Variability of aerosol optical depth over Swat in Northern Pakistan based on satellite data

Two satellite-born sensors, the moderate resolution imaging spectroradiometer (MODIS) and the Multi-Angle Imaging Spectroradiometer (MISR), were used to investigate annual and seasonal variations in aerosol optical depth (AOD) in the Swat district, Khyber Pakhtunkhwa (KP), Pakistan. This study was carried out for 6 years during the period from 2005 to 2011. The results showed that the maximum annual mean AOD was found in 2008 during the period of observation for both satellite sensors. The maximum AOD during 2008 was attributed due to local sources, long-range transported sources, and the emissions due to explosions during the operation of Pakistan army against the militants (Taliban). The seasonal mean AOD was highest during the summer season (0.44 ± 0.1) and lower during winter (0.24 ± 0.2). AOD was determined to be strongly dependent on wavelength, with shorter wavelength AOD values being higher than at larger wavelength values. A hybrid single particle Lagrangian integrated trajectories (HYSPLIT) model was used for trajectory analysis in order to determine the origin of air masses and understand the variability of AOD. The back trajectories analysis revealed that air masses mostly originated from India and Afghanistan. Apart from the distant sources, the local pollution due to biomass burning, vehicular emissions, resuspension road dust, emission due to soil erosion, and remains of blasting materials can be seen to have contributed to an increase in AOD concentrations over Swat.