Ramachandran Srikanthan

This study utilizes new, high-quality ground-based observations to investigate seasonal variations in key aerosol optical and physical properties across entire Indo-Gangetic Plain (IGP), Himalayan foothills, and central Himalayas including southern Tibetan Plateau, for the first time. Seasonal average aerosol optical depth (AOD) is ≥ 0.3 across most of the region, indicating heavy pollution almost throughout the year, except at high altitude locations where AOD is 2–3 times lower. Pre-monsoon AOD levels are elevated due to additional emissions from seasonal sources such as agro-residue burning and forest-fires, and continental aerosols advected from polluted regions. Monsoon AODs are lower because rains suppress emissions from forest fires and garbage burning, significant seasonal sources such as brick production are almost absent as well as regional transport of emissions. Fine-mode aerosols contribute > 80% to AOD in post-monsoon and winter at almost all sites. Clean continental and clean marine aerosols dominate Himalayas, while urban-industrial and biomass burning aerosols are prevalent in Himalayan foothills and IGP, except in Karachi and Lahore where mixed aerosols are present in significant quantity. Aerosol volume size distributions are bimodal in all seasons, with modal radius at 0.1–0.3 μm (fine mode) and at 2–5 μm (coarse mode). A comparison revealed commonality of aerosols between Himalayas, Minsk (Belarus) and Dushanbe (Central Asia), at the same time demonstrating intra-regional variability of dominant aerosol types unique to Himalayas and IGP, providing valuable constraints for high-resolution chemistry-transport modelling, inputs to improve satellite retrievals, and deepen understanding of aerosol impacts on climate and ecosystems in this region.