Effect of aerosol types from various sources at an urban location on spectral curvature of scattering and absorption coefficients

Abstract

This study analyzes the spectral characteristics of in situ measured aerosol properties, namely scattering, absorption and single scattering albedo (SSA), explicitly examining the curvature characteristics of their wavelength dependence. Measurements were performed at an urban background site in Athens, Greece, during December 2017 and March 2018 characterized by contrasting aerosol sources and types, i.e. residential wood burning (RWB) and desert dust, respectively, which modulate the urban conditions, that are typically dominated by fossil-fuel combustion. Aerosol types were classified via the Scattering Ångström Exponent (SAE) vs. Absorption Ångström Exponent (AAE) matrix, with the black carbon (BC)-dominated type displaying the highest fraction (~40%) in both periods. The spectral optical properties were examined for each aerosol type and month, revealing notable differences, which are closely linked to the contrasting emission sources, atmospheric dynamics and mixing processes. BC from fossil-fuel emissions and regional background aerosols mostly presented similar characteristics in both months. Emphasis was given on the spectral curvature effect, since the different aerosol types display notable changes in the spectral dependence of scattering and absorption in logarithmic coordinates. The enhanced presence of brown carbon resulted in negative curvature for scattering (concave curves) and highly positive for absorption (convex curves), while the presence of BC was mostly translated in a better fit of the Ångström formula and small curvature effects. Intense dust events resulted in positive curvature for absorption and mostly negative for scattering. A scatterplot between the wavelength dependence of SSA and the scattering curvature may further differentiate key aerosol types i.e. BC, brown carbon and dust. This study highlights the curvature effects of the scattering, absorption and SSA, which have not been adequately addressed yet. The approach provides new insights in the differentiation of source-related aerosol types, although more analysis is needed to examine if findings are reproduced in other environments.