findings will improve the understanding of the critical roles and interactions of chemical composition, size and mixing state of atmospheric particles that manipulate the chemistry of an urban atmosphere.