The study aimed to understand the optical properties of Black Carbon (BC) and radiative forcing over a data deficient Himalayan region focusing on critical zone observatory employing ground-based measurements by Aethalometer for BC and satellite retrieval techniques for optical properties during mid-May-June 2022 and January-May 2023. BC mass concentration ranged from 0.18 to 4.43 mu gm- 3, exhibit a mean of 1.47 +/- 0.83 mu gm- 3 with higher summer concentration (1.51 +/- 0.94 mu gm- 3) than winter (1.39 +/- 0.61 mu gm- 3). The average Absorption & Aring;ngstrom Exponent observed to be significantly higher than unity (1.77 +/- 0.31) over the studied high-altitude Himalayan region, suggesting the dominance of biomass-burning aerosol. Higher aethalometer derived compensation parameter (K) in winter suggesting locally originated BC while, lower K value in summer suggesting aged BC transported from Indo-Gangetic Plains. Optical properties calculated from Optical Properties of Aerosol and Cloud (OPAC) model are used in the Santa Barbara DISORT Atmospheric Radiative Transfer (SBDART) model to calculate the aerosol Direct Radiative Force (DRF). The entire studied period is characterized by the predominance of absorbing aerosols, particularly BC, increasing Aerosol Optical Depth, Asymmetric Parameters and decreasing Single Scattering Albedo, leading to a considerable increase in atmospheric radiative forcing (+0.9 Wm-2, top of atmosphere) and Heating Rate (0.36 KDay- 1). The mean radiative forcing within atmosphere during summer was higher (+14.29 Wm-2) relative to the winter (+12.00 Wm-2), emphasizing the impact of absorbing aerosols on regional warming and potential glacier melting in the Himalayas at a faster rate. Urgent policy consideration for the reduction of absorbing aerosols is highlighted, recognizing the critical roles of Black Carbon in the changing behaviour of Critical Zone observatory. The study's data serve as a valuable resource to understanding and addressing uncertainties in climate models, aiding effective policy implementation for Black Carbon reduction.

This study investigates the long-term (2003-2019) variations of high aerosol loading days and their radiative impacts over the western Indo-Gangetic Plain (IGP) and eastern IGP during pre-monsoon season (March-April-May-June). The Aerosol Optical Depth (AOD) climatology from MODIS (Terra and Aqua) and MERRA-2 reanalysis shows high aerosol burden across the IGP region during the pre-monsoon season. The high aerosol loading days are identified based on a standardized AOD anomaly approach, from MODIS and MERRA-2. The frequency of high aerosol loading days over the western IGP is roughly twice that of the total number of high aerosol loading days over the eastern IGP. The area-averaged percentage differences in AOD between high aerosol loading days and normal days over western IGP is always higher, about 6-8%, than eastern IGP from Terra, Aqua and MERRA-2. The natural (mainly dust) and anthropogenic aerosols (particularly sulfate, black carbon and organic carbon) are majorly contributed to total AOD over western IGP and eastern IGP. Furthermore, the MERRA-2 and ERA5 composite surface and 850 hPa wind anomalies show that strong westerly winds dominate, transporting dust aerosols from arid regions to the western IGP. On the other hand, weak prevailing winds and background pre-monsoonal cyclonic circulations over eastern IGP favor the accumulation of regionally emitted aerosols. During high aerosol loading days, the decrease in ventilation coefficient indicates the high aerosol burden (less dispersion) over both the regions, leading to the deterioration of air quality. The enhanced aerosol loading induced potential atmospheric radiative forcing (19.78 Wm(-2) over western IGP and 20.77 Wm(-2) over eastern IGP) during high aerosol loading days compared to normal days (11.12 Wm(-2) and 12.9 Wm(-2)).