Characterizing variations in ambient PM concentrations at the U.S. Embassy in Dhaka, Bangladesh using observations and the CMAQ modeling system.

We analyze hourly PM (particles with an aerodynamic diameter of ≤ 2.5 μm) 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 concentrations reached ~165-175 μg/m while monsoon concentrations remained ~30-35 μg/m. Annual mean PM concentration reached ~5-6 times greater than the Bangladesh annual PM standard of 15 μg/m. The number of days exceeding the daily PM standard of 65 μg/m in a year approached nearly 50%. Daily-mean PM concentrations remained elevated (>65 μg/m) 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 in winter months with a normalized mean bias of 13-32%. In the model, organic aerosol is the largest component of PM, of which secondary organic aerosol plays a dominant role. Transboundary pollution has a large impact on the PM concentration in Dhaka, with an annual mean contribution of ~40 μg/m.