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

Abstract

The presence of clear coatings on atmospheric black carbon (BC) particles is known to enhance the magnitude of light absorption by theBCcores. Based on calculations using core/shell Mie theory, we demonstrate that the enhancement of light absorption (EAbs) by atmospheric black carbon (BC) when it is coated in mildly absorbing material (CBrown) is reduced relative to the enhancement induced by non-absorbing coatings (CClear). This reduction, sensitive to both theCBrown 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 ofCClear is therefore reduced if the coating is absorbing. Additionally, the need to explicitly treatBCas an internal, as opposed to external, mixture withCBrown is shown to be important to the calculated single scatter albedo only when models treatBCas large spherical cores (>50 nm). For smallerBCcores (or fractal agglomerates) consideration of theBCandCBrown 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 Angström exponent (AAE)>1 indicates absorption by a non-BCaerosol. Here, it is shown thatBCcores coated inCClear can reasonably have anAAEof up to 1.6, a result that complicates the attribution of observed light absorption toCBrown within ambient particles. However, anAAE<1.6 does not exclude the possibility ofCBrown; ratherCBrown cannot be confidently assigned unlessAAE>1.6. Comparison of these model results to various ambientAAEmeasurements demonstrates that large-scale attribution ofCBrown is a challenging task using current in-situ measurement methods. We suggest that coincident measurements of particle core and shell sizes along with theAAEmay be necessary to distinguish absorbing and non-absorbing OC. © Author(s) 2010.