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

Atmospheric Environment Vol. 32, No. 11, pp. 1991—2005, 1998( 1998 Elsevier Science Ltd. All rights reserved

Printed in Great Britain1352—2310/98 $19.00#0.00PII: S1352–2310(97)00508–6

CHARACTERISTICS OF THE AIR POLLUTION IN THE CITYOF 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 SO2

and NO2

were measured using molecular diffusiontubes in Dhaka, Bangladesh, in winter 1995—96, and their spatial distribution and temporal variation weresimulated using an Eulerian transport/chemistry/deposition model. Conversion rates for SO

2and NO

2were also estimated. The measurements first elucidated the characteristics of the spatial distribution of SO2and NO

2in this area, and showed significant spatial variations and extremely high SO

2concentrations in

the southeastern industrial zone of Dhaka where the highest value was about 100 ppb. The polluted zone,defined as the average SO

2concentration 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 themain roads of Dhaka. An estimation of anthropogenic SO

2and NO

xemissions in Dhaka for winter

1995—96 was made to see their importance to SO2

and NO2

concentration distributions; estimated totalemissions over greater Dhaka area (about 1700 km2) were 72 and 70 ton day~1 for SO

2and NO

x,

respectively. Motor vehicles and brick fields were speculated two major emission sources. The model wellreproduced the observed concentrations. The conversion rate coefficient for SO

2to SO2~

4, and the

conversion rate for NO2

to other N-species showed large diurnal and spatial variations. The averagepseudo-first-order-reaction rate coefficient of SO

2to SO2~

4at 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 atground level in winter was about 7.3]106 and 1.6]107 molecule cm~3 s~1, respectively. ( 1998 ElsevierScience Ltd. All rights reserved

Key word index: Measurement, simulation, air pollution, sulfur dioxide, nitrogen dioxide developingcountry, Dhaka.

1. INTRODUCTION

Atmospheric pollution in urban area is a major issuein many developing countries all over the world. Sul-fur dioxide (SO

2) and nitrogen dioxide (NO

2) are

major pollutants in the ambient atmosphere becauseof their adverse effects on human health and vegeta-tion, their contributions to the acidification of theenvironment (Rodhe, 1989; Legge and Krupa, 1990)and the role of NO

xin the formation of photochemi-

cal oxidants. NOx

contributes to the build-up oftropospheric ozone (O

3) 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 speciesfor global warming.

The rates of increase of pollutant concentrations inthe cities of developing countries are higher thanthose 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 dueto high influx of people from rural areas. Emissionsfrom various kinds of diesel traffic vehicle and badlymaintained 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 (whichuse coal as main fuel and operate only in winter due tometeorological condition) have grown up aroundDhaka, especially in the northwest and southeast sideof the city. These brick fields are another major con-tributors to the severe air pollution in winter inDhaka. The adverse meteorological conditions inwinter further aggravate the situation. But no system-atic measurements of air pollution in Dhaka havebeen done until now due to limited measuring facili-ties and economic constraints.

To develop a reliable control strategy it is necessaryto know the present pollution levels. Diffusion tubesamplers were used for monitoring SO

2and NO

2concentrations in Dhaka. The samplers require nomaintenance or power supply since they collectSO

2/NO

2via molecular diffusion. This method has

been extensively used for measurements of ambient

1991

SO2/NO

2concentrations in both urban and rural

areas (Atkins et al., 1986; Bower et al., 1991; Gairet al., 1991; Maeda et al., 1994).

The development of efficient control strategies forair pollution problem in urban area should also bebased on a better understanding of the physical andchemical processes that govern the formation, trans-port, diffusion, chemical transformation and removalof the pollutants. A mathematical model incorporatesthe necessary analytic framework for describing thephysics and chemistry of polluted urban airsheds andsimulating urban air pollution episodes. So that, themathematical model has been extensively used to cal-culate the distribution of air pollutants and to formu-late effective strategies for controlling the air pollution(Juda, 1986; Pilinis et al., 1993; Kumar and Russell,1996).

In this study, we have measured the concentrationsof SO

2and NO

2in Dhaka city on a large scale using

diffusion tube samplers, derived their spatial distri-butions, and compiled emission source inventoriesover Dhaka. The paper also reports the applicationof an Eulerian transport/chemistry/deposition model(Kitada et al., 1984; Carmichael et al., 1986; Kitadaet al., 1993) to simulate SO

2and NO

2concentrations

over greater Dhaka. An investigation on the chemicalconversion of SO

2to SO2~

4, and NO

2to other nitro-

gen species over Dhaka is also performed. These arethe first results of systematic measurements andmodeling efforts in Dhaka. This study will give use-ful information for emission-control strategies anddecision-making processes such as planning andmanagement for preserving preferable atmosphericenvironment in the large cities of the developingcountries like Dhaka.

2. MEASUREMENTS

2.1. ¹he method

The molecular diffusion tube method, described in Maedaet al. (1994), was used to measure the concentration distribu-tions of SO

2and NO

2in Dhaka city and its suburbs during

the period from mid-December, 1995 to mid-January, 1996.The sampler, a plastic tube, 33 mm long and 36 mm indiameter, had a cap at one end. One piece of aluminium foil,polyethylene sheet, absorption medium (filter paper coatedwith reagent), and two pieces of polyflon filter paper wereplaced into the tube in the order, and fixed in position witha rubber O-ring (see Fig. 1). The cap was removed and thesampler was exposed by suspending it vertically with theopen end downside in unrestricted air.

The filter paper (2 cm]2.5 cm in size, Toyo Roshi Co.,No. 51A) was washed two times in highly pure water for30 min at 60°C and soaked in the reagent solution (2.5%sodium carbonate for SO

2and 6.6% triethanolamine for

NO2) for 30 min followed by drying at 50—60°C for one hour

in an oven with pure air supply. The sulfate/nitrite absorbedin each filter paper was extracted by wetting it for 30 min at30°C into 15 ml highly pure water with addition of 0.3 ml of30% H

2O

2followed by oxidation for sulfate, and into 25 ml

highly pure water followed by appropriate dilution of a frac-tion of the extracted solution for nitrite. The extracted solu-tions were analyzed by Dionex ion-chromatograph (model

Fig. 1. Diagram of a diffusion tube.

