Characteristics of the air pollution in the city of Dhaka, Bangladesh in winter

Ten-day-average concentrations of SO2 and NO2 were measured using molecular diffusion tubes in Dhaka, Bangladesh, in winter 1995–96, and their spatial distribution and temporal variation were simulated using an Eulerian transport/chemistry/deposition model. Conversion rates for SO2 and NO2 were also estimated. The measurements first elucidated the characteristics of the spatial distribution of SO2 and NO2 in this area, and showed significant spatial variations and extremely high SO2 concentrations in the southeastern industrial zone of Dhaka where the highest value was about 100 ppb. The polluted zone, defined as the average SO2 concentration over 40 ppb, extended along the major route running from northwest to southeast, and also parallel to the Buriganga river in Dhaka area. In the case of NO2, the highest 10-day-average was 35 ppb, and higher concentrations appeared in the city center and along the main roads of Dhaka. An estimation of anthropogenic SO2 and NOx emissions in Dhaka for winter 1995–96 was made to see their importance to SO2 and NO2 concentration distributions; estimated total emissions over greater Dhaka area (about 1700 km2) were 72 and 70 ton day-1 for SO2 and NOx, respectively. Motor vehicles and brick fields were speculated two major emission sources. The model well reproduced the observed concentrations. The conversion rate coefficient for SO2 to SO2-4, and the conversion rate for NO2 to other N-species showed large diurnal and spatial variations. The average pseudo-first-order-reaction rate coefficient of SO2 to SO2-4 at ground level in winter was about 0.3% h-1, resulting in 7% conversion in a 24 h period. The average conversion rate of NO2 to HNO3 and PAN at ground level in winter was about 7.3×106 and 1.6×107 molecule cm-3 s-1, respectively.