Dhaka and its neighboring areas suffer from severe air pollution, especially during dry season (November-April). We investigated temporal and directional variations in particulate matter (PM) concentrations in Dhaka, Gazipur, and Narayanganj from October 2012 to March 2015 to understand different aspects of PM concentrations and possible sources of high pollution in this region. Ninety-six-hour backward trajectories for the whole dry season were also computed to investigate incursion of long-range pollution into this area. We found yearly PM10 concentrations in this area about three times and yearly PM2.5 concentrations about six times greater than the national standards of Bangladesh. Dhaka and its vicinity experienced several air pollution episodes in dry season when PM2.5 concentrations were 8-13 times greater than the World Health Organization (WHO) guideline value. Higher pollution and great contribution of PM2.5 most of the time were associated with the north-westerly wind. Winter (November to January) was found as the most polluted season in this area, when average PM10 concentrations in Dhaka, Gazipur, and Narayanganj were 257.1, 240.3, and 327.4 μg m(-3), respectively. Pollution levels during wet season (May-October) were, although found legitimate as per the national standards of Bangladesh, exceeded WHO guideline value in 50 % of the days of that season. Trans-boundary source identifications using concentration-weighted trajectory method revealed that the sources in the eastern Indian region bordering Bangladesh, in the north-eastern Indian region bordering Nepal and in Nepal and its neighboring areas had high probability of contributing to the PM pollutions at Gazipur station.
Recently, the World Health Organization ranked Narayanganj, Chittagong, and Dhaka among the top 25, 40, and 45 cities, respectively, for high ambient PM 2.5 concentrations. Bangladesh has instituted an air quality monitoring system operated by the Department of Environment. PM 2.5 and PM 10 were measured hourly from January 2014 through December 2017 in Dhaka, Gazipur, Narayanganj, Chittagong, Sylhet, and Barisal. All sites registered concentrations for both pollutants that exceeded the 24-h Bangladesh National Ambient Air Quality Standards. The particulate matter (PM) concentrations varied significantly seasonally and with different diel patterns from city to city. The highest concentrations were observed during the winter, typically when wind speeds and mixed layer heights are low and pollutant concentrations are increased by transport from the northwest. The PM 2.5 concentrations from the 1 st quarters of 2014 and 2015 were compared to assess whether political unrest that appeared to reduce vehicular moment to very low levels affected the observed values. However, the PM 2.5 concentrations were statistically similar at the Dhaka, Narayanganj, and Sylhet sites and different for the Gazipur, Chittagong, and Barisal locations. Thus, the PM 2.5 concentrations during the political unrest in the 1 st quarter of 2015 were not consistently lower across the measurement sites. These results indicate that vehicular emission contributions to PM 2.5 concentrations are smaller than in the past, which agrees with recent source apportionment studies showing that brick kilns have become the dominant source of PM.
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