DX-300) for sulfate and by spectrophotometer (U-2000 at540 nm) for nitrite, and the results were converted to airconcentrations of SO

2/NO

2using the exposure period and

calibration equation. The contamination of unexposed sam-plers under field conditions was determined, and the value ofthe blank test was subtracted from the measurements madeby the diffusion tube samplers. The samplers were preparedand analyzed by the authors themselves. Care was taken toensure that blank value was always minimized (Miller, 1988).The effects of wind turbulence and temperature were reducedusing polyflon filters.

The diffusion tube samplers were calibrated using 6 auto-mated air pollution monitoring analyzers (Electro-ChemicalInstrument Co. ¸td, Model GRH-76M and GPH-74M) inAichi-prefecture, Japan since there was no automatedmonitoring system in Dhaka. ¹he automated analyzers werechosen from the air quality surveillance network of the Depart-ment of Environment of Aichi-prefecture Government, andwere on operation in good condition. Three samplers weredeployed at each automated air pollution monitoring stationfor 10 days to measure the concentration of one pollutantspecies. All three samplers at each monitoring station gaveconsistent result. The sulfate/nitrite extracting rate of eachsampler was calculated using the amount of sulfate/nitriteabsorbed in the filter paper of the sampler and the exposureperiod of that sampler. The average sulfate/nitrite extractingrate of the three samplers at each station was regressedagainst ambient SO

2/NO

2concentration at that station.¹he

regression line and the distribution of SO2/NO

2concentration

data acquired by automated analyzer at each site showed thatthe error range of measurements made by the diffusion tubesamplers after calibration was 2—25%. The calibration equa-tions, which were ½"0.423X for SO

2and ½"0.300X for

NO2; where ½ denotes ambient SO

2/NO

2concentration

(ppb) and X represents sulfate/nitrite extracting rate(Nnl h~1), were used to convert the sulfate and nitrite ex-tracting rates of the samplers to air concentrations of SO

2and NO

2, respectively, in Dhaka.

2.2. Sampling protocol

The sampling was carried out from mid-December of 1995to mid-January of 1996 at 64 sites in the highly populatedcity of Dhaka and its suburbs, including Savar, Tongi, Kach-pur, Futulla and Keraniganj (Fig. 2b). Dhaka (latitude 23°43@N, longitude 90°24@ E) is in the middle of Bangladesh (latit-ude 20°34@—26°38@ N, longitude 88°01@—92°41@ E), a countrywhich lies in the eastern part of south Asia and is boundedby India on the west, the north and the northeast, andBurma on the southeast, and the Bay of Bengal on the south

1992 A. K. AZAD and T. KITADA

Fig. 2. (a) Location of Dhaka in Bangladesh. (b) Locations of sampling sites in Dhaka.

Air pollution in Dhaka, Bangladesh 1993

Table 1. Daily fuel consumption and estimated SO2

and NOx

emissions by various sources in Dhaka in winter 1995—96!

Fuel Emission factor (kg t~1)" Amount of emission (t dv1)

Source Type Amount (t d~1) SO2

NOx

SO2

NOx

Traffic vehicle% Diesel oil$ 1395 28.8 27.4 40.17 38.22Percent (%) 55.8 54.5

Brick field# Coal 1430 9.61 7.5 13.74 10.72Wood 536 0.86% 6.0% 0.15 1.05Furnace oil 107 64 5.09 6.85 0.54

Subtotal 20.7 12.3Percent (%) 28.8 17.5

Industry& Natural gas 390]103' 9.6]10~6) 8800]10~6) 0.00374 3.43Coal 156 9.61 7.5 1.5 1.17Kerosene 49 4 7.46 0.196 0.365Diesel oil 98 28.8 9.62 2.82 0.942Residual oil 49 64 5.84 3.14 0.286


Residential& Natural gas 744]103' 9.6]10~6) 8800]10~6) 0.0071 6.54Kerosene 10 4 2.49 0.04 0.025Diesel oil 20 28.8 3.21 0.576 0.064


Navigation# Diesel oil 100 28.8 54.1 2.88 5.41Percent (%) 4 7.7

Commerce Natural gas 160]103' 9.6]10~6) 8800]10~6) 0.0015 1.41Percent (%) 0.1 2

Total 72 70Percent (%) 100 100

!Energy consumption data were compiled from reports and personal communications to the respective departments."Kato and Akimoto (1992), except natural gas emission factors of which were adopted from EPA (1985).#Estimated from total number of cars, brick fields and navigation vessels, and energy consumption by one unit of each type

(data were collected from Bangladesh Road Transport Authority, Petro-Bangla, Brick Field Union and Bangladesh InlandWater Transport Corporation).

$Taken as average for petroleum oil, diesel oil, gasoline and other fuels since the amount of diesel fuel used was highest.%kg toe~1; toe means ton oil equivalent.&Estimated from total energy consumption by each sector (data were collected from Titas Gas Authority and BBS (1995)).'m3 d~1.)kg m~3.

(Fig. 2a). Dhaka, located on a flat plain with no mountains, issurrounded by rivers at all sides and is the center of com-merce and industry in Bangladesh. The city is growing ra-pidly, but not in a well-planned way.

The sampling sites were selected to reflect ambient concen-trations of the pollutants over all types of area and environ-ment in Dhaka. One set of samplers was exposed within15—50 m from major roads, another set was set up far away('100 m) from major roads. The third set of samplers (lar-gest set) was deployed in the planned and unplanned housingarea, commercial area, industrial area, suburban and othertypes of area to measure ambient concentrations of pollu-tants. The locations of the sites are shown in Fig. 2b. Itshould be noted that we could not expose the samplers fullyto those sites where we desired (e.g. kerbside) due to thesafety of samplers such as very high probability of loss ordamage of the samplers. The samplers were attached towalls, windows, fences, poles and leafless trees at man height.Three samplers were exposed at a site for 7—10 d to measurethe concentration of one pollutant species. The samplerexposure time should be dependent on pollutant concentra-tion. As we expected high pollutant concentration in Dhakafrom few existing data collected from the Department ofEnvironment, Dhaka, hence a shorter exposure period waschosen. The samplers were sealed into plastic bags after

recapping at the end of the exposure period. Data from threesamplers were averaged to give concentration at a site. Typi-cal exposure variance of co-located tubes was about0.4 (ppb)2 for SO

2and 0.7 (ppb)2 for NO

2.

