Observation and Model Simulation of Aerosol Optical Properties and Size Distribution Over the Hilly Terrain of Northeast India

The aerosol optical properties are studied for all seasons at northeast India’s high-altitude station. In this study, total scattering and backward scattering data of Integrating Nephelometer 3563 is utilized for computation of scattering Ångström exponent (α_450-700 nm), backscatter fraction (bf), and asymmetric parameter (g). Noteworthy, the asymmetric parameter and particle size cannot be inferred directly from the Integrating Nephelometer. Theoretical approximation (Kokhanovsky and Nauss Atmos Chem Phys 6:5537–5545, 2006; Sviridenkov et al. Atmos Ocean Opt 30:435–440, 2017) and model simulation (MieTab and Mieplot) are utilized to estimate the g and particle size. The α_450-700 nm varies from 1.47 to 1.88, indicating that the fine aerosol particles with a radius of < 0.5 µm are dominant at the station. 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 the estimated g value with the model simulated particle size, observed that the radius varies from \(\approx \) $\approx$ 0.17 µm to 0.21 µm. Here, the aerosol particles are a homogeneous mixture of graphite-air and dry ash with a size range of 0.17 µm ≥ radius ≥ 0.21 µm. The bf decreased from winter to monsoon season, while the g values enhanced and demonstrated a negative correlation. The bf value decreased owing to the lower 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.