Aerosol chemical characteristics of a mega-city in Southeast Asia (Dhaka–Bangladesh)

Elemental carbon (EC), organic carbon (OC), organic acids, major inorganic ions and trace elements were determined in aerosol samples collected under pre-monsoon conditions (March–April 2001) in Dhaka (Bangladesh). Using the Fe content of the aerosol to reconstruct the aerosol mass from soil-type mineralic material, a mass balance of the Dhaka aerosol was achieved. From this follows that on the average around 76% of the aerosol is from soil-type material, around 18% of carbonaceous material, and around 6% soluble ions and trace elements (without iron) <0.3%. Enrichment factors (Fe as a reference element) indicated that coal fly ash is a likely main source for Cd, Pb and Zn in the Dhaka aerosol, while As appears to be of geogenic origin. Organic acids contributed only 0.72% C to OC and were much less abundant relative to OC than at European sites. The trace elements levels in Dhaka were much lower than at comparable Southeast Asian mega-cities (e.g. Lahore, Pakistan), but considerably higher than reported for European and US cities under present day conditions. The correlation between EC and OC was quite high (R2=0.81) indicating a potential joint source of emission for carbonaceous aerosols. The EC/total carbon (TC) and K/EC ratios indicated that biomass combustion was not a main contributor to EC in Dhaka, which implicates that fossil fuel combustion is the major contributor to EC levels in the Dhaka aerosol. The differences in the EC/TC and K/EC ratios in the three mega-cities in Southeast Asia (data available from Dhaka, Bangladesh; Lahore, Pakistan; and Mumbai, India) indicate that the aerosol source mix in Southeast Asian cities varies considerably at a national or even regional scale.