3. EMISSION ESTIMATION

Emissions from various sources in Dhaka wereestimated to understand their relative contributionsto SO

2and NO

2concentrations. The anthropogenic

emissions of SO2

and NOx

in Dhaka in winter1995—96 were computed using fuel consumption andemission factors for the unit consumption. Majorsources of SO

2and NO

xover Dhaka are listed in

Table 1 with their emission factors and the totalamounts. These emission data were compiled usingpublished reports, journals, and personal communica-tion with government and private organizations asdescribed in Table 1.

Table 1 shows that the primary source of SO2

istraffic vehicle (55.8%); followed by brick field (28.8%)


Table 2. Comparisons of the estimated emissions of SO2

and NOxin this study with

those adapted from other studies

Studies SO2

emissions (t d~1) NOx

emissions (t d~1)

This study (winter, 1995—96) 72 70Kato and Akimoto (1992) 8.7! 11.7!

Arndt et al. (1997) 123.8 —

!Calculated by multiplying the total emissions of Bangladesh estimated in Kato andAkimoto (1992) with the population ratio of Dhaka to whole of Bangladesh.

Table 3. Average meteorological conditions in Dhaka in different seasons, 1991—1995

Avg.Average Relative Prevailing Avg. Wind

temp. (°C) Avg. rainfall Humidity Wind speed % Calm % WindSeason Max. Min. (mm) (%) Direction (m s~1) ((1 m s~1) ('2 m s~1)

Winter 27 13 68 72 NW 0.8 70 10(Dec.—Feb.)Pre-monsoon 34 21 528 72 S, SW 1.6 30 40(March—May)Monsoon 32 26 1283 83 S, SE 1.6 27 35(June—Sept.)Post-monsoon 31 20 220 77 W, SW 0.72 71 7(Oct.—Nov.)

and industry (10.5%). The remainder (about 5%) aredue to navigation vessel in the Buriganga river, resi-dential activity and commerce. Using high-sulfur-con-taining petroleum products as fuel in car, togetherwith the lower maintenance quality, lead to high emis-sion of SO

2from traffic vehicle. The primary source of

NOxemission is also traffic vehicle (54.5%), indicating

its most liability to severe air pollution of Dhaka city.Brick field is the second most important source, con-tributing 17.5%, residential 9.5%, industry 8.8%,navigation 7.7% and commerce 2%. The overall perday emissions in Dhaka city in winter 1995—96 are 72tonnes for SO

2and 70 tonnes for NO

x.

Specific published data on Dhaka city are not avail-able for the comparisons of the emissions of SO

2and

NOx. Comparisons to the data adapted from the

study of Kato and Akimoto (1992), and Arndt et al.(1997) are shown in Table 2. We calculated the datafrom the study of Kato and Akimoto on the basis ofthe ratio of the population of Dhaka to wholeBangladesh.

The SO2

and NOx

emissions calculated in thisstudy for Dhaka city are 7 and 5 times higher thanthose of Kato and Akimoto. This may be ascribed tothe higher per capita fuel consumption in Dhaka citythan the rural area, where the only fuel consumptionis the oil burning for lighting at night, and biofuelsusually used for cooking. According to Bartone(1995), the cities in the developing countries consumeas much as seven times more energy per resident thanthe rural population, which gives support for higherSO

2and NO

xemissions found in this study than

those of Kato and Akimoto. The higher emissions inDhaka might be expected because of: (i) the more

urbanized/industrialized nature of Dhaka comparedto Bangladesh as a whole, (ii) the greater use of fossilfuels in Dhaka, and (iii) seasonal activities such asbrick burning. In the study of Kato and Akimoto thetotal emission of NO

x(11.7 t d~1) is higher than the

total emission of SO2

(8.7 t d~1), which is quite oppo-site to our study. Traditional vegetal fuels, which arenot used in Dhaka, but extensively used in rural areafor cooking, could be the reason for the higher NO

xemission in their study. The estimated SO

2emission

in our study compares well with that in Arndt et al.(1997), but slightly lower which could be seen fromTable 2.

4. METEOROLOGY AT THE STUDY AREA

Meteorologically, the year of Bangladesh can bedivided into four distinct seasons; pre-monsoon (March—May), monsoon (June—September), post-monsoon(October—November) and winter (December—February).A brief summary of the average meteorological condi-tions (1991—1995) in Dhaka for different seasons isgiven in Table 3. From Table 3, the winter is charac-terized by 13—27°C average temperature. During thesewinter months, wind blows very weakly from north-west direction with an average speed of 0.8 m s~1 andwith a high percentage of calm (avg. 70%). This ex-tremely low wind speed and slight precipitation havemade winter the most severe season for air pollution.

Meteorological data for the study period werecollected from the Dhaka national observatory (seeFig. 2b). During the measurement period, the after-noon temperatures were moderately warm (&26°C)


Fig. 3. Diurnal patterns of wind speed in Dhaka in winter1995 for five days, i.e. 20—24 December (we assumed

0.45 m s~1 wind speed in calm condition).

and relatively cooler temperatures were observed dur-ing the night (&13°C). The maximum temperaturewas 28°C on 25 and 31 December 1995, and theminimum temperature was 9.6°C on 14 January 1996.The average temperature was 20°C. Very low windspeeds averaging 0.52 m s~1 with 68% calm (windspeed less than 1 m s~1) were observed during thestudy period. The northwesterly was the predominantwind. The diurnal patterns of wind speed in Dhaka inthe study period for 5 d beginning from 20 Decemberare depicted in Fig. 3, in which relatively high windcan be seen at noon in the 2nd day (21 December).There was no rain in the measurement period.

