MONITORING AND MEASUREMENT OF ABSORBING AEROSOL AND ITS CLIMATIC IMPLICATIONS IN KARACHI, PAKISTAN

Abstract

This thesis is a collection of studies focusing on absorption characteristics of aerosol particles using satellite and ground data as well as model simulations at an urban environment in Karachi. First of all, identification of absorbing aerosols through various aerosol optical properties was performed. After that, a study of temporal variation of absorbing Black Carbon (BC) aerosol and their impact on meteorological parameters and surface reflectance was carried out. Finally, the radiative effect of absorbing BC aerosols was investigated. In order to seasonally characterize the absorbing aerosols into different types, the optical properties of aerosol retrieved from AErosol RObotic NETwork (AERONET) and Ozone Monitoring Instrument (OMI) were utilized. Firstly, the OMI Absorption Aerosol Optical Depth (AODabs) was validated with AERONET AODabs and was found to have a high degree of correlation. Then, based on this validation, characterization was conducted by analyzing aerosol Fine Mode Fraction (FMF), Angstrom Exponent (AE), Absorption Angstrom Exponent (AAE), Single Scattering Albedo (SSA) and Aerosol Index (AI) and their mutual correlation, to identify the absorbing aerosol types and also to examine the variability in seasonal distribution. The absorbing aerosols were characterized into Mostly Black Carbon (BC), Mostly Dust and Mixed BC & Dust. The results revealed that Mostly BC aerosols contributed dominantly during winter and postmonsoon whereas, Mostly Dust was dominant during summer and premonsoon. These types of absorbing aerosol were also confirmed with the MODerate resolution Imaging Spectroradiometer (MODIS) and Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) observations. Furthermore, BC mass concentration was measured continuously for every five-minute interval with ground-based Aethalometer at an urban site in Karachi. In this study, the temporal (diurnal, monthly and seasonal) variations of BC and its relationship with meteorological ix variables were analyzed. Monthly averaged concentrations of BC ranged from 2.2 to 12.5 μg/m3, with maximum in the month of January, 2007 and minimum in the month of June, 2006. BC showed higher concentrations during the months of January, February and November while lower during the months of May, June, July and August throughout the years. It also displayed comparatively high concentrations during winter and postmonsoon, while moderate during premonsoon and low during summer. Diurnal analysis of BC concentration showed sharp peaks between 07:00 and 09:00 LST and again around 22:00 during all the months. Moreover, the relationship between BC concentration and meteorological variables such as Temperature (Temp), Relative Humidity (RH), Wind Speed (WS), Wind Direction (WD), VISibility (VIS) and RainFall (RF) was found and it was observed that BC concentration showed an inverse relationship with all these meteorological variables. The results revealed that BC aerosol concentration showed significant inverse relationship with surface reflectance with correlation coefficient (R) of -0.77 which implies that rise in BC aerosol concentration strongly reduces the surface reflectance. In contrast, the lower the BC aerosol concentration, the higher the surface reflectance. In addition, a significant negative correlation was recorded during all seasons showing that increase in BC aerosol concentration was accompanied with reduction in surface reflectance. The opposite relation between BC aerosol concentration and surface reflectance is relatively higher during the premonsoon and winter followed by postmonsoon, while it is lower during summer. With observations of BC aerosol concentrations, optical and radiative properties were obtained over the urban city of Karachi during the period of March 2006-December 2008. BC concentrations were continuously measured using an Aethalometer, whereas optical and radiative properties were estimated through the Optical Properties of Aerosols and Clouds (OPAC) and Santa Barbra DISORT Atmospheric Radiative Transfer (SBDART) models, respectively. The measured BC concentrations were higher during January, February and x November, while these were found to be lower during May, June, July and August throughout the duration of the study. A maximum peak value was observed during January 2007 while the minimum value was observed during June 2006. The Short Wave (SW) BC Aerosol Radiative Forcing (ARF) at Top of the Atmosphere (ToA) and within the ATMOSphere (ATMOS) were positive during all the months, whereas negative BC ARF values were found at the SurFaCe (SFC). Overall, all three RF components at SW indicated that the heating by absorption of BC aerosols is higher during January, February and November mostly due to their enhanced presence, higher BC Aerosol Optical Depth (AOD) in combination with low SSA, though the surface reflectance was low. While relatively lower values of ARF were found during May, June, July and August, these were attributed to lower BC concentrations and BC AOD coupled with higher SSA even though surface reflectance was much higher in these months as compared to other months. Conversely, the Long Wave (LW) BC ARF at ToA remained positive, and BC ARF at SFC was positive whereas, BC ARF in ATMOS shifted towards positive values (heating effect) during June-August when an increase in water vapor content was found. Finally, the net (SW + LW) BC ARF at ToA and in ATMOS were found to be positive while BC ARF at SFC were found to be negative. It should be noted that a systematic increase in Atmospheric Heating Rate (AHR) was found during October to January underlining the significant effect of absorbing BC aerosols. Moreover, we found the highest correlation between AODabs and BC ARF in ATMOS followed by correlation with SFC and then with ToA. Similar to BC ARF, the values of all the three BC Absorption Forcing Efficiency (AFE) were found to be at a minimum during June 2006 while these were maximum during January 2007. On an average, the contribution of BC to the total ARF was found to greater than 80% for the whole observational period and contribute up to 99 % during January 2007.