Aerosol Optical Properties and its radiative effects over two topographically different locations of the Indian Himalayan Region

This study investigates aerosol characteristics using ground-based measurements at two distinct regions, Mohal-Kullu (31.9°N, 77.12°E; 1154 m amsl) and Kosi-Katarmal (29.64°N, 79.62°E; 1225 m amsl), from July 2019 to June 2022. The average Black Carbon (BC) concentrations were 1.5 ± 1.0 μg m⁻³ at Mohal and 1.1 ± 1.4 μg m⁻³ at Katarmal. BC showed strong seasonal variability, with maxima during post-monsoon (2.6 ± 1.0 μg m⁻³) and pre-monsoon (1.8 ± 0.5 μg m⁻³) seasons. The diurnal variation displayed distinct morning and evening peaks in all the seasons. High pre-monsoon AOD₅₀₀ (0.30 ± 0.06 to 0.54 ± 0.08) and low values of Ångström exponent (0.67 ± 0.10 to 0.95 ± 0.30) indicated dominance of large particles, whereas lower AOD₅₀₀ (0.21 ± 0.07 to 0.25 ± 0.03) in post-monsoon and winter, along with larger Ångström exponent (1.05 ± 0.74 to 1.13 ± 0.11), indicated smaller particles. Satellite-derived (OMI and MAIAC) AOD₅₀₀ showed weak to moderate correlation with ground-based measurements at Mohal (R = 0.4639 for MAIAC, R = 0.1402 for OMI) and Katarmal (R = 0.3976 for MAIAC, R = 0.2980 for OMI). Using optical properties of aerosols and clouds (OPAC) and Santa Barbara discrete ordinate radiative transfer (SBDART) models, the short-wave aerosol radiative forcing (SWARF) was found negative at the surface and top of the atmosphere but positive in the atmosphere, suggesting significant surface cooling and atmospheric warming leading to high heating rates, respectively. Annual mean atmospheric radiative forcing was 27.36 ± 6.00 Wm⁻² at Mohal and 21.87 ± 7.26 Wm⁻² at Katarmal. These findings may have consequences for planning air pollution strategies and understanding the effects of regional climate change.