5. MEASUREMENT RESULTS AND DISCUSSION

5.1. Analysis of the concentration distributions of SO2

and NO2

over Dhaka

Figure 4a shows the spatial distribution of themeasured 10-day-average SO

2concentration over

December 1995/January 1996 period in Dhaka. InFig. 4a extremely high SO

2can be found in the south-

eastern industrial and brick field zone, where thehighest concentration is over 100 ppb. The pollutedzone, in which the average SO

2was over 40 ppb,

extended along the major road running from north-west to southeast, and also parallel to the Burigangariver in the Dhaka area. Formation of this pollutedzone can be explained by the characteristics of emis-sion source and topography: (1) major emission sour-ces such as brick fields, industries, and traffic andnavigation routes are located by the Buriganga river,and (2) weak winds, northwesterly, tend to blow alongthe river in winter. In the case of NO

2(Fig. 4b), the

highest 10-day-average concentration was 35 ppb,and higher concentration appeared in the city centerand along main roads of Dhaka, indicating traffic asmajor NO

2source. It is noteworthy that in the south-

east brick field area the highest SO2

concentrationwas 104 ppb, whereas NO

2concentration was only

Fig. 4. Distributions of measured (a) SO2

and (b) NO2

con-centrations in Dhaka averaged over Dec.’95/Jan.’96 sam-pling period (the number represent measured SO

2/NO

2concentrations in ppb).


Table 4. 10-day-average concentrations of SO2

and NO2

over Dec.’95/Jan. ’96 period in differentareas of Dhaka

Area No. of sites SO2

(ppb) NO2(ppb)

Lalbagh—Sadarghat—Gandaria 4 15 24Mothijheel—Sci. Laboratory—Maghbazar 5 16 25Farmgate—Mohakhali—Tejgaon—Sangsad Bhavan 5 16 26Dhanmondi—Mohammadpur—Hazaribagh 4 28 18Gabtali—Dhaka Aricha Road 4 36 28Savar Area 6 4 14Mirpur—Agargaon 7 6 16Gulshan—Banani 4 3 16Airport—Uttara 4 8 18Tongi 2 9 21Merul—Rampura—Goran 4 13 21Kamalapur—Jatrabari—Postogola 5 25 28Kachpur—Soniakra 4 23 22Pagla—Futulla 3 70 19Keranigonj 3 6 16

16 ppb (Figs. 4 a and b); this can be attributed to thehigher SO

2emission compared with that of NO

xin

brick field which result from extremely high SO2

emission factor of ‘‘furnace oil’’ (see Table 1); the brickfields account 28.8% for SO

2emission, but only

17.5% for NOx. The SO

2and NO

2concentrations in

Figs. 4 a and b were mapped with eye using ourmeasurement data and also observational experienceon emission sources.

Measured SO2

in Fig. 4a show large variability intheir values, i.e. from merely 1 ppb to more than100 ppb, and also show a relatively localized nature ofextremely high-concentration zone, while the NO

2concentrations in Fig. 4b vary within a rather narrowrange between 10 and 35 ppb, and form broad moder-ately high-value zone over the city center and majorroad area. This distinct nature in the distribution ofSO

2and NO

2can be attributed to the difference of

the type of major emission sources for the two species:brick field and other industry are the second largestSO

2emission sources, i.e. around 40% of the total

emission, which are located to the northwest andsoutheast of downtown Dhaka, and they dischargeSO

2mostly as point sources, while automobiles are

the most important emission sources for NO2

andthose traffic are widely distributed over the downtownDhaka. A summary of the 10-day-average SO

2and

NO2

concentrations obtained over the December1995/January 1996 sampling period in different loca-tions of Dhaka is shown in Table 4.

It should be noted that SO2

value at backgroundsite of Savar (northwest of downtown Dhaka) foundin our measurement in winter 1995 is similar to thatfound in the measurement of Rains Asia AtmosModule, Phase I Project in winter 1994 in the samearea (Carmichael et al., 1995; Ahmad et al.,1996).

5.2. Comparison with air quality standards

In this subsection air quality over Dhaka isevaluated by comparing observed SO

2and NO

2

Table 5. WHO air quality guidelines for SO2

and NO2

Substance Exposure level Exposure time

Sulfur dioxide 500 kg m~3 10 min350 lg m~3 1 h

Nitrogen dioxide 400 kg m~3 1 h150 kg m~3 24 h

concentrations with WHO guidelines and Japaneseenvironmental standard values.

The WHO guidelines for SO2

and NO2, listed in

Table 5, do not show values for 10 days’ exposure.Thus those were evaluated by logarithmic extrapola-tion of 10 min and 1 h values for SO

2, and 1 and 24 h

values for NO2. They are 188 kg m~3 or 66 ppb for

SO2

and 143 kg m~3 or 70 ppb for NO2

for 10 days’exposure. This WHO-standard-derived-value for SO

2(66 ppb) was exceeded at 2 sites in the southeast in-dustrial and brick field zone, but the NO

2value

(70 ppb) did not exceed at any site in Dhaka.According to the ambient air quality standards set

by the Japanese Environmental Agency (JEA), dailyaverage of hourly values shall not exceed 40 ppb forSO

2(JEA, 1973), and daily average of hourly values

must be within the range between 40 and 60 ppb orbelow for NO

2(JEA, 1978). The limit value for SO

2(40 ppb) was exceeded at 7 sites (and possibly at aneighth site with a 10-day-average of 39 ppb), where10-day-average concentration was above 40 ppb (seeTable 6). These sites are located especially in thesoutheast, and northwest industrial areas. The highestNO

2over Dhaka was 35 ppb for 10-day-average, and

may reach the allowable range (40—60 ppb) for dailyaverage.

