Composition and radiative forcing of aerosols across the Indo-Gangetic Plain and Himalayas: A seasonal analysis

The chemical composition determines the influence atmospheric aerosols exert on environment, radiation budget, and climate. In this study, for the first time, using Aerosol Robotoc Network (AERONET) columnar aerosol observations at ten locations across the Indo-Gangetic Plain (IGP) and the Himalayan foothills, a comprehensive regional and seasonal analyses of aerosol chemical characteristics and radiative effects is performed. Single scattering albedo (SSA) increases during monsoon, with varying magnitude across the sites depending on the strength and the advancement of monsoon from west to east. SSA over Kathmandu in the Himalayan foothills during pre-monsoon is the lowest among all sites. Black carbon (BC) dominates aerosol absorption over the eastern IGP and the Himalayan foothills, whereas dust dominates the western IGP. Absorbing aerosol types are ‘Mostly BC’ and ‘Mixed’ which originate from urban/industrial and biomass burning emissions, and ‘Mostly Dust’ is absent over the eastern IGP and the Himalayan foothills revealing a spatial gradient in dust. Aerosol radiative forcing at the surface (ARFSFC) is ≤ -50 Wm-2 throughout the year contributing to a significant surface cooling across the entire region. ARF efficiency at the surface and in the atmosphere are highest for Kathmandu. Aerosol-induced atmospheric solar heating rate (HR) is ≥ 0.4 K day−1 throughout the year at all sites. The HR is highest over Kathmandu (∼1.5 K day−1) during pre-monsoon. In present day conditions, a significantly regionally coherent higher HR over the Himalayas can enhance the total atmospheric warming in the large region. Quantitative knowledge obtained from the present analysis heretofore unavailable are crucial for global climate models and regional chemistry-transport models to more accurately assess the climate impact of aerosols over this ecologically sensitive and climatically vulnerable region.