Impact of brown and clear carbon on light absorption enhancement, single scatter albedo and absorption wavelength dependence of black carbon

The presence of clear coatings on atmospheric black carbon (BC) particles is known to enhance the magnitude of light absorption by the BC cores. Based on calculations using core/shell Mie theory, we demonstrate that the enhancement of light absorption (E-Abs) by atmospheric black carbon (BC) when it is coated in mildly absorbing material (C-Brown) is reduced relative to the enhancement induced by non-absorbing coatings (C-Clear). This reduction, sensitive to both the C-Brown coating thickness and imaginary refractive index (RI), can be up to 50% for 400 nm radiation and 25% averaged across the visible radiation spectrum for reasonable core/shell diameters. The enhanced direct radiative forcing possible due to the enhancement effect of C-Clear is therefore reduced if the coating is absorbing. Additionally, the need to explicitly treat BC as an internal, as opposed to external, mixture with C-Brown is shown to be important to the calculated single scatter albedo only when models treat BC as large spherical cores (> 50 nm). For smaller BC cores (or fractal agglomerates) consideration of the BC and C-Brown as an external mixture leads to relatively small errors in the particle single scatter albedo of < 0.03. It has often been assumed that observation of an absorption Angstrom exponent (AAE)> 1 indicates absorption by a non-BC aerosol. Here, it is shown that BC cores coated in C-Clear can reasonably have an AAE of up to 1.6, a result that complicates the attribution of observed light absorption to C-Brown within ambient particles. However, an AAE < 1.6 does not exclude the possibility of C-Brown; rather C-Brown cannot be confidently assigned unless AAE > 1.6. Comparison of these model results to various ambient AAE measurements demonstrates that large-scale attribution of C-Brown is a challenging task using current in-situ measurement methods. We suggest that coincident measurements of particle core and shell sizes along with the AAE may be necessary to distinguish absorbing and non-absorbing OC.