5.3. Influence of meteorology

The influence of meteorology on the pollutant con-centrations in Dhaka was assessed using mean windspeed, predominant wind direction, mean mixing


Table 6. Sites with 10-day-average SO2

concentration ex-ceeding 40 ppb

SO2

Concentration NO2

concentrationSite (ppb) (ppb)

Futulla 104 16Pagla 78 21Amin Bazar 46 35Kachpur 43 28Zinzira 43 20Mohammadpur 43 18Gabtali 41 29

height, humidity and rainfall. The strength of north-westerly wind from the Himalayas to the Bay ofBengal is low to transport the pollutants from thesources to a long distance in winter. Mixing height isan important factor to assess the influence of meteoro-logy on the dispersion of the pollutants. In Dhaka at24°N, mixing height in winter is expected to be lowerthan that in summer, as can be logically derived andalso as shown in the study of Gamo et al. (1994) inIndia, due to small values of sensible heat flux anda large lapse rate owing to the strong stable surfacelayer due to radiative cooling (Gamo et al., 1994).There are slight rainfall in Dhaka in winter due tonorthwesterly dry air coming over the land surface,and this acts in favor to the severe air pollution. Insummer, the higher sunlight, wind speed, rainfall andmixing heights are all likely to lead to lower concen-trations. The greater solar flux would promote theefficiency of atmospheric chemical reactions, leadingto greater conversion of SO

2and NO

xto sulfate and

nitrate. This would reduce the concentrations of gas-eous pollutants (SO

2and NO

x), but could increase the

relative amount of particulate matter formed.

5.4. ¹rends in SO2/NO

xemission and concentration in

Dhaka

No previous data of emission and concentration ofSO

2/NO

xin Dhaka are available to show their trends.

An attempt has been made here using populationdata. The population of Dhaka city were about 2million in 1970, 7.5 million in 1990 and will be 17.5million in 2010 (Bartone, 1995). The projected emis-sions in these years were estimated on the basis of thepopulation data in the respective year with the help ofthe present (1995) emissions and population data (as-suming per capita fuel consumption, emission controlsituation and the ratio of using fuel types did not andwill not change significantly). The trends of estimatedprojected emissions show that the total emission ofSO

2in Dhaka in winter has increased by a factor of

4.5 from 16 t d~1 in 1970 to 72 t d~1 in 1995 with anaverage increase rate of 4.9% yr~1, and will be in-creased by a factor of 1.9 from 72 t d~1 in 1995 to140 t d~1 in 2010 with an average increase rate of4.6% yr~1. The factors and rates of increase for NO

xemission are same to those for SO

2. The factors

and rates of increase for SO2/NO

xemission provide

sufficient evidence for an upward trend in SO2/NO

2concentration.

6. MODELING STUDY OF SO2

AND NO2

POLLUTION

IN DHAKA

6.1. Modeling description

A three-dimensional Eulerian photochemicalmodel (Kitada et al., 1984; Carmichael et al., 1986;Kitada et al., 1993) that accounts for the transport,chemical conversion and removal of pollutant wasused to simulate SO

2and NO

2concentration distri-

butions over Dhaka. The chemical mechanism used inthis model is adapted from the work of Lurmann et al.(1986), the mechanism which includes 148 reactionsamong 68 species; the modified chemical mechanismis based on the condensed model by Lurmann et al.,and additionally includes reactions due to biogenichydrocarbons which were not applied in this simula-tion. Of these species, 42 long-lived species are treatedas advected species while the remaining 26 short-livedspecies such as free radicals are modeled using thepseudo-steady-state approximation. The mechanismincludes major photochemical smog reactions as wellas production processes of acidic pollutants. The drydeposition of different species is described by their drydeposition velocities. In order to parameterize the drydeposition velocities of gases to surfaces a ‘‘big-leaf ’’multiple resistance-based scheme is used (see, e.g.Kitada and Ueda, 1989).

Horizontal eddy diffusivity, KH, was given by the

following equation after Pielke (1974):

KH"a2*x*y CA

Lv

Lx#

Lu

LyB2

#

1

2 GALu

LxB2#A

Lv

LyB2

HD1@2

. (1)

A lower limit of KH"1000 m2 s~1 was imposed.

The vertical diffusivity (K7) was calculated using the

following relation (Tanaka, 1991), which is based onthe output of k—e turbulence model for inland bound-ary layer (Kitada, 1987):

K7"A

10.417

h2!

0.374

h B z2#A0.36!5.625

h Bz

#0.013z#0.208 (2)

where h is the mixing height and z is the height abovethe ground, z)h; for z'h, K

7"1 m2 s~1 was used.

The upper limit of KV

was restricted to 50 m2 s~1.The modeling area, greater Dhaka (latitude

23°36@—24°N, longitude 90°9 @—90°32@E), covers 44 kmin the north—south direction and 39 km in the east—west direction, and includes urban, suburban andrural area. A detail description about Dhaka is pre-sented in Section 2.2. The domain was chosen so thatthe concentrations at the outside of the domain


boundary can be assumed to be small compared tothe concentrations in the domain. The modeling do-main was discretized into 40]45 grid points witha spacing of 1 km in the horizontal direction, and inthe vertical, the domain was divided into 17 layerswith different thickness of 12, 18, 26, 34 m in the 1st,2nd, 3rd, 4th layer, and 280 m at the uppermost modellayer (17th layer), up to 1.5 km above the ground level.

Meteorological fields used in the model were pre-pared from the surface (10 m) data measured at every3 h at the Meteorology Observatory, Dhaka, duringthe study period. Because of nearly flat terrain in theGreater Dhaka area and relatively narrow calculationdomain, i.e. 39 km]44 km, we have assumed samewind speed at a height of 10 m above ground for thewhole area as that at this meteorological observatory.The upper wind fields were extrapolated from thesurface wind data using the log wind profile up to100 m. Above the heights of 100 m, winds were setequal to those at 100 m height.

Typical diurnal variation of the mixed layer, h inequation (2), was given using information on themixed layer in eastern India where the climate issimilar to that of Dhaka area, i.e. dry and moderatetemperature in winter. The maximum mixing heightwas estimated using aerological data in Dhaka.

