Characterizing variations in ambient PM2.5 concentrations at the US Embassy in Dhaka, Bangladesh using observations and the CMAQ modeling system

We analyze hourly PM2.5 (particles with an aerodynamic diameter of <= 2.5 mu m) 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 PM2.5 concentrations reached similar to 165-175 mu g/m3 while monsoon concentrations remained similar to 30-35 mu g/m(3). Annual mean PM2.5 con-centration reached similar to 5-6 times greater than the Bangladesh annual PM2.5 standard of 15 mu g/m(3). The number of days exceeding the daily PM2.5 standard of 65 mu g/m(3) in a year approached nearly 50%. Daily-mean PM2.5 concentrations remained elevated (>65 mu g/m(3)) 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 PM2.5 in winter months with a normalized mean bias of-13 to -32%. In the model, organic aerosol is the largest component of PM2.5, of which secondary organic aerosol plays a dominant role. Transboundary pollution has a large impact on the modeled PM2.5 concentration in Dhaka, with an annual mean contribution of similar to 40 mu g/m(3).