Bangladesh is situated in the eastern part of south Asia. The climate in Bangladesh is characterized by high temperature and high humidity most of the year with a distinct seasonal variation of precipitation [22] [24] [25] . The year can be divided into four seasons, pre-monsoon (March-May), monsoon (June-September), post monsoon (October-November) and winter (December-February) in Bangladesh. Dhaka ((Latitude: N23˚43'40"; Longitude: E90˚23'52"; Elevation: 34 meter a.s.l.), the capital of Bangladesh, is the center of commerce and industries for the country. Dhaka is growing rapidly and faces all the problems associated with mega-city. Dhaka is situated on flat land surrounded by rivers. The exact sampling location is situated on the roof of the Mukarram Hussain Science Building, Department of Chemistry, University of Dhaka (DU), Bangladesh (Figure 1). Wind direction in Dhaka city is mainly from south and southwest direction during pre-monsoon and from north and north-west during winter. The average wind speed is 1.39 meters・second⁻¹ (m・s⁻¹) in January, 1.39 m・s⁻¹ in

February. There was no rain event during this sampling period. The average temperature was 18˚C in January, 21˚C in February. Backward air trajectories studies (http://ready.arl.noaa.gov/HYSPLIT_traj.php) indicated that air masses were transported from north to south.

Aerosol particulate matters were collected with Envirotech (Model AFM 550) on Quartz filters (Gelman, Membrane Filters, Type TISSU Quartz 2500QAT-UP, 47 mm diameter). The instrument was installed at about 34 m height on the roof of Mukarram Hussain Science Building, Department of Chemistry, University of Dhaka, Bangladesh. The particulate matter sampler (Envirotech APM550) was programmed to sample through PM_2.5 particle size separator (impactor) and then through 2.5 mm pore quartz filters. The PM_2.5 samples were collected for both day and night time. The average sampling time was about 12:00 hours (7.00 pm to 7.00 am) considered “nighttime” and about 9:00 hours (7.30 am to 6.30 pm) considered “daytime”. All the filters were pretreated at about 800˚C for 200 min to reduce the organic species background level from filters. Particulate mass (PM) was determined at both night and day time form the differences between loaded and unloaded filters with a digital balance after conditioning them in a desiccator for about 3 hours. The loaded quartz filters were kept in the refrigerator at 4˚C until chemical analysis to limit the losses of the components.

Ions were extracted from one portion of each filter with 20 mL of deionized ultrapure water with automatic shaker followed by sonicator. Portion of these extracted solution were used for sulfate, nitrate, phosphate, chloride, bromide analysis with Ion Chromatography (Model 881, Metrohm Ltd., Switzerland). Another portion was used for sodium, potassium and calcium analysis with Flame photometer (Model PFP7, Jenway, UK). Concentrations of the each element were determined from a calibration curve of five different standard solutions. One filter (field blank) was also treated as same procedure of the loaded filters for blank correction.

The daily average PM_2.5 mass values from January 27 to February 7, 2013 in Dhaka, Bangladesh have given in Table 1. The night time average PM_2.5 concentration varied from 157.6 µg・m⁻³ to 379.3 µg・m⁻³ with the total

average of 246.8 µg・m⁻³. The day time average concentration of PM_2.5 ranges from 37.9 µg・m⁻³ to 233.0 µg・m⁻³ with an average of 134.5 µg・m⁻³. The highest value for night time average was observed on January 31, 2013 (233.0 µg・m⁻³), and the lowest value (37.9 µg・m⁻³) was found on February 5, 2013. The highest PM_2.5 concentration was observed on January 27, 2013 (379.3 µg・m⁻³) and the lowest concentration was on January 28, 2013 (157.6 µg・m⁻³) for the day time. The very high concentrations were observed for PM_2.5 at both day and night in Dhaka, Bangladesh. The average night time PM_2.5 mass was about 2.4 times higher than the day time PM_2.5 mass. The reasons may be the very high emissions from heavy traffics running only at night (between 8:00 pm and 6:00 am) in Dhaka city with high humidity and low temperature. The total average PM_2.5 mass (average of day and night time) was about 8 times higher than WHO (25 µg・m⁻³) guideline value [26] and about 3 times higher than DoE, Bangladesh (65 µg・m⁻³) guideline value. The elevated concentrations of PM_2.5 observed in Dhaka, Bangladesh are still lower than those reported for Asian sites [27] . The night and day time ratio of PM_2.5 mass varies between 4.55 and 1.04 with an average of 1.85 (Table 1). The highest ratio between night and day time PM_2.5 mass concentrations was about 4.55 on February 5, 2013, whereas the lowest ratio was about 1.04 on January 31, 2013. There were no evidences for the day time higher values compared than time during this sampling period. One can easily see the evidence of the very high mass loading during night time at the following Figure 2.

Water soluble anions (sulfate, nitrate, phosphate, chloride and bromide) were determined with ion chromatogram, and cations (sodium, potassium and calcium) were determined with flame photometer at PM_2.5 in Dhaka, Bangladesh. Water soluble ions concentrations were much higher (ratio from 1.17 to 2.86 in Table 2) during night time compared than day time except nitrate, bromide and potassium. The mass of the total water soluble ions contributed about 15% of the total PM_2.5 mass during this winter sampling in Dhaka, Bangladesh. Potassium, nitrate and sulfate are the main constituents in water soluble ionic species at the atmospheric fine particulate matters in Dhaka, Bangladesh. Calcium also has contributed significantly to PM_2.5 mass. The total 24-hour average concentration of sulfate, nitrate, phosphate, bromide, chloride, sodium, potassium and calcium were 5.30, 7.75, 0.62, 0.16, 1.19, 1.30, 8.11, and 3.09 µg・m⁻³, respectively. The detail of the results of the ionic species has given below.

The correlation coefficient of the determined water soluble ions in Dhaka, Bangladesh has given in the Table 4 (r² < 0.62 are in bold). Excellent corrections were observed between and (r² = 0.91) indicating joint sources of origin. also showed very good correlation with Cl⁻ (r² = 0.82), and Na⁺ (r² = 0.72). Na⁺ and Cl⁻ have good correlation (0.63) indicating sea salt origin. K⁺ and Ca²⁺ have good correlation (0.63) indicating joint sources of crustal and biomass burning origin. The results are indicating that determined water soluble ion species in Dhaka aerosols have significant contributions from traffic emissions, sea salts biomass burning and also crustal contribution. Sodium chloride is a signature for the sea salts origin aerosol and K is the tracer of the of biomass burning aerosols.

The water soluble ion species (K⁺, Ca²⁺, Cl⁻, Br⁻, ,) concentration in the fine particulate matters was much higher in Dhaka, Bangladesh compared to other European locations but slightly lower than polluted China and India (Table 5). Dhaka values were also much higher than other developed Asian countries like Japan and Singapore. Comparing with Salam et al. 2003 measurements in Dhaka, potassium concentration is about 5 times higher, sulfate and calcium concentrations are about half, whereas nitrate concentration is about double.