The SOx

and NOx

emission fields were preparedfrom the estimated emissions described in Section 3 ofthis text. The emissions for non-methane hydrocar-bon (NMHC) and carbon monoxide were derivedfrom NO

xemission field. Emission of each NMHC

was specified in proportion to fuel-combustion-de-rived NO

xemission: in molar basis, [NO

x] : [C

2H

6] :

[C3H

8] : [ALKA] : [C

2H

4] :[ALKE] : [AROM]"1 :

0.25 : 0.45 : 2.33 : 0.48 : 0.37 : 1.62, where [C3H

8] stands

for propane and benzene, [ALKA] for lumped *C4

alkanes, [ALKE] for *C3alkenes, and [AROM] for

alkylbenzenes. The coefficients of proportion listedabove were derived on the basis of the data for AichiPrefecture, Japan (Nakanishi, 1996; Nakanishi andKitada, 1997). The emission sources were classifiedinto two groups, i.e. surface and elevated sources. Thesurface sources included emissions from automobile,industry, residential activities, navigation vessel andcommercial activities, while the elevated sources wereassigned for brick fields. The effective stack height of30 m (3rd vertical grid level) was considered in thisstudy. The brick fields are especially located in thenorthwest and southeast suburban of Dhaka (see Fig.2b). The inventories were compiled over a 39]44 km2

region with 1]1 km2 grid cells. The time depen-dences of the emission rates were given so that theycan reflect the diurnal activities of the major emissionsources in Dhaka.

The horizontal boundary conditions are as follows:for the inflow case, the advective flux at the boundaryis set equal to that due to the prescribed concentrationat the outside of the domain and zero concentration-gradient normal to the boundary for the outflow case.The top boundary is treated in the same way as the

horizontal. At the bottom boundary, diffusional massflux of chemical species are equated to the sum of theiremission and deposition fluxes.

Initial concentrations, 1 ppb for SO2

and 5 ppb forNO

2, were assumed from the measured concentration

distributions described in Section 5.1. To minimizethe impact of background concentrations on themodel results, background concentrations were takenas initial concentrations. As the modeling domain wassurrounded by relatively clean environments espe-cially in the northwest sector, the inflow boundaryconditions were not a significant problem in this situ-ation. The influence of the boundary conditions wastested directly by comparing results from two simula-tions where the only difference was that the concen-trations at the outside of the boundary were doubled.The concentration was increased by about 2.5% forSO

2and about 4.8% for NO

2at surface level in the

city center of Dhaka.

6.2. Model results and discussion

6.2.1. Estimated SO2

and NO2

concentrations, andtheir comparison with observations. Simulations wereperformed for 20—30 December 1995 to compare cal-culated 10-day-average concentrations of SO

2and

NO2

in Dhaka with those observed. The results areshown in Figs 5a and b for SO

2and NO

2, respect-

ively. Characteristics found in Fig. 5 may be sum-marized as follows. (i) The computed 10-day-averageconcentration distributions of SO

2in Fig. 5a show

that the southeastern part of Dhaka, i.e. the area ofbrick fields and other industries, exhibits the highestSO

2concentration, and the city center also forms

a high concentration zone. (ii) High NO2is seen in the

city center of Dhaka in the computed spatial distribu-tions of NO

2in Fig. 5b. (iii) The computed concentra-

tions in Fig. 5 show lower values in northern andwestern parts of the domain, reflecting the effect ofpredominant northwesterly bringing clean air to thedomain. These characteristics found in the SO

2and

NO2

concentrations in Fig. 5 agree well with theobservation in Fig. 4. The above feature (iii) alsosuggests that long-range trans-boundary transport ofSO

2and NO

2from India was not significant under

weak wind with its direction ranging from north towest; no large emission sources exist to the northwestof Dhaka in India. Thus, the SO

2and NO

2pollution

in Dhaka in winter season may not be affected by theIndian SO

xand NO

xemissions since the prevailing

wind in this season is northerly to westerly blowingfrom the Himalayas to the Bay of Bengal and is weak(see Table 3).

The predicted ground-level SO2

and NO2

valuesare in good qualitative and quantitative agreementwith observations. Approximately 80% of the sitesshow agreement between modeled and observed con-centrations within $50%, and at about 50% of thesites the agreement is better than $25%. About 90%of the modeled values lay within a factor of 2 of theobserved values. Scatter diagrams of the measured


Fig. 5. Computed 10-day-average concentrations of (a) SO2

and (b) NO2

in the surface level of Dhaka in winter (20—30December, 1995). X in (a) denotes the highest SO

2concentra-

tion at 70 ppb.

and calculated SO2

and NO2

concentrations at 64sites are shown in Figs 6a and b. The solid linerepresents the regression line, while the dotted linesrepresent its 95% confidence limits about the regres-sion line. It is seen from these scatter plots that onlyfive points for SO

2and one point for NO

2lie outside

the confidence interval. The correlation coefficientbetween the calculated and measured values was 0.91for SO

2and 0.87 for NO

2. However, the model under-

predicted the highest SO2

concentration level (obser-vation: 104 ppb) in the southeast suburban area andoverpredicted the NO

2concentration (observation:

Fig. 6. Scatter diagram of measured and computed (a) SO2

and (b) NO2

concentrations.

16 ppb) in the same area (see Figs 4 and 5). Mostprobably this is due to the lack of sufficient informa-tion regarding very local emission sources. Amongthe other possible reasons are inaccuracies in theinterpolated wind fields and stack heights for pointsources.

6.2.2. Estimated conversion rate coefficient of SO2

to SO2~4

over Dhaka area. The development of effec-tive policies to control SO2~

4deposition requires an

understanding of how SO2~4

is produced in theatmosphere. Based on the good agreement betweencomputed and observed SO

2concentrations (see pre-

vious subsection), we have computed and analyzedthe SO

2to SO2~

4conversion rate coefficient in the

atmosphere of Dhaka. Three lumped chemical reac-tions are considered for the SO

2to SO2~

4conversion

in the chemical mechanism (Lurmann et al., 1986)used in this study. We have estimated the relativeimportance of those in the conversion mechanism.These reactions are:

SO2#OH K29&" SO2~

4#HO

2(3)

SO2#CHO

2K86&"SO2~

4#HCHO (4)

SO2#CRO

2K87&"SO2~

4#ALD

2(5)


Fig. 7. (a) Spatial distributions of 3-day-averaged ground level total conversion rate coefficient (% h~1) ofSO

2to SO2~

4in winter in Dhaka. (b) Vertical profiles of conversion rate coefficients (% h~1) of SO

2to

SO2~4

by OH and by OH#CHO2#CRO

2radicals together with OH conc. in the city center of Dhaka at

noon of 22 December 1995. (c) Diurnal patterns of the total conversion rate coefficient of SO2

to SO2~4

and[SO2~

4/(SO

2#SO2~

4)] ratio at ground level in the city center of Dhaka (starting from 20 December 1995).

where OH and HO2

denote hydroxyl and hydro-peroxyl radicals, and CHO

2and CRO

2stand for

CH2O

2~ and CH3CHO

2~ Criegee biradicals, respec-tively; ALD

2means acetaldehyde, CH

3CHO.

