Observation and model simulation of aerosol optical properties and size distribution over the hilly terrain of Northeast India

Abstract The aerosol optical properties are studied at northeast India's high-altitude station. The aerosol total scattering at the station is influenced by the mountain valley wind circulation. The backward trajectory shows the pre-monsoon season is a transition period when the air mass transportation alters the direction from northwest to south direction. The dominance of biomass burning and other emissions over long-range transportation of particles contributed strongly to the aerosol loading, which caused the higher range of scattering Ångström exponent (α450−700 nm > 1.5) in the pre-monsoon, monsoon, and post-monsoon seasons. The α450−700 nm varies from 1.47 to 1.88, indicating that the fine aerosol particles with a radius of < 0.5 µm have dominance at the station. To estimate the backscatter fraction (bf) and Asymmetry Parameter (g), theoretical approximation (Sviridenkov 2017 and Kokhanovsky 2006) and model simulation (MieTab and Mieplot) are utilized. The bf and g are found to be in the range of 0.11–0.13 and 0.68 to 0.74, after comparison of this value with the model simulated particle size, observed that the radius varies from \(\approx\) 0.17 µm − 0.21 µm. The bf decreased from winter to monsoon season, while the g values enhanced, demonstrating a negative correlation. The bf value decreased owing to the less backscatter and higher forward scatter for bigger particles from winter to monsoon. Thus, the g values were smaller for higher bf values and associated with smaller aerosol particles and vice-versa.