Wintertime Air Quality in Megacity Dhaka, Bangladesh Strongly Affected by Influx of Black Carbon Aerosols from Regional Biomass Burning

Clean air is a key parameter for a sustainable society, and currently, megacity Dhaka has among the worst air qualities in the world. This results from poorly constrained contributions of a variety of sources from both local emissions and regional influx from the highly polluted Indo-Gangetic Plain, impacting the respiratory health of the 21 million inhabitants in the Greater Dhaka region. An important component of the fine particulate matter (PM2.5) is black carbon (BC) aerosols. In this study, we investigated the combustion sources of BC using a dual carbon isotope (δ13C and Δ14C) in Dhaka during the high-loading winter period of 2013/14 (regular and lockdown/hartal period) in order to guide mitigation policies. On average, BC (13 ± 6 μg m-3) contributed about 9% of the PM2.5 (145 ± 79 μg m-3) loadings. The relative contribution from biomass combustion under regular conditions was 44 ± 1% (with the rest from fossil combustion), while during periods of politically motivated large-scale lockdown of business and traffic, the biomass burning contribution increased to 63 ± 1%. To reduce the severe health impact of BC and other aerosol pollution in Dhaka, mitigation should therefore target regional-scale biomass/agricultural burning in addition to local traffic.