The spatial distribution of the 3-day-averaged totalconversion rate coefficient from SO

2to SO2~

4,

K29

[OH]#K86

[CHO2]#K

87[CRO

2], at ground

level in Dhaka in % h~1 is shown in Fig. 7a. Thevertical profiles of the conversion rate coefficients ofSO

2to SO2~

4together with OH concentration in the

city center of Dhaka at noon on 22 December areshown in Fig. 7b. The diurnal patterns of the totalconversion rate coefficient and [SO2~

4/(SO

2#

SO2~4

)] ratio at ground level in the city center areshown in Fig. 7c. Figure 7a indicates high conversionrate at the emission source area, i.e. southeast indus-trial zone (which is the downwind of the main city)and city center, and relatively low conversion rate atthe area with almost no emission, i.e. near north andwest boundary. Figure 7b depicts increasing trends ofconversion rate of SO

2to SO2~

4at mid-day in the

vertical direction indicating the dependence of con-version rate on radical concentrations (especially OHconcentration) via daytime solar radiation, which isreduced by high aerosol concentration at near surface.The dense aerosol layer near the surface scatters andabsorbs solar radiation which controls the photodis-sociation rates of chemical species. The present simu-lation used photodissociation rate coefficients forNO

2, O

3, HNO

3, etc., which were derived in Kitada

and Peters (1980), based on the actinic flux estimatedby Peterson (1976) by including higher aerosol con-centration near surface in typical polluted area. About87% of conversion from SO

2to SO2~

4occurred by the

reaction of SO2

with OH (equation (3)) in the surfacelevel of central Dhaka at noon (see Fig. 7b). Thenear-surface level conversion rate in the city center ofDhaka (23°43@N latitude) at noon on 22 December,1995, which was 1.2% h~1 (see Fig. 7b), is similar to1% h~1 at mid-day of November at 25°N latitude inthe study of Altshuller (1979). High value of conver-sion rate during day and low value at night is


Fig. 8. (a) Diurnal variations of the formation rate of HNO3at the surface level in the city center of Dhaka.

(b) Vertical profiles of the formation rate of HNO3

at midnight of Dec. 21 in the city center of Dhaka. Therate of HNO

3formation is defined, e.g. as 2K

16[H

2O][N

2O

5] for equation (8b) in the text.

seen from Fig. 7c, indicating its dependence onradical concentrations. The diurnal patterns of[SO2~

4/(SO

2#SO2~

4)] ratio is generally influenced

by diurnal patterns of emission and conversionrate. The difference in the total conversion ratefrom day to day at noon (see Fig. 7c) is due to thechanging concentrations of precursors of OHradical such as NO

xand non-methane hydrocar-

bons. These differences of concentrations from day today were mainly caused by difference of wind speed(see Fig. 3), since other conditions such as solar radi-ation and activity of emission source were almostsame.

The mean and maximum value of the 3-day-aver-age total rate coefficient at ground level at Dhaka was0.3 and 0.4% h~1 , respectively. Using this mean value

as the rate coefficient of pseudo-first-order chemicalreaction the conversion rate of SO

2to SO2~

4at near-

surface level in Dhaka was about 7% in a 24-h period,which is larger than that reported in Eastern NorthAmerica in the same season. The conversion rate inEastern North America was within 3.1—4.7% in a 24 hperiod in midwinter (Hidy, 1994). The higherconversion rate in Dhaka is most probably due tostrong emission sources of NO

xand hydrocarbons

in the city area, larger intensity of actinic fluxand other meteorological parameter such as windspeed.

6.2.3. Estimated conversion rate of NO2

to othernitrogen compounds over Dhaka area. More than 90%of NO

xemission is initially NO, which is subsequently

oxidized to NO2

by O3, HO

2, RO

2, etc. with different


conversion rates. NO2

is further oxidized to a varietyof products such as HNO

3(NO~

3), PAN, NO

3, N

2O

5,

and some portion is removed by dry deposition at thesurface, under no precipitation condition. Some por-tion of NO

3radical reacts with NO

2to form N

2O

5,

which further produces HNO3. Most of the NO

3reacts with the organics to form HNO

3.

There are three lumped routes for HNO3formation

from NO2

(Lurmann et al., 1986); one is by directreaction with OH and H

2O as follows:

NO2#OH K13&"HNO

3(6)

NO2#H

2O K26&"HNO

3#HONO!NO

2. (7)

The two other routes for HNO3formation are from

N2O

5and NO

3. The major reactions related to

HNO3

formation from N2O

5and NO

3are (Lurmann

et al., 1986):

NO2#NO

3K5&"N

2O

5(8a)

N2O

5#H

2O K16&" 2HNO

3(8b)

NO3#HO

2K9&"HNO

3#O

2(9)

NO3#HCHO K38&"HNO

3#HO

2#CO (10)

NO3#ALD

2K40&"HNO

3#MCO

3(11)

NO3#CRES K103&"HNO

3#b

10NO

2#b

11OH (12)

where MCO3denotes CH

3CO

3, CRES means Cresol,

and b10

and b11

are stoichiometric coefficients.The temporal variations of HNO

3formation rates

through these three routes are illustrated in Fig. 8a,where ‘‘HNO

3’’ formation rate is defined, for example,

as K13

[OH][NO2] for equation (6). Figure 8a shows

large diurnal variations of HNO3formation rate from

NO2

directly, i.e. by equation (6). This large diurnalvariations of the formation rate are due to large tem-poral variations of OH concentration (see Fig. 8a) asall reactions related to OH production are dependenton sunlight. Formation of HNO

