Understanding sources of airborne water soluble metals in Lahore, Pakistan

Lahore, the second largest city of Pakistan with a population of over 10 million, is growing at a rate of 3.32 percent per year. This is accompanied by a large increase in the number of motor vehicles in recent years (over 37 % average annual increase during last the 5 years) and some industrialization. This air quality study, undertaken during March-April 2006, indicated high levels of PM sub(10) which ranged between 158 and 733 kg/m super(3) with an average of 360 kg/m super(3). All measurements exceeded the USEPA as well as EU 24-hour standard values. Concentrations of 23 elements in water extracts of PM sub(10) were determined by ICP-OES along with ions chlorides, nitrates, sulfates, ammonium and potassium by ion chromatography. The concentrations were generally low except for crustal elements of calcium, magnesium and sodium, and toxic metals cadmium, lead and zinc. Higher concentration of crustal elements was expected due to the arid nature of the area, suggesting greater contribution of wind-blown and re-suspended road dust to PM sub(10). Nitrates, sulfates and ammonium were also measured in relatively high concentrations indicating the contributions by secondary aerosols. Principal Component Analysis showed that greatest contributors to PM sub(10) were industrial sources followed by secondary aerosols and mobile sources.