Investigation of physical and optical properties of aerosol over high altitude stations along the sub-Himalayan region of North-Eastern India

The present study focuses on exploring the spectral dependency of atmospheric extinction, the size distribution of aerosols, black carbon concentration and absorption and compensation parameter along the sub-Himalayan region of northeastern India, a field campaign was conducted from 1st February to 20th February 2018. Measurements were taken at three different stations in Arunachal Pradesh i.e., Tawang (2990 m a.s.l), New Palin (1300 m a.s.l.), and Geku (200 m a.s.l.). Techniques of Ångström exponent's curvature effect has been used to determine the aerosol size distribution over the stations. The mean black carbon concentration was 3.98 ± 1.03 μg m⁻³, 4.27 ± 1.22 μg m⁻³ and 2.38 ± 1.05 μg m⁻³ at Tawang, New Palin, and Geku respectively. The daily mean absorption coefficient at 370 nm (σ₃₇₀) was (1.46 ± 0.56) × 10⁻⁴, (1.84 ± 0.42) × 10⁻⁴, and (1.85 ± 1.8) × 10⁻⁴ at Tawang, New Palin, and Geku. Whereas daily mean absorption coefficient at 880 nm (σ₈₈₀) was (0.33 ± 0.08) × 10⁻⁴, (0.42 ± 0.05) × 10⁻⁴, and (0.2 ± 0.17) × 10⁻⁴. The σ₃₇₀ was 126%, 125%, and 164% higher than the absorption at the longer wavelength (880 nm) which indicates a higher biomass burning BC. The average Absorption Ångström Exponent (AAE) at 370–880 nm were 1.59 ± 0.23, 1.63 ± 0.23 and 2.28 ± 0.57 at Tawang, New Palin, and Geku. In addition, at Tawang, New Palin, and Geku, the average AAE in the shorter spectral range (370–520 nm) were 2.43 ± 0.97, 2.41 ± 1.22 and 3.09 ± 1.54, whereas in longer spectral range (520–880 nm) were 1.05 ± 0.05, 1.13 ± 0.18 and 1.44 ± 0.33, respectively.