3in the daytime is

predominantly caused by the hydroxyl radical (equa-tion (6)), but other reaction pathways also participate.At night, ozone aloft oxidizes NO

2to NO

3, which

rapidly reacts with NO2

to form N2O

5. Then N

2O

5further reacts with water vapor to form HNO

3. The

predominancy of this pathway for the formation ofHNO

3at night can be seen from Fig. 8b. This path-

way is unimportant in the daytime because N2O

5is in

equilibrium with NO2

and NO3, which is photolyzed

as well as rapidly destroyed by NO, which in turn ispresent whenever there is NO

xand sunlight. On aver-

age, 92% of HNO3

was formed by reaction of NO2

with OH (equation (6)) at noon in the surface level and90% by reaction of N

2O

5with H

2O (equation (8b)) at

midnight at about 500 m height from surface in thecity center of Dhaka (see Figs. 8a and b, respectively).The relatively low formation rates of HNO

3at noon

on the 2nd day (21 December) compared to those atnoon on the 1st day (20 December) and 3rd day (22

Fig. 9. Spatial distributions of 2-day-averaged ground levelconversion rate for NO

2to PAN (molecule cm~3 s~1).

December) in Fig. 8a are due to relatively high windspeed at noon on the 2nd day (see Fig. 3).

A group of PAN is also a major product of NO2. In

the present model, where reactions involved in biogenichydrocarbons were inactivated, based on the condensedmechanism by Lurmann et al. (1986), PAN and TPANare formed by the reactions of the radicals CH

3CO

3and CHOCH"CHCO

3with NO

2, respectively. The

spatial distribution of the 2-day-averaged total PANformation rate in the surface level of Dhaka is depic-ted in the contour map in Fig. 9, which shows grad-ually increased formation rate in the downwind of thecity center. A significant spatial variation of theformation rate is also seen from this contour map.

7. CONCLUSIONS

Ambient SO2

and NO2

concentrations have beenmeasured in Dhaka city in winter at 64 sites usingdiffusion tube samplers. Emissions of SO

2and NO

xhave been estimated and their relations to the SO

2and NO

2concentration distributions over Dhaka

have been analysed, together with meteorological con-ditions. An Eulerian transport/chemistry/depositionmodel has also been used to estimate SO

2and NO

2concentrations, and their conversion rates to otherspecies in Dhaka in winter. The model predictionswere verified by measurements. The overall results ofthe study can be summarized as follows:

(1) Dhaka city is highly polluted by SO2. At some

places SO2concentrations are more than two times to

the Japanese ambient SO2

concentration standard,which is 0.04 ppm for daily average of hourly values;but NO

2concentrations are moderately high. So it is

necessary to do realistic extensive sensitivity study forSO

2emission reduction and find out optimized emis-

sion control system after cost—benefit analysis.


(2) The measurement results showed large spatialvariations of SO

2and NO

2concentration. SO

2con-

centrations are high in the southeastern industrial andbrick field zone together with the route running fromnorthwest to southeast, and also parallel to theBuriganga river; whereas NO

2concentrations are

high in the city center and along the major roads.(3) The major sources for SO

2emissions are traffic

vehicle (55.8%) followed by brick field (28.8%), indus-try (10.5%) and navigation vessel (4%); for NO

x, they

are traffic vehicle (54.5%) followed by brick field(17.5%), residential activity (9.5%), industry (8.8%)and navigation vessel (7.7%). One of the main causesof the severe SO

2pollution in Dhaka in winter is the

emission from brick fields, which operate only inwinter and use coal and furnace oil as main fuel. Theoverall SO

2and NO

xemissions in Dhaka in winter

1995—96 were 72 and 70 t d~1, respectively.(4) The discrepancy in the concentration distribu-

tions of SO2

and NO2

over Dhaka (Fig. 4) is due tothe difference of relative contributions of source typesof brick field (28.8% for SO

2and 17.5% for NO

x) and

residential activity (0.8% for SO2

and 9.5% for NOx)

in total emissions of SO2

and NOx.

(5) The reaction rate for conversion of SO2

andNO

2to other sulfur and nitrogen species is not con-

stant but shows marked temporal- and spatial-vari-ation which is clearly indicated in Figs 7b, 8b and9 (for spatial variations) and Figs 7c and 8a (fortemporal variations). The conversion rates are signifi-cantly dependent on the meteorological conditions(especially wind speed) which can be observed fromFigs. 7c and 8a with the help of Fig. 3. Estimatedpseudo-first-order chemical reaction coefficient ofSO

2to SO2~

4conversion was 0.3%h~1 (averaged

value at surface level) for Dhaka area in winter.

The estimation of the contributions of various sour-ces to the SO

2and NO

2concentration distributions

and the evaluation of source reduction strategies arethe subjects of future studies.

Acknowledgements—The authors acknowledge Mr H. Moriat Techno Chubu Co. Ltd and Prof. Y. Kiso at ToyohashiUniversity of Technology, Japan for providing laboratoryfacilities and invaluable assistance with the chemical analy-sis. We also acknowledge the help of Drs M. Gamo andS. Yamamoto at National Institute for Resources and Envir-onment, Japan for providing us aerological data of Dhakaand valuable information on atmospheric diffusion in east-ern India. Profound thanks are also due to Profs S. Ohta andN. Murao at Hokkaido University for useful discussion onthe measurement procedures. We also thank Prof. G. R.Carmichael at the University of Iowa, U.S.A for his valuablecomments on the initial version of this manuscript. Thanksare also extended to Bangladesh Meteorological Depart-ment for providing meteorological data, Bangladesh RoadTransport Authority for traffic vehicle data, Petro-Banglafor fuel consumption data and Titas Gas Authority for gasconsumption data in Dhaka, and Department of Environ-ment in Aichi Prefectural Office, Japan for providing usenvironmental data in Nagoya, etc. The authors are indebtedto the many individuals in Dhaka who managed the sites

and helped them in the sampling process. This work wassupported in part by the Environmental Agency of Japanthrough the research grant for ‘‘Preliminary Study on Sus-tainable Development of Urban Area’’ within the GlobalEnvironment Research Fund.

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

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