Spectral Derivatives of Optical Depth for Partitioning Aerosol Type and Loading

Tang-Huang Lin,Wei-Hung Lien,Neng-Huei Lin,Ta-Chih Hsiao,Si-Chee Tsay

doi:10.3390/rs13081544

Tang-Huang Lin, Wei-Hung Lien + Show 3 more

Open Access

https://doi.org/10.3390/rs13081544

Copy DOI

Abstract

Quantifying aerosol compositions (e.g., type, loading) from remotely sensed measurements by spaceborne, suborbital and ground-based platforms is a challenging task. In this study, the first and second-order spectral derivatives of aerosol optical depth (AOD) with respect to wavelength are explored to determine the partitions of the major components of aerosols based on the spectral dependence of their particle optical size and complex refractive index. With theoretical simulations from the Second Simulation of a Satellite Signal in the Solar Spectrum (6S) model, AOD spectral derivatives are characterized for collective models of aerosol types, such as mineral dust (DS) particles, biomass-burning (BB) aerosols and anthropogenic pollutants (AP), as well as stretching out to the mixtures among them. Based on the intrinsic values from normalized spectral derivatives, referenced as the Normalized Derivative Aerosol Index (NDAI), a unique pattern is clearly exhibited for bounding the major aerosol components; in turn, fractions of the total AOD (fAOD) for major aerosol components can be extracted. The subtlety of this NDAI method is examined by using measurements of typical aerosol cases identified carefully by the ground-based Aerosol Robotic Network (AERONET) sun–sky spectroradiometer. The results may be highly practicable for quantifying fAOD among mixed-type aerosols by means of the normalized AOD spectral derivatives.

Highlights

As the Ångström exponent showed, the spectral gradient was primarily related to the particle size information, indicating that the radius of DS particles was much greater than the counterparts of anthropogenic pollutants (AP) and BBs
It is worthy of notice that the gradient of spectral aerosol optical depth (AOD) could vary with aerosol loading (AOD(0.55μm) value of 0.4, 0.8, 1.2, 1.6 and 2.0) even for the same type
AOD derivatives are shown to be highly sensitive to the aerosol particle size distribution and complex index of refraction (SSA)

Summary

Introduction

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. The Fifth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC, 2013) stated that the variation (−0.85 to +0.15 Wm−2 ) of aerosol radiative forcing is relatively significant compared with its counterpart (2.54 to 3.12 Wm−2 ) of well-mixed greenhouse gases [1]. The large fluctuation in radiative forcing (RF) is primarily related to the poor characterization of the microphysical and optical properties of atmospheric aerosols [2,3]. These results suggest that each type of aerosol does not weigh with regard to the total amount of RF. Black carbon (the main component of biomass burning (BB)) exhibits a positive RF (0.0 ± 0.2 Wm−2 ) in the atmosphere, while the RFs of sulfate and nitrate (the main components of anthropogenic pollutants (AP))

Methods

Results

Conclusion