The work reported in this paper was carried out to study the trends of PM2.5 (particles with an aerodynamic diameter of 2.5 µm or less) concentrations and source apportionment of PM2.5 monitored at an urban residential site in Lahore, Pakistan. PM2.5 aerosol samples were collected for 2 days in a week at 12 h interval in a day, both in dry and wet seasons, on ZefluorTM filter papers using Thermo-Electron Corporation Reference Ambient Air Sampler (RAAS). Total 310 samples were collected during the period under study, i.e., from November 2005 to December 2007. High PM2.5 loads were observed in winter, which were approximately 4 times greater than those observed in the summer, spring, fall and monsoon seasons in the yearlong measurements. Source apportionment was performed on short duration analysis results of November 2005 to March 2006 using Positive Matrix Factorization (PMF) model. The results derived from PMF model indicated that the major contributors to PM2.5 in Lahore are: soil/road dust, industrial emissions, vehicular emissions and secondary aerosols. It is, therefore, concluded that in addition to local vehicular and industrial emissions, the city is also affected from trans-boundary air pollutants particularly due to secondary aerosols (especially SO4(2-)) during winter which increase PM2.5 concentrations manifold when relatively less mixing height exists. The sulfate particles also facilitate in haze/fog formation during calm highly humid conditions, thus reduce visibility and increase the incidents of respiratory diseases encountered in the city every year.
airborne particulate matter; source apportionment; PM2.5; Lahore
