Occurrence and Characterization of Atmospheric Trace Gases in Dhaka City, Bangladesh

Abstract

Trace gases (CO, NO, NO 2 , TVOC, O 3 , NH 3 , H 2 S and PH 3 ) concentrations were measured at the Dhaka University campus by using automatic direct sense toxic gas monitoring instrument in Dhaka City, Bangladesh from June 2013 to May 2014. Measurements were also done at Karwan Bazar (highly traffic congested and commercial area), Dhaka City; Dhamrai (semi-urban area) in Dhaka district; Jamalganj (rural area) in Sunamganj district and a remote location at Badla Haor (haor area, seasonally submerged for 6 to 7 months) in Kishoreganj district for the comparison. The concentrations of the measured gases at Dhaka University campus varied both within the day and across the year (four seasons). The average concentrations of CO, NO, NO 2 , TVOC, O 3 , NH 3 , H 2 S and PH 3 were 2603.6 ± 1216.4, 281.5 ± 158.0, 182.7 ± 69.4, 10,068.2 ± 5296.1, 36.6 ± 23.6, 133.8 ± 94.9, 84.8 ± 38.8, and 133.1 ± 37.7 µg/m 3 , respectively in Dhaka City. The concentrations of the measured trace gases showed strong monthly and seasonal variations – (a) CO and NO concentrations were found the highest in winter, (b) TVOC, PH 3 concentrations were the highest in monsoon, and (c) NO 2 , H 2 concentrations were the highest in pre-monsoon season. The average hourly diurnal concentrations of the measured trace gases (CO and NO 2 ) were higher in the morning and evening hours due to the emission from a large number of vehicles and atmospheric processes. The diurnal concentrations of NH 3 , PH 3 and H 2 S were higher in the morning and it might be due to the release of these gases from communal wastes. Concentrations of total volatile organic compounds (TVOC) increased during the day time. From backward air mass trajectory analysis and wind rose plots of two seasons (monsoon vs. winter) indicated that air masses from the south and the southeast direction in monsoon had effects on decreasing the concentrations of most of the trace gases as they pass over the Bay of Bengal. On the other hand, air masses from the north, the northwest and the west direction in winter season had greater influence on the higher concentration of trace gases since they pass over the Indo Gangetic Plain. The ratios of the trace gases of two days (D 1 , D 2 ) from two wind directions (D 1 from the Indo Gangetic Plain, D 2 from the Bay of Bengal) were 2.91 (CO), 1.71 (NO), 1.04 (NO 2 ), 0.4 (TVOC), 1.11 (O 3 ), 0.12 (H 2 and O 3 S and PH 3 S), 1.08 (PH 3 ) and 1.85 (NH 3 ). Among measured gases, NO 2 concentration (182.7 μg/m 3 ) exceeded annual guidelines value of United States Environmental Protection Agency (USEPA), WorldHealth Organization (WHO), European Union (EU) and Department of Environment(DoE), Bangladesh. Overall, the concentrations of the major trace gases (CO, NO,NO 2 , H 2 S and O 3 ) in the Dhaka City (Dhaka University campus and Karwan Bazar) were relatively higher than that of other places (Dhamrai, Jamalganj and Badla Haor) in Bangladesh. Positive matrix factorization (PMF 5.0) model was used to identify the source profile of NOx, TVCO and O 3 gases. Fossil fuel burning and atmosphericprocesses were identified as the largest contributors of NOx and O 3 , while atmospheric process was exclusively recognized as the source of TVOC concentration. Since the atmosphere is a dynamic system, and the concentration of trace gases and their chemistry vary over time, it would need further research to form effective long-term air quality management strategies.