Articles | Volume 16, issue 3
Atmos. Chem. Phys., 16, 1587–1602, 2016
Atmos. Chem. Phys., 16, 1587–1602, 2016

Research article 11 Feb 2016

Research article | 11 Feb 2016

Remote sensing of soot carbon – Part 2: Understanding the absorption Ångström exponent

G. L. Schuster1, O. Dubovik2, A. Arola3, T. F. Eck4,5, and B. N. Holben5 G. L. Schuster et al.
  • 1NASA Langley Research Center, Hampton, VA, USA
  • 2Laboratoire d'Optique Atmosphérique, Université de Lillé 1, CNRS, Villeneuve d'Ascq, France
  • 3Finnish Meteorological Institute, P.O. Box 1627, 70211 Kuopio, Finland
  • 4Universities Space Research Association, Columbia, MD, USA
  • 5NASA Goddard Space Flight Center, Greenbelt, MD, USA

Abstract. Recently, some authors have suggested that the absorption Ångström exponent (AAE) can be used to deduce the component aerosol absorption optical depths (AAODs) of carbonaceous aerosols in the AERONET database. This AAE approach presumes that AAE ≪ 1 for soot carbon, which contrasts the traditional small particle limit of AAE = 1 for soot carbon. Thus, we provide an overview of the AERONET retrieval, and we investigate how the microphysics of carbonaceous aerosols can be interpreted in the AERONET AAE product. We find that AAE ≪ 1 in the AERONET database requires large coarse mode fractions and/or imaginary refractive indices that increase with wavelength. Neither of these characteristics are consistent with the current definition of soot carbon, so we explore other possibilities for the cause of AAE ≪ 1. AAE is related to particle size, and coarse mode particles have a smaller AAE than fine mode particles for a given aerosol mixture of species. We also note that the mineral goethite has an imaginary refractive index that increases with wavelength, is very common in dust regions, and can easily contribute to AAE ≪ 1. We find that AAE ≪ 1 can not be caused by soot carbon, unless soot carbon has an imaginary refractive index that increases with wavelength throughout the visible and near-infrared spectrums. Finally, AAE is not a robust parameter for separating carbonaceous absorption from dust aerosol absorption in the AERONET database.

Short summary
Some authors have recently suggested that the spectral dependence of aerosol absorption may be used to separate soot carbon absorption from the aerosol absorption associated with organic carbon and dust. We demonstrate that this approach is inconsistent with the underlying assumptions that are required to infer aerosol absorption through remote sensing techniques, and that carbonaceous aerosols can not be differentiated from dust by exclusively using spectral absorption signatures.
Final-revised paper