Radiative characteristics of near-surface aerosols at a tropical site: An estimation based on concurrent measurements of their physico-chemical characteristics

Abstract

This study is an attempt to estimate the radiative characteristics of aerosols, namely, the scattering coefficient (βsc), absorption coefficient (βab), extinction coefficient (βex), single scattering albedo (ω) and the phase function P(θ), on a seasonal basis, incorporating the concurrent measurements of aerosol mass loading, size distribution and chemical composition at the tropical coastal site, Thiruvananthapuram. The software package Optical Properties of Aerosols and Clouds (OPAC) has been made use for the estimation of the radiative parameters. This paper presents the seasonal features of aerosol chemical composition and their source characteristics also. Along with this, the association between size-resolved number density of aerosols and their chemical characteristics were also investigated through correlation analysis. The location is significantly influenced by human activities as seen from the dominance of the anthropogenic component which is highest in winter (22%) with comparable values in pre-monsoon and post-monsoon and minimum in monsoon (13%). The sea-salt contribution is found to peak in monsoon (~40%) and attain a minimum in winter. The source characterization using principal component analysis along with back-trajectory analysis showed the seasonally changing mixed aerosol sources over the region. Accordingly, the radiative properties of aerosols also exhibit significant seasonal variations. βsc varied from 0.04 to 0.14 km−1 and βab between 0.01 and 0.05 km−1 over a year. The single-scattering albedo exhibited significant seasonal differences being ~0.71 for winter and ~0.89 (0.55 μm) for monsoon season, indicating the presence of more absorbing aerosols in winter.