A new mathematical treatment of light scattering by soot particles of fractal complexity suggests current models may instead underestimate its effects by about one sixth.
AEROSOL SCIENCE & TECHNOLOGY
Constraining the complex refractive index of black carbon particles using the complex forward-scattering amplitude
Nobuhiro Moteki, Sho Ohata, Atsushi Yoshida & Kouji Adachi
The complex refractive index, m = mr + imi, the primary determinant of the absorbed and scattered energies of incident radiation per unit volume of particulate material, has not been accurately known for atmospheric black carbon material. An accurate value at visible wavelengths has been difficult to obtain due to the black carbon’s wavelength-scale irregularity and variability of aggregate shape, distribution in particle size, and mixing with other aerosol compounds.
Here, we present a method to constrain a plausible (mr, mi) domain for black carbon from the observed distribution of the complex forward-scattering amplitude S(0°). This approach suppresses the biases due to the above-mentioned complexities. The S(0°) distribution of black carbon is acquired by performing single particle S(0°) measurements in a water medium after collecting atmospheric aerosols into water. We demonstrate the method operating at λ = 0.633 μm for constraining the refractive index of black carbon aerosols in the north-western Pacific boundary layer. From the plausible (mr, mi) domain consistent with the observed S(0°) distributions and the reported range of mass absorption cross-section, we conservatively select 1.95 + 0.96i as a recommendable value of the refractive index for uncoated black carbon at visible wavelengths.
Figure 2. Examples of the geometric arrangement of spherical monomers in the AGGREGATE and SPHPACK models. (a) AGGREGATE with fractal dimension 2.0, (b) AGGREGATE with fractal dimension 2.5, (c) SPHPACK with packing density 0.05, (d) SPHPACK with packing density 0.30. The number of monomers Npp is 1024 in each example. The length unit of each Cartesian coordinate is the monomer radius.
The recommendable value is 0.17 larger in mi than the widely used value 1.95 + 0.79i in current aerosol-climate models, implying a ∼16% underestimate of shortwave absorption by black carbon aerosols in current climate simulations.
