Remote sensing of aerosol characteristics and radiative forcing in Pakistan

This thesis investigates the aerosol characteristics over different cities of Pakistan through satellite borne sensors, namely the Total Ozone Mapping Spectrometer (TOMS), the Moderate Resolution Imaging Spectroradiometer (MODIS), and the Multi-angle Imaging Spectroradiometer (MISR), and ground-based instruments such as Aerosol Robotic Network (AERONET) and GRIMM 1.109 dust monitor. A Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model was used for trajectory analysis in order to visualize the origins of air masses and understand the spatio-temporal variability of aerosol concentrations. An assessment of seasonal variability in aerosol optical depth (AOD) for industrial, urban, semi-urban, rural, and semi-arid areas revealed maximum AOD values during summer over all the areas under investigation. The correlation between AERONET and MODIS/MISR AODs during 2007 was also analyzed for Karachi and Lahore. The correlation coefficient for Karachi was found to be relatively high between AERONET and MISR and lower between AERONET and MODIS. In contrast, the correlation coefficient for Lahore was higher between AERONET and MODIS than between AERONET and MISR. The results suggest that the MISR sensor provides better AOD estimates near the ocean while AOD estimates from the MODIS sensor are better over terrestrial regions (especially over vegetated surfaces). The assessment of aerosol optical properties and aerosol radiative forcing (ARF) through the ground-based Aerosol Robotic Network (AERONET) over Lahore and Karachi has also been investigated in this study. The monthly mean of AOD at 500 nm over Lahore and Karachi ranges from 0.39 to 0.76, and the monthly mean Angstrom Exponent ranges from 0.29 to 1.22. The relationship between the Absorption Angstrom Exponent and the Extinction Angstrom Exponent provided an indication of relative proportions of urban-industrial and mineral dust aerosols over both sites. The single scattering albedo (SSA) ranged from 0.83 (440 nm) to 0.97 (1020 nm) over Lahore and Karachi. The SSA and imaginary parts of the refractive index values reveals that absorbing aerosols due to human activities (like industries and transport) are more dominant over Lahore than Karachi. The average ARF values over Lahore and Karachi were -22.5±5.9 Wm-2 and -18±2.2 Wm-2 at the top of atmosphere and -96±13 Wm-2 and -60±6.8 Wm-2 at the surface, respectively. The corresponding average atmospheric forcing is 74.56±16.8 Wm-2 over Lahore and 41.85±6.4 Wm-2 over Karachi - indicating significant heating of atmosphere at both sites. The relationships between AOD and four other cloud parameters, namely water vapour (WV), cloud fraction (CF), cloud top temperature (CTT) and cloud top pressure (CTP) are also investigated. The analyses showed strong positive correlations between AOD and WV for all selected cities. The correlation between AOD and CF was positive for cities where the air masses were predominantly humid, but negative for those where the air masses were relatively dry and carried low aerosol abundance. AOD showed a positive correlation with CTP and CTT in the northern areas of Pakistan and a negative correlation in southern areas. Aerosol inventories were also conducted in terms of particle size distributions (N), mass (M) concentrations, and particulate matter (PM) concentrations. These M and PM concentrations were obtained for Karachi, Lahore, Rawalpindi, and Peshawar from N concentrations using a native algorithm based on the Grimm model 1.109 dust monitor. PM10 and PM2.5 (24 hour average) concentrations at all sampling points were between 2 and 10 times higher than the maximum PM concentrations recommended by the WHO guidelines. Considering the high ambient aerosol concentrations, regional lung deposition of aerosol particles in the human respiratory tract was calculated using the stochastic lung deposition code IDEAL (Inhalation, Deposition and Exhalation of Aerosols in the Lung) to assess the extent of exposure. The results derived from this model disclose that extrathoracic deposition is in the range of 20 to 25% with total lung deposition ranging from 40 to 44 % for the scanned particle size range. Owing to the elevated alveolar deposition and longer retention times, deposited particles must be considered as a health risk to the personnel working on or near road side junctions in cities of Pakistan. (author)