Estimation of Aerosol Complex Refractive Index over a tropical atmosphere using a synergy of in-situ measurements

Abstract

Aerosol complex refractive index (ACRI) is a key micro-physical parameter used for modeling their radiative effects. In this study, an approach is developed to estimate the ACRI using a combination of surface-based measurements (2008–2015) from the Aerodynamic Particle Sizer (APS), the integrating Nephelometer, and the Aethalometer over a tropical rural location in the southern peninsular India. Around 98% of the real part of the estimated ACRI varied in the range of 1.3–1.8 while 99% of the imaginary part was found to vary in the range of 0.01–0.2 at 550 nm. The estimated imaginary parts are found to be high during pre-monsoon season suggesting the dominance of the increased absorption type aerosols. This is in line with the high concentration of black carbon aerosol observed during the pre-monsoon. The spectral variation of estimated ACRI is also studied and finally the significance of refractive index of surface aerosols in columnar properties is analysed by using the columnar single scattering albedo (SSA) and aerosol optical depth (AOD) observations from Sky radiometer. During winter season, the contribution to the total columnar scattering and absorption are mainly from surface or below the atmospheric boundary layer aerosols, whereas in monsoon season 80–90% contribution is from aerosols above.