This thesis examines the effects of black carbon (BC) and other aerosols on the regional climate, focusing on their climate-relevant optical properties and implications for radiative effects. We focus on South Asia, home to one-third of the worldwide population. The abrupt societal changes during the COVID-19 pandemic led to an unprecedented reduction in anthropogenic emissions and provided a unique opportunity to study the impact of aerosol demasking on global warming. The thesis revolves around insights obtained from three atmospheric receptor observatories intercepting the outflow from South Asia, complemented by remote sensing data and aerosol modeling. These are:  Bangladesh Climate Observatory at Bhola (BCOB), positioned at the exit of the highly polluted Indo-Gangetic Plain (IGP), Maldives Climate Observatory at Hanimaadhoo (MCOH) in the Northern Indian Ocean, and Maldives Climate Observatory at Gan (MCOG) near the equator in the Indian Ocean. These observatories hence facilitate the measurements of aerosol properties from source regions to distant receptor environments and provide a comprehensive framework for assessing BC's regional climate impacts.

The thesis offers insights into the changes in the sources of BC and the resultant climatic impact. The thesis quantifies the changes in the regional aerosol properties resulting from reduced anthropogenic emissions during the COVID-19 pandemic societal shutdown. The study reveals a substantial decrease in atmospheric aerosol loading, resulting in a reduction of the aerosol forcing roughly three-fourths of the magnitude of CO₂-induced radiative forcing at the top of the atmosphere and a ~7% increase in surface-reaching solar radiation. This work also demonstrates a notable shift in relative contribution of BC sources at BCOB during the COVID period: fossil fuel contributions decreased from 49% to 35%, while C₃ biomass burning increased from 31% to 55%, with C₄ biomass remaining a minor source. Similarly, for MCOH, reflecting the outflow from the greater South Asian subcontinent, the contribution from fossil combustion decreased while C₃ combustion increased.

Further, the thesis explores the optical properties of BC, coating effects, and changes in mass absorption cross section (MAC) during long-range transport. The study shows a uniform MAC enhancement from water-soluble coatings of 1.6±0.5 across sites BCOB, MCOH and MCOG. However, the BC MAC increased by 80% in two separate studies from the IGP outflow at BCOB to the Indian Ocean receptor stations, suggesting a common underlying phenomenon.  This mechanism is likely linked to scavenging fractionation occurring during the long-range transport of BC, which selectively filters out larger, lower BC MAC particles. This results in a residual population of finer, hydrophobic BC particles characterized by a notably higher MAC₆₇₈ at the receptor observatories in Northern Indian Ocean. Additionally, throughout the transport from the BCOB to the MCOH, BC particles undergo a transformation, becoming more absorbent—akin to becoming "darker." This transformation is facilitated by the photochemical bleaching of organic carbon (OC) and in-cloud processes, which further contribute to the increase in BC- BC-MAC₆₇₈. On the other hand, BrC-MAC decreased due to photochemical bleaching.

The findings of this thesis, based on observations, enhance our understanding of the constantly changing optical and radiative impact of human-made aerosols in the highly significant South Asian emission region. This, in turn, enables better descriptions of the evolving aerosol characteristics in climate models and offers guidance for informed policy-making and the development of effective climate mitigation strategies.