A study on the spatio-temporal Distribution, Properties and Transport of Atmospheric Aerosols over the Pakistan using Remote Sensing

Abstract

Atmospheric aerosols are an important part of the atmosphere and are recognized as the main source of uncertainty for Earth’s radiation budget and climate modeling. To reduce these uncertainties, accurate and continuous observations of aerosols are required. Due to diverse topographic patterns and meteorological conditions, aerosol pollution over Pakistan is composed of various types of aerosols such as desert dust, biomass burning, and urban/industrial. Aerosol Robotic Network (AERONET) system, ground-based remote sensing instruments, provides detailed and authentic information about the optical and physical properties of aerosols. Currently, there are only two operational AERONET sites in Pakistan: Lahore and Karachi. In this thesis, AERONET data (level 2.0) have been used to study the aerosol optical and physical properties such as aerosol optical depth (AOD), Angstrom exponent (440/870) (AE), and volume size distribution over the cities of Lahore (2009-2011) and Karachi (2006-2014). The disparity of the time periods for Lahore and Karachi is due to the availability of the data. The aerosol properties are also investigated during intense crop-residue burning and haze events during October 2010 and October 2013 respectively. Satellite based observations from Terra-MODIS, Aqua-MODIS and CALIPSO have also been analyzed to understand the spatio-temporal distribution of aerosols over Pakistan. Monthly mean values of AOD were found to be higher than 0.47 over Lahore. The highest value (1.02) of AOD was found in the month of July while the lowest value (0.47) was observed in February. AE was found to be maximum (1.20) in winter representing the dominance of fine mode aerosols and minimum in spring (0.68) indicating the dominance of coarse mode particles. Due to high monsoon rainfall amounts highest monthly average value of water vapor content (WVC) was observed in the month of August (4.80 cm). A bimodal lognormal structure in monthly mean volume size distribution of aerosols was observed. The maximum value of fine mode volume concentration was found at a radius of ~0.15 μm in summer, fall and winter and at a radius of ~0.11μm in spring. On the other hand, the maximum value of monthly mean coarse mode volume concentration was observed at a radius of ~2.9 μm in spring summer and fall and at a radius of ~3.8 μm in winter. SSA exhibited an increasing trend in the wavelength interval 440 nm-675 nm in all the seasons. The seasonal average SSA was found to be maximum (0.933) in summer and lowest (0.892) in spring. The peak value of real part of refractive index (RI) occurred in spring (1.53) while imaginary part of RI peaked in winter (0.0100). Seasonal average values of ASY were found to be 0.65 (winter), 0.69 (spring), 0.70 (summer) and 0.68 (fall). The mean annual AOD and AE over Karachi were found to be 0.48±0.20 and 0.59±0.29 respectively. The highest value ii (0.88±0.31) of mean monthly AOD was observed in July along with AE value of 0.30±0.22 indicating relatively higher amount of coarse aerosols over Karachi. The highest volume concentrations of coarse mode and fine mode aerosols were observed in summer and autumn respectively. Considerable increasing trend in spectral single scattering albedo indicates that desert dust aerosols dominate the atmosphere of Karachi during summer and spring. Real part of RI was found to be highest during spring (1.53) and lowest during winter (1.50). Imaginary part of RI was recorded to be consistently higher during winter and lower during spring over Karachi. During intense crop-residue burning period in October 2010, daily average AOD ranged from 0.28 to 2.75 (on 20 October 2010) with the monthly average AOD value of 1.09±0.70. Fine mode AOD ranged from 0.14 to 2.68 with monthly average value of 0.87 during October 2010 indicating the major contribution of fine mode aerosols in total aerosol burden. Cluster analysis using scatter plot of absorption AE and extinction AE revealed the dominance of biomass burning and urban-industrial aerosols during the crop-residue burning period. The maximum value of fine-mode volume concentration was observed on 19 and 20 October 2010 indicating the dominance of fine-mode aerosol particles. Very high values (~0.95) of SSA were observed on 19 and 20 October 2010 due to scattering of incoming solar radiation by intense smoke. The HYSPLIT model backward trajectories revealed that winds came from southeast and northwest directions. Very high value (3.70) of instantaneous AOD was found during intense haze event on 9 October 2013. Lower temperature with relatively higher relative humidity and higher mean sea level (MSL) pressure favored the formation of haze. The daily average value of AOD was found to be 2.36 on 9 October 2013, which was about 8 times higher than AOD value (0.29) on a normal day. The volume concentration of fine mode aerosols was observed to be more than 1.5 times greater than the volume concentration of coarse mode aerosols on the high aerosol burden day. SSA showed decreasing trend with wavelength on 9 October 2013 indicating the dominance of absorbing aerosols such as urban/industrial and biomass burning. Scatter-plot of AOD and AE revealed that biomass burning/urban-industrial aerosols prevailed on the heavy aerosol loading day over Lahore. MODIS fire activity image showed that most of the biomass burning activities occurred in the southeastern areas of Lahore across the border with India. HYSPLIT model backward trajectory showed that winds came from southeast direction on high aerosol burden day. Vertical profile of sub-types of aerosols derived from CALIPSO revealed wide spread layer of smoke, polluted continental and dust aerosols.