A Method to Derive an Aerosol Composition from Downward Solar Spectral Fluxes at the Surface

Abstract

Aerosol properties are highly variable in space and time which makes the aerosol study more complex. The sources and production mechanism of aerosols decide the properties of the aerosols. Aerosol radiative forcing is defined as the perturbation to the radiative fluxes of the earth atmosphere system caused by the aerosols. High uncertainty in the aerosol radiative forcing values exists today due to the lack of the exact chemical composition data of the aerosols everywhere. There are previous studies which have introduced methods to estimate ‘optical equivalent’ composition of aerosols using spectral aerosol optical depth measurements at the surface. The impact of aerosols on the solar radiative flux depends on its size distribution and composition. Hence, measurements of downward solar spectral fluxes at the surface can be used to infer ‘optically equivalent’ composition of aerosols. Measurements of downward solar spectral flux at Bangalore were made on clear days using a spectroradiometer. This data has been used to infer the aerosol composition following an iterative method with the help of the Santa Barbara DISORT Atmospheric Radiative Transfer (SBDART). Aerosols have been classified as water soluble, black carbon and three types of dust. Influence of the different aerosol types on spectral down welling irradiance at the surface have been simulated using Optical Properties of Aerosols and Clouds (OPAC) and SBDART models. The strong spectral dependence influence of water soluble aerosols and the dust aerosols on the spectral irradiance is shown. Aerosol composition was inferred following least square error minimization principle. This method can be used to estimate near-surface aerosol concentration if the vertical profile of aerosols is known a priori. This method also enables derivation of spectral single scattering albedo. The aerosol spectral radiative forcing has been estimated using downward spectral flux at the surface and compared with modeled fluxes. The contribution to the total forcing by the wavelength band 360 – 528 nm is around 60% of the total forcing. The wavelength band of 453-518 nm contributes maximum to the total forcing and it is seen that the shape of the spectral forcing is a major function of shape of the incoming solar spectrum. Aerosol spectral radiative forcing from observations of radiative fluxes agreed with modeled values when derived aerosol chemical composition was used as input. This study demonstrates that spectral flux measurements at the surface are useful to infer aerosol composition (which is optically equivalent) when and where the conventional chemical analysis is unavailable.