Aerosol optical and radiative properties over Asia: Ground-based AERONET observations

Aerosols continue to contribute the largest uncertainty in quantifying Earth’s climate change. The uncertainty associated with aerosol radiative forcing is found to be higher over Asia. The simulation and future projection of aerosol impact on climate may not be highly accurate over Asia due to rapid changes in aerosol emissions, limitations in simulating the observed aerosol trends, and the non-availability of regional distribution of columnar aerosol parameters based on high-quality observational datasets on a seasonal scale. For the first time, this comprehensive study examines the spatial and regional variations of aerosol columnar optical and physical properties (aerosol optical depth (AOD), fine mode fraction (FMF), and single scattering albedo (SSA)) and their associated radiative effects (aerosol radiative forcing (ARF) and heating rate (HR)) using high-quality Aerosol Robotic Network (AERONET) datasets on seasonal and annual scales over Asia. This study is performed over a total of 44 selected AERONET observational sites covering different regions of Asia, e.g., Central, South, South-East, and East Asia. AOD, ARF at the surface and in the atmosphere, and aerosol-induced atmospheric HR are observed to be the highest over South Asia, followed by South-East, East, and Central Asia in each season. SSA is found to be lower over South and Central Asia compared to South-East and East Asia. The combined influence of both fine anthropogenic aerosol emissions (e.g., carbonaceous aerosols) from biomass burning and fossil fuel combustion, and coarse mode dust aerosols from seasonal transport lead to higher AOD (0.6) and lower SSA (0.90), which overall result in higher ARF (~−70 Wm^-2 at surface and 40 Wm^-2 in atmosphere) and HR (0.80 Kday^-1) over South Asia. South-East and East Asia are dominated by fine aerosols (higher FMF) due to higher contributions from forest fire and anthropogenic emissions, respectively, and relatively less dominance of dust aerosols compared to Central and South Asia. In addition, the seasonal aerosol optical and radiative parameters over Asia are also compared and contrasted with other regions of the globe, e.g., North America, South America, Europe, Africa, and Australia, where aerosol emissions are significantly different and mostly lower than in Asia. These findings provide observational constraints that are crucial for the improvement in model simulations for accurately assessing the radiative and climatic impacts of aerosols over a global aerosol hotspot region, Asia, where the uncertainty associated with aerosol radiative forcing is found to be higher. Details of the spatiotemporal variations in aerosol characteristics over Asia will be presented, compared and contrasted with the rest of the world, and inferences will be drawn.