Characterizing variations in ambient PM_2.5 concentrations at the U.S. Embassy in Dhaka, Bangladesh using observations...

We analyze hourly PM_2.5 (particles with an aerodynamic diameter of ≤ 2.5 microm) concentrations measured at the U.S. Embassy in Dhaka over the 2016 - 2021 time period and find that concentrations are seasonally dependent with the highest occurring in winter and the lowest in monsoon seasons. Mean winter PM_2.5 concentrations reached ~165-175 microg/m³ while monsoon concentrations remained ~30-35 microg/m³. Annual mean PM_2.5 concentration reached ~5-6 times greater than the Bangladesh annual PM_2.5 standard of 15 microg/m³. The number of days exceeding the daily PM_2.5 standard of 65 microg/m³ in a year approached nearly 50%. Daily-mean PM_2.5 concentrations remained elevated (>65 microg/m³) for more than 80 consecutive days. Night-time concentrations were greater than daytime concentrations. The comparison of results obtained from the Community Multiscale Air Quality (CMAQ) model simulations over the Northern Hemisphere using 108-km horizontal grids with observed data suggests that the model can reproduce the seasonal variation of observed data but underpredicts observed PM_2.5 in winter months with a normalized mean bias of 13-32%. In the model, organic aerosol is the largest component of PM_2.5, of which secondary organic aerosol plays a dominant role. Transboundary pollution has a large impact on the PM_2.5 concentration in Dhaka, with an annual mean contribution of ~40 microg/m³.