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

Abstract

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.

Mie Scattering Captures Observed Optical Properties of Ambient Biomass Burning Plumes Assuming Uniform Black, Brown, and Organic Carbon Mixtures

“…A heterogeneous internal mixture is usually represented by a core containing BC surrounded by a shell of less absorbing material (e.g., Kanngiesser & Kahnert, ). Although this core‐shell model seems to be in agreement with our mental picture of the aged biomass burning aerosols, its disadvantage is an increasing number of free parameters, and results which do not seem to rank this model much better than a simple homogeneous sphere model (Cappa et al, ; China et al, ; Curci et al, ; Kahnert et al, ; Lesins et al, ; Schuster et al, ). Although theoretical method to obtain scattering characteristics of nonspherical particles and particle groups are available (e.g., He et al, ; Mishchenko, ; Mishchenko et al, ; Videen et al, Yang et al, ), we concentrate in the following analysis on the simple Mie scattering model.…”

Black and brown carbon over central Amazonia: Long-term aerosol measurements at the ATTO site

“…The light absorption of BC has a wavelength dependence that is conditioned by the BC mixing state, its size distribution and the composition of co-emitted particles Kirchstetter et al, 2004;Schuster et al, 2016). The wavelength dependence is described by the absorption Ångström exponent (å abs ) (Ångström, 1929), which can vary from low values (å abs = 1.0 ± 0.1, weak spectral dependence), usually associated to fossil fuel emitted BC (Bond and Bergstrom, 2006), up to high values (å abs = 6-7, strong spectral dependence) for organic-rich aerosol, e.g., humic-like substances (HULIS) (Hoffer et al, 2006).…”

Multiyear Ground‐Based Measurements of Aerosol Optical Properties and Direct Radiative Effect Over Different Surface Types in Northeastern China

“…The AERONET algorithm retrieves the size distribution and complex refractive indices under the assumption of internal homogenous mixing for all particles in the atmosphere. This assumption could introduce a bias in the estimates of AAE especially in urban regions as discussed in Schuster, Dubovik, and Arola () and Schuster, Dubovik, Arola, Eck, et al ().…”

“…The highest uncertainty in the AAEs is for size distributions dominated by coarse mode particles with the lowest AAE values. Schuster, Dubovik, and Arola () and Schuster, Dubovik, Arola, Eck, et al () have concluded that the AAE value is not useful for discriminating dust from carbonaceous aerosols. Acknowledging that there is still some controversy about using AAE = 1 as a way to distinguish among carbonaceous particles, we recognized the need to obtain more information about the AAEs of BC and other particles in further studies.…”