Air pollution has become a serious challenge for developing countries like Pakistan.Very scarce information is available regarding pollution levels in this geographic region.This study presents the first modelling work to simulate the spatial distribution and temporal variation of aerosol concentrations over Pakistan for using the Weather Research and Forecasting Model coupled with chemistry (WRF-Chem).Simulated aerosols species include sulfate, nitrate, ammonium, organic carbon, black carbon, and PM2.5 (particles with a diameter of 2.5 μm or less), which are evaluated against ground-based observations and satellite measurements.In year 2006, simulated PM2.5 concentrations averaged over northeastern Pakistan (71-74.5°E, 28-34°N) are 55, 48.5, 31.5, and 98 μg m-3 in January, April, July, and October, respectively.The lowest precipitation and relatively low temperatures lead to the simulated highest PM2.5 concentration in October, whereas the simulated lowest concentration of PM2.5 in July can be attributed to the largest precipitation associated with the South Asian summer monsoon.Sensitivity studies show that transboundary transport contributes to PM2.5 aerosol levels in northeastem Pakistan by 10-20％ in January and April and by 10-40％ in July and October of year 2006.Wind over India and Pakistan is found to be the major meteorological parameter that determines the transboundary aerosol transport.Concentrations from sensitivity simulations show that transport, industrial and energy sectors contribute to about 5-10％ of sulfate concentrations in northeastem Pakistan while transport, residential and energy sectors contribute to about 20-40％ of NO3-and 10-30％ of NH4+, as the concentrations are averaged over the four months.Residential sector is major source of BC and OC in Pakistan and contributes to about 50-80％ of BC and OC mass concentrations in northeastern and southern Pakistan.