Assessing the PM2.5 impact of biomass combustion in megacity Dhaka, Bangladesh.

In Dhaka, Bangladesh, fine particulate matter (PM2.5) air pollution shows strong seasonal trends, with significantly higher mean concentrations during winter than during the monsoon (winter = 178.1 μg/m3 vs. monsoon = 30.2 μg/m3). Large-scale open burning of post-harvest agricultural waste across the Indo-Gangetic Plain is a major source of PM2.5 air pollution in northern India during the non-monsoon period. This study evaluates the extent to which the seasonal differences in PM2.5 pollution concentrations in Dhaka are accounted for by biomass-burning vs. fossil-fuel combustion sources. To assess this, an index was developed based on elemental potassium (K) as a marker for biomass particulate matter, after adjusting for soil-associated K contributions. Alternatively, particulate sulfur was employed as a tracer index for fossil-fuel combustion PM2.5. By simultaneously regressing total PM2.5 on S and adjusted K, the PM2.5 mass for each day was apportioned into: 1) fossil-fuels combustion associated PM2.5; 2) biomass-burning associated PM2.5; and, 3) all other PM2.5. The results indicated that fossil-fuel combustion contributed 21.6% (19.5 μg/m3), while biomass contributed 40.2% (36.3 μg/m3) of overall average PM2.5 from September 2013 to December 2017. However, the mean source contributions varied by season: PM2.5 in Dhaka during the monsoon season was dominated by fossil-fuels sources (44.3%), whereas PM2.5 mass was dominated by biomass-burning (41.4%) during the remainder of the year. The contribution to PM2.5 and each of its source components by transport of pollution into Dhaka during non-monsoon time was also evaluated by: 1) Conditional bivariate (CBPF) and pollution rose plots; 2) Concentration weighted trajectories (CWT), and; 3) NASA satellite photos to identify aerosol loading and fire locations on high pollution days. The collective evidence indicates that, while the air pollution in Dhaka is contributed to by both local and transboundary sources, the highest pollution days were dominated by biomass-related PM2.5, during periods of crop-burning in the Indo-Gangetic Plain.