Seasonal Variations of Aerosols in Pakistan: Contributions of Domestic Anthropogenic Emissions and Transboundary Transport

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 by 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 mu m 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 degrees E, 28-34 degrees N) are 55, 48.5, 31.5, and 98 mu g/m(3) in January, April, July, and October, respectively. The simulated highest PM2.5 concentration in October results from the relatively low temperatures that favor nitrate formation as well as the lowest precipitation that leads to the smallest wet deposition of all aerosol species. 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 northeastern 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.