Atmospheric Environment Vol. 32, No. 11, pp. 1991—2005, 1998  1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 1352—2310/98 $19.00#0.00 PII: S1352 – 2310(97)00508 – 6 CHARACTERISTICS OF THE AIR POLLUTION IN THE CITY OF DHAKA, BANGLADESH IN WINTER A. K. AZAD and T. KITADA* Department of Ecological Engineering, Toyohashi University of Technology, Tempaku-cho, Toyohashi, 441—8580, Japan (First received 3 April 1997 and in final form 15 November 1997. Published May 1998) Abstract—Ten-day-average concentrations of SO  and NO  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 SO  and NO  were also estimated. The measurements first elucidated the characteristics of the spatial distribution of SO  and NO  in this area, and showed significant spatial variations and extremely high SO  concentrations in the southeastern industrial zone of Dhaka where the highest value was about 100 ppb. The polluted zone, defined as the average SO  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 NO  , 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 SO  and NO  emissions in Dhaka for winter 1995—96 was made to see their importance to SO  and NO  concentration distributions; estimated total emissions over greater Dhaka area (about 1700 km) were 72 and 70 ton day for SO  and NO  , respectively. Motor vehicles and brick fields were speculated two major emission sources. The model well reproduced the observed concentrations. The conversion rate coefficient for SO  to SO  , and the conversion rate for NO  to other N-species showed large diurnal and spatial variations. The average pseudo-first-order-reaction rate coefficient of SO  to SO  at ground level in winter was about 0.3% h, resulting in 7% conversion in a 24 h period. The average conversion rate of NO  to HNO  and PAN at ground level in winter was about 7.310 and 1.610 molecule cm s, respectively.  1998 Elsevier Science Ltd. All rights reserved Key word index: Measurement, simulation, air pollution, sulfur dioxide, nitrogen dioxide developing country, Dhaka. 1. INTRODUCTION Atmospheric pollution in urban area is a major issue in many developing countries all over the world. Sul- fur dioxide (SO  ) and nitrogen dioxide (NO  ) are major pollutants in the ambient atmosphere because of their adverse effects on human health and vegeta- tion, their contributions to the acidification of the environment (Rodhe, 1989; Legge and Krupa, 1990) and the role of NO  in the formation of photochemi- cal oxidants. NO  contributes to the build-up of tropospheric ozone (O  ) and to the change of the concentration of hydroxyl radical (OH) which deter- mines the lifetime of reactive greenhouse gases (Houghton et al., 1990), and thus is also a key species for global warming. The rates of increase of pollutant concentrations in the cities of developing countries are higher than those of developed countries (Kato et al., 1991). Dhaka, the capital of Bangladesh, having a popula- tion of about 9 million is one of the biggest cities of the *Author to whom correspondence should be addressed. developing countries. It is expanding very rapidly due to high influx of people from rural areas. Emissions from various kinds of diesel traffic vehicle and badly maintained automobiles contribute most to air pollu- tion problems. As brick is the main material for build- ing construction in Dhaka, a lot of brick fields (which use coal as main fuel and operate only in winter due to meteorological condition) have grown up around Dhaka, especially in the northwest and southeast side of the city. These brick fields are another major con- tributors to the severe air pollution in winter in Dhaka. The adverse meteorological conditions in winter further aggravate the situation. But no system- atic measurements of air pollution in Dhaka have been done until now due to limited measuring facili- ties and economic constraints. To develop a reliable control strategy it is necessary to know the present pollution levels. Diffusion tube samplers were used for monitoring SO  and NO  concentrations in Dhaka. The samplers require no maintenance or power supply since they collect SO  /NO  via molecular diffusion. This method has been extensively used for measurements of ambient 1991