Aerosol are solid or liquid particles suspended in the atmosphere ranging in size from 10 3 to 102 m. Aerosol influence both the regional and global climate of the earth by its direct and indirect effect. Role of atmospheric aerosols on the radiative forcing of atmosphere is a matter of serious research for past few decades and still it remains highly uncertain as acknowledged by Intergovernmental Panel on Climate Change. Heterogeneous nature of aerosol both spatially and temporally makes it more complex in estimating radiative forcing compared to that of greenhouse gases. Compounding to the existing difficulty in determining the climate effects, changing aerosol concentration and nature of the aerosol further increases the complexity in determining its effects in both regional and global climate. Increasing aerosol loading is emerging to be an issue of major concern over several regions. The first step towards achieving this goal is by determining the trends in the physical and optical properties of aerosol over the globe. Main objective of the thesis is the determination of the recent trends in aerosol loading over the globe and then to focus specifically on the properties of aerosol over an urban site in southern India. Specific objectives are (a) to determine the trend in aerosol physical and optical properties over the globe using AERONET surface observations (b) to characterize the chemical properties of water soluble inorganic aerosol over an urban site in Southern India, Bangalore (c) to have a better estimation of aerosol radiative forcing over Bangalore with measured aerosol chemical concentration, black carbon concentration and aerosol optical depth (AOD). To quantify the recent trends in aerosol loading over the globe, we have used the surface observations from AERONET and the study provided the first step in giving a global picture of the recent trends in the fundamental optical and physical property of aerosol. Trend analysis showed a significant spatial inhomogenity, and Asian continent clearly showed an increasing trend in AOD compared to other continents. Solar village (24.9oN, 46.4oE) of Saudi Arabia showed a maximum with a value of 0.04/yr and Bac -Giang (21oN, 106oE) of Vietnam showed the minimum value of -0.04/yr. North American study region included 18 sites in which eastern US (E.US) exhibited a decreasing trend while the scenario in western US (W.US) is different with more of sites with increasing AOD trend. Single scattering albedo (SSA) trend in W. US showed a decreasing trend irrespective of the AOD trend. Study sites in South America include Cordoba -CETT (31.5oNS, 64.5 oN W) Alta Floresta (9.8oS, 56.1 o W), Riobranco (9.9oNS, 67.8o W) and Soa Poulo (23.5oS, 46.7o W).Except Riobranco which has a positive trend in AOD, all other sites exhibited a statistically signi cant negative trend. Over Australia, there is an inclination towards increasing AOD in sites and all the three sites in Australia exhibit a statistically significant increasing trend in SSA. According to the recent trends in AOD over African region, there exists a significant decrease in AOD compared to that reported for few years earlier, showing the high temporal in homogeneity and need for continuous observation of aerosol over the regions. European study region included 15 stations, among them only 3 sites showed an increasing trend in AOD, remaining 12 sites showed a significant decreasing trend in aerosol loading over the period of study. SSA was also observed to be decreasing over most of the European sites, even with a decreasing AOD over most of the sites. A Comparison study carried out to determine the relation of population growth rate and aerosol loading, and it revealed that the increasing AOD trend not always coincided with the sites having high population growth rate. Having determined the trends in AOD and other aerosol parameters over the globe and seeing an alarmingly increasing trend over most of the Asian sites, especially over Indian region, we have then focused over work on the aerosol properties of one of the rapidly growing urban location in southern India, Bangalore. While physical properties of aerosol have been extensively studied over Bangalore, chemical characteristics are still an unexplored area. Extensive information on aerosol chemical composition is not available over Indian region except for a few locations based on campaign mode. Even available data is of very coarse temporal resolution, since hours or full day sampling is needed to gather enough samples for chemical analysis. High temporal resolution data of aerosol chemical characteristics, especially for all season is completely lacking over Indian subcontinent. Among aerosol, water soluble aerosol form an important component in particulate matter, since it can change its size, composition, can easily mix with other aerosols and can act as cloud condensation nuclei, based on its hygroscopic nature. Present study provided the rst time results from a high temporal resolution water soluble inorganic aerosol chemical data over Indian region, which is first step towards estimating aerosol climate impacts more accurately. Water soluble inorganic aerosol ions over Bangalore namely, sulphate, nitrate, chloride, potassium, calcium, magnesium, sodium and ammonium are measured using Particle Into Liquid Sampler Ion Chromatograph (PILS-IC). PILS is an online sampling technique for quantitatively measuring the chemical concentration of ion in water soluble aerosol particles. PILS IC used in the present study is developed in Georgia Institute of Technology. Instrument samples ambient air at a flow rate of 16.7Lmin 1. Particles below PM 2.5 micron are collected for the analysis using cyclone impactor. Two annular glass denuders are used to remove inorganic gases which else will interfere with the aerosol ion concentration. Ambient air which is deprived of the inorganic gases is then mixed with steam vapours at 150oC, eventually high supersaturated atmosphere is produced with rapid adiabatic mixing of steam and ambient air. High supersaturated air allows droplets to grow enough to be collected by inertial impaction onto a quartz impactor plate. Entire PILS condensation unit is kept at a slight tilt of 15o, to remove all condensate through drain tube connected to the end of the PILS condensate body. Condensed liquid sample is collected from the impaction chamber and known concentration LiF is allowed to mix with the collected sample at a constant rate. LiF known as carrier liquid is added to know the dilution occurring to the collected sample. Sample with carrier liquid is then collected to a debubbler and is supplied to the IC through peristaltic tubings for determining the ion chemical concentration. Seasonal variation of mass concentration of water soluble aerosol species and the influence of long range transport is carried out using HYSPILT back trajectory analysis. Marine air mass from Arabian Sea dominated the air parcel reaching the site for both SW monsoon and summer. Continental air mass dominated the site during both NE monsoon and winter with slight contribution from marine atmosphere. Source characteristics of sulphate, potassium, calcium and magnesium ions are carried out based on sea salt (ss) and non sea salt (nss) origin and it is observed that the nss contribution is dominant over the site for all these ions except magnesium where ss component comparatively dominates the source. SO24 and NO3 form the dominant anions while NH+4 makes the dominant cation species. Monthly variation of the ratio of ammonium to nss -sulphate is carried out to determine the possible cation -anion relation existing between these two major ions. During later winter and summer months ammonium bi sulphate is found to be the existing chemical form and ammonium sulphate during other seasons. High temporal resolution data enabled us to study the diurnal variation of aerosol ions and it is influenced by various mechanisms from boundary layer to local emissions. Optical properties of aerosols depend upon the size and the relative abun-dance of each components. It is usual practice to assume default aerosol chemical composition in radiative transfer models due to unavailability of data, which can lead to errors in forcing estimates. Incorporating realistic aerosol chemical composition in models is essential to reduce the uncertainty in aerosol radiative forcing. Hence we have included measured aerosol chemical compositions, black carbon and AOD to improve the determination of radiative forcing of aerosol. OPAC and SBDART models were used for estimating the aerosol radiative forcing over Bangalore. We have used mainly four components namely, soot, water soluble, sea salt and dust. Except dust all are other components are measured over the site and formed a constrain for the calculation. Dust concentration was altered so that the OPAC AOD matched the measured AOD within 5%. Mineral dust shows the highest contribution in AOD among the four components, however water soluble and soot even being less is mass concentration compared to mineral dust, has significant impact on the AOD. This clearly indicate the influence of both water soluble and soot aerosol over the regional climate of the site. Sea Salt exhibited low AOD compared to other three constituents. The results presented in the thesis highlights the importance of varying trends in the aerosol properties and its effects on a global picture and speci - cally over an urban site in Indian region , we explored the temporal variations of water soluble inorganic aerosol ions and its effects on regional climate. Hence the thesis addressed some of the unexplored areas in aerosol science. This study also suggests the need of continuous observation of aerosol over both spatial and temporal scale, which is essential to estimate their effects